Question 11

[kazuma]

11. Reaction I is endothermic and hence the actual temperature of the reaction vessel may be different from that expected. Given that the initial temperature of the reaction vessel was used in the calculations, how would this affect the predicted value of n?

1. It would be greater than the actual value. [x]
2. It would be less than the actual value. [x]
3. It would be the same as the actual value. [x]
4. This cannot be determined from the information given. [x]

INCORRECT:
Your Answer: A
Correct Answer: B
Explanation

CHM 4.1.6, 8.2; decrease T(endo) → increase in nactual

Since the reaction is endothermic, the final temperature of the reaction vessel would be lower than the initial temperature. From the equation obtained in the answer for question 48, if the temperature used in the calculation is greater than the actual temperature, the value of n predicted will be less than the actual value.



This concept always confuses me, I understand that when an exothermic reaction happens, it gives off heat, so if you touch a container it will feel hot. and endothermic reaction makes it feel cooler, but isnt an endothermic reaction 'absorbing heat', so shouldn't it raise the temperature?

[admin]

Imagine a solar powered freezer: all day it would absorb heat from the sun in order to reduce temperature. That is analogous to what happens in endothermic chemical reactions.

BTW, just because a reaction is exothermic does not necessarilly mean that the reaction container would be hot. Exo/endothermic are relative terms. If the container was -60 C and after the reaction it is -30, that would be exothermic but it would still be pretty cold!

[kazuma]

makes sense!

so the reaction container has a lower temperature, but what about the actual solution>? the temperature must've gone up right? because it is after all endothermic

[admin]

Endo/exothermic is not referring to the container, they refer to the reaction. If it is exothermic, heat is released and thus the temperature increases (of the reaction mixture and then secondarily of the container if the container can conduct heat). But clearly endothermic is the opposite in all regards: it absorbs or "removes" heat and thus the temperature of the reaction mixture must go down because there is less heat.

[kazuma]

thanks, ill stick to that, i dunno why the concept bothers me so much. I think of ice melting, obviously heat needs to be put into it, and at first the temperature doesn't change, the heat goes into breaking up the chemical bonds (potential energy), than it turns into liquid at which point, putting in more heat (endothermic) will continually raise the temperature. maybe this analogy is bad for reactions, who knows

[admin]

Yes, it is a poor analogy for chemical reactions. It is better to imagine a solar powered freezer. But as you know, no analogy is perfect.

[kazuma]

thx i feel much better about this concept

[fergus_to5501]

What is equation from "question 48" exactly?

[admin]

This thread is for question 11 which is really concept based (it refers indirectly to the ideal gas equation PV = nRT).

[fergus_to5501]

Thanks...that's what I thought this question was about. The answer says "From the equation obtained in the answer for question 48..." for question 11.

[admin]

You are correct. The Explanation had a typo: it said question 48 but it was referring to an earlier question, #9. It has been updated.

[sbains3528]

So, if the since the reaction is endothermic, the reaction temperature that is used in the calculation will be lower than before. So do you just assume based off of the ideal gas law equation that n and T will be inversely proportional when n is solved for in the equation?

[hua8986059]

[mopilas1171]

Sorry, I'm still a little confused. Maybe I'm thinking about this wrong. For this question, I got answer A because I used n=PV/RT. Since the temperature of the reaction was lowered (endothermic), doesn't that meant that T was lower, therefore making n greater?

[mohsinalam5969]

Endothermic means that they take in heat. So if the reaction is taking in all the heat, the heat is decreasing.

Think of the oposite, a reaction that is giving off heat. If something were giving off heat, you would expect it to feel hot. If something is taking in heat, it's going to feel cool.

So it must be kept in mind that we should think about these processes very carefully as these are bit confusing bt are still understandable if carefully thought