Equilibrium - Le Chatelier's Principle help?

[b^3]

ok so my main question is why does an increase in temperature at a constant volume create a back reaction for exothermic reactions? La Chaterlier's principle will tell us that it will try to do the opposite of what we do right? so if we are putting energy in shouldn't it to try to get to equilibrium put energy out? i.e. a forward reaction?

This is the question it relates to.
How will the concentration of hydrogen gas in each of the following equilibrium mixtures change when the mixtures are heated and kept at constant volume?
a)
The answer says the concentration of H₂ will increase.

[chris_1993]

Your logic is mostly correct. By increasing the concentration of H2 it is opposing the system. It is an exothermic reaction meaning that it gives out energy. However, this is only the lets call it, forward, reaction. The equilibrium sign means it can go both ways. Naturally, if you reverse the way the system is going, delta H value will become inverse so you get:
2NH3 = (pretend its an equilibrium sign) N2 + 3H2      Delta H = +91 kj/mol

Hence if you increase temperature, at a constant pressure, you will increase the concentration of H2.

[b^3]

So are you saying that because we increase the temperature there is energy for the reaction to absorb and undertake the endothermic reaction which is in the opposite direction?

[andy456]

so exothermic reactions produce heat, i think of it as an extra product. If you add more heat the balance is thrown off and the system has to regain equilibrium. So it will favour a reverse reaction to get rid of the excess heat... Therefore a net backwards reaction

[b^3]

Ok thanks you guys, the help is appreciated.