# Enthalpy change what moles do I use?! Help!

I am doing a level chemistry and I'm trying to revise energetics.
I don't get what to use for the mass in q=mct.
Is it mass of solution? But what if it isn't in solution?

Also what moles do I divide the value of q by to get enthalpy change? In combustion it is the moles of fule I think? But what about for other reactions?

Hi!

My exam board is WJEC so I don't know if its different for us but we were taught:

E = -mcΔt
where: m = total mass of liquid (in g assuming 1cm3 = 1g - make sure to add up the total liquid if more than one solution is being mixed together), c = specific heat capacity and Δt = change in temperature to find energy in J (divide by 1000 if you need the answer in kJ)

Then divide the answer by moles using moles = mass/Mr (use the mass of the substance that is dissolved in solution - this is a solid on all the questions I've done) to get the answer in kJ mol-1

Hope this makes sense and good luck!
Original post by ellliejane
I am doing a level chemistry and I'm trying to revise energetics.
I don't get what to use for the mass in q=mct.
Is it mass of solution? But what if it isn't in solution?

Also what moles do I divide the value of q by to get enthalpy change? In combustion it is the moles of fule I think? But what about for other reactions?

I do OCR A and here is my condensed notes for the 3 ways you could use the equation: Please excuse my handwriting i did it with a mouse on PowerPoint lmao.

Lemme know if u need any clarifications

Original post by ellliejane
I am doing a level chemistry and I'm trying to revise energetics.
I don't get what to use for the mass in q=mct.
Is it mass of solution? But what if it isn't in solution?

Also what moles do I divide the value of q by to get enthalpy change? In combustion it is the moles of fule I think? But what about for other reactions?

The 'm' in q=mcDT is the mass of the thing that you stick the thermometer into, i.e. the thing wot gets hot.

The n in DH = -q/n (thank to you charco for that minus sign) is usually the amount of the limiting reagent, that will no doubt be aligned to the 'per mol' bit of the units.

I say 'usually' as DneutH is 'per mol of water', so not necessarily the same as the amount of limiting reagent.
Original post by ellliejane
I am doing a level chemistry and I'm trying to revise energetics.
I don't get what to use for the mass in q=mct.
Is it mass of solution? But what if it isn't in solution?

Also what moles do I divide the value of q by to get enthalpy change? In combustion it is the moles of fule I think? But what about for other reactions?