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)

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

System Equilibria

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

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 – If the reaction is exothermic, heat is written on the

product side of the equation

– If the reaction is endothermic, heat is written on the reactant side of the equation

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

The Solubility Product Constant

Dissolution and Precipitation

Remember: ionic substances separate into their ions in solution and become uniformly distributed in the sol’n

Dissolution- the process in which an ionic solid dissolves in a polar liquid

Can write an equation for dissociation– Only dissociated substances are written as ions in

equations

– Must balance numerically and electrically

Precipitation

Precipitation- the process in which ions leave a sol’n and regenerate an ionic solid

Precipitate- insoluble solid formed Dissolution and precipitation are opposite

processSolubility equilibrium- rate of dissolution=

rate of precipitation

What is a solubility product constant, and what is it used for?

An equilibrium constant for slightly soluble ionic substances– symbolized Ksp

Used to determine solubility of sparingly soluble compounds

Cannot be applied successfully to salts that are more soluble

How is a solubility constant written?

The equation for a slightly soluble ionic substance in a saturated sol’n can be written in the following general form:

AaBb (s) aA+(aq) + bB-

(aq)

The solubility product constant is Ksp = [A+]a[B-]b

Association Equations and Solubility Product Constants

Write the dissociation equation and solubility product constant for each of the following substances.– Strontium arsenite– Calcium oxalate– Barium sulfide– Magnesium hydroxide

Solubility Product

At 25°C, the concentration of Pb+2 ions in a saturated sol’n of PbF2 is 1.9 x 10-3M. What is the value of Ksp for PbF2?

PbF2 (s) Pb+2 (aq) + 2F- (aq)For every molecule of PbF2, there will be

one Pb+2 ion and two F- ions. If [PbF2] = x, then [Pb+2] = x and [F-] = 2x

Ksp = [Pb+2] [F-]2

Ksp = x (2x)2

Ksp = 4x3

Solubility Product

A sample of Cd(OH)2 (s) is added to distilled water and allowed to come to equilibrium at 25°C. The concentration of Cd+2 is 1.7 x 10-5M at equilibrium. What is the value of Ksp for Cd(OH)2?

Solubility

What will be the equilibrium concentrations of lithium and phosphate ions in a saturated solution of lithium phosphate? (Ksp = 3.2 x 10-9)

Solubility

What will be the equilibrium concentrations of strontium and phosphate ions in a saturated solution of strontium phosphate? (Ksp = 1.0 x 10-31)

Precipitates

Supersaturated solutions are unstable Non equilibrium state achieved by manipulating

conditions Precipitates will form in a supersaturated solution To determine supersaturated solution calculate Q,

the ion product

– Ksp < Q = Supersaturated

– Ksp > Q = Unsaturated

– Ksp = Q = Saturated

Precipitation Reactions

Reaction in which 2 solutions are mixed and a precipitate is formed– Described by a chemical equation– Remember ionic substances dissociate in solution– The precipitate that forma is a combination of ions

present– The precipitate formed can be identified by using

solubility rules but can only be truly confirmed experimentally

What is the common ion effect?

Common ion: an ion that comes from two or more substances making up a chemical reaction– example: BaSO4 and Na2SO4; common ion is SO4

-

2

Common ion effect: a process in which an ionic compound becomes less soluble upon the addition of one of its ions by adding another compound

Why does the common ion effect work?

The common ion effect is an example of Le Chatelier’s principle

When a product is added to a system in equilibrium, it will cause the equilibrium to shift to the left, making more insoluble reactant

a) Saturated silver sulfate solution, Ag2SO4 (aq), is colorless. A schematic of the solution is shown above, omitting the water for simplicity.(b) Following the addition of Na2SO4 (aq), most of the Ag+ ions originally present (about 7 of 8 shown) have precipitated. The schematic shows the only remaining silver ion as a silver + ball.