Gen chem question about the effect of temperature on the rate of reaction

[LawNonTrad]

Hey guys,

I have a general chemistry question for you regarding temperature effect on rate of reaction. It's not an MCATquestion (it's actually from my gen chem 2 exam that I got back today) but I figure this is the best place for it because I can get a good answer, and it will probably benefit you guys at least a little bit if you're prepping for the MCAT. I know I re-inforce my understanding of concepts when I explain them to someone else.

The rates of most chemical reactions are sensitive to a change in the temperature of the reaction system. The increase in rate as the temperature increases is best explained by

a. an increase in the activation energy
b. a decrease in the activation energy
c. an increase in the collision frequency
d. an increase in the number of high energy molecules
e. a decrease in the collision frequency.

I eliminated choices a and b right off the bat because there's no catalyst or anything present that would affect the activation energy. I also eliminated e as an answer choice because the temperature is increasing so I knew that something would increase. I chose choice C, that the increase in rate as the temp increases is best explained by an increase in the collision frequency. The scantron says that the answer is choice D, that the increase in the rate as the temp increases is best explained by an increase in the number of high energy molecules.

I remember learning and reading that an increase in temperature increases the rate because both C and D happen. I'm probably missing something though and I'm hoping you guys can point it out.

Thanks.

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[tartrate]

Interesting question, as one of the answer choices is true, but does not best explain the result of increased reaction rates (c).

Basically, increase in temperature produces more molecules that have enough energy to meet the activation energy levels required for a successful reaction.

Why C doesn't best explain what's going on: So imagine you have 5 lanes of marbles, and try to roll them up an incline. If they don't have enough initial velocity, then they will not go over the hump. Just because you roll a higher number of balls, let's say 10 now instead of 5, that does not mean you will have a significantly higher number of balls going over (each lane is like a collision).

Why D is the best answer: Imagine a room full of balloons. If two balloons bump into one another and explode, that is a successful reaction. Now you increase the temperature, which in balloon-land, makes the balloons move around faster. Note that not all balloons will have this faster movement, and the balloons have a minimum bump speed requirement to make it explode. Now that there are more balloons moving at a faster rate, there are more chances to make balloons explode.

 

[Rabolisk]

I don't know if you learned the Arrhenius equation relating rate and temperature. The typical form that you see is k=Ae^-Ea/RT. A reaction occurs when two or more molecules collide with a certain energy that exceeds the activation energy. The -Ea/RT exponent quantitatively shows this. As temperature increases, the number of molecules with sufficient energy increases exponentially. This is why you sometimes hear the rule of thumb that a reaction doubles in rate when you increase the temperature by 10 degrees.

Now, you are also right that the collision frequency increases as a result of higher speed. But this effect is very small. That's why often times we consider the A, or the pre-exponential factor, to be constant and independent of temperature. This factor is assumed to include the collision frequency as well as steric factors. tartrate's explanation is excellent.

 

[loveoforganic]

D is more inclusive. Consider a rxn that's rate dependent on only a single reagent (SN1)

 

[LawNonTrad]

D is more inclusive. Consider a rxn that's rate dependent on only a single reagent (SN1)

Thanks guys for all the help!

Loveoforganic, I haven't had organic yet. Should I know what an SN1 is in gen chem?

 

[Rabolisk]

SN1 is an organic concept. Don't worry about it.

 

[loveoforganic]

What he said. Sorry. Read your whole post but didn't think when making an example