CI 4.5CI 4.5

Energy changes in solutionsEnergy changes in solutions

Why do some ionic substances dissolve in Why do some ionic substances dissolve in water, whilst others are insoluble?water, whilst others are insoluble?

If there is enough energy to separate the If there is enough energy to separate the ions in the lattice, the substance will be ions in the lattice, the substance will be solublesoluble

Ions in solid lattice Ions in solution

Lattice enthalpy, Lattice enthalpy, HHLELE

The enthalpy change when 1 mole of solid The enthalpy change when 1 mole of solid is formed from the separate ionsis formed from the separate ions

This is always an exothermic processThis is always an exothermic process

HHLE LE is is alwaysalways negative negative

The energy to break up a lattice = - The energy to break up a lattice = - HHLELE

--HHLELE is a positive is a positive

valuevalue

Which will give more negative Which will give more negative HHLELE??

Small ionic chargeSmall ionic charge

Large ionic chargeLarge ionic charge

Small ionic radiusSmall ionic radius

Large ionic radiusLarge ionic radius

Where does the energy come from Where does the energy come from to break up the lattice?to break up the lattice?

HydrationHydration

3+ 3-

Enthalpy of hydration,Enthalpy of hydration,HHhydhyd

HHhydhyd is the enthalpy change when a is the enthalpy change when a

solution of ions is made from 1 mole of solution of ions is made from 1 mole of gaseous ionsgaseous ions

Is hydration exothermic or endothermic?

Exothermic – because bonds are made

Example of hydrationExample of hydration

NaNa++ (g)(g) + aq + aq NaNa++ (aq)(aq)

HHhydhyd = = - 406 kJ mol- 406 kJ mol-1-1

Different ionsDifferent ions

An ionic compound contains cations and An ionic compound contains cations and anionsanions

Total Total HHhydhyd = = HHhyd (cation) hyd (cation) + + HHhyd (anion)hyd (anion)

When NaCl is dissolved:When NaCl is dissolved:

Total Total HHhydhyd = = HHhyd (Na+) hyd (Na+) + + HHhyd (Cl-)hyd (Cl-)

Which will give more negative Which will give more negative HHhydhyd??

Small ionic chargeSmall ionic charge

Large ionic chargeLarge ionic charge

Small ionic radiusSmall ionic radius

Large ionic radiusLarge ionic radius

Water is not the only solventWater is not the only solvent

Enthalpy of solvation, Enthalpy of solvation, HHsolvsolv , is used for , is used for

other solventsother solvents

Enthalpy change of solutionEnthalpy change of solution

This is the enthalpy change when 1 mole of a This is the enthalpy change when 1 mole of a solute dissolves to form an infinitely dilute solute dissolves to form an infinitely dilute solutionsolution

HHsolutionsolution = = HHhyd (cation) hyd (cation) + + HHhyd (anion) hyd (anion) - - HHLELE

If If HHsolutionsolution is negative, it is more likely that is negative, it is more likely that

the substance will dissolvethe substance will dissolve

What decides solubility?What decides solubility?

Energy is put in to separate the ions in the Energy is put in to separate the ions in the lattice [ - lattice [ - HHLELE]]

Energy is released by hydration of the ionsEnergy is released by hydration of the ions

If more energy is released than used up, If more energy is released than used up, then then HHsolution solution will be negative and the will be negative and the solute is more likely to dissolve.solute is more likely to dissolve.

[[HHhyd (cation)hyd (cation) + + HHhyd (anion)hyd (anion)]]

Enthalpy cycle for solutionEnthalpy cycle for solution

Ionic lattice + solvent solutionHsolution

Gaseous ions + solvent

- - HHLELE HHhydhyd (cation)(cation)

+ + HHhyd (anion)hyd (anion)

HHsolutionsolution = = HHhyd (cation) hyd (cation) + + HHhyd (anion) hyd (anion) - - HHLELE

Enthalpy level diagramsEnthalpy level diagrams

Make it easier to compare the Make it easier to compare the sizes of the enthalpy changes sizes of the enthalpy changes

enthalpy

Solute + solvent

Gaseous ions

Solution

ΔHsolution

-ΔHLE ΔHhyd(cat)

ΔHhyd(an)

Is ΔHsolution

Endothermic or exothermic ?

-ΔHLE + ΔHhyd(cat) + ΔHhyd(an)ΔHsolution =

enthalpy

Solute + solvent

Gaseous ions

Solution

ΔHsolution

-ΔHLE

ΔHhyd(cat)

ΔHhyd(an)

-ΔHLE + ΔHhyd(cat) + ΔHhyd(an)ΔHsolution =

Is ΔHsolution

exothermic or endothermic?

Solubility Solubility

The more negative the value for The more negative the value for ΔHsolution

the more likely the solute is to dissolve.

If ΔHsolution is very large and positive, the solute will not dissolve.

If ΔHsolution is small and positive, the solute may dissolve, if there is sufficient increase in entropy.

Over to you!Over to you!

Time to try the Time to try the problems.problems.

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