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# GCSE Chemistry Question Help? :3 watch

1. Hello ,
I was wondering if someone could help me understand how to solve this.
Using the formula: Q= mcΔT
(where Q is energy release, m is water or in this case, solution mass, c being the specific heat capacity of water, and for solutions, 4.2'C, and ΔT being temp change).

So what I want to know is HOW to calculate this question:

When 5.6g of iron filings reacted completely with 200cm^3 of copper (II) sulphate solution the temperature of the solution increased by 17'C. Calculate the energy change in KJ/mole of iron. (RAM of Fe- 56).

I don’t see where I’m going wrong? I have been told to assume the S.H.C of any aqueous solution as 4.2K/g'C, and I have also been told to assume the solutions volume doesn’t change when adding a solid to it.

I did:
Q= 200 x 4.2 x 17,
& then divided it by 56 to get it per mole, but that was incorrect.

'142.8 KJ/mol of iron'
With absolutely no explanation as to why?

Any help would be gratefully received
Many thanks-
2. You're using 5.6 grams of Iron... moles = mass / RFM!
3. ahhhhhh! Of course where is my brain today?
Thanks alot
4. Hi - I was also having difficulty with this question.
In the book it says:
Energy change in kJ/mol can be calculated by multiplying the energy change by the relative formula mass of the substance.

200x4.2x17 = 14,280J/g
Moles in 5.6g of iron = 5.6/56 = 0.1
If the answer is 142.8, why has 14,280 been divided by 0.1?

Thanks
5. (Original post by sparrows)
Hi - I was also having difficulty with this question.
In the book it says:
Energy change in kJ/mol can be calculated by multiplying the energy change by the relative formula mass of the substance.

200x4.2x17 = 14,280J/g
Moles in 5.6g of iron = 5.6/56 = 0.1
If the answer is 142.8, why has 14,280 been divided by 0.1?

Thanks
The energy is worked out for only 0.1 moles of reacting iron. The answer is required "per mole",. so it must be scaled up by multiplying by 10 (or dividing by 0.1, which is the same operation)

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