KNC2133 Chap 4 Chemical Equilibrium

8/3/2019 KNC2133 Chap 4 Chemical Equilibrium

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Chapter 4

Chemical Equilibrium

KNC2133 Analytical ChemistryDr Ivy Tan Ai Wei1



Learning ObjectivesCalculation of equilibrium constant.Activity and activity coefficients.

Systematic approach to equilibriumcalculations.Mass balance and charge balance

equations.

KNC2133 Analytical Chemistry Dr Ivy Tan Ai Wei

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Chemical Reactions: The Rate Concept

• Consider the chemical reaction

aA + bB cC + dD

• The rate forward is equal to

ratef = k f [A]a[B]b

where ratef is the rate of the forward reaction andk f is the rate constant

• Kf is dependent on factors such as temperature

and the presence of catalysts

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• The rate reverse is equal to

rateb = k b[C]c[D]d

where rateb is the rate of the reverse reaction and k b is

the rate constant

• For a system equilibrium, the forward and reverse

rates are equal

k f [A]

a

[B]

b

= k b[C]

c

[D]

d

• The molar equilibrium constant will be[ ] [ ]

[ ] [ ]

c d

f

a b

b

k C D K

A B k

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A and B disappearing

Equilibrium Concentration

C o n c e n t r a t i o n

Time

C and D appearing

0

Note:

The equilibrium constantdoes not provide any info.

on how fast the reaction

will occur

• K can be evaluated empirically by measuring the

concentration of A, B, C and D at equilibrium


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Fig. 6.1. Progress of a chemical reaction.

The rate of the forward reaction diminishes with time, while that of thebackward reaction increases, until they are equal.

A large K means the reaction lies far to the right at equilibrium.

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

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Equilibrium constants may be written for dissociations, associations, reactions

or distributions.

Equilibrium Reaction Equilibrium Constant

Acid-base

dissociation

HA + H2O H3O+ +

A-

Ka, acidity constant

Solubility MXMn+ + An- Ksp, solubility product

Complex formation Mn+ + aLb- MLa(n-

ab)+Kf, formationconstant

Reduction-oxidation Ared + Box Aox + Bred Keq, reaction eq.

constantPhase distribution AH2O Aorganic KD, distribution

coefficient

Types of Equilibria


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Gibbs Free Energy and Equilibrium Constant

• The tendency of a reaction to occur is defined

thermodynamically from its change in enthalpy (∆H) and

entropy (∆S).

• A system always favor lower energy and increased randomness,that is lower enthalpy and higher entropy.

• The combined effect is given by the Gibbs free energy, G:

G = H – TS

• The change in energy of the system at constant T:

ΔG = ΔH – TΔS

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• At standard condition (1 atm, 298 K)

ΔGo

= ΔHo

– TΔSo

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When stress is applied to a system at

chemical equilibrium, the

equilibrium will shift in a directionthat tends to relieve or counteract

that stress.

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What are the factors (stress) ?

• Temperature (T)

Affects the individual rate constants.

the equilibrium constant Also affects the free energy.

Depends on the magnitude of the heat of reaction of the

system.

Q: for endothermic reaction, in which direction the reaction

will occur if heat is added?


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• Pressure

Larger influence for reaction in gaseous phase

An increase of pressure will favor a shift in the

direction that results in a reduction in the volume of the

system. Negligible effect for reactions in liquid phase.


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• Concentration

Does not affect the value of equilibrium constant.

Affects the position of equilibrium for the reaction.

Consider:

3I- + 2Fe3- I3- + 2Fe2+

What will happen if Fe2+ is removed from the reactor?

What will happen if it is added to the reactor?


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• Catalysts

Catalysts do not affect the equilibrium constant or the

position of equilibrium.

• Completeness of Reactions

For quantitative analysis, equilibria should be at least

99.9% to the right for precise measurements.


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Review of PrinciplesChemical reactions are never

“complete” Chemical reactions proceed to a state

where ratio of products to reactants isconstant

NH3 + HOH NH4+ + OH-

[NH4+][OH-]/[NH3][HOH] = Kb

o

If Kb << 1 (little ionization)

H2SO4 + HOH H3O+ + HSO4- [H3O+][HSO4

-] / [H2SO4][HOH] = KaIf Ka

>> 1 (mostly ionized)

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Equilibrium

is not reached instantaneously can be approached from either direction is a dynamic state amounts of reactants/products can be changed

by “mass action”

(adding/ deleting products/reactants) HCO3

- + H+ CO2(g) + HOH

Ke = [CO2][HOH]/[HCO3-][H+]


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Equilibrium Constants 2 A + 3 B C + 4 D

Ke = [C][D]4/[A]2[B]3

Concentrations [ ] : molar for solutes partial pressures (atm) for gases [1.0] for pure liquid, solid, or solvent


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Equilibrium constants for dissociating or

combining species

When a substance dissolves in water, it will often

partially or completely dissociate or ionize.

• Weak electrolytes: partially dissociate

Ex: acetic acid

• Strong electrolytes: completely dissociate

Ex: hydrochloric acid

The dissociation of weak electrolytes or the solubility of

slightly soluble substances can be quantitatively described

by equilibrium constant.

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Consider the dissociating species AB,

ABA + B

The equilibrium constant can be written as:

[ ] [ ]

[ ] eq

A B

K AB

The larger the Keq, the

greater will be the

dissociationSome species dissociate stepwise,

A2BA + AB

ABA + B

1

2

[ ][ ]

[ ]

A AB K

A B

2

[ ] [ ]

[ ]

A B K

AB

Overall:

Keq = K1.K2

= [A]2[B]/[A2B]

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Activity and Activity Coefficients

Activity of an ion,ai = Ciƒi

Ci = concentration of the ionƒi = activity coefficient ( @ Ci < 10-4M )= 1

Ionic Strength,

= ½ CiZi2

)Zi = charge on each individual ion


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Activity and Activity Coefficients

Calculation of Activity CoefficientsDebye-Huckel Equation:

-log ƒi = 0.51Zi2 ½

i½

i = ion size parameter in angstrom (Å)1 Å = 100 picometers (pm, 10-10 meters)

Limitations: singly charged ions = 3 Å

-log ƒi = 0.51Zi2 ½

½

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Systematic Approach to EquilibriumCalculations

How to Solve Any Equilibrium Problem?

1. Write balanced chemical reactions2. Write equilibrium constant expressions

3. Write all mass balance expressions4. Write the charge balance expression5. Equations >= Chemical Species solpossible6. Make assumptions where possible7. Calculate answer 8. Check validity of assumptions

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Tutorial 3

Christian, G.D., [2004] AnalyticalChemistry, 6th Edition, John Wileyand Sons.

Page 216-217:Problems 2, 4, 12, 14, 16

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KNC2133 Chap 4 Chemical Equilibrium