AP Chemistry Question of the Day--68

I was tutoring a student and came across the following question, I found it to be a good question regarding several concepts in chemistry.

A 2cm Mg ribbon was reacted with 100ml 0.10M HCl and at the end of the reaction, all the Mg ribbon has dissolved. The temperature of the reaction mixture increased by 20 degrees. Another 1cm Mg ribbon was reacted with 0.1M HCl in a separate reaction. How much HCl solution is required to make the temperature increase 10 degrees?

A. 50mL
B. 100mL
C. 200mL
D. Could not be decided.

The correct answer is B.
Mg reacts with HCl and produces MgCl2 and Hydrogen gas. If at the end of the reaction, all the Mg has dissolved, it means that Mg was the limiting agent. If half the length of Mg was used in the second experiment, then the amount of heat released should also be halfed.
According to q=mc(deltaT), both the q and delta T were half of the original, the m should stay the same. m here is the mass of the total reaction mixture. If we assume the reaction mixture has the same density as water, then we can deduce that the mass of the reaction mixture depends on the volume of the HCl solution. As a result, the volume of the HCl solution should remain as 100mL.
A common mistake would think the m in q=mc(deltaT) refers to the mass of the reactants. Because the temperature increase occurs to the overall reaction mixture, the mass should always be the mass of the total reaction mixture, not the mass the reactants themselves.

Hope this helps with your AP review. Good luck in your exam in May.

AP Chemistry Question of the Day --67

It has been a while since I last posted an AP Chem question. I recently got an email acknowledging me that this blog is helpful. I am glad to hear it and decided to resume it. Here comes question 67.

The equilibrium constants for the following reaction:

N2(g) +O2(g) ⇄2NO(g)

are Kp=1.1e^3 at 2200K and Kp=3.6e^3 at 2500K respectively. Which of the following statement is true:

A. Higher total pressure shifts the equilibrium to the left.
B. The partial pressure of NO(g) is less at 2200K than at 2500K
C. The total pressure at 2200K is the same as at 2500K
D. The reaction is exothermic
E. Kp is less than Kc by (RT)

The correct answer is B.

Since both the reactant side and the product side have 2 moles of gas. The change in pressure or total volume will not shift the equilibrium, which eliminate A.
C sounds right but is actually wrong because as the total mole of the gases and the volume of the gases stay unchanged, the total pressure will increase as a result of the temperature change. The total pressure should be higher at 2500K than at 2200K.
As the temperature increases from 2200K to 2500K, the equilibrium constant increases, indicating the  forward reaction is endothermic.
Since there are equal number moles of gases on both sides, the Kp should be equal to Kc

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