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Electrolytes and Colligative Properties

Mr. Shields Regents Chemistry U12 L07

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ElectrolytesThere are a number of different kinds of pureSubstances that can be dissolved in solvents to Produce solutions.

-Some of these substance involve dissolving covalent molecules in liquids

examples: O2, C6H14, C6H12O6, CCL4

-Some of them involve dissolving ionic compounds in liquids examples: NaCl, NH4Cl, KI

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There is a difference between how these twotypes of solutes dissolve in a polar liquid such assuch as water.

Molecular compounds…- remain as single entities in solution

Ionic Compounds …- separate into the ions from which they are

made

The Separation of a compound into it’s IONS is called DISSOCIATION

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Electrolytes

Compounds that produce ions in solution by dissociation areCalled… ELECTROLYTES

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Notice that when a compound dissociates the resultingSolution has more particles in it than what we started with:

Dissolving equations:

NH4Cl + H20 NH4+ (aq) + Cl- (aq) K2SO4 + H20 2K+ (aq) + SO4

-2 (aq)

1 particle 2 particles 1 particle 3 particles1 mole 2 moles 1 mole 3 moles

How many moles of particles do I have if I dissolve 1 mole ofAl(NO3)3 in water? What is the dissolving Equation?

How many moles of particles do I have if I dissolve 1 mole ofSucrose in water? (sucrose = C12H22O11)

1 C12H22O11 +H20 C12H22O11 (aq)

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An important property of ionic compounds in solution is thatThey Conduct electricity.

- Molecular compounds in solution DO NOT

In other words, Electrolytes form conductive solutions but Non-electrolytes do not

When the ends of an electric circuit are placed in a Solution of an Electrolyte 2 things happen…

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Conduction of electricity

+ -

+ -

+ ions

- ions Na+

Cl-

1) Positive ions will flow towards the negative electrode

2) Negative ions will flow toward the positive electrode

This free flow of ionsThis free flow of ionsAllows electricity toAllows electricity toBe conducted throughBe conducted throughThe solutionThe solution

Molecules in solutionMolecules in solutionCan’t do thisCan’t do this!

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Problem:

1) Which of the following are electrolytes and whichare not? 2) How many particles does each produce in Solution?

1)Ethyl Alcohol (C2H5OH)2) NaCl3) Sugar

4) CuSO45) AlCl36) NH4NO37) Al2(SO4)3

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Colligative Properties

Now… When substances dissolve in liquids we change someOf the physical properties of the liquid

These changes are DEPENDENT only on the Number ofSolute particles present

And … are INDEPENDENT of the nature or kind of soluteParticles

CCL4NaCl

NaNO3

CH3CH3

CH3CH2OH

IonicMolecular

Polar

Non-polar

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Colligative PropertiesFor example:

So, if changes in properties are dependent only on thenumber of solute particles in solution why is the above true?

2 moles of sugar (C12H22O11) in water would have the same impact on the solvent’s properties as 1 mole of NaCl.

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Properties that are dependent on the # of particles butAre Independent of the nature of the particle are known as COLLIGATIVE PROPERTIES

When comparing the properties of a pure solvent vs. theProperties of solution made with the same solvent there are2 colligative properties of special interest that are Observed to change:

2) The boiling pt. - which increases

1) The freezing pt. - which decreases

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Problem:

a)Which sol’n will have the greatest effect on FP & BP?how much greater? How many moles of ions are in solutionfor each?

1) 2M KBr2) 0.5M NaCl

b) Which sol’n will have the greatest effect on FP & BP?

1) 1M CuSO42) 2M NaCl3) 3M Sugar4) 2M CaF25) 5 M C2H5OH

2M particles4M “3M “6M “5M “

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BP Elevation

The Elevation of Boiling Point (BPE) is one of the ColligativeProperties of solutions

So … What’s the definitionof the Normal Boiling Point?

The NBP is the temperature at which the VP of the liquidEquals Standard Pressure (1 atm)

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By adding solute to solvent the VP of the solvent is reduced.Therefore, it takes a higher temperature for the Vapor Pressure of the liquid To reach 1 Atm (i.e. the Boiling Point)

Normal BP of Solvent w/o solute

Normal BP ofSolvent w/solute

T1

T2

BP T2 > T1

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Freezing Point DepressionDepression of Freezing Point is the second of the ColligativeProperties of solutions we’ll discuss.

First … what is the definition of the freezing Point?

The FP is the temperature atwhich both solid and Liquid exist in equilibrium.

This means that liquid becomes solid and solid becomesLiquid and this oscillates back and forth – in equilibrium

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As the temperature is lowered the attractive forcesbetween molecules become more significant. These forces organize liquid molecules into regular, repeating patterns.In other words into solids. This is what we call solidification(freezing).

Solutes interfereWith this process.

In order to organize theSolvent molecules a lowerTemperature is now Required.

As the temp dec. molecularMotion slows down. The Attractive forces can nowOvercome the disruptive force of the solute.

T1

T2

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The freezing point of seawater is about 28.4°F (-2°C), instead of the 32°F (0°C) freezing point of ordinary water

Seawater is about 3.5% salts(ionic compounds) in water

As antifreeze is addedTo water in your carsRadiator the freezingPt. is markedly depressed

A molecule

And…

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Now you can appreciate why we add antifreeze(ethylene glycol) to our car’s cooling systems.

By adding solute we Raise the BP of the water in theCooling system AND at the same time Lower the FP ofThe water.

So we can protecting our enginesin both winter and summer.