Chapter 13

Water as a Solvent

Water the Life Force• More than 70% of the Earth’s surface is

covered by water.

• Through the water cycle, water is recycled in the ocean and the atmosphere, and on the landmass.

• The human body also consists largely of water.

Physical Properties of Water• Water is a polar, discrete molecular compound.• At 25°C, water is a colourless, odourless liquid.• It freezes at 0°C and boils at 100°C• It is found in all forms – solid, liquid and gas• It evaporates at all temperatures, so there is much

water vapour in the Earth’s atmosphere• Water has many properties that have resulted in the

development of life on Earth in the form that we know it.

• It needs a great deal of energy to heat up (high melting temp 0°C and a high boiling temp 100°C and it holds its heat longer than most other substances

Latent Heat

When a substance changes phase energy is needed.

This energy comes from the surrounding atmosphere

The energy needed to change the phase of a substance at its melting or boiling temperature is called the latent heat of that substance

Latent Heat of Fusion

Latent heat of fusion of water is the amount of energy needed to change a fixed amount of water from a solid to liquid phase at 0°C.

The latent heat of fusion of water is 6.02kJ mol-1, so 6.02kJ of energy must be supplied for each mole of water to change from solid to liquid

Latent Heat of Vapourisation

The latent heat of vapourisation of water is the amount of energy needed to change a fixed amount of water from liquid to a gas at 100°C.

The remaining forces holding the water molecules together must be overcome to that the molecules may move around freely as a gas.

Latent Heat

Latent heat measures the amount of energy required to change the state of a given amount of substance.

The latent heat of fusion measures the change from solid to liquid, while the latent heat of evaporation measures the change from liquid to gas.

Water has a relatively high latent heat for both these processes.

Specific Heat Capacity

• The specific heat capacity of a substance is the amount of energy required to raise one gram of it by 1C.

• It is therefore a measure of how easy or how difficult it is to change the temperature of a substance.

• Water has a high specific heat capacity (4.15kJ g-1), which means that its temperature is difficult to alter.

DENSITY Water has an unusual property:

Its solid form, ice, is less dense than its liquid form.

The molecules in the solid form are arranged in a more structured and hexagonal way. This means that ice will float on water and that water will freeze from the surface down.

DENSITY

Electrical Conductivity

• A substance will conduct electricity only if it contains charged particles that are able to move.

• The covalent bonding in water, which results in the formation of neutral molecules, means that it should not conduct electricity – even though these molecules move freely in the liquid state.

• However, pure water does conduct electricity very slightly.• Water self-ionises

– H2O(l) + H2O(l) ---> H3O+(aq) + OH-(aq)

• The small extent of this reaction means that only a few ions are formed and that the resulting conductivity is virtually zero

Water As A Solvent

• Water is sometimes called the universal solvent, because it will dissolve so many different substances – in particular, salts

• Questions – – Why does water dissolve salts?– Why does oil not dissolve in water?– What substances will dissolve in water?

• The answers lie in the forces that hold the molecules of water and each of the other substances together.– Unequal sharing of electrons resulting in partial positive

and negative charges

• Review the properties of water

Water As A Solvent

Hydrogen Bonding• When a molecule consists of atoms with different

electronegativities, the bonding electrons are attracted toward the atoms with the stronger electronegativity

• The atom with the greater share of the bonding electrons will become slightly more negatively charged than the atom that had the lower electronegativity.

• Differences in charges at either end of a molecule produce a polar molecule

• These slight negative and positive regions cause the molecules to attract other polar molecules quite strongly

• The stronger the attraction, the closer the molecules can get, and the stronger the resulting intermolecular bond.

Water As A Solvent

Hydrogen Bonding• Hydrogen has only one electron, and when this

electron has moved toward the non-metal making up the molecule, there are no other electrons left to shield the hydrogen nucleus from the negative region it is approaching

• Such molecules can therefore attract other molecules very closely, resulting in a stronger bond than would be the case if hydrogen were not involved

Effects of Hydrogen Bonding on the Properties of Water

Hydrogen bonding significantly affects the physical properties of a substance

Relatively high melting and boiling points of water compared to other substances

Expansion of water upon freezing Ability of water to dissolve other

substances

Review

• Complete the revision questions 1, 2 page 295

Solubility

• Solubility measures the extent to which a solute dissolves in a solvent.

• Whether or not a solute dissolves can be predicted by the ‘like dissolves like’ rule.

• A polar solvent will generally dissolve a polar solute

• Non-polar substances usually dissolve in non-polar solvents

• Solubility• Solute and Solvent

Solubility• When a salt is placed in water and

dissolves, the positive and negative ions become surrounded by water molecules, which pull them apart.

• When the individual ions are surrounded by water, they are said to be hydrated ions

• For those about to dissolve

Solubility• Summary – behaviour of substances in water

– They may remain undissolved. These substances are insoluble

– They may dissociate. This means that the water will pull the particles from the solid structure, and they will then move apart and spread throughout the water to form a solution. These substances are soluble. Ions present become surrounded by water molecules. The resulting solution will conduct electricity

– The polar molecules may react with water to form ions. This is called ionisation, and is a feature of many acids and bases. Substances that ionise in water, conduct electricity

– They may themselves form hydrogen bonds with water. Sugar and ethanol are in this category. These solutions do not conduct electricity.

Solubility of solids in water• Heterogeneous mixtures – In chemistry, a mixture is when

two or more different substances are mixed together but not combined chemically. The molecules of two or more different substances are mixed in the form of mechanical mixtures, solutions, suspensions, and colloids. ...

• Homogeneous mixtures – solutions• Saturated solution – contains max amount of solute for the

volume of solution at a particular temperature• Unsaturated solution – less than the max amount of solute

added to the solvent• Supersaturate solution – prepared by slowly cooling a

saturated solution.• Aqueous solutions – water samples containing dissolved

substances

Solubility of solids in water

• Most salts are soluble in water to some extent.• Solubility is influenced by the temperature of the

solution. Most salts become more soluble at high temperature, but a few become less soluble as the temperature is increased

• Salts with a solubility greater than 0.1mol L -1 of water are classed as soluble and salts with a solubility less than 0.1 mol L -1 of water are classed as insoluble

• Solubility curve – solubility of a substance against temperature.

Rules for SolubilitySolubility Rules

1 All compounds containing alkali metal cations and the ammonium ion are soluble.

2 All compounds containing NO3-, ClO4

-, ClO3-, and C2H3O2

- anions are

soluble.3 All chlorides, bromides, and iodides are soluble except those containing

Ag+, Pb2+, or Hg22+.

4 All sulfates are soluble except those containing Hg22+, Pb2+, Sr2+, Ca2+, or

Ba2+.5 All hydroxides are insoluble except compounds of the alkali metals, Ca2+,

Sr2+, and Ba2+.6 All compounds containing PO4

3-, S2-, CO32-, and SO3

2- ions are insoluble

except those that also contain alkali metals or NH4+.

Review

• Complete the revision questions 3 – 8 pages 298, 299

Solubility of gases in water• Gases can also dissolve in water.• As a general rule, the solubility of gases decreases as

temperature increases• Polar gases dissolve in water easily (eg NH3 and HCl

gas – both composed of polar molecules)• Non-polar gases do not dissolve as easily (eg O2 and N2)

• An increase in pressure in the gas-water mixture causes more of the gas to dissolve in water.

• What happens when you remove the lid from a carbonated drink?

• An increase in temperature generally decreases the solubility of gases in liquids.

Mixing Solutions

• When solutions are mixed, chemical reactions may occur.

• Two common such reactions are neutralisation reactions and precipitation reactions

• Neutralisation and precipitation reactions are carried out in water – electrolysis

The concentration of substances

• Concentration is usually defined in terms of the amount of solute per volume of solvent.

• The greater the quantity of solute added to the solvent, the more concentrated the solution becomes.

• The concentration of a solution is a measure of how much solute has been dissolved in a certain volume of solvent.

• It is often expressed as a ratio of the solvent amount unit to the volume unit

• A number of different units therefore exist.

Gram conversions

• 1 g = 106 μg

• 1 μg = 10-6 g

• 1 mg = 10-3g

Volume Conversions

• 1 m3 = 1000 L = 1 000 000 mL

• 1 m3 = 103 L = 106 mL

• 1 mL = 10-3 L

Review

• Work through the sample problems pages 301, 302

• Complete the revision questions 9 – 11 pages 302, 303

Micrograms per Gram (ppm)

• When very small quantities of solute are dissolved to form a solution, the concentration can be measure in parts per million (ppm).

• This can be written as mg L -1 or μg g-1• Concentration of chemicals in an

environment and trace elements in the soil are often expressed in ppm

Review• Work through the sample problem page 303

• Complete the revision questions 12 - 14 page 304

• Work through Percentage by Mass page 304 and the sample problem

• Complete the revision questions 15 – 18 pages 304, 305

• Work through Percentage mass/volume (%w/v) and the sample problem page 305

• Complete the revision questions 19 – 22 page 305

• Work through Percentage by volume page 305 and the sample problem page 306

• Complete the revision questions 23 – 25 page 306

Water Quality

• The excellent solvent properties of water mean that it is difficult to obtain pure water and that it is also easily polluted.

Water Quality - Micro-organisms

• Micro-organisms (page 307) – how is nitrogen released back into the atmosphere?

• What do nitrifying bacteria do?• Where is Escherichia coli commonly found?

Water Quality - Chlorine

• Chlorine is added to water to kill bacteria

• Calcium hypochlorite reacts with carbon dioxide to form calcium carbonate and release chlorine:

• 2 Ca(ClO)2 + 2 CO2 → 2 CaCO3 + 2 Cl2 + O2

Water Quality – Heavy Metals

• Mercury, lead and cadmium are some of the heavy metals that pose a major disposal problem.

• Inorganic and organic lead compounds are extremely poisonous and are difficult to dispose of

• Many shellfish and other fish species accumulate heavy metals.

Can We Drink It?

• Pages 308, 309• Where can we get pure water?• Which minerals are often found in our

drinking water?

Improving Water QualityMethods of Desalination

• Desalination is the process of removing salt and other unwanted substances from sea water and waste water.

• It is anticipated that this will become an important method for producing water that is fit for humans to use in the future

• Two important methods for desalination are distillation and reverse osmosis.

Review

• Read the summary and note the important terminology

• Complete the multiple choice revision questions 1 – 16 pages 312, 313

• Complete the review questions 7, 8, 9, 11, 13, 15 page 314