Chemical Equilibrium

Chemical equilibrium refers to a state in which products of a reaction can be converted back to reactants at the same rate in which reactants are converted to products. It can be applies to reactions that occur in both directions. For example take the reaction:  CH ₄   +    H₂O     ⇔     CO   +   3H₂   .    The reaction can occur in both ways. Reactants will collide to form products and after a particular time products will react to form reactants. The process will continue but the amount of reactants or products will not change. The reaction is said to be in equilibrium when the net change of reactants and products is nullified (zero).

Equilibrium Constant

Equilibrium constant is essential to calculate the quantity of each compound present at equilibrium.

Consider the generic equation: aA + bB  ⇔ cC   + dD ; here A, B, C & D represents the molar concentrations of reactants and products. a, b, c & d are the coefficients that balance the equation. Using the given equation equilibrium constant (K_c) can be calculated.

K_c  = (C)^c  (D)^d / (A) ^a (B) ^b

Le Chatlier’s Principle and chemical equilibrium

 According to Le Chatlier’s Principle, a chemical equilibrium is disturbed by change in temperature, pressure or a concentration ,the equilibrium composition of the system shifts in such a way that it tends to counteract this change of variable. The chemical equilibrium can be affected in three ways.

• By changing concentration by adding or removing products
• By changing partial pressure of gaseous reactants and products
• By changing the temperature

These changes can affect equilibrium differently and, so you have to determine what needs to happen for the reaction to get back in equilibrium.

Example for change in concentration:

2NO (g) +    O₂ (g)  ⇔   2NO₂(g)

• When the concentration of  NO (g) increases the equilibrium will shift to the right producing more  NO₂ (g)
• When the concentration of O₂ (g) increases the equilibrium will shift to the right producing more NO₂ (g).
• When the concentration of  NO₂ (g) increases the equilibrium will shift to left producing more reactants;  2NO (g)  &   ₂  (g).

Example showing change in Pressure:

2SO₂ (g)+   O₂ (g)  ⇔   2SO₃(g)

When pressure increases the equilibrium will shift in the direction that can reduce pressure (side with less number of gas molecules).So in this reaction an increase in pressure will cause a shift in equilibrium to the right, which means the products formed will be more.

Questions:

• Define chemical equilibrium?
• Explain Le Chatlier's Principle?