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# Enthalpy change of neutralisation and reaction watch

1. When working out the enthalpy change of neutralisation, do you work out the number of moles for the acid or alkali? Our teacher told us to use whatever isn't in in excess, but surely using the molar ratio, you can work out the number of moles of what is in excess? Same with enthalpy of neutralisation.

eg. In an experiment to determine the enthalpy of neutralisation of sodium hydroxide with sulphuric acid, 50cm3 of sodium hydroxide, concentration 0.4M was added to 21cm2 of sulphuric acid. A temperature rise of 3.9C was observed. (HINT: assume the acid is in excess)

The ratio of sodium hydroxide to sulphuric acid is 2:1

Because the acid is in excess, you would find the number of moles of NaOH, which is 50/1000 x 0.4 = 0.02.

But surely now you can half the number of moles of sodium hydroxide to find the number of moles of sulphuric acid to get 0.01? So what do you use, 0.02 or 0.01? Thanks in advance!
2. Rewind, what is the definition of 'enthalpy change of neutralisation' ?
3. (Original post by Farseer)
Rewind, what is the definition of 'enthalpy change of neutralisation' ?
oops... one mole of water formed! so you would use the sodium hydroxide as the sodium hydroxide to water ratio is 1:1?

What about for enthalpy change of reaction? how would you know which value of number of moles to use for that?
4. (Original post by krishthakrar)
oops... one mole of water formed! so you would use the sodium hydroxide as the sodium hydroxide to water ratio is 1:1?
Exactly.

What about for enthalpy change of reaction? how would you know which value of number of moles to use for that?
That would depend on the type of reaction/question/calculation, like we tend to think about the enthalpy of a reaction in terms of other enthalpies e.g. bond enthalpy and enthalpy of formation.
5. (Original post by Farseer)
Exactly.

That would depend on the type of reaction/question/calculation, like we tend to think about the enthalpy of a reaction in terms of other enthalpies e.g. bond enthalpy and enthalpy of formation.
Ok! thank you so much for your help I really appreciate it.

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