which of the following stresses will not cause a shift in equilibrium?

Question

H2[g] + I2[g] <-> 2HI[g] + energy

a) decrease [I2]
b) increase [H2]
c) decrease volume
d) increase temperature

and why?

Answer 1

(c) decrease volume.

If you decrease volume, you increase the pressure. When a system (reaction) experiences an increase in pressure, it will favour the side of the reaction where the least amount of mols of gas is produced. The left side has 2 mols of gas (1 mol of H2 and 1 mol of I2) and the right side also has 2 mols of gas (2 mols of HI), therefore neither side is favoured by an increase in pressure. That is, both sides of the reaction have the same amount of gas produced so favouring one side cannot decrease the amount of gas present. Therefore, no shift in equilibrium.

Answer 2

Any change in the concentration of one of the components would cause a shift (Le Chatelier's principle) so a and b are rejected.

An increase in temperature would shift the equilibrium in favour of the endothermic part. In this case you have energy released on the right-hand side thus an increase in temperature would shift the equilibrium to the left.

C is the correct answer and not just because weeliminated all the others. For the equilibrium you have the concentration equilibrium constant

Kc= [HI]^2/[H2][I2] = (moleHCl/V)^2 / (moleH2/V)*(moleI2/V)
as you can see the stoichiometry is such that volume is simplified away from the equation and the Kc for this reaction is independent of the volume.

Answer 3

Well decreas in volume will not cause a shift
because you should remeber or know that
volume/pressure causes a shift only if the number of gas molecules are unbalance

increase pressure High -> Low
decrease pressure Low-> High

In this case there are two molecules of gas each side.
(Count up the coefficient)

Therefore, there will be no shift even if you decrease volume.

Answer 4

well I know it isn't d because increasing temp will favor to the left and decreasing temp will favor to the right.

I know it isn't a or b because decrease/increase in concentrations always affect the equilibrium because of Le Chatelier Principle...

So I would say it is C but I don't have a reason why...hmmm...