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Chemistry question help?

Hey, I've been practising chemistry calculations however I just can't seem to get this question right:

Zinc will displace copper from copper (II) sulphate solution according to the following equation:
CuSO4(aq) + Zn(s) ( Cu(s) + ZnSO4(aq)
If an excess of zinc powder is added to 50 cm3 of 1.0 moldm-3 copper(II) sulphate, the temperature increases by 6.3 oC. Calculate the enthalpy change for the reaction.

According to the mark scheme the answer is -26.3 kJmol-1. I got 26334KJmol-1, which is very very wrong.

I did q=mc∆t so 50x4.19x6.3 which equalled 1316.7
To find n I did 1x(50/1000) which equalled 0.05dm-3 and then times that by 1.
Finally I did ∆h=q/n so 1316.7/0.05.

Can anyone see where I went wrong? I think it could be mu second step, finding n as I divided 50 by 1000 but the conc is in moldm-3...
Original post by AlohaCharlotte
Hey, I've been practising chemistry calculations however I just can't seem to get this question right:

Zinc will displace copper from copper (II) sulphate solution according to the following equation:
CuSO4(aq) + Zn(s) ( Cu(s) + ZnSO4(aq)
If an excess of zinc powder is added to 50 cm3 of 1.0 moldm-3 copper(II) sulphate, the temperature increases by 6.3 oC. Calculate the enthalpy change for the reaction.

According to the mark scheme the answer is -26.3 kJmol-1. I got 26334KJmol-1, which is very very wrong.

I did q=mc∆t so 50x4.19x6.3 which equalled 1316.7
To find n I did 1x(50/1000) which equalled 0.05dm-3 and then times that by 1.
Finally I did ∆h=q/n so 1316.7/0.05.

Can anyone see where I went wrong? I think it could be mu second step, finding n as I divided 50 by 1000 but the conc is in moldm-3...


Work out Q=mcdeltaT=50*4.19*6.3=1319.85J= 1.31985 KJ

delta H= -1.31985/(50/1000)=-26.4 KJ/mol (give or take rounding)

REMEMBER deltaH=q/moles Q must be in KJ not J

and since reaction is exothermic, we need the negative sign.
(edited 8 years ago)
Original post by samcrossley28
Work out Q=mcdeltaT=50*4.19*6.3=1319.85J= 1.31985 KJ

delta H= -1.31985/(50/1000)=-26.4 KJ/mol (give or take rounding)

REMEMBER deltaH=q/moles Q must be in KJ not J

and since reaction is exothermic, we need the negative sign.


Thank you! I feel so dumb not picking up something like that! :colondollar:
Original post by AlohaCharlotte
Thank you! I feel so dumb not picking up something like that! :colondollar:


don't worry it's a common mistake.

I personally remember Q=m*c*deltaT*10^-3

that way you won't get it wrong
Original post by samcrossley28
don't worry it's a common mistake.

I personally remember Q=m*c*deltaT*10^-3

that way you won't get it wrong


Okay I'll try that in the future, thank you so much!

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