Suppose you have the following reaction at equilibrium:
N2(g) + 3H2(g) = 2NH3(g), ΔH = −22 kcal/mol
What changes are expected to occur if the temperature of the system was decreased. You may select multiple answers.
The concentration of NH3(g) would decrease
The concentration of NH3(g) would increase
The concentration of N2(g) would decrease
The concentration of H2(g) would decrease
No change in the equilibrium would occur.
The concentration of H2(g) would increase
Le Chatelier's Principle tells us that if we do something to a system at equilibrium that the system will react so as to undo what we did to it. I will rewrite the equation like this,
N2(g) + 3H2(g) ==> 2NH3(g) + heat.
So if we decrease T, it will try to increase T. How can it do that. More N2 and H2 can react to produce more NH3 and heat. So N2 will decrease, H2 will decrease, NH3 will increase, and the system will shift to the right. Pick out the choices that meet this reaction.
To determine the effect of decreasing temperature on the given reaction, we can consider Le Chatelier's principle. According to this principle, if a stress is applied to a system at equilibrium, the equilibrium will shift in a way that counteracts the stress.
In this case, decreasing the temperature is considered a stress on the system. To understand the effect, we need to determine whether the reaction is exothermic (releases heat) or endothermic (absorbs heat).
The given value for ΔH is -22 kcal/mol. Since the ΔH value is negative, it indicates that the reaction is exothermic, meaning it releases heat.
When the temperature of an exothermic reaction is decreased, the system will shift in the direction that produces more heat to counteract the temperature decrease. In this case, the system would shift to the right to produce more heat because the forward reaction is exothermic (2NH3(g) → N2(g) + 3H2(g)).
Therefore, the concentration of NH3(g) would increase, indicating that one of the expected changes is "The concentration of NH3(g) would increase."
Additionally, since the reaction shifts to the right, the concentration of N2(g) and H2(g) would decrease because they are reactants in the forward reaction. Therefore, the expected changes also include "The concentration of N2(g) would decrease" and "The concentration of H2(g) would decrease."
On the other hand, since the reaction shifts in the direction that produces more NH3(g), the concentration of NH3(g) would not decrease. Therefore, the option "The concentration of NH3(g) would decrease" is incorrect.
As for the concentration of H2(g) increasing, decreasing the temperature would not cause an increase in the concentration of H2(g) since it is a reactant in the forward reaction. Therefore, the option "The concentration of H2(g) would increase" is incorrect.
To summarize, the expected changes if the temperature of the system was decreased are:
- The concentration of NH3(g) would increase.
- The concentration of N2(g) would decrease.
- The concentration of H2(g) would decrease.