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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AP Chemistry Question of the Day --65

Welcome back to my blog. Since most schools are doing the unit on acid and base at this time of the year and here is another question related to this topic.

Question 65: Four acids HA, HB and HC and HD solutions have the same pH, which of the following statements is correct:
A. They all have the same concentration
B. They are all strong acids
C. They are all weak acid
D. not enough information to make any conclusion

The correct answer is D. pH represents the concentration of hydronium ion in solution. The pH if an acidic solution is dependent on both the Ka and the concentration of the acid. A solution of a strong acid can have a higher pH due to lower concentration and a solution of a weak acid cna have a lower pH due to higher concentration.

Hope this helpful.

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AP Chemistry Question of the Day --64

I have not written here for a long time. AP season is coming soon in a couple of months. I will resume my series and hopefully it is helpful to anyone who come across to visit this site. Instead of posting a question and answer it the next day, I am going to post the question and answer at the same time.

Question 64: Which one of the following cidic solutions requires the most 0.1000M NaOH aqueous solution to neutralize completely:
A. 0.1000M HCl 25ml
B. 0.1000M HF 25ml
C. 0.05000M H2SO4 25ml
D. 0.05000M H2CO3 25ml
E. They are all the same

The answer to this question is E. When an acid and a base react, whenever there is a strong acid or base involved, the reaction will go to completion. The amount of base required will be totally determined by the mole number of the protons in the acid. It does not matter the acid is strong or weak, monoprtotic or diprotic. It is the number of moles of H+ that is in the acid that determines how much base will be needed to completely neutralize.

Hope this helps with your understanding on acid and base and thank you for visiting my blog.