Gen chem II -DAT questions

[purplepanda]

Thank you guys in advance.

1. The gas-phase reaction, A(2) + B(2)-->2AB proceeds by bimolecular collisions between A2 and B2 molecules. If the concentrations of both A2 and B2 are doubled, the reaction rate will change by a factor of:
ANSWER: 4

How do you know that this is the rate equation: rate=k[A] + ? Where is it given away in the question? Bimolecular?

2. The activity of a radioisotope is 3000 counts per minute at one time and 2736 counts per minute 48 hours later. What is the half-life of the radioisotope?
ANSWER: 361 hr

What are they talking about? Counts per minute?




3. The half-life for the first order conversion of cyclobutane to ethylene, is 22.7 s at a particular temperature. How many seconds are needed for the partial pressure of cyclobutane to decrease from 100mmHg to 10mmHg?
ANSWER: 75.4 s............I know how to do this one. But is there anything like this on the DAT because you have to use natural log to figure this one out.

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[ethanyoo726]

For #1, rate = k [A2][B2]. So if let's say both concentrations are 1, so rate would be 1 (ignore the constant k). If you double both A2 and B2, then you have 2 for each, so the rate increases by a factor of 4.

For #3, you don't need to use any hard natural logs. You just need to deduce to the answer. Through each half life, the sample decays/degrades by 50% or 1/2 of its former weight. So in order to get from 100mmHg to 10mmHg...the half life weights are as follows....
- 0 half life = 100 mmHg
- 1 half life = 50 mmHg
- 2 half life = 25 mmHg
- 3 half life = 12.5 mmHg
- 4 half life = 6.25 mmHg

So from this chart, you can see that in order to get to 10mmHg, you need to fall somewhere between 3-4 half lives. So 22.7s * 3 half life = 68.1s and 22.7 * 4 half life = 95.8s. Thus you know that your answer falls between 68.1s - 95.8s. Scan the answer choices and 75.4s falls nicely in that range.

 

[purplepanda]

Thank you so much

How did you know that the first one was second order? I don't see where they tell you that in the question...does "bimolecular" mean second order?

 

[ethanyoo726]

The problem gives you the equation of how both A2 and B2 are needed to make the product. It's bimolecular because two molecules are involved - gas A2 and gas B2.