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# Energetics question? watch

1. why is there another - sign on the -248?
2. (Original post by gomc)
why is there another - sign on the -248?
ΔHr = Σ[ΔHf(products)] - Σ[ΔHf(reactants)] = ΔHf(CH3CHOHCH3) - [ΔHf(CH3COCH3) + ΔHf(H2)]ΔHf(H2) = 0 by definition

Therefore ΔHr = ΔHf(CH3CHOHCH3) - ΔHf(CH3COCH3)
ΔHf(CH3CHOHCH3) = -318 kJ mol-1
ΔHf(CH3COCH3) = -248 kJ mol-1

ΔHr = -318 -(-248)
= -318 + 248
= -70 kJ mol-1

There's two minus signs because you have to subtract the enthalpy of formation of CH3COCH3 which is negative itself.

Hope this helps you out.
3. (Original post by Jpw1097)
ΔHr = Σ[ΔHf(products)] - Σ[ΔHf(reactants)] = ΔHf(CH3CHOHCH3) - [ΔHf(CH3COCH3) + ΔHf(H2)]ΔHf(H2) = 0 by definition

Therefore ΔHr = ΔHf(CH3CHOHCH3) - ΔHf(CH3COCH3)
ΔHf(CH3CHOHCH3) = -318 kJ mol-1
ΔHf(CH3COCH3) = -248 kJ mol-1

ΔHr = -318 -(-248)
= -318 + 248
= -70 kJ mol-1

There's two minus signs because you have to subtract the enthalpy of formation of CH3COCH3 which is negative itself.

Hope this helps you out.
Thank you!

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