Chemical Equilibrium

A chemical reaction is said to be in equilibrium if the reactants react together to form the products, and the products then react together to form the reactants and these two processes are equal in their rates.

Example Consider the following equation At the beginning of this reaction, the concentrations of reactants A and B are very high. So, the reaction goes from left to right (A, B to C, D).

Gradually the reactants are consumed and the direct reaction (to the right) slows down. The concentrations of C and D increase. The inverse reaction (to the left) begins very slowly and accelerates. After a certain time the direct reaction and the inverse reaction occur at the same rate. This is chemical equilibrium.

Concentrations of A and B versus Time, for the reaction A↔B Dynamic equilibrium occurs when the rates of the forward and reverse reactions are the same (when the two curves are parallel).

Characteristics of the Equilibrium Condition: 1.A system at equilibrium is dynamic, with both forward and reverse reactions occurring at the same rate. 2. Macroscopically, no observable property changes are occurring. Concentration, pressure, temperature, colour, etc. remain constant.

Characteristics of the Equilibrium Condition: 3.Equilibrium is reached spontaneously from either direction. 4.A system reacts spontaneously to reduce an imposed stress in order to re-establish a state of equilibrium.

Requirements for Chemical Equilibrium: In order to attain chemical equilibrium: 1. The temperature must remain constant. 2. The external pressure must remain constant. 3. Once the reaction has begun, no substances are added to the system. So, the system must be isolated and sealed.

Factors That Can Modify Chemical Equilibrium: 1. Effect of concentration changes Example If KSCN is added to the system in equilibrium, the reaction will proceed from left to right. The [Fe(NO 3 ) 3 ] decreases while the [KSCN], [Fe(SCN) 3 ] and [KNO 3 ] increase. So, the red colour will get darker. This indicates that the reaction is proceeding towards the right.

Factors That Can Modify Chemical Equilibrium: 2. Effect of temperature changes Example

When adding energy to this system in equilibrium, the reaction will go from left to right. The [N 2 O 4 ] decreases and the [NO 2 ] increases. The colour in this system will become a darker brown, which means that there are more NO 2 molecules. Once the temperature stabilises, the system will again be in the equilibrium state.

Factors That Can Modify Chemical Equilibrium: 3. Effect of external pressure changes Note: Changing the pressure of a system only affects the substances involved that are gases.

There is 1 mol of reactants on the left and 2 mol of products on the right. The pressure of the system can be increased by decreasing the volume. The side with the lowest number of moles of gases will be favoured. So the reaction will produce more N 2 O 4 and the colour will become lighter.

Changing the external pressure does not necessarily affect a system in equilibrium Example In this reaction, 1 mol of H 2 and 1 mol of I 2 are needed to make 2 mol of HI. This means that there is the same amount of moles per litre, on each side of the equation. Therefore, the equilibrium of the system is not affected.

Factors That Can Modify Chemical Equilibrium: 4. Effect of a catalyst A catalyst does not affect the equilibrium state of a system. It only permits the system to attain its equilibrium state faster.

Check Your Understanding 1.Sodium chromate, Na 2 CrO 4, can be dissolved in water to form a yellow solution. Sodium dichromate, Na 2 Cr 2 O 7, can be dissolved in water to produce an orange solution. The equilibrium equation is: a) What happens when you add hydrochloric acid to a solution of sodium chromate?

b) What happens if Cr 2 O 7 2- is added to the system?

2.What happens when the system described by the following reaction is heated?