Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: stevemont7 on November 10, 2015, 03:34:22 PM
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Hello,
On a test, the following information preceded a question:
H+(aq) + OH-(aq) :rarrow: H2O(l)
A student is asked to determine the molar enthalpy of neutralization, ∆Hneut , for the reaction
represented above. The student combines equal volumes of 1.0 M HCl and 1.0 M NaOH in an open polystyrene cup calorimeter. The heat released by the reaction is determined by using the equation q = mc∆T.
The last of the questions that followed asks:
Suppose that a significant amount of heat were lost to the air during the experiment. What effect would this have on the calculated value of the molar enthalpy of neutralization, ∆Hneut ? Justify your answer.
I wrote, "the calculated value of ΔHneut would increase because the heat lost to the air will cause the value of ΔT to be smaller."
This is incorrect, apparently, and the correct answer is supposed to be that ΔHneut decreases. However, since the reaction is exothermic, would ΔH not be negative? So since the question does not ask for change in the magnitude of ΔH, why is saying it increases incorrect?
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You're correct (I think; if someone else can argue otherwise please do so).
The loss of heat to the air would decrease the magnitude of ΔT and subsequently decrease the magnitude of ΔHneut (or make it a less negative number which is an "increase" in the value). Perhaps you can bring it up to your teacher/professor and let him or her know that you understand the concepts behind the question but got tripped up on semantics. Hopefully once they see you understand the concept they might give you some/all of the points back.
*Edited to change neut to neut
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Thank you for your response, mikasaur. I will talk to my teacher about it. I did not lose terribly much credit because of this particular question; however, I made a few careless errors (something I very rarely do), which further reduced my score and damaged my 99 average, so I would like to get any credit back that I can. ;)
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I agree that you are technically correct, but I would always in such a situation be quite explicit, to avoid any possibility of misunderstanding. So rather than saying
"the calculated value of ΔHneut would increase because..."
I would say
"the calculated value of ΔHneut would increase (i.e. become less negative) because..."
or maybe
"the calculated value of ΔHneut would decrease in magnitude because..."
Then the reader is clear that you have understood the situation correctly, even if you haven't used their preferred form of words.
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I see what you're saying, mjc123. From now on I'll be sure to specify what I mean on any similar questions where confusion might occur.