AP Chemistry Question of the Day --17

Welcome back to my blog. Here comes the answer to question 16 I posted yesterday. The correct answer is B. 

Chemical equilibrium is dynamic equilibrium. When the reaction is at equilibrium, both the forward and backward reactions are still going on. They are at the same rate and both of them are not zero. Answer A can be eliminated.
One key concept here is that the reaction rate increases when the concentration of the reactants increases. When there is a sudden increase in the concentration of chlorine gas, the equilibrium will be broken and there will be a sudden increase of the backward reaction rate. As chlorine concentration lowers, the backward reaction rate drops down too. The increase in reaction rate is a sudden increase, not a gradual increase. Answer D can be eliminated.The difference between answer B and C is that the equilibrium rate is higher than the original rate in B and lower than the original rate in C. How can we figure that out? If we consider from the backward direction, both chlorine and PCl3 is being consumed. We do not know how the final equilibrium concentration will be compared to the original. But one thing to remember is that when the equilibrium is regained, the final concentrations of PCl5 is higher than original, which means the forward reaction rate is higher than the original forward reaction rate. Since at equilibrium, the forward and the backward reaction rates are equal, we can conclude when the equilibrium  is regained, the backward reaction rate will rest at a speed higher than the original backward reaction rate. Answer B is the right answer.

Following is Question 17 of the series. 
17. The elements K and Cl react directly to form the compound KCl according to the equation below.

How much heat is released or absorbed when 0.050 mol of Cl2(g) is formed from KCl(s)?


(A) 87.4 kJ is released
(B) 43.7 kJ is released
(C) 43.7 kJ is absorbed
(D) 87.4 kJ is absorbed

I will post the answer and explanation tomorrow. 

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