# enthalpy change

##### Member
10+ Year Member
Calculate the enthalpy change for following reaction:C(s) + 2H2(g)--> CH4(g)

Bond diss energy: H-H is 436 and for C-H is 415

Hf of C (g) is 715KJ/mol

#### Nasem

##### Full Member
10+ Year Member
well... this is off the top of my heat, but I think this is how you do it:

The C(s) is in its standard state so its formation is zero.

Enthalpy change = sum[bonds broken] - sum[bonds formed]

[0 + 2(436)] - [4(415)] = 872 - 1660
= -788

*someone double check my work plz, I might be wrong but I think Im on the right track*

#### iistoogood4u

##### Full Member
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5+ Year Member
[715 + 2(436)] - [4(415)] = 715+872 - 1660
Because C(s) -> C(g)
and the Hf is 715
thus the H rxn would be -73kj/mol

#### Lonely Sol

##### cowgoesmoo fan!
10+ Year Member
[715 + 2(436)] - [4(415)] = 715+872 - 1660
Because C(s) -> C(g)
and the Hf is 715
thus the H rxn would be -73kj/mol

I dont understand why Hf of C(s) should be added. anything in a solid stage doesnt aid in final hf.

#### iistoogood4u

##### Full Member
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5+ Year Member
yes but going from solid to gas reqires the Hf to be added. Same concept as when u compute the total Q reqired to take an ice cube at -10 C to 110 C. You compute the energy needed to bring ice from -10 C to 0, then the H at 0 (the plateau), then 0 to 100, then the H at 100, then 100 to 110. The solid state of C is 0, but the H of formation of C gas reqires energy, a change of state.

#### Lonely Sol

##### cowgoesmoo fan!
10+ Year Member
yes but going from solid to gas reqires the Hf to be added. Same concept as when u compute the total Q reqired to take an ice cube at -10 C to 110 C. You compute the energy needed to bring ice from -10 C to 0, then the H at 0 (the plateau), then 0 to 100, then the H at 100, then 100 to 110. The solid state of C is 0, but the H of formation of C gas reqires energy, a change of state.

Thanks alot for the explanation

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