3.1.6 Chemical equilibria, Le chatelier’s principle and Kc

Dynamic equilibrium

When a reversible reaction happens in a closed system, is may reach dynamic equilibrium

In dynamic equilibrium:

• Forward and backward reactions occur at equal rates

• The concentration of the products and reactants stay constant

Characteristics of a system in equilibrium

Microscopic activity: Bonds continuously break and reform

Macroscopic activity: properties like colour, pressure and concentration appear unchanged

Dynamic nature: balanced until the system becomes an open system

Effect of pressure on equilibrium

Pressure changes only apply to gaseous equilibria

• Increasing the pressure: equilibrium shifts to the side with fewer molecules of gas to oppose the increase in pressure

Decreasing pressure: equilibrium shifts to the side with more gas molecules to oppose the decrease in pressure

Lower pressure means more time to reach equilibrium as the reactants are further apart so successful collisions become less frequent

Effect of temperature on equilibrium

• Increasing temperature: equilibrium shifts in the endothermic direction to absorb excess heat and oppose the increase in temperature

• Decreasing temperature: equilibrium shifts to the exothermic direction to release heat an oppose the decrease in temperature

Effect of concentration on equilibrium

Increasing the concentration of reactant:

• Equilibrium shifts towards the products, so to the right, to use up the extra reactant, so more product is formed

Increasing the concentration of product:

• Equilibrium shifts towards the reactants, so to the left, to use up the extra product, so less product is formed

Opposite effect of decreasing concentration

Catalyst effect on equilibrium

Does not effect the position of equilibrium

A catalyst speeds up the rate of the forward and reverse reactions equally, and therefore the yeild is not effected

Allows for equilibrium to be reached faster

Maximising production: compromise

Factors to consider:

• Yeild

• Rate

• Energy costs

• Equipment costs

• Safety considerations

Considerations:

• Temperature: yield vs rate

• Pressure: yeild vs cost of energy

• Catalyst: cost saving

Kc

When a reversible reaction reaches the state of dynamic equilibrium, we can calculate a value called the equilibrium constant, KC using molar concentrations of each of the reactants and products at equilibrium

Kc gives a quantitative measure of where the equilibrium lies, whether there are more products or reactants at equilibrium

A large Kc indicates that equilibrium favours the products

A small Kc shows that equilibrium favours the reactants

A Kc of one indicates that at equilibrium, the concentrations of reactants and products are equal when raised to their respective stoichiometric coefficients

The Kc expression

The products must go on the top, and reactants at the bottom

Square brackets represent concentration

Only include substances that are in the same phase, gas liquid, solid, aq

For complex Kc calculations, an ice table may need to be used

Effect of temperature on Kc

Kc is only influenced by temperature

When the position of equilibrium is altered due to a temperature shift:

• A decrease in the amount of product at equilibrium will causes Kc to decrease

• An increase in the amount of product causes Kc to increase

Do other factors affect Kc

Concentration shifts the position of equilibrium, but Kc stays the same at the equilibrium already shifts the system to counteract the change, so the ration of products to reactants stays the same

Adding a catalyst increases the rate of the forward and reverse reactions equally but does not affect Kc