Chemistry Olympiad question

Hi, please could i have help on part c ii of this question? I just wanted to check whether the answer to this question should be negative? The mark scheme says it’s positive?
Here is the question: https://app.gemoo.com/share/image-annotation/607825281284747264?codeId=DGaGG1Vw68ErY&origin=imageurlgenerator

Thanks!

Reply 1

bl0bf1sh

15

Original post by anonymous294

Hi, please could i have help on part c ii of this question? I just wanted to check whether the answer to this question should be negative? The mark scheme says it’s positive?
Here is the question: https://app.gemoo.com/share/image-annotation/607825281284747264?codeId=DGaGG1Vw68ErY&origin=imageurlgenerator

Thanks!

The question's asking for the energy released – a negative value would suggest that energy has been taken in; due to the wording of the question and the fact that it's a combustion reaction it'll probably be a positive value.

Note the difference between enthalpy and energy – enthalpy essentially refers to the system, i.e. the hydrogen molecules. So whilst you'd calculate the enthalpy change to be negative (a loss from the system), the value for the energy released would be positive (just change the ± sign from negative to positive) :smile:

Reply 2

anonymous294

OP

11

Original post by bl0bf1sh

The question's asking for the energy released – a negative value would suggest that energy has been taken in; due to the wording of the question and the fact that it's a combustion reaction it'll probably be a positive value.

Note the difference between enthalpy and energy – enthalpy essentially refers to the system, i.e. the hydrogen molecules. So whilst you'd calculate the enthalpy change to be negative (a loss from the system), the value for the energy released would be positive (just change the ± sign from negative to positive) :smile:

Sorry could you explain what you mean when you said how the question is worded, isn’t enthalpy released always a negative value? Thanks!

Reply 3

bl0bf1sh

15

Original post by anonymous294

Sorry could you explain what you mean when you said how the question is worded, isn’t enthalpy released always a negative value? Thanks!

Enthalpy encompasses the internal energy of the system (enthalpy = internal energy + (pressure x volume); I don't think you need to know this at A level), so you could think it as energy in a molecule. Energy released refers to the energy that is lost from the system into the surroundings.

This reaction is a combustion reaction, which means that energy is released in the "break bonds, make bonds" process – the reaction is exothermic; heat (energy) is lost to the surroundings. This suggests that the products have a lower internal energy than the reactants, hence the negative value for the enthalpy change.

Bond enthalpy basically tells you how much energy is needed to break a bond. So if the sum of the bond enthalpies of the reactants is greater than the sum of the bond enthalpies of the products, there's going to be some "spare" energy that gets released. The ΔH value will be negative, because (bond enthalpy of products) < (bond enthalpy of reactants). But because energy is released into the surroundings, then the "energy released" value must be positive because if it were negative it would suggest that energy has been lost from the surroundings/taken up into the molecules.

If the bond enthalpy of the products is 100 kJ lower than the bond enthalpy of the reactants, then enthalpy change = -100 kJ, and therefore energy released = +100 kJ.

If that makes any sense?

https://www.chemguide.co.uk/physical/energetics/basic.html
https://www.chemguide.co.uk/physical/energetics/bondenthalpies.html

Chemistry Olympiad question

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