Reaction rates, temperature, and thermal energy

[brake4country]

Homework Statement

Hi there. My question is more for clarification than a homework question. In my studies, I noticed that my MCAT study manual provides problems relating reaction rates and the effect that temperature and thermal energy have on a reaction. For example, a question asks: in an exothermic reaction, what affects the ratio of the forward rate to the reverse rate?

Homework Equations

The Attempt at a Solution

The correct answer is: removing thermal energy from the system. This answer makes sense. But, in another similar problem, the question is based on what would happen if the temperature is increased. One would think that it would shift the equilibrium to the reactants in an exothermic reaction. What am I missing here? How can the rates of both the forward and reverse be increased when temperature is increased? Thanks in advance.

 

[honlin]

It is true that the reaction will shift to the reactants due to Le Chatelier's principle, however when you increase the temperature, the reactant and product molecules will gain energy and move more vigorously. The rate of collision between the molecules increases. Thus even though the equilibrium does shift to the left, the rate increases because the molecules gain energy and make the chemical reaction occurs more rapidly.

 

[brake4country]

I see. Temperature will increase the the forward rate of a reaction even if it is exothermic. Removing thermal energy from an exothermic reaction will affect the ratio of the forward rate to the reverse rate by shifting it toward the products, right? I think I am getting the hang of this. I was confusing what affect temperature would have on a reaction and how removing thermal energy would affect it. Two different questions with different consequences.

 

[honlin]

Yup you are right =) le chatelier principle is all about achieving equilibrium while rate of reaction can be explained through kinetic theory of particles.

 

[Borek]

How can the rates of both the forward and reverse be increased when temperature is increased?

It is not absolute values that matter, but their ratio. If both are increased by the same amount such that the ratio doesn't change, equilibrium doesn't shift at all. However, typically changes are different and the ratio changes as well.

 

[brake4country]

It is not absolute values that matter, but their ratio. If both are increased by the same amount such that the ratio doesn't change, equilibrium doesn't shift at all. However, typically changes are different and the ratio changes as well.

Got it. Equilibrium can be maintained as long as the ratios are not affected. Thanks for the clarification!