CH₄ + 2 O₂ → CO₂ + 2 H₂O   (all gases)

In this example, the bonds to break are: 4(C-H) and 2 (O=O)
The bonds to make are 2(C=O) and 4(H-O)

That translates into 4(414) + 2(498.7) = +2653.4 kJ
And 2(799) + 4(460) = -3438 kJ

The net enthalpy change for the reaction is thus +2653.4 - 3438 = -785 kJ

The actual value is -802 kJ.

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