Question

hydrogen and fluorine react to form hydrogen fluoride, like this:

h₂(g) + f₂(g) → 2hf(g)

the reaction is exothermic. suppose a mixture of h₂, f₂ and hf has come to equilibrium in a closed reaction vessel. predict what change, if any, the perturbations in the table below will cause in the composition of the mixture in the vessel. also decide whether the equilibrium shifts to the right or left

perturbation change in composition shift in equilibrium

the temperature is lowered. the pressure of f₂ will select to the right

to the left

(none)

the temperature is raised. the pressure of hf will select to the right

to the left

(none)

Explanation:

According to Le - Chatelier's principle, for an exothermic reaction (\(H_2(g)+F_2(g)
ightleftharpoons2HF(g)\), \(\Delta H<0\)):

• When the temperature is lowered: The system will try to counteract the change. Since the forward reaction is exothermic (releases heat), lowering the temperature favors the forward reaction. As the forward reaction proceeds, more \(HF\) is formed and the concentrations of \(H_2\) and \(F_2\) decrease. So the pressure of \(F_2\) will decrease and the equilibrium shifts to the right.
• When the temperature is raised: The system will try to absorb the added heat. The reverse reaction (endothermic) is favored. As the reverse reaction proceeds, \(HF\) decomposes into \(H_2\) and \(F_2\). So the pressure of \(HF\) will decrease and the equilibrium shifts to the left.

Answer:

perturbation	change in composition	shift in equilibrium
The temperature is raised	The pressure of \(HF\) will decrease	to the left