UNIT 13: CHEMICAL EQUILIBRIUM

Section 1: The Nature of Equilibrium

Reaction Reversal

Theoretically, every reaction can proceed in two directions, forward and backwards. A chemical reaction in which the products can

react to re-form the reactants is called a reversible reaction…

2HgO(s) 2Hg (l) + O2(g)

or…2Hg(l) + O2(g) 2HgO(s)

These reactions are said to be in equilibrium.

So… What does it mean for

something to be at

“equilibrium”?

2HgO(s) 2Hg(l) + O2(g)

Chemical Equilibrium

A reversible reaction is said to be in chemical equilibrium when the rate of its forward reaction equals the rate of its reverse reaction and therefore the concentration of products and reactants stays the same…

Chemical Equilibrium

Shifting Equilibrium

Equilibrium is dynamic – meaning it may shift one direction or the other in a reaction to counteract effects of the environment.

This is supported by Le Chatelier’s principle which states that if a system at equilibrium is subjected to a stress, the equilibrium is shifted in the direction that tends to relieve the stress.

Stress can be applied in the form of increased pressure, concentration of reactants, or temperature.

Shifting Equilibrium

Example:

Pressure

N2(g) + 3H2(g) 2NH3(g)

When pressure is applied, the reactants more readily combine to form NH3, shifting the reaction to the side of the products (away from the stress) to regain equilibrium.

System Shifts