8.4 Water8.4 Water

Focus 1: Water is distributed Focus 1: Water is distributed on Earth as a solid, liquid and on Earth as a solid, liquid and gasgas

The significance of water on The significance of water on EarthEarth• Necessary for living thingsNecessary for living things

– A transport medium for A transport medium for nutrients in cellsnutrients in cells

– A raw material for plants in A raw material for plants in photosynthesisphotosynthesis

– A solvent for nutrients and A solvent for nutrients and O2 in bloodO2 in blood

– A solvent for wastes (e.g. A solvent for wastes (e.g. CO2, sweat)CO2, sweat)

• A natural resourceA natural resource– Drinking and food Drinking and food

preparationpreparation– Washing of clothes, dishes, Washing of clothes, dishes,

etc.etc.– Irrigation of cropsIrrigation of crops– RecreationRecreation– Transportation (ferries)Transportation (ferries)– HydroelectricityHydroelectricity

• Alters landformsAlters landforms– Moving water in rivers Moving water in rivers

forms canyonsforms canyons– Water can erode rocks by Water can erode rocks by

dissolving mineralsdissolving minerals– Freezing water expands in Freezing water expands in

small cracks creating small cracks creating fragmentsfragments

– Glaciers slowly change the Glaciers slowly change the landscapes from mountains landscapes from mountains to the seato the sea

• A habitat for lifeA habitat for life– A place where aquatic flora A place where aquatic flora

and fauna live (e.g. fish, and fauna live (e.g. fish, coral, algae, coral, algae, phytoplankton)phytoplankton)

– There is much less There is much less fluctuation in temperature fluctuation in temperature within an aquatic within an aquatic environment than a environment than a terrestrial habitat. This is terrestrial habitat. This is due to the high heat due to the high heat capacity of watercapacity of water

Distribution on EarthDistribution on Earth

• Biosphere Biosphere – Liquid (as a solvent for nutrients, etc.)Liquid (as a solvent for nutrients, etc.)– 60-90% in most living things (50-75% in humans)60-90% in most living things (50-75% in humans)

• LithosphereLithosphere– Solid, liquid or chemically bound as waters of Solid, liquid or chemically bound as waters of

hydration (e.g. CaSOhydration (e.g. CaSO44..2H2H22O)O)

– Variable percentages in groundwater, aquifers and Variable percentages in groundwater, aquifers and rocksrocks

• HydrosphereHydrosphere– Liquid and solidLiquid and solid– Approx. 95% in the oceans and greater in lakes, Approx. 95% in the oceans and greater in lakes,

rivers and the polar icecapsrivers and the polar icecaps• AtmosphereAtmosphere

– Gas and liquid dropletsGas and liquid droplets– 0-5% in the air, variable depending upon 0-5% in the air, variable depending upon

environmentenvironment

SolutionsSolutions

• SolutionsSolutions are homogeneous mixtures are homogeneous mixtures that contain a solvent and solute.that contain a solvent and solute.– SolventSolvent: a substance that dissolves a : a substance that dissolves a

solute in a solution.solute in a solution.– SoluteSolute: a substance that is dissolved by : a substance that is dissolved by

a solvent in a solutiona solvent in a solution

• Examples:Examples:– Sea water (solvent-water; solute-salt)Sea water (solvent-water; solute-salt)– Blood (solvent-water; solutes-oxygen, Blood (solvent-water; solutes-oxygen,

nutrients)nutrients)

DensityDensityThe density of any substance is The density of any substance is

defined as:defined as:

Most substances contract as they Most substances contract as they coolcool due to a decrease in kinetic due to a decrease in kinetic energy of the particles. This energy of the particles. This generally leads to an increase generally leads to an increase in density.in density.

Water behaves differentlyWater behaves differently. As the . As the temperature of water decreases temperature of water decreases to around 5to around 500C, the H-bonds C, the H-bonds arrange themselves so that arrange themselves so that there is more space between there is more space between water molecules. This means water molecules. This means that the density drops until solid that the density drops until solid ice is formed.ice is formed.

The lower density of ice means The lower density of ice means aquatic organisms can survive aquatic organisms can survive under floating sheets of ice.under floating sheets of ice.

Density of water

0.90000.91000.92000.93000.94000.95000.96000.97000.98000.99001.00001.0100

-20 0 20 40 60 80 100 120

Temperature (0C)

De

ns

ity

(g

/mL

)

Density Density ==

MassMassvolumevolume

There is a sharp decrease in the density of water due to the rigid hexagonal shape that is formed when water freezes, leaving more space between molecules.

8.4 Water8.4 Water

Focus 2: The wide distribution and importance Focus 2: The wide distribution and importance of water on Earth is a consequence of its of water on Earth is a consequence of its molecular structure and hydrogen bondingmolecular structure and hydrogen bonding

Property ComparisonsProperty Comparisons

WaterWater AmmoniaAmmonia Hydrogen Hydrogen SulfideSulfide

H HH

Nxo

oo

xo xoOHH

xo xo

oo oo

SHH

xo xo

oo oo

Construct Lewis dot structures of water, ammonia, and hydrogen sulfide to identify the distribution of electrons

The structure of water can be understood by comparing it with some isoelectronic molecules (i.e. the same number of electrons)

Property ComparisonsProperty Comparisons

WaterWater AmmoniaAmmonia Hydrogen Hydrogen SulfideSulfide

Shape of Shape of moleculemolecule

bentbent pyramidalpyramidal bentbent

m.p./b.p.m.p./b.p.((00C)C)

0/1000/100 -78/-33-78/-33 -86/-60-86/-60

Construct Lewis dot structures of water, ammonia, and hydrogen sulfide to identify the distribution of electrons

Hydrogen BondingHydrogen Bonding

Hydrogen bondingHydrogen bonding occurs between occurs between hydrogen atoms and hydrogen atoms and unshared pairs of unshared pairs of electrons on electrons on NN, , OO or or FF atoms. atoms.

This results in an This results in an unequal sharing of unequal sharing of electrons leading to a electrons leading to a partial positive charge partial positive charge on the H atom.on the H atom.

These bonds are These bonds are stronger than dipole-stronger than dipole-dipole forces and dipole forces and dispersion forces.dispersion forces.

H HH

Nxo

oo

xo xo

H HH

Nxo

oo

xo xo Hydrogen bond

OH

Hxo

xo

oooo

OHH

xo xo

oo oo

Hydrogen bond

ammoniaammonia

waterwater

Dipole-dipoleDipole-dipole

Previously, we defined electronegativity as an atoms tendency to attract electrons to Previously, we defined electronegativity as an atoms tendency to attract electrons to itself. The top right of the periodic table has the highest values. itself. The top right of the periodic table has the highest values. Fluorine is the Fluorine is the most electronegative atom.most electronegative atom.

Polar covalent bondingPolar covalent bondingWhen two atoms that have a difference in electronegativity bond with each other, When two atoms that have a difference in electronegativity bond with each other,

the one with the higher value tends to have a partially negative charge due to the one with the higher value tends to have a partially negative charge due to the stronger attraction to the electrons. For the same reason, the other atom will the stronger attraction to the electrons. For the same reason, the other atom will tend to have a partially positive charge. This bond is said to be tend to have a partially positive charge. This bond is said to be polar covalent.polar covalent.

For example, Cl is more electronegative than H. This means that Cl will have a slight For example, Cl is more electronegative than H. This means that Cl will have a slight negative charge given the symbol negative charge given the symbol δδ (Greek letter ‘delta’) (Greek letter ‘delta’) meaning ‘small’ or meaning ‘small’ or ‘slight’.‘slight’.

δδ++ H --- ClH --- Cl δδ--

Chlorine is partially negative due to a higher attraction to e-

Hydrogen is partially positive due to a lower attraction to e-

Dipole-dipoleDipole-dipole

Net dipoleNet dipole

An HCl molecule has an obvious negative end and positive end. However, An HCl molecule has an obvious negative end and positive end. However, not all molecules with polar covalent bonds are polar overall, or have a not all molecules with polar covalent bonds are polar overall, or have a net dipole. net dipole. Some of these molecules contain polar bonds that cancel out Some of these molecules contain polar bonds that cancel out the effect of any single polar bond. For example, BeFthe effect of any single polar bond. For example, BeF22 : :

δδ-- F Be F δδ--2δδ++ This molecule contains two

polar covalent bonds, but no overall net dipole.

- + + -- + + -

Questions:Questions:1. Draw out the structures for water, ammonia and beryllium

trifluoride. 2. Indicate any polar bonds with the arrow head pointing towards

the negative end.3. Which of these molecules has a net overall dipole?

Dipole-dipoleDipole-dipole

HH

OO

HH

22δδ--

δδ++ δδ++

33δδ--

NN

HH

HH HHδδ++

δδ++

δδ++

BeBe

FF

FF FF

33δδ--

δδ++

δδ++δδ++

Net dipole towards O Net dipole towards N No net dipole

AnswersAnswers

Dipole-dipoleDipole-dipole

Dipole-dipole interactionsDipole-dipole interactions

When molecules have an overall net dipole, the interactions between When molecules have an overall net dipole, the interactions between them is largely electrostatic. As one might expect, the positive end of them is largely electrostatic. As one might expect, the positive end of one of these molecules is attracted to negative end of another one of these molecules is attracted to negative end of another molecule.molecule.

If we imagine a polar molecule as a positive and negative end, the If we imagine a polar molecule as a positive and negative end, the interaction between molecules becomes simple to visualise.interaction between molecules becomes simple to visualise.

Note: dipole-dipole interactions are generally not as strong as hydrogen bonds

+ -+ - + -+ - + -+ - + -+ -

Explaining the properties of Explaining the properties of waterwater

Surface TensionSurface TensionAt the surface, water appears to have a ‘skin’ that resists At the surface, water appears to have a ‘skin’ that resists

deformation. This can be seen when water striders sit deformation. This can be seen when water striders sit on top of the water (see left). This is due to a property on top of the water (see left). This is due to a property known as surface tension. known as surface tension.

Water molecules are attracted to one another in all Water molecules are attracted to one another in all directions. This is known as directions. This is known as cohesive forces.cohesive forces. At the At the surface, the molecules can only be attracted to either surface, the molecules can only be attracted to either side and down. This results in an overall force in side and down. This results in an overall force in towards the liquid. This unbalanced force results in towards the liquid. This unbalanced force results in surface tensionsurface tension. This also explains why water forms . This also explains why water forms droplets as all of the force is directed inwards.droplets as all of the force is directed inwards.

Water molecules attract each other in all directions except at the surface leading to surface tension.

QuestionQuestion: What are adhesive forces and how do they help to explain a meniscus in a glass cylinder?

Explaining the properties of Explaining the properties of waterwaterViscosityViscosity

The resistance of a liquid to flow is The resistance of a liquid to flow is known as its viscosity.known as its viscosity.

Viscosity can be high if the molecules Viscosity can be high if the molecules of the liquid are quite large, for of the liquid are quite large, for instance in motor oil.instance in motor oil.

Viscosity is also directly related to Viscosity is also directly related to the strength of the intermolecular the strength of the intermolecular forces.forces.

Water has a relatively high viscosity Water has a relatively high viscosity due to strong hydrogen bonding.due to strong hydrogen bonding.

Melting/Boiling pointMelting/Boiling point

Generally, the larger the molecule, the Generally, the larger the molecule, the higher the melting and boiling higher the melting and boiling point.point.

Water has unusually high melting and Water has unusually high melting and boiling points relative to its low boiling points relative to its low molecular weight.molecular weight.

As stated previously, hydrogen bonds As stated previously, hydrogen bonds between water molecules are between water molecules are relatively strong.relatively strong.

Since melting and boiling involve Since melting and boiling involve breaking intermolecular forces, the breaking intermolecular forces, the strength of hydrogen bonds in strength of hydrogen bonds in water leads to higher relative water leads to higher relative melting and boiling points.melting and boiling points.

This explains why hydrogen sulphide This explains why hydrogen sulphide and other similar sized molecules and other similar sized molecules have much lower mp/bp. There are have much lower mp/bp. There are no hydrogen bonds. (See slide 8)no hydrogen bonds. (See slide 8)

8.4 Water8.4 Water

Focus 3: Water is an important Focus 3: Water is an important solventsolvent

Soluble ionic compoundsSoluble ionic compoundsSolubility of table salt (NaCl)Solubility of table salt (NaCl)Common table salt (NaCl) is made up of +ve Na ions and –ve Cl ions. Their Common table salt (NaCl) is made up of +ve Na ions and –ve Cl ions. Their

attraction to one another is what holds them together in a lattice.attraction to one another is what holds them together in a lattice.Water is polar and has positive and negative ends as well. Therefore, water is Water is polar and has positive and negative ends as well. Therefore, water is

attracted to the charged ions in the salt lattice. The water molecules attracted to the charged ions in the salt lattice. The water molecules surround the ions one at a time, overcoming the forces between the solute surround the ions one at a time, overcoming the forces between the solute particles until the salt is dissolved. This process is known as particles until the salt is dissolved. This process is known as dissociationdissociation where ions in a lattice are dispersed in a solvent. where ions in a lattice are dispersed in a solvent.

It is important to note that not all ionic compounds are soluble in water. More It is important to note that not all ionic compounds are soluble in water. More on this later.on this later.

-- --

-- -- --

--

-- --

---- --++++

++

++++

++

++

++++++

Water molecule

Cl ion

Na ion

Salt lattice

Soluble molecular Soluble molecular compoundscompounds

Solubility of SucroseSolubility of Sucrose

Sucrose is common table sugar and readily dissolves in water. As can Sucrose is common table sugar and readily dissolves in water. As can be seen below, the sucrose molecule contains many polar OH be seen below, the sucrose molecule contains many polar OH groups that attract water molecules. This breaks the crystal groups that attract water molecules. This breaks the crystal structure of the sugar and distributes the individual sugar structure of the sugar and distributes the individual sugar molecules through the water. Normally, the only soluble molecular molecules through the water. Normally, the only soluble molecular compounds are highly polar or can form hydrogen bonds with compounds are highly polar or can form hydrogen bonds with water.water.

Source: www.visionlearning.com/ library/module_viewer....

Note the polar OH groups that attract polar water molecules.

Sucrose

Soluble or partially soluble Soluble or partially soluble molecular molecular

elements/compoundselements/compoundsCovalent molecular elements and compounds are held together by either dipole-dipole Covalent molecular elements and compounds are held together by either dipole-dipole

interactions or dispersion forces. These are relatively weak compared to the hydrogen interactions or dispersion forces. These are relatively weak compared to the hydrogen bonds in water. These species are therefore generally insoluble or only slightly soluble.bonds in water. These species are therefore generally insoluble or only slightly soluble.

Solubility of iodineSolubility of iodine

Iodine (IIodine (I22) is a non-polar substance that are held together by weak dispersion forces compared ) is a non-polar substance that are held together by weak dispersion forces compared to water’s strong hydrogen bonds. For this reason, to water’s strong hydrogen bonds. For this reason, iodine is not very soluble in wateriodine is not very soluble in water. . Rather, it is more soluble in non-polar solvents such as liquid bromine (BrRather, it is more soluble in non-polar solvents such as liquid bromine (Br22).).

Solubility of oxygen gasSolubility of oxygen gas

Many gases such as OMany gases such as O22, N, N22 and CO and CO22 are non-polar and, therefore, are are non-polar and, therefore, are not very soluble in waternot very soluble in water. . In fact, oxygen has a solubility of 0.004 g in a 100 g of water.In fact, oxygen has a solubility of 0.004 g in a 100 g of water.

Solubility of hydrogen chlorideSolubility of hydrogen chlorideHydrogen chloride is a covalent molecular gas that is Hydrogen chloride is a covalent molecular gas that is highly soluble in waterhighly soluble in water. Hydrogen chloride . Hydrogen chloride

molecules undergo a process known as molecules undergo a process known as ionisationionisation because the HCl reacts with water to because the HCl reacts with water to produce H and Cl ions in solution. Acids produce Hproduce H and Cl ions in solution. Acids produce H++ ions and bases produce OH ions and bases produce OH-- ions in ions in water.water.

HClHCl(g)(g) H H++(aq)(aq) + Cl + Cl--(aq)(aq)

Whether a covalent substance is soluble in water is largely dependant upon whether it reacts Whether a covalent substance is soluble in water is largely dependant upon whether it reacts with water.with water.

Solubility of covalent Solubility of covalent networknetwork

Solubility of silicon dioxideSolubility of silicon dioxideCovalent network compounds Covalent network compounds

such as silicon dioxide such as silicon dioxide (sand) have very strong (sand) have very strong covalent bonds that hold covalent bonds that hold the atoms in a rigid lattice.the atoms in a rigid lattice.

The strong covalent bonds in The strong covalent bonds in silicon dioxide cannot be silicon dioxide cannot be broken by attraction to broken by attraction to water molecules. water molecules.

Therefore, Therefore, covalent network covalent network compounds such as silicon compounds such as silicon dioxide are not soluble in dioxide are not soluble in water.water.

Sand (mostly silicon dioxide) is insoluble in water. Source: www.thesand.net/ csalva/Index.htm

Solubility of large moleculesSolubility of large moleculesSolubility of celluloseSolubility of cellulose

Cellulose is a very large molecule that Cellulose is a very large molecule that contains repeating units of Ccontains repeating units of C66HH1010OO55. .

There are polar OH groups on this There are polar OH groups on this molecule, but the long chains lie molecule, but the long chains lie beside each other and the OH groups beside each other and the OH groups are involved in hydrogen bonding. are involved in hydrogen bonding. Therefore, the OH groups are not Therefore, the OH groups are not accessible to the water molecules.accessible to the water molecules.

Solubility of polyethene Solubility of polyethene

Polyethene is a polymer, meaning that it Polyethene is a polymer, meaning that it has repeating units called monomers.has repeating units called monomers.

It is used to make plastic drink bottles and It is used to make plastic drink bottles and other containers.other containers.

The diagram below shows the repeating The diagram below shows the repeating unit for this molecule. unit for this molecule.

In general, the larger the molecule, the less likely it will be soluble in water unless there are a large number of accessible polar groups (i.e. OH, NH2, COOH)

Cellulose is insoluble in waterCellulose is insoluble in water..

Polyethene is non-polar and is Polyethene is non-polar and is therefore insoluble in water.therefore insoluble in water.

Water as a solvent Water as a solvent summarysummary

• Water is a polar moleculeWater is a polar molecule, with a slight negative charge on the , with a slight negative charge on the oxygen end and a slight positive charge on the hydrogen end.oxygen end and a slight positive charge on the hydrogen end.

• Like dissolves likeLike dissolves like – polar solvents (e.g. water) tend to dissolve – polar solvents (e.g. water) tend to dissolve polar solutes and non-polar solvents (e.g. hexane) tend to dissolve polar solutes and non-polar solvents (e.g. hexane) tend to dissolve non-polar solutes.non-polar solutes.

• DissolutionDissolution is a process where ions in a solid are dispersed in a is a process where ions in a solid are dispersed in a solvent such as water e.g. NaClsolvent such as water e.g. NaCl(s)(s) Na Na++

(aq)(aq) + Cl + Cl--(aq)(aq)

• IonisationIonisation is a process where a covalent molecule reacts with is a process where a covalent molecule reacts with water to form ions in solution e.g. HClwater to form ions in solution e.g. HCl(g)(g) H H++

(aq)(aq) + Cl + Cl--(aq)(aq)

• Covalent networkCovalent network molecules are highly insoluble in water due to molecules are highly insoluble in water due to strong covalent lattice structures.strong covalent lattice structures.

• Large moleculesLarge molecules tend to be insoluble in water unless they have a tend to be insoluble in water unless they have a large number of accessible polar sites.large number of accessible polar sites.

• Most non-polar moleculesMost non-polar molecules are insoluble or are only slightly soluble are insoluble or are only slightly soluble in water e.g. Oin water e.g. O22 and I and I22..

8.4 Water8.4 Water

Focus 4: The concentration of salts will Focus 4: The concentration of salts will vary according to their solubility, and vary according to their solubility, and precipitation can occur when the precipitation can occur when the ions of an insoluble salt are in ions of an insoluble salt are in solution togethersolution together

Solubility of ionic Solubility of ionic compoundscompounds

Not all ionic compounds are soluble in water. If the ionic bonds are Not all ionic compounds are soluble in water. If the ionic bonds are stronger than the interaction with water, the compound will be stronger than the interaction with water, the compound will be insolubleinsoluble

If two solutions of different ionic compounds are mixed, an insoluble If two solutions of different ionic compounds are mixed, an insoluble compound may form. This insoluble compound falls to the bottom of compound may form. This insoluble compound falls to the bottom of the container and is called a the container and is called a precipitateprecipitate. .

For example:For example:barium nitratebarium nitrate(aq)(aq) + sodium sulphate + sodium sulphate(aq)(aq) barium sulphate barium sulphate(s)(s) + sodium nitrate + sodium nitrate(aq)(aq)

(solution) (solution) (precipitate) (solution)

Ba2+

NO3-

SO42-

Na+

NO3-

Na+

BaSO4+ +

Activity: Write a net ionic equation for this reaction.Ba2+(aq) + SO4

2-(aq) BaSO4(s)

Solubility Rules for common Solubility Rules for common saltssalts

summarysummarySoluble Ionic compoundsSoluble Ionic compounds Important exceptionsImportant exceptions

Compounds containingCompounds containing NONO3-3- NoneNone

CC22HH33OO22- - (acetate)(acetate) NoneNone

ClCl-- Cmpds of AgCmpds of Ag++, Hg, Hg222+2+, Pb, Pb2+2+

BrBr-- Cmpds of AgCmpds of Ag++, Hg, Hg222+2+, Pb, Pb2+2+

II-- Cmpds of Ag Cmpds of Ag++, Hg, Hg222+2+, Pb , Pb 2+2+

SOSO442-2- Cmpds of SrCmpds of Sr2+2+, Ba, Ba2+2+, Hg, Hg22

2+,2+, Pb Pb2+2+

Insoluble Ionic compoundsInsoluble Ionic compounds Important exceptionsImportant exceptions

Compounds containingCompounds containing SS2-2- Cmpds of NHCmpds of NH44++, IA cations, Ca, IA cations, Ca2+2+, ,

BaBa2+2+

COCO332-2- Cmpds of NHCmpds of NH44

++, IA cations, IA cations

POPO443-3- Cmpds of NH Cmpds of NH44

++, IA cations, IA cations

OHOH-- Cmpds of IA cations, CaCmpds of IA cations, Ca2+2+, Ba, Ba2+2+

Some precipitation Some precipitation examplesexamples

Aluminium ion reacts with aqueous ammonia to produce a white Aluminium ion reacts with aqueous ammonia to produce a white gelatinous precipitate of Al(OH)gelatinous precipitate of Al(OH)33::Al Al 3+3+

(aq)(aq) + 3NH + 3NH3 (aq)3 (aq) + 3H + 3H22O O (aq)(aq) Al(OH) Al(OH)3(s)3(s) + 3NH + 3NH44+ +

(aq)(aq)

Lead iodideLead iodide Copper hydroxideCopper hydroxide

Activity: write out possible reactions that could have formed the two precipitates above.Activity: write out possible reactions that could have formed the two precipitates above.

Aluminium hydroxideAluminium hydroxide

EquilibriumEquilibriumWe normally think of chemical reactions as reactants forming products We normally think of chemical reactions as reactants forming products

(i.e. from left to right). However, sometimes the products can also (i.e. from left to right). However, sometimes the products can also react to form the reactants (i.e. right to left). These are known as react to form the reactants (i.e. right to left). These are known as reversible reactionsreversible reactions. We use a double arrow, such as the one below, . We use a double arrow, such as the one below, to indicate these reactions.to indicate these reactions.

In reversible reactions, both are taking place simultaneously. Once the In reversible reactions, both are taking place simultaneously. Once the forward and reverse reactions are happening at the same rate, we forward and reverse reactions are happening at the same rate, we have what is known as have what is known as dynamic equilibriumdynamic equilibrium. At this stage there are . At this stage there are no observable changes in the system.no observable changes in the system.

Essentially all reactions are somewhat reversible, but some favour one Essentially all reactions are somewhat reversible, but some favour one direction so strongly that they seem to go in one direction only.direction so strongly that they seem to go in one direction only.

Equilibrium is also possible where a vapour is in equilibrium with a liquid Equilibrium is also possible where a vapour is in equilibrium with a liquid or when a precipitate is in equilibrium with a saturated solution.or when a precipitate is in equilibrium with a saturated solution.

Saturated solution Saturated solution equilibriumequilibrium

Recall that a Recall that a saturated solutionsaturated solution is when the maximum amount of is when the maximum amount of solute is dissolved in a given amount of solvent.solute is dissolved in a given amount of solvent.

Solution equilibriaSolution equilibriaIn a saturated solution, the dissolved solute is in dynamic equilibrium In a saturated solution, the dissolved solute is in dynamic equilibrium

with any undissolved solid. Any excess solid that is added to the with any undissolved solid. Any excess solid that is added to the system will not affect the dynamic equilibrium. The rate of system will not affect the dynamic equilibrium. The rate of crystallisation and dissolution will be the same.crystallisation and dissolution will be the same.

Dissolved soluteDissolved solute

Undissolved soluteUndissolved solute

Saturated solutionSaturated solution

In a saturated In a saturated solution, dissolved solution, dissolved and undissolved and undissolved solute particles solute particles are in dynamic are in dynamic equilibrium.equilibrium.

Concentration of solutionsConcentration of solutions

For quantitative analysis it is important to know exactly how much solute is For quantitative analysis it is important to know exactly how much solute is dissolved in a given amount of solution. This is known as the dissolved in a given amount of solution. This is known as the concentrationconcentration of the solution. There are many ways of expressing concentration of the solution. There are many ways of expressing concentration depending upon the nature of the solution and the information that is depending upon the nature of the solution and the information that is available.available.

Some ways of expressing concentration are:Some ways of expressing concentration are:• g/Lg/L mass per unit volume of solution (normally g/L) mass per unit volume of solution (normally g/L)• %(w/v)%(w/v) meaning percent weight per volume of solution (g/100mL) meaning percent weight per volume of solution (g/100mL)• %(v/v)%(v/v) meaning percent volume per volume of solution (mL/100mL) meaning percent volume per volume of solution (mL/100mL)• %(w/w)%(w/w) meaning weight percent (g/100g) meaning weight percent (g/100g)• ppm ppm or parts per million (g/million g solution)or parts per million (g/million g solution)• Molarity (M)Molarity (M) or moles per litre of solution or moles per litre of solution

Questions: Questions: 1.1. Which of these concentrations would be most useful when the solute is a Which of these concentrations would be most useful when the solute is a

liquid? liquid? 2.2. Which units would be most useful if the concentration was very low, such as Which units would be most useful if the concentration was very low, such as

heavy metal concentrations in drinking water?heavy metal concentrations in drinking water?3.3. What if we wanted to calculate quantities used in chemical reactions?What if we wanted to calculate quantities used in chemical reactions?

MolarityMolarity

Recall that molarity (M) (read as ‘molar’) is defined as:Recall that molarity (M) (read as ‘molar’) is defined as:

M = # moles of solute / L of solutionM = # moles of solute / L of solution or or M = n/VM = n/V

Remembering that the number of moles is:Remembering that the number of moles is:

Moles = mass / molar massMoles = mass / molar mass

We use molarity in the laboratory because the basic unit of measure We use molarity in the laboratory because the basic unit of measure in chemical reactions is the mole. We give concentrations for in chemical reactions is the mole. We give concentrations for solutions in solutions in mol/Lmol/L. Let’s consider some examples.. Let’s consider some examples.

Molarity examplesMolarity examples

Solution:  Note that in this particular example, where the number of moles of Solution:  Note that in this particular example, where the number of moles of solute is given, the identity of the solute (KBr) has nothing to do with solute is given, the identity of the solute (KBr) has nothing to do with solving the problem.solving the problem.

                                        # of moles of solute# of moles of soluteMolarity = ----------------------Molarity = ----------------------                  Liters of solution                   Liters of solution

Given:  # of moles of solute = 10.0 molesGiven:  # of moles of solute = 10.0 moles           Liters of solution = 5.00 liters           Liters of solution = 5.00 liters

                                    10.0 moles of KBr10.0 moles of KBrMolarity = --------------------------  = 2.00 MMolarity = --------------------------  = 2.00 M                 5.00 Liters of solution                 5.00 Liters of solution

Answer = 2.00 MAnswer = 2.00 M

Example 1.  What is the molarity of a 5.00 liter solution that was made Example 1.  What is the molarity of a 5.00 liter solution that was made with 10.0 moles of  KBr ?with 10.0 moles of  KBr ?

Molarity examplesMolarity examples

Solution:Solution:First find the molar mass of NaCl.First find the molar mass of NaCl.Na = 23.0 g x 1 ion per formula unit = 23.0 gNa = 23.0 g x 1 ion per formula unit = 23.0 gCl = 35.5 g x 1 ion per formula unit  = 35.5 gCl = 35.5 g x 1 ion per formula unit  = 35.5 g

                                                 ----------                                                 ----------                                                                                                    58.5 g58.5 g

Now find out how many moles of NaCl you Now find out how many moles of NaCl you have: have:

                                          mass of samplemass of sample# of moles = -----------------# of moles = -----------------                      Molar mass                      Molar mass

Given:  mass of sample = 526 gGiven:  mass of sample = 526 g           Molar mass = 58.5 g            Molar mass = 58.5 g

                                                                      526 g526 g# of moles of NaCl = ------------# of moles of NaCl = ------------                                 58.5 g                                 58.5 g

Answer:  # of moles of NaCl = Answer:  # of moles of NaCl = 8.99 moles8.99 moles  

Finally, go back to your molarity formula Finally, go back to your molarity formula to solve the problem:to solve the problem:

                                                            # of moles of solute# of moles of soluteLiters of solution =   --------------------Liters of solution =   --------------------                                     Molarity                                     Molarity

Given:  # of moles of solute = 8.99 molesGiven:  # of moles of solute = 8.99 moles Molarity of the solution = 3.0 M Molarity of the solution = 3.0 M (moles/L)(moles/L)

                                                                            8.99 moles8.99 moles# of Liters of solution = -------------# of Liters of solution = -------------                                   3.0 moles/L                                   3.0 moles/L

Final Answer = 3.0 LFinal Answer = 3.0 L

Example 2.  What is the volume of 3.0 M solution of NaCl made with 526g of solute?Example 2.  What is the volume of 3.0 M solution of NaCl made with 526g of solute?

DilutionsDilutions

It is often important in the laboratory to dilute solutions to obtain a desired It is often important in the laboratory to dilute solutions to obtain a desired concentration. This is known as a concentration. This is known as a dilutiondilution and can be accomplished using a and can be accomplished using a straightforward calculation.straightforward calculation.

MM11VV11 = M = M22VV22

Where MWhere M11 is the initial concentration in mol/L and V is the initial concentration in mol/L and V11 is the initial volume. M is the initial volume. M22 and Vand V22 are the final concentration and volume respectively. are the final concentration and volume respectively.

Example:Example:

What volume of 25M NHWhat volume of 25M NH33 is needed to prepare 500mL of 1M solution? is needed to prepare 500mL of 1M solution?

MM11VV11 = M = M22VV22

25mol/L X V1 = 1mol/L X 0.5L25mol/L X V1 = 1mol/L X 0.5LV1 = 0.02LV1 = 0.02LV1 = 20 mLV1 = 20 mL

8.4 Water8.4 Water

Focus 5: Water has a higher heat Focus 5: Water has a higher heat capacity than many other liquidscapacity than many other liquids

Specific Heat CapacitySpecific Heat CapacitySome materials resist changes in Some materials resist changes in

temperature more than others. Water temperature more than others. Water is a substance that has a high is a substance that has a high resistance to changes in temperature. resistance to changes in temperature.

This can be seen on a hot summer day. This can be seen on a hot summer day. The air temperature may increase to The air temperature may increase to over 40over 40OOC, while the temperature of a C, while the temperature of a large pool of water will rise only large pool of water will rise only slightly. slightly.

This resistance to changes in temperature This resistance to changes in temperature is due to water having a large is due to water having a large specific specific heat capacity (C). heat capacity (C). In other words, water In other words, water can absorb a large amount of heat can absorb a large amount of heat energy before changing temperature.energy before changing temperature.

Specific heat capacitySpecific heat capacity (C), is the amount of heat energy in Joules (J), (C), is the amount of heat energy in Joules (J), required to raise the temperature of 1g of a substance by 1 kelvin required to raise the temperature of 1g of a substance by 1 kelvin (K).(K).

On a hot summer day, water stays cool due to a high specific heat capacity.

Specific Heat CapacitySpecific Heat CapacityCompared to other solvents, the relatively high Compared to other solvents, the relatively high

specific heat of water makes it very useful in specific heat of water makes it very useful in the removal of heat energy.the removal of heat energy.

Radiator coolantRadiator coolantWater can absorb 10 times as much heat as cast Water can absorb 10 times as much heat as cast

iron and five times as much heat as iron and five times as much heat as aluminium. aluminium.

Ethylene glycol has a lower heat capacity than Ethylene glycol has a lower heat capacity than water, but it lowers the freezing point and water, but it lowers the freezing point and raises the boiling point of the coolant making it raises the boiling point of the coolant making it a useful additive to car cooling systems.a useful additive to car cooling systems.

SubstanceSubstance

Specific Specific Heat Heat CapacityCapacity

J KJ K-1-1gg-1-1 SubstanceSubstance

Specific Specific Heat Heat CapacityCapacity

J KJ K-1-1gg-1-1

WaterWater 4.184.18 EthanolEthanol 2.412.41

Ethylene Ethylene glycolglycol

2.392.39 HexaneHexane 2.262.26

50/50 50/50 water/E. water/E. glycolglycol

2.862.86 ChloroformChloroform 0.960.96

AcetoneAcetone 2.172.17 AluminiumAluminium 0.900.90Aquatic ecosystemsAquatic ecosystemsBodies of water maintain stable temperatures Bodies of water maintain stable temperatures allowing aquatic organisms to thrive. These allowing aquatic organisms to thrive. These organisms can only survive and reproduce organisms can only survive and reproduce within very narrow temperature ranges.within very narrow temperature ranges.

Living systems/ClimateLiving systems/ClimateThe water in living cells is used to regulate The water in living cells is used to regulate temperature in organisms due to waters high temperature in organisms due to waters high heat capacity and high thermal conductivity heat capacity and high thermal conductivity (ability to remove heat). Climates are (ability to remove heat). Climates are moderated by large bodies of water acting as moderated by large bodies of water acting as thermal reservoirs. thermal reservoirs.

Energy ChangesEnergy ChangesRecall that changes in temperature are related to changes in Recall that changes in temperature are related to changes in

the amount of heat energy that is being absorbed or the amount of heat energy that is being absorbed or released. This change in heat energy, or released. This change in heat energy, or enthalpyenthalpy (H), can (H), can be calculated using the following equation:be calculated using the following equation:

∆∆ H = H = mm C C ∆T∆T

Heat energy Heat energy released or absorbed released or absorbed in Joules (J)in Joules (J) Mass (g)Mass (g) Specific heatSpecific heat

Capacity Capacity

Change in Change in TemperatureTemperature(final – initial)(final – initial)

Example:Example:What quantity of energy is required to raise the temperature What quantity of energy is required to raise the temperature of 0.5L of water from 20of 0.5L of water from 2000C to 100C to 10000C?C?∆∆ H = H = mm C ∆T C ∆T = 500g X 4.18 J g= 500g X 4.18 J g-1-1KK-1-1 X 80 X 8000C or KC or K = 167,200 J= 167,200 J = 167 kJ= 167 kJ NB: NB: 00C = K - 273C = K - 273

Energy changes in chemical Energy changes in chemical reactionsreactions

In addition to heat changes in a particular substance, it is also possible to In addition to heat changes in a particular substance, it is also possible to measure the changes in heat content in chemical reactions. In a chemical measure the changes in heat content in chemical reactions. In a chemical reaction, the change in enthalpy is known as the heat of reaction:reaction, the change in enthalpy is known as the heat of reaction:

∆∆ HHrxnrxn = H = H (products)(products) – H – H (reactants)(reactants)

From this equation, we can see that there are two possibilities for the heat of reaction values. These are:

1. The enthalpy of the reactants is higher than the enthalpy of the products.

2. The enthalpy of the products is higher than the enthalpy of the reactants.

Considering the above information, Considering the above information, 1.1. Which will have a negative value and which will have a positive Which will have a negative value and which will have a positive

value.value.2.2. Which will result in a rise in temperature and which a fall in Which will result in a rise in temperature and which a fall in

temperature.temperature.

Energy changes in chemical Energy changes in chemical reactionsreactions

Exothermic reactionsExothermic reactions

The enthalpy of the reactants is The enthalpy of the reactants is higher than the products.higher than the products.

ReactantsReactants

ProductsProducts

Endothermic reactionsEndothermic reactions

The enthalpy of the reactants is The enthalpy of the reactants is lower than the products.lower than the products.

ProductsProducts

ReactantsReactantsEnth

alp

y (

H)

Enth

alp

y (

H)

∆∆ H is -ve ∆∆ H is +veHeat releasedHeat released Heat absorbedHeat absorbed

Reaction progression Reaction progression

CalorimetryCalorimetryWhat is calorimetryWhat is calorimetryWater can be used to measure the change in Water can be used to measure the change in

heat energy in a chemical reaction due heat energy in a chemical reaction due to its ability to absorb heat. This is to its ability to absorb heat. This is known as known as calorimetrycalorimetry..

A A calorimetercalorimeter is a device that is used to is a device that is used to measure the enthalpy change that measure the enthalpy change that occurs during a chemical reaction. occurs during a chemical reaction.

Measuring heat in the laboratoryMeasuring heat in the laboratory

If a known quantity of water is placed in the If a known quantity of water is placed in the calorimeter and a reaction is carried out, calorimeter and a reaction is carried out, the change in temperature due to the the change in temperature due to the reaction is transferred to the water and is reaction is transferred to the water and is measured using a thermometer placed in a measured using a thermometer placed in a hole in the lid.hole in the lid.

The heat of reaction is: The heat of reaction is:

∆∆ HHrxnrxn = - = -mm C ∆T C ∆T

The amount of heat lost or gained in the The amount of heat lost or gained in the reaction is equal in size but opposite in sign reaction is equal in size but opposite in sign to the amount of heat lost or gained by the to the amount of heat lost or gained by the water, thus the negative sign in the above water, thus the negative sign in the above equation. equation.

Coffee cup calorimeterCoffee cup calorimeterThe The simplest calorimetersimplest calorimeter makes use of two makes use of two polystyrene (Styrofoam) cups, one inside polystyrene (Styrofoam) cups, one inside the other with a lid on top. This minimises the other with a lid on top. This minimises the amount of heat lost to the the amount of heat lost to the surroundings. This is the most significant surroundings. This is the most significant source of error.source of error.

Heat of solutionHeat of solutionWhen an ionic substance dissolves, it may be endothermic (absorbs heat/gets When an ionic substance dissolves, it may be endothermic (absorbs heat/gets

cold) or it may be exothermic (releases heat/warms up). The change in heat cold) or it may be exothermic (releases heat/warms up). The change in heat (enthalpy) when one mole of a solute dissolves in a solvent is called the (enthalpy) when one mole of a solute dissolves in a solvent is called the molar heat of solution, molar heat of solution, ∆∆ HHsolnsoln. . The values for these dissolutions are given in The values for these dissolutions are given in kJ/mol.kJ/mol.

Energy changes involved in dissolutionEnergy changes involved in dissolution• Breaking the bonds that hold ions together requires energy (endothermic)Breaking the bonds that hold ions together requires energy (endothermic)• Forming bonds with water (hydration) releases energy (exothermic)Forming bonds with water (hydration) releases energy (exothermic)So, So, ∆∆ HHsoln soln = heat absorbed when breaking bonds – heat released when forming = heat absorbed when breaking bonds – heat released when forming

bondsbonds

Exothermic examplesExothermic examples (when the solvent-solute interactions are stronger)(when the solvent-solute interactions are stronger)NaOHNaOH(s)(s) CaCl CaCl2(s)2(s) H H22SOSO4(l)4(l) HCl HCl(g)(g)

Endothermic examplesEndothermic examples (when the solvent-solute interactions are weaker)(when the solvent-solute interactions are weaker)

NHNH44ClCl(s)(s) KCl KCl(s)(s) NH NH44NONO3(s)3(s)

CalorimetryCalorimetryexperimentexperiment

1.1. Prepare the coffee cup Prepare the coffee cup calorimeter by placing one calorimeter by placing one polystyrene cup inside another. polystyrene cup inside another. Trim off the top of a third cup to Trim off the top of a third cup to make a lid. (See diagram)make a lid. (See diagram)

2.2. Make two holes in the lid for the Make two holes in the lid for the thermometer and stirrer thermometer and stirrer just big just big enough to fit (no gaps).enough to fit (no gaps).

3.3. AccuratelyAccurately weigh 100mL of weigh 100mL of water and pour into the water and pour into the calorimeter. Record initial calorimeter. Record initial temperature.temperature.

4.4. AccuratelyAccurately weigh approximately weigh approximately 5g of reagent for dissolution.5g of reagent for dissolution.

5.5. Add the reagent to the Add the reagent to the calorimeter and calorimeter and immediatelyimmediately replace the lid and start stirring.replace the lid and start stirring.

6.6. Record the temperature every Record the temperature every 10 sec until no change is 10 sec until no change is observed.observed.

QuestionsQuestions1.1. Calculate the heats of solution (Calculate the heats of solution (∆∆

HHsolnsoln = = --mm C ∆T C ∆T) for each for each reagent in kJ/mol.reagent in kJ/mol.

2.2. Which dissolution was Which dissolution was endothermic/exothermic?endothermic/exothermic?

3.3. Explain why the sign is reversed Explain why the sign is reversed in the above calculation.in the above calculation.

4.4. What are the sources for error in What are the sources for error in this procedure? How could you this procedure? How could you reduce them? Calculate the % reduce them? Calculate the % error.error.

5.5. Find the accepted values for Find the accepted values for molar heats of solution and molar heats of solution and compare with the experimental compare with the experimental values.values.

ProcedureProcedure

CalorimetryCalorimetryexperiment experiment ((SolutionsSolutions))1.1. ∆∆ Hsoln = Hsoln = --mm C ∆T C ∆T

1.1. M = mass of waterM = mass of water2.2. C = specific heat of water (4.18 C = specific heat of water (4.18

J/g.K)J/g.K)3.3. ∆∆T = final temp – initial tempT = final temp – initial temp4.4. To calculate heat of solution, To calculate heat of solution,

multiply by the reagents molar massmultiply by the reagents molar mass2.2. Depends upon reagent (see table)Depends upon reagent (see table)3.3. Because we are measuring the quantity Because we are measuring the quantity

of heat change of the water and we of heat change of the water and we want to know the heat of solution for want to know the heat of solution for the reagent. The quantity is the same, the reagent. The quantity is the same, but the sign is reversed because as one but the sign is reversed because as one gives up heat the other absorbs the gives up heat the other absorbs the same quantity.same quantity.

4.4. The sources for error include any The sources for error include any potential for loss of heat to the potential for loss of heat to the surroundings (e.g. gaps, inadequate surroundings (e.g. gaps, inadequate insulation). These can be reduced by insulation). These can be reduced by reducing the gaps, using better reducing the gaps, using better insulating material.insulating material.

5.5. Accepted Heats of Solution (kJ/mol) for Accepted Heats of Solution (kJ/mol) for some substances in table at right.some substances in table at right.

CompoundCompound Heat of soln Heat of soln (kJ/mol)(kJ/mol)

calcium calcium chloride, chloride, anyhydrousanyhydrous

- 81.3- 81.3

sodium sodium hydroxide hydroxide

-44.0-44.0

sodium sodium chloridechloride

+3.9+3.9

ammonium ammonium chloridechloride

+ 14.8+ 14.8

ammonium ammonium nitratenitrate

+25.7+25.7

Thermal pollutionThermal pollution• When human activities significantly change the When human activities significantly change the

temperature of waterways, it is known as temperature of waterways, it is known as thermal pollutionthermal pollution. A significant change can be . A significant change can be as little as 2as little as 200C.C.

• The most common example of thermal The most common example of thermal pollution is the use of natural bodies of water pollution is the use of natural bodies of water for industrial cooling such as in power plants. for industrial cooling such as in power plants.

• Thermal pollution can have detrimental effects Thermal pollution can have detrimental effects on aquatic life.on aquatic life.– Solubility of oxygen decreases as temperature Solubility of oxygen decreases as temperature

increases. Less than 5 ppm oxygen can threaten increases. Less than 5 ppm oxygen can threaten the survival of fish. the survival of fish.

– Sharp increases in temperature may directly kill Sharp increases in temperature may directly kill fish and other aquatic organisms.fish and other aquatic organisms.

– High temperatures can prevent the High temperatures can prevent the development/hatching of fish eggs.development/hatching of fish eggs.

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Compiled by: Robert Slider (2006)Compiled by: Robert Slider (2006)

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