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High School Chemistry Rapid Learning Series - 19

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HS Ch i t R id L i S i

Rapid Learning Centerwww.RapidLearningCenter.com/© Rapid Learning Inc. All rights reserved.

HS Chemistry Rapid Learning Series

Wayne Huang, PhDKelly Deters, PhDRussell Dahl, PhD

Elizabeth James, PhD



Learning Objectives

How solutions form.

F t ff ti l bilit

By completing this tutorial you will learn…

Factors affecting solubility.

Concentrations of solutions.

Calculations with solution concentrations.

Electrolyte solutions.

Colloids.

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Concept Map

Chemistry

Studies

Previous content

New content

Matter

Solutions

One type ofSolution FormationSolution

Formation

Solute & Solvent properties govern

Various factors affect

Solute & Solvent ratios described by

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Solubility Concentration

Dilutions Stoichiometry

affect

Used in calculations



Forming Solutions

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Definition: Solution, Solute & Solvent

Solution – Homogeneous mixture, solution = solute + solvent.

S l t S b t b i di l d t iSolute – Substance being dissolved, present in a smaller quantity (solute dissolves into solvent).Solvent – Substance (in a greater quantity) doing the dissolving.

Water is called the “Universal Solvent” because it’s used for so

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Solvent because it s used for so many solutions.

Solutions can be any combination of solids, liquids and gases!

Solute vs Solvent Mnemonic: Dissolve solute into solvent = “The police came, the thief hide the lute in the vent.”



Steps for Solution Formation

Intermolecular forces in the solvent are broken (“expanding the solvent”): Solvent - - X- - Solvent1

In order for a solution to form:

( p g )

Intermolecular forces in the solute are broken (“expanding the solute”): Solute - - X - - Solute

Solute and solvent particles form new intermolecular forces together: Solute –– Solvent

2

3

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intermolecular forces together: Solute –– Solvent

Energy of Solution FormationIntermolecular forces in the solvent are broken.

Intermolecular forces in the solute are broken.

1

2

Breaking IMF’s requires energy Forming IMF’s releases energy

Solute and solvent particles form new intermolecular forces together.3

If energy required >> energy released…

IMF – Intermolecular Force

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gy q gythe solution will not form.

If energy required similar or < energy released…

The solution will form.



Why Don’t Oil and Water Mix?Let’s look at the IMF’s broken and formed.

IMF’s Broken IMF’s Formed

Oil & W t t thWater—London Dispersion Oil & Water together—London Dispersion

Water—Dipole-Dipole

Water—Hydrogen bonding

Oil—London Dispersion

The two are more stable separate than together.

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4 IMF’s are broken.Only 1 IMF can be formed (and it’s the weakest kind).Much more energy would be required than released.Therefore, the solution doesn’t form.

“Like Dissolves Like”This general rule that can be followed for solution formation stems from:

If similarCompounds that

have similar properties and

bond characteristics

form similar intermolecular

forces.

If similar intermolecular forces can be

formed between 2 compounds as

within each compound, a

solution is likely to form.

Things with “Like” bond

types will dissolve other

things with “Like” bond

types.

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Things dissolves other things with “Like” bond properties (“Like dissolves Like”):Examples: Polar compounds (HCl) dissolve polar compounds (H2O). Non-polar (C5H12) dissolves non-polar (C6H6), etc.



Factor’s Affecting Solubility

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Pressure - 1How does pressure affect solubility of a gas in a liquid?

Gas molecules above the solvent cause pressure.

When a gas molecules comes into contact with the surface of the solvent, it can be “dissolved”.

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If th

Pressure - 2How does pressure affect solubility of a gas in a liquid?

If there are more gas particles above the solution (a higher pressure)…

More will come in t t ith th

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contact with the solvent and “dissolve”.

Higher pressure Higher solubility for gases

(Pressure has no affect on solids or liquids dissolving!)

Temperature-Dissolving Gases - 1When dissolving gases:

A ti l d hAs gas particles move around, when they reach the surface of the solution, they can “escape” if they have enough energy.

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Temperature-Dissolving Gases - 2When dissolving gases:

If ti l hIf gas particles have more energy (higher temperature)…

They will move faster, reach the surface more often, and have more energy to

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more energy to “escape” when they do reach the surface.

Higher temperature Lower solubility for gases

Dissolving GasesGases dissolve in a liquid best at:

High PressureHigh Pressure

Low Temperature

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Temperature—Dissolving SolidsFor most solids dissolving, increasing the temperature of the solvent:

Increase in solvent

temperature increases energy

of solvent particles.

Increased energy allows for more

IMF’s of the solvent to be broken (the

solvent is more “expanded”).

There is more “room” for

solute particles in between the

solvent particles.

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For most solids, as temperature increases, solubility increases.

Solubility CurvesThe solubility of a solid at different temperatures is shown in a solubility curve:

Solubility is barely affected

Solubility changes a lot as temperature is increased.

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barely affected by change in temperature.



Saturation of Solutions

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Definitions: Saturation

Unsaturated Solution – The solvent can still hold more solute particles.

More solute can dissolve.

solute particles.

Saturated Solution – The solvent is holding as many solute particles as it can possibly hold.

Supersaturated Solution – The

No more solute can dissolve.

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Supersaturated Solution The solvent is holding more solute particles (crystals) than it should be able to at that temperature.

More solute can crystallize.



How are Supersaturated Solutions Formed?

How can a solution hold more solute particles than it usually can at that temperature?

T t fSolution is saturated.

Temperature of solution is

raised.

Solution is no longer saturated

at the new temperature.

More solute

Solution is slowly and carefully cooled back

down.

Solute particles remain dissolved.

Solution is now

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More solute particles are

added.

supersaturated to form crystals.

Concentrations of Solutions

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Definition: Concentration

Concentration – Measure of the quantity of solute per quantity qua t ty o so ute pe qua t tyof solvent or total solution.

Common Units:Percent Composition by Mass (%)Molarity M (Moles of solute per volume

solution L)Molality m (Moles of solute in 1kg solvent)

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y ( g )Mole Fraction (ratio of moles in one

component over moles of all components)

Definition: Concentrated & Dilute

Concentrated – Large ratio of solute : solvent.

Dil t S ll ti f l t l tDilute – Small ratio of solute : solvent.

Do not use the terms “weak” or “strong” to describe concentrations -those words have very specific meanings in chemistry.

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Percent by MassTo determine concentration in % by mass:

100% ×=solutionmasssolutemassmass The mass units must

match!

Example: A sample contains 1.25 g NaCl in 100 mL of water. (1 mL water = 1 g water). Determine the % by mass of the solution.

Mass solute = 1.25 g NaClMass solution = 1.25 g NaCl + 100 g water = 101.25 g% by mass = ?

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% by mass ?

NaClsolutiongNaClgmass %23.1100

25.10125.1% =×=

Percent by VolumeTo determine concentration in % by volume:

100% ×=solutionvolumesolutevolumevolume The volume units

must match!solutionvolume

Example: What volume of water is needed to make a 15% by volume solution of alcohol if you have 5 mL of alcohol?

Volume solute = 5 mLVolume solution = 5 mL solute + x mL solvent = (5+x) mL% by volume = 15%

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100)5(

5%15 ×+

=mLx

mL 100%15

5)5( ×=+mLmLx

mLmLmLx 5100%15

5−×=

x = 28 mL solvent



Percent Mass/VolumeTo determine concentration in % mass/volume:

100/% ×=solutionvolumesolutemassvolumemass Use mass in grams.

Use volume in milliliters.

Example: Vinegar is a 5% solution of acetic acid in water (by mass/volume). What mass of acetic acid (CH3COOH) is in 45 mL of vinegar?

Mass solute = ? gVolume solution = 45 mL% mass/volume = 5%

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10045

%5 ×=mL

acidaceticmass acidaceticmassmL=

×100

%5)45(

2.25 g CH3COOH

MolarityOne of the most commonly used concentration units is Molarity (M):

nM =M = Molarityn = moles solute

V

Example: Find the molarity if 12.5 g NaCl is dissolved in 500 mL of water.

Solute = 12.5 g NaClSolution = 500 mLMolarity = ? M

NaClmoleMolarity 214.0=

= 0.214 mole NaCl= 0.500 L

V = volume solution (in Liters)

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Molarity = ? ML

Molarity500.0

=

0.428 M NaCl

12.5 g NaCl

58.44 g NaCl

1 mole NaCl= 0.214 mole NaCl

500 mL

1 mL

0.001 L= 0.500 L



MolalityTo determine molality (m):

solventkgnm = m = molality

n = moles solute

Example: How many moles NaNO3 are needed to make a 0.24 m solution with 1.5 L (1.5 kg) of water?

Solute = ? mole NaNO3Solvent = 1.5 kgmolality = 0 24 m kg

NaNOmolem5.1

24.0 3=

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molality = 0.24 m kg5.1

0.36 mole NaNO3

324.05.1 NaNOmolemkg =×

Molarity vs Molality Mnemonic: MolaRity (moles to Liter solution) and MolaLity (moles to kg solvent) = “Rose to Lover and Loyal to King!”

CalculationsCalculations with Concentrations

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Dilutions - 1Often, you are supplied with a more concentrated solution than you need.In order to dilute a solution to a lower concentration, more solvent is dd dadded.

The moles of solute did not change:moles solute before = moles solute after

SolventSolute

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Dilutions - 2Often, you are supplied with a more concentrated solution than you need.In order to dilute a solution to a lower concentration, more solvent is addedadded.The moles of solute did not change:moles solute before = moles solute after 21 nn =

And ifVnM = Then VMn ×=

VMVM ×=×

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2211 VMVM ×=×

The original molarity times the volume of the original solution = the new molarity times the new volume of the solution.



Dilution ExampleA dilution calculations example:

2211 VMVM ×=× 2 volume units must match!

Example: You need 55 mL of 0.10 M HCl solution. You currently have 12 M HCl solution. What volume of the concentrated solution will you dilute to 55 mL?

M1 = 12 M HClV1 = ? mLM 0 10 M HCl

mLMVM 5510.012 1 ×=×

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M2 = 0.10 M HClV2 = 55 mL

V1 = 0.46 mL

MmLMV

125510.0

1×

=

Using Molarity in ConversionsMolarity is used to convert between moles and liters.Example: If 0.85 moles NaOH are needed and you have a 1.5 M

solution, how many liters of the solution do you need?

0.85 mole NaOH

mole NaOH

L = ________ L1.5

1 0.57

From concentration: 1.5 mole NaOH = 1 L

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From balanced equation:

Example:

Solution StoichiometryIf you need 15.7 g Ba(OH)2 to precipitate, how many liters of 2.5 M NaOH solution is needed? 2 NaOH + BaCl2 Ba(OH)2 + 2 NaCl

q2 mole NaOH 1 mole Ba(OH)2

15.7 g Ba(OH)2 mole Ba(OH)21

Concentration of NaOH:2.5 mole NaOH = 1 L

mole NaOH2 L NaOH1

Molar Mass of Ba(OH)2:1 mole Ba(OH)2 = 171.35 g

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g ( )2

g Ba(OH)2

mole Ba(OH)2

= ________ L NaOH

171.35

0.0733

mole Ba(OH)21 mole NaOH

L NaOH

2.5

1

Electrolyte Solutions

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Definition: Electrolyte

Electrolyte – Ionic compound which dissolves in water, producing free-floating ions.

Free-floating ions can conduct electricity (hence “electro”).

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e.g. When dissolved in water:NaCl Na+ + Cl-Ca(NO3)2 Ca2+ + 2 NO3

-

Definition: Strong, Weak & Non-Electrolytes

Strong Electrolyte – Most of the ions dissociate and are free floating.g

Weak Electrolyte – Only some of the ions dissociate and are free floating (weakly conducts electricity).

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Non-Electrolyte – Dissolved substance does not produce ions at all (does not conduct electricity).



Breaking Compounds into Electrolytes

How do you break up a compound when forming electrolytes?

Do not break up polyatomic ions.1

Example: Break up the following strong electrolytes:

Use subscripts that are not a part of a polyatomic ion as coefficients.2

e.g. CaCl2 doesn’t have “Cl2” ions, it has 2 “Cl” ions.

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Na3PO4

(NH4)2CO3

3 Na+ + PO43-

2 NH4+ + CO3

2-

Colloids

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Definition: Colloid

C ll id S l ti ithColloid – Solution with solute particles large enough to deflect light as it travels through the solution.

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Definition: Tyndall Effect

Tyndall Effect – Property exhibited by colloids. The scattering light is visible through the solution.

Light coming in Solution

Light going out

The light is not scattered, and is not seen traveling through the solution.

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Light coming in Colloid

Light going out

The light is scattered, and is seen traveling through the colloid.



Solutions are composed Solutions are composed Colloids are mixtures

which have large enough Colloids are mixtures

which have large enough

Learning Summary

Concentration is an i f th ti f

Concentration is an i f th ti f

pof solute and solvents.

pof solute and solvents.

g gsolute particles to scatter light (exhibit the Tyndall

Effect).

g gsolute particles to scatter light (exhibit the Tyndall

Effect).

Th l tiTh l ti

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expression of the ratio of solute to solvent particles. Concentrations are used

in dilution and stoichiometry calculations.

expression of the ratio of solute to solvent particles. Concentrations are used

in dilution and stoichiometry calculations.

The solution process is governed by energetics of solution formation and factors

affecting solubility.

The solution process is governed by energetics of solution formation and factors

affecting solubility.

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You have successfully completed the core tutorial

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