# Enthalpy

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If the change in enthalpy is calculated for a reaction at two different temperatures and pressures, and afterwards compared to another reaction for which the chamge in enthalpy is also calculated at the same pressures and temperatures, will it still be the same reaction that has the highest change in enthalpy?
Thanks

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In case this is homework , ?

here is a hint

Enthalpy is a state function so this should answer some question.

The question as written in meaningless.
What did you mean to say?

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Well I think it can be boiled down to whether the change in enthalpy is independent of pressure and temperature as long as they are constant?

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Let me ask again for a proper statement of the question which reads

30 minutes ago, africanhorse said:

If the change in enthalpy is calculated for a reaction at two different temperatures and pressures, and afterwards compared to another reaction for which the chamge in enthalpy is also calculated at the same pressures and temperatures, will it still be the same reaction that has the highest change in enthalpy?

What do you mean by "will it still be the same reaction highest change in enthalpy"  ?

You haven't said which one has the "highest chnage in enthalpy"

So the same as what?

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well, I have two unknown reactions at two sets of conditions. All i know is that one has a higher change in enthalpy at the first set of conditions, but I dont know whether it would be the same at the other set of conditions.

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I gave you a hint about enthalpy as a state function (for this question) what does that mean  and what is your definition of enthalpy?

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It isn't homework but rather preparation for an exam.

well if it is state function then the change in enthalpy should be independent of the steps in between the reactants and the products. And the definition of enthalpy is the amount of heat transferred to or from the system. Right?

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Posted (edited)

OK. I'll let Studiot press you for more data, but AFAICT, you're trying to keep too many things constant. Enthalphy is useful for processes at constant P, in which case the change in enthalpy can be identified with the heat (isobaric,) but those processes are not at constant T, in general. If you keep P, T, n constant, V is determined and thus constant too (by the equation of state,) and there goes your process.

Also, a detailed specification of which processes you would like to compare would be very useful, if not absolutely necessary. The more specific you are, the better answer you're going to get.

Edited by joigus
correction: work --> heat

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Posted (edited)
4 hours ago, africanhorse said:

It isn't homework but rather preparation for an exam.

Why didn't you say so before?

So we could stop all this shilly-shallying about?

5 hours ago, africanhorse said:

well, I have two unknown reactions at two sets of conditions. All i know is that one has a higher change in enthalpy at the first set of conditions, but I dont know whether it would be the same at the other set of conditions.

So I take it these are chemical reactions?

In Physics the substances in the system are taken as non interactive with each other.
That is they don't react chemically because chemical reactions have heats (enthalpies) of reaction to consider.
So chemical reactions are more complicated.

So I would expect to see statements of the reactants and products.

Are the reactants in the two systems the same ?
What ends up at the same P1 ,T1 and P2, T2 ?

The whole question is best served by some examples.

Say for instance mixtures of oxygen and hydrogen, oxygen and carbon monoxide, oxygen and methane......

?

So you have system 1 with reactants A and B in state α  and products C and D in state β

and system 2 with reactants E and F in state α and products G and H in state β

Are P and T sufficient state variables to specify these states ?

Edited by studiot

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