Enthalpy question

[IntelInside]

EK 1001 #369:

Under standard conditions, which of the following reactions will have the greatest difference between enthalpy and internal energy of reaction?

A. 2C(graphite) + O2(g) --> 2CO(g)
B. C(graphite) + O2(g) --> CO2(g)
C. C(graphite) --> C(diamond)
D. CO(g) + NO2(g) --> CO2(g) + NO(g)

Answer is A.

Now because it says standard conditions do I assume then that we are dealing with constant pressure and if so, I understand then that dH=q at constant P and U = q - w at constant P. This obviously would be expansion for reaction A and U would be less than d.H, correct?

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[lorenzomicron]

what do u mean by energy of reaction? i thought enthalpy change was energy of reaction?

 

[IntelInside]

They mean change in internal energy according to them.

 

[lorenzomicron]

deltaH= deltaE + PdeltaV (work)

deltaH-deltaE= P*deltaV (work)

so, under standard conditions, the one with the greatest volume change will have the greatest difference. so ur reasoning is right

 

[Rosalindbungs]

Lorenzomicron, why would the volume change be greatest for answer choice a? Why not answer choice b?

 

[MCAT guy]

typically the internal energy is the heat of reaction or enthalpy BUT it isn't always true. this is the whole point of a bomb calorimeter that keeps the volume constant. The idea here is that the entire energy inside of the molecule will be converted into either HEAT or energy to change the volume.

So, if we fix the volume or hold it constant, then the total energy will be the same as the heat of reaction, but if we allow the volume to change then some of the energy can be used to perform work (which means changing the volume of the system).


The question by the OP is correct. It would be expansion and U is greater than the delta H

Choice A is the better choice because there is an increase in volume from reactants to products while in choice B there is an equal amount of gas volume on each side.