Question

Hydrogen gas and chlorine gas react according to the following equation:

H2(g) + Cl2(g) Double arrow yields 2HCl(g) + Energy

Hydrogen gas and chlorine gas are placed in a sealed container and are allowed to reach equilibrium. What effect will heating the system have?

It will add more HCl(g) to the system.
It will decrease the concentration of H2(g).
It will increase the rate of both the forward and the reverse reactions.
It will shift the equilibrium to the left.

Answer 1

Think about energy as being a product. If you increase the heat of the system you are effectively increasing the energy. In terms of le chatelier's principle if a stress is added to a system, the system will attempt to relieve this stress. The stress added here is the increased stress of more product (in terms of energy). So if increased stress is added to the product (right) side how will the system relieve the stress?

Answer 2

The forward reaction is exothermic. When heat is added to the system, the equilibrum counteracts the increase in heat by favouring the backward endothermic reaction to remove the excess heat. The equilibrum position shifts left and the yield of H2 and Cl2 increase.

The third option is technically correct, but it will only increase the rate of the forward and backward reactions for a very short period of time, before the equilibrum counteracts the increase in temperature by favoring the backward endothermic reaction.