## Chem – Le Chatelier’s Principle: Temperature or Energy

How do you relate temperature or energy to Le Chatelier’s Principle?

200kJ + N2(g) + 3 H2(g) <—-> 2 NH3(g)

If we talk about Le Chatelier’s Principle in terms of temperature changes or energy it works much the same as concentration. I will use the same chemical equation example above to explain. The part you want to focus on for temperature or energy changes is the part in my example that says 200kJ. In other examples they can present this part to make it appear different. They can talk about calories instead of joules or say the equation is endothermic or exothermic or use the representation for Δ H. For the example I have above the chemical equation example is endothermic and the Δ H is positive.

In my example above, if we increase the temperature we increase energy available. Since energy is on the left side of the chemical equation in this case, we will shift toward the products (right). Just like concentration if you add or increase one side the shift will be away from that side. On the other hand if we decrease the temperature we decrease the energy available. Since energy is on the left side of the chemical equation in this case, we will shift toward the reactants (left). Again just like the concentration if you take away from one side it will shift toward that side.

Examples: A) Which way will the equilibrium shift when we increase the temperature?

B) Which way will the equilibrium shift when we decrease the temperature?

2 C4H10(l) + 13 O2(g) <—-> 8 CO2(g) + 10 H2O(g) +300KJ

A) Shift Left (toward reactants) B) Shift Right (toward products)

500Kcal + 4 CO2(g) <—-> C4(s) + 4 O2(g)

A) Shift Right (towards products) B) Shift Left (towards reactants)

Examples: Which way will the equilibrium shift when we change the temperature or energy as mentioned?

Increase the temperature

This reaction is endothermic:
2 Fe3+(aq) + 3 CO32-(aq) <—-> Fe2(CO3)3(s)

Decrease the temperature

This reaction is exothermic:
H2SO4(aq) <—–> H2(aq) + S(s) + 2 O2(aq)

Decrease the temperature

TheΔ H of this reaction is positive:
2 Fe3+(aq) + 3 CO32-(aq) <—-> Fe2(CO3)3(s)

Increase the temperature

The Δ H of this reaction is negative:
H2SO4(aq) <—–> H2(aq) + S(s) + 2 O2(aq)

PRACTICE PROBLEMS: Which way will the equilibrium shift when we change the temperature or energy as mentioned?

Increase temperature

350KJ + MgBr2(aq) + 2 NaI(aq) <——> MgI2(aq) + 2 NaBr(aq)

Decrease temperature

The ΔH of this reaction is positive:

6 Ag(s) + Ca3(PO4)2(s) <—-> 3 Ca(aq) + 2 Ag3PO4(s)

Decrease temperature

H2SO4(aq) <—–> H2(aq) + S(s) + 2 O2(aq) + 50 Kcal

Decrease temperature

The reaction is exothermic:

C4(s) + 4 O2(g) <—-> 4 CO2(g)  