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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?

Please help!!! Thank you!
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?

Please help!!! Thank you!
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 :smile:

Q=MCDELTAT.png
Reply 3
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?

Please help!!! Thank you!

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?

Please help!!! Thank you!

If you are doing enthalpy of combustion, use the mass of WATER ONLY. Don't include the mass of fuel in your q=mcat calculation.

1cm3 of water = 1g, so e.g 250cm3 of water you would use 250g for the mass.

There is no solution in this scenario, its just mass of water being heated. At the end you do q/moles to get the enthalpy. Moles is calculated using the mass of FUEL ONLY. Do the mass of fuel divided by its Mr to get the moles and voila theres your answer.
Reply 6
if you arent given the mass of water that is heated when trying to find the specific heat capacity from the enthalpy of solution, do you keep mc together? if so, why? (even when given the mass of the fuel used why cant we use that mass?)

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