Enthalpies of combustion: C(s) -394, H2(g) -286, CH4(g) -890 kJ mol-1. What is the enthalpy of formation of methane, C + 2H2 -> CH4?
Answer and explanation
Correct answer: C. -76 kJ mol-1
- Step 1: Hess's law: . Step 2. . Answer: -76 kJ mol-1. Why not the others: +76 reverses the sign; -1570 adds all three values, using hydrogen once; -210 uses the hydrogen value once and so mis-balances the equation.
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