Question

What is the effect on the concentration of Fluorine when stress is added to this equilibrium system?

Describe the effect on the concentration of the bold substance by the following changes:
2HF(g) ⇌ F2 (g) + 2H2(g) ΔH = 536kJ
a. Decrease the temperature - I think it would be decrease
b. Decrease [H2] - I think it would be increase
c. Increase the volume - I think it would be increase

Answer 1

Yes, you have it correct. Here's how I would think about it.

`2"HF"(g) rightleftharpoons "F"_2(g) + 2"H"_2(g)`,

`DeltaH_(rxn) = "536 kJ"`

`a)`

This is an endothermic reaction. So, if you decrease the temperature, you decrease the amount of heat present. That means there is less available for the reaction to absorb and use to proceed.

If the forward reaction absorbs heat, the reverse reaction releases heat.

Therefore, decreasing the temperature forces the reaction to shift to the left, decreasing `["F"_2]`.

`b)`

If there is less `["H"_2]` than there was at equilibrium, then the reaction wants to replenish it, so it must also generate more `"F"_2(g)` in the process, and `["F"_2]` increases.

(You cannot shift towards the products side and only make one of the products.)

`c)`

The way I would think about it is in terms of increasing pressure first.

That decreases the volume, and makes it more "cramped" for the side of the reaction that has more mols of gas. The reaction then shifts towards the side with fewer mols of gas so that it does not have to produce as many gas particles.

In your case, it's the reverse.

You increased the volume, so you decreased the pressure, and the reaction shifts towards the side with more mols of gas. Hence, `["F"_2]` increases.