CHEMICAL EQUILIBRIUM

Chapter 17

Reversible Reactions

Most reactions do not proceed to completion.

N2(g) + H2(g) 2NH3(g)

2NH3(g) N2(g) + H2(g)

What is Equilibrium

Equilibrium is when the concentrations of the reactants and products of a chemical reaction are constant.

Chemical equilibrium happens when the rate of the forward reaction is equal to the rate of the reverse reaction.

Law of Chemical Equilibrium

aA + bB cC + dD

The equilibrium constant expression:

If Keq > 1 the products are favored If Keq < 1 the reactants are favored

Types of Equilibrium

Homogeneous: CO2(g) 2CO(g) + O2(g)

Heterogeneous: H2O(l) H2O(g)

CaCO3(s) CaO(s) + CO2(g)

Write the equilibrium constant expression fo the following reactions

N2(g) + H2(g) 2NH3(g)

N2O4(g) 2NO2(g)

2H2S(g) 2H2(g) + S2(g)

CO(g) + 3H3(g) CH4(g) + H2O(g)

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

Equilibrium Constants

When equilibrium is reached the concentrations of the reactants and the products will not change.

At any given temperature the value of keq will always be the same no matter what the equilibrium concentrations are.

Practice

Calculate the value of keq for the following reaction and concentrations.

N2 + 3H2 2NH3

[N2] = 0.533 M [H2] = 1.600 M [NH3] = 0.933 M

0.399

Calculate keq for the reaction: N2O4(g) 2NO2(g) If [N2O4] = 0.0185

mol/L and [NO2] = 0.0627 mol/L

Calculate keq for the reaction CO(g) + 3H2(g) CH4(g) + H2O(g) if [CO]

= 0.0613 mol/L, [H2] = 0.1839 mol/L, [CH4] = 0.0387 mol/L and [H2O] = 0.0387 mol/L

For the reaction COCl2(g) CO(g) + Cl2(g)

Factors Affecting Equilibrium

The by-products of an industrial process are CO and H2. These two gasses can combine to produce CH4 and H2O in equilibrium.

Use the following data to find keq for this reaction. [CO] = 0.300M, [H2] = 0.100M, [CH4] = 0.059 M,

[H2O] = 0.020 M The chemists in charge of this industrial process

would like to make use of the methane (CH4) that is being produced. In order for them to have a usable amount of methane the concentration must be 0.100 M or higher. How can they achieve this?

Le Chatelier’s Principle

If a stress is applied to a system at equilibrium, the system will shift in the direction that relieves that stress.

Suppose additional CO is injected into the reaction vessel of the industrial process we discussed. How would this affect the equilibrium of the system?

Le Chatelier’s Principle

Along with adding reactants or removing products, what other stresses can we apply to a system?

Temperature Pressure

Calculating Equilibrium Concentrations

The reaction: CO(g) + 3 H2(g) CH4(g) + H2O(g)

Has a Keq of 3.933 at 1200 K. If the [CO] = 0.850 M, [H2] = 1.33 M, and

[H2O]= 0.286 M, what is the concentration of CH4?

Solubility Product Constant

Some ionic compounds dissolve completely in water.

Ex: NaCl(aq) Na+(aq) + Cl-(aq) Some ionic compounds do not dissolve

completely in water. Ex: BaSO4(s) Ba2+(aq) + SO4

2-(aq) This process is happening in equilibrium. So there is an equilibrium constant

associated with it.

Calculate the solubility of AgI(s). ksp = 8.5 x 10-17

Examples Calculate the solubility in mol/L of CuCO3

if it’s ksp is 2.5 x 10-10

1.6 x 10-5 mol/L Calculate the solubility PbCrO4 if it’s ksp is

2.3x10-13.

4.8 x 10-7 mol/L Calculate the solubility of CaF2 if it’s ksp is

3.5x10-11. 4.18x10-6 mol/L

Calculating Ion Concentrations

Sometimes the concentrations of the ions are not the same as the solubility.

Mg(OH)2(s) Mg2+(aq) + 2OH-(aq) ksp = 5.6 x 10-12