Chemical Equilibrium

Complete and Reversible Reactions

Complete – Forms a precipitate or evolves gas, all reactants are used up

Reversible - When products formed in a chemical reaction decompose back to the original reactants

Reversible Reactions

The arrows go in both directions

– forward reaction

– reverse reaction Must be in a closed system where nothing can

escape

Equilibrium

Occurs when the forward and reverse reactions happen at an equal rate: there is no net change– Based on a specific temperature and pressure– The total amount of particles remains the same and

therefore so does the concentration– The concentration of a substance is denoted by the

use of brackets around the formula [H2]

– The reaction is dynamic - in constant motion

Equilibrium Constant

For the reaction: aA + bB cC + dD,

Keq = [C]c[D]d

[A]a[B]b

– Keq = equilibrium constant

– [ ] = concentration in M (mol/L)

Do not include any solids or liquids in the Keq

expression– Both solids and liquids are pure substances, their

concentration cannot change by definition

Write the formula for the equilibrium constant for each of the following reactions:

1. H2 (g) + I2 (g) 2HI (g)

2. As4O6 (aq) + 6C (s) As4 (g) + 6CO (g)

3. Hg (l) Hg (g)

4. NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

Equilibrium Constant Calculations

At a temp of 25°C, the following concentrations of the reactants and products for the reaction involving carbonic acid and water are present: [H2CO3] = 3.3 x 10-2M; [H3O+] = 1.1 x 10-6M; and [HCO3

-] = 7.1 x 10-1M. What is the Keq value for the following reaction at equilibrium in a dilute aqueous solution?

H2CO3 (aq) + H2O (l) H3O+ (aq) + HCO3- (aq)

Equilibrium Constant Calculations

What is the equilibrium constant of formic acid, HCHO2? In water, the equilibrium concentrations are [HCHO2] = 2.00M; [H3O+] = 6.00 x 10-6M; and [CHO2

-] = 6.00 x 10-6M.

HCHO2 (aq) + H2O(l) H3O+ (aq) + CHO2- (aq)

What is favored:ProductsReactants

Reactants ⇋ Products

X ⇋ Y Time

Time

Time

System EquilibriaEquilibria can favor the formation of reactants or

productsKeq can determine which direction is favored in a rxn

– Keq > 1 means forward rxn favored– Keq < 1 means reverse rxn favored– Keq = 1 means neither is favored

If conditions of the reaction are changed, the reaction will shift from its original equilibrium state to compensate for the change

Le Chatelier’s Principle

When a system at equilibrium is disturbed it attains a new equilibrium position to accommodate the change– Used in industry to increase efficiency

A Rddway

emoveeplace

System Equilibria – 3 Factors

Factors that alter chemical equilibrium:– Concentration of reactants or products

– Pressure

– Temperature

Concentration

Adding a substance to a system at equilibrium drives the system to consume that substance

Removing a substance from a system at equilibrium drives the system to produce more of that substance

N2 (g) + 3H2 (g) 2NH⇋ 3 (g) + 22.0 kcal

Stress Equilibrium Shift

[N2] [H2] [NH3] Keq

Add N2

Add H2

Add NH3

Remove N2

Remove H2

Remove NH3

Increase Temp

Decrease Temp

Increase Pressure

Decrease Pressure

Temperature

Only factor that affects the value of the equilibrium constant

Affects how completely a reaction proceeds to products

Remember– exothermic: releases heat– endothermic: absorbs heat

Temperature

Heat can be treated as a product or a reactant – EXOTHERMIC if ∆H is negative

• A + B ⇋ C + HEAT

– ENDOTHERMIC if ∆H is positive• A + HEAT ⇋ C + D

Adding heat to an exothermic reaction will shift the equilibrium towards the reactants

Adding heat to an endothermic reaction will shift the equilibrium towards the products

Pressure

Increase system pressure - the system will shift to reduce that pressure by proceeding in the direction that produces fewer molecules of gas

An equilibrium reaction that has the same # of moles of gas on both sides of the equation will not be affected by changes in pressure