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Energetics
Exothermic and Endothermic reactions
Exothermic Reactions

Heat energy is given out to the surroundings so the H is negative
The energy needed to break the bonds is less than the energy needed to make the bonds

Endothermic Reactions

Heat energy is taken in from the surroundings so the H…

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Calculate the overall enthalpy change for the following reaction:
C (s) + 2H 2(g) CH 4 (g)
Hc

C (s)= -393 H 2 (g)= -286 CH 4= -890

H=
(-393 + (2 x -286))­ (-890)
H=
-75


Hess's Law
Hess's Law - The enthalpy change of a reaction depends only…

Page 3

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Bond H­H O=O C­C C=C C­H F­F H­O Br ­ Br H ­ Br

KJ mol-1 436 496 348 612 412 158 463 193 366


H­H + Br ­ Br 2 H ­ Br

H = 463 + 193
= 656

H = 2 x 366
= 732

H =…

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