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Author Topic: Positive or negative enthalpy change?  (Read 453 times)

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jkq2

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Positive or negative enthalpy change?
« on: December 11, 2018, 07:34:08 PM »

I recently took a test in which I got two enthalpy related questions wrong, and I still don’t understand why even after talking to the professor. Even the assistant teachers didn’t understand why I got the questions wrong. I’ve attached images of the two questions below, with the correct answer circled in red.





I emailed my professor explaining my reasoning: For #11, the raise in temperature should make the enthalpy change positive instead of negative. For #16, the reaction has a decrease in temperature, indicating an exothermic reaction and therefore a negative enthalpy change. However, the answer key says it should be a positive change in enthalpy.

And this was my professor’s response: The thermometer is always reporting the temperature change of ‘surroundings’. When a system is consisting of molecules or ions, even the closest solvent molecule belongs to ‘surroundings’.

Does this reasoning make sense? The thermometer was never even mentioned in the question. Is a change in temperature always assumed to be measured with a thermometer?
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mjc123

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Re: Positive or negative enthalpy change?
« Reply #1 on: December 11, 2018, 10:52:42 PM »

In examples like these you are assuming that the insulation is such that there is no significant transfer of heat to the surroundings on the experimental timescale. Your teacher is wrong. The system itself is increasing or decreasing in temperature.

If the reaction is exothermic, heat is given out. In an isothermal set-up, this heat would escape to the surroundings. In an adiabatic (no heat exchange) set-up, this heat can't escape, so it heats up the system. In effect, the heat content of the system remains the same, but some is converted from chemical energy to thermal energy. Or to put it another way,
Q = ΔHrxn + mCpΔT = 0
Therefore ΔHrxn = -mCpΔT
Thus a rise in temperature means an exothermic reaction and a fall in temperature means an endothermic reaction. This has nothing to do with how you measure the temperature.
As to work, chemists and physicists have different conventions as to whether positive work is work done by the system or work done on the system. Make sure you know which convention you are working with.
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