Dynamic Equilibrium

Objectives

Describe chemical equilibrium in terms of equilibrium expressions

Use equilibrium constants

Describe how various factors affect chemical equilibrium

Explain Le Chatelier’s principle

EquilibriumSome chemical systems have little tendency to react, while others go to completion

In between these extremes are chemical systems that reach a state of equilibrium with varying amounts of reactants unconsumed

For example:

N2 + 3H2 ⇔ 2NH3

This reaction reaches equilibrium when fewer than 2 moles of ammonia produced

Law of Chemical Equilibrium

At a given temp, ratio of concentrations of reactants and products has a constant value

General equation:

aA + bB ⇒ cC + dD

Equilibrium constant (Keq)

Example

Hydrogen and Iodide react to produce Hydrogen Iodide

H2(g) + I2(g) ⇔ 2HI(g)

This is known as homogeneous equilibrium = all reactants & products in same physical state

Write Equilibrium Expression Keq for this reaction

Example

Ammonia gas production:

N2 (g) + 3H2 (g) ⇔ 2NH3 (g)

Write equilibrium expression for this reaction

Practice

Write equilibrium expressions for the following reactions:

N2O4(g) ⇔ 2NO2(g)

CO(g) + 3H2(g) ⇔ CH4(g) + H2O(g)

2H2S(g) ⇔ 2H2(g) + S2(g)

Heterogeneous Equilibrium

Not all reactants and products in same physical state

leave out reactants and products in solid or liquid state

Just include gas concentration or solute concentration

Heterogeneous Equilibrium Example

C(s) + H2O(g) ⇔ CO(g) + H2(g)

Keq = [CO] [H2] [H2O]

Why use equilibrium constants?

If you know the equilibrium constant for a particular chemical reaction at a particular temperature, you can determine the concentration of one of the reactants or products given the concentration of the remaining reactants and products

Factors Affecting Chemical Equilibrium

When manufacturers make products, they want to minimize waste or leftover materials

Principles of chemical equilibrium can help determine the conditions that favor the most cost effective and environmentally friendly production of a chemical product

Le Châtelier’s Principle

If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress

How can we apply Le Châtelier’s Principle?

For example - we are trying to produce methane using the following reaction:

CO(g) + 3H2(g) ⇔ CH4(g) + H2O(g) + heat

Unfortunately, using our current manufacturing techniques, at equilibrium we produce only 0.05900 mol of CH4 - way too low a yield to be cost effective

What can we do to increase our yield of methane?

What is a stress?

Any kind of a change in a system that upsets equilibrium

Le Châtelier’s principle = predict how system will change

Going back to CO(g) + 3H2(g) ⇔ CH4(g) + H2O(g) + heathow to stress system to produce more product?

Stresses that affect equilibrium

pressure/volume

temperature

concentration

Changes in concentration

CO(g) + 3H2(g) ⇔ CH4(g) + H2O(g) + heat

If we increase [CO] or [H2], system will respond by producing more product (shift to the right). If we decrease [CH4] or [H2O], system will respond by producing more product.

If we increase [CH4] or [H2O], system will respond by producing more reactant (shift to the left)

Change in Volume/Pressure

N2 (g) + 3H2 (g) ⇔ 2NH3 (g)

Increase pressure (reduce volume) → reduce number of moles (shift to the right)

Reduce pressure (increase volume) → increase number of moles (shift to the left)

Changes in Temperature

CO(g) + 3H2(g) ⇔ CH4(g) + H2O(g) + heat

Increase temperature, system will respond in endothermic direction (shift to the left)

Decrease temperature, system will respond in exothermic direction (shift to the right)

Practice Problems

Use Le Châtelier’s Principle to predict how each of these changes would affect CO(g) + 3H2(g) ⇔ CH4(g) + H2O(g) + heat

Increase Temperature

Remove Hydrogen gas

Increase Volume

Increase Pressure

Decrease Temperature

Remove water vapour

Practice problems

How would decreasing the volume of the reaction vessel affect each of these equilibria?

2SO2(g) + O2(g) ⇔ 2SO3(g)

H2(g) + Cl2(g) ⇔ 2HCl(g)

2NOBr(g) ⇔ 2NO(g) + Br2(g)