Help with this chemistry question please?

Question

Choose which direction (right, left, or 'no affect') the equilibrium will shift when the following stressors (changes) are placed on the equilibrium.

Given:    heat + 2 NH_3 (g) <--> N_2 (g) + 3 H_{2 (g)}

1)Add NH₃ gas :

2) Decease the temperature :

3) Remove N₂ gas:

4) Add H₂ gas:

5) Decrease volume: 

Thank you

Answer 1

Hello, Lauren,

I’m answering these quickly, so please review my analysis carefully,

Given:    heat + 2 NH_3 (g) <--> N_2 (g) + 3 H_{2 (g)}

K_c = [N₂][H₂]³/[NH₃]²

1)Add NH₃ gas :

Moves the reaction to the right, since K_c is a constant and the numerator in the equilibrium equation would have to increase to keep K_c constant.

2) Decease the temperature :

This is hard for me to predict. The loss of energy would move the reaction to the left, since heat is needed for the NH₃ decomposition. But if we assume a fixed volume container, the pressure will decrease, pushing the reaction to the right, since it is the partial pressures of the gasses that are used for concentration values. This is particularly impacted by the fact there are 4 molecules on the product side vs. 2 on the reactant side. The numerator of [N₂][H₂]³ would be sensitive to this. I can’t predict, especially since there are no quantitative values, just “decrease the temperature.” I just can't say for sure which effect will predominate. [Perhaps someone else will see what I'm missing here. If so, please answer in the comments section. Thanks.]

3) Remove N₂ gas:

Moves the reaction to the right, since K_c is a constant and the value for [H₂] would need to increase to compensate.

4) Add H₂ gas:

Moves the reaction to the left, since K_c is a constant and [NH₃] will need to increase.

5) Decrease volume: 

Moves the reaction to the left, since there are more molecules of gas on the product side than on the reactant side, the system will react in a manner that will reduce the pressure that occurs as the container is shrunk. That means reassembling 3H₂ and 1N₂ into two NH₃ molecules, which will reduce the pressure.

I hope these help,

Bob