LE CHATELIER’S PRINCIPLE

Chapter 13Section 13.5

Le Chatelier’s Principle

If a system at equilibrium is subjected to a stress, a reaction proceeds in that direction that tends to relieve the disturbing force.

Let’s look at three important stresses!

Change in Concentration

Example: H2 (g) + I2 (g) ↔ 2HI (g)

What happens if the concentration of I2 increases?

The reaction will shift to the right.(More I2 will have to be converted to HI and

the [H2] will decrease)

Change in Pressure

Only affects gases!Example: CO (g) + 2H2 (g) ↔ CH3OH (g)

What happens if we increase the pressure on the system?

The reaction will shift in the direction that produces the fewest number of moles of gas.

The reaction will shift right! (3 moles vs. 1 mole)

Changes in Temperature

Example: 2SO2 (g) + O2 (g) ↔ 2SO3 (g) + 180 Kj/mol

What if we increase the temperature (add heat) to the system?

The reaction will shift to the left! Away from the heat

Addition of a Catalyst

A catalyst is a substance that is added to a reaction to increase its rate. It does not effect the equilibrium. It just helps the reaction to reach equilibrium faster.

Homework

Complete the “Le Chatelier’s Principle” Worksheet