# Calculate the standard enthalpy of formation for diamond, given?

Calculate the standard enthalpy of formation for diamond, given that

C(graphite) + O2(g) → CO2(g) ΔH0 = –393.5 kJ/mol

C(diamond) + O2(g) → CO2(g) ΔH0 = –395.4 kJ/mol

Is it 1.9 or -1.9 and why?

### 5 Answers

- 1 decade agoBest Answer
It has to be 1.9 kJ/mol.

The reason being, you are looking for the enthalpy of FORMATION of DIAMOND.

So when you apply Hess' Law, the second equation MUST be flipped so you get this equation...remember when you reverse the reaction, you must change the sign of the enthalpy (H).

CO2(gas) -> C(diamond) + O2(gas) , H=+395.4 kJ/mol

Then you retain your graphite equation and add these two equations together.

C(graphite) + O2(g) -> CO2 (g) , H = -393.5 kJ/mol

CO2(g) -> C(diamond) + O2(g) , H = +395.4 kJ/mol

The end result when you add these two together is this equation...note that the CO2 and O2 gas drops out of the equation:

C(graphite) --> C(diamond) , H = -393.5 + 395.4 = +1.9 KJ/mol

Remember, you flipped the second equation and not the first because you wanted to FORM , i.e. you are looking for the enthalpy of FORMATION, of C(diamond)

- Anonymous4 years ago
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Calculate the standard enthalpy of formation for diamond, given?

Calculate the standard enthalpy of formation for diamond, given that

C(graphite) + O2(g) → CO2(g) ΔH0 = –393.5 kJ/mol

C(diamond) + O2(g) → CO2(g) ΔH0 = –395.4 kJ/mol

Is it 1.9 or -1.9 and why?

Source(s): calculate standard enthalpy formation diamond given: https://tr.im/Qj6AS - wibblytumsLv 51 decade ago
If you reverse the second reaction, then add the two togther, the CO2 and O2 cancel out

C (graphite) + O2 (g) ---> CO2 (g)

CO2 (g) ----> C (diamond) + O2 (g)

and you get

C(graphite) ---> C(diamond)

By Hess's Law, you change the sign of the enthalpy for the second reaction (because you reversed it), and add it to the enthalpy of the first.

So, the enthalpy change is -393.5 kJ/mol + (+395.4 kJ/mol)

or +1.9 kJ/mol.

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