Chapter 2: Further Chemical Equilibria

Reversible reactions at equilibrium are denoted by the equilibrium symbol. Many chemical reactions are reversible. Reversible reaction is a reaction which can go forward and backward depending on the condition.

 

Dynamic Equilibrium Occurs when the rate of forward and backward reaction are equal. The concentration of products and reactant remain constant despite the fact particles are continually reacting.

Equilibrium Constant (K)

Homogeneous equilibria have all the reactants and products in same physical state.

Heterogeneous equilibria, contains reactant and products in different physical state.

The amount of product and reactants in a particular equilibrium process can be measured by the equilibrium constant. For the given equation below:

aA + bB——->cC + dD

The equation used below shows how to calculate Kc;

Kc = ([C]c[D]d)/([A]a[B]b)

Gaseous Equilibria

To calculate gaseous equilibria Kp, the following are required:

Mole Fraction: In a mixture of gases, the mole fraction of gas (XA) is:

XA = (number of moles of gas A)/(total number of moles of gas)

Partial Pressure: In mixture of gases, the partial pressure is the pressure thet one gas would exert if it occupied the whole container:

PA = mole fraction of gas A × total pressure.

KP can be calculated by;

Kp =(PCc × PDd)/(Paa × PBb)

Change in equilibrium Conditions

Le Chatelier’s Principle: When a system in dynamic equilibrium is subjected to change, the position of the equilibrium will shift to oppose the change.

Changing Concentration

Changing the concentration of a reactant or product will cause the position of equilibrium to shift to oppose this change;

Increasing the concentration of the reactant will cause the position of equilibrium to shift to make more of the product. Increasing the concentration of the products will cause the position of equilibrium to shift to remove the extra product, making more reactant.

Changing Temperature

The effect of a change in temperature on the position of equilibrium will depend on the enthalpy change of the reaction. If the forward reaction is exothermic and the backward reaction is endothermic.

Increasing the temperature will cause the position of equilibrium to shift in the endothermic direction to absorb the added heat. Decreasing the temperature will cause the position of equilibrium to shift in exothermic direction to add more heat.

Changing Pressure

Changing pressure of the system will only change the position of the equilibrium to shift to oppose tis change,

Increasing the pressure of the system will cause the position of equilibrium to shift to the side with the fewest moles of gas. Decreasing the pressure of the system will cause the position of the equilibrium to shift to the side with more moles of gas.

Using Catalyst

Adding a catalyst does not change the position of equilibrium, but increases the rate at which the equilibrium is established.

Common ion effect

The effect observed on the position of equilibrium for a sparingly soluble salt when an ionic compound which contains one of the ions involved in the equilibrium is added.