AP Chemistry Question of the Day --18

Welcome back to my blog. Here comes the answer to question 17 I posted yesterday. The correct answer is C. 

From the reaction we know, the delta (H) value is negative, which means the reaction is exothermic and energy is released. As a matter of fact, this is a synthesis reaction. All synthesis reactions are exothermic.
From the reaction, we know that if 0.5mol of Cl2 is consumed, 437kJ of heat would be released. According to Hess's Law, if you reverse a reaction, the sigh of the Delta H would be reversed too. If we reverse the reaction, to produce 0.5mol Cl2 from KCl(s), 437kJ of heat needs to be absorbed. Now the question is asking about the energy change when 0.05mol Cl2 is formed. Since the amount of Cl2 is 1/10 of the amount in the reaction equation, the heat that is going to be absorbed would be 1/10 of 437kJ, which is 43.7kJ. So. C is the correct answer.


Following is Question 18 of the series. It is in continuation of Question 17. 
18. The elements K and Cl react directly to form the compound KCl according to the equation below.

What remains in the reaction vessel after equal masses of K(s) and Cl2(g) have reacted until either one or both of the reactants have been completely consumed?



(A) KCl only
(B) KCl and K only
(C) KCl and Cl2 only
(D) KCl, K, and Cl2

I will post the answer and explanation tomorrow. 

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