ChemistryA LevelEnergetics & Thermodynamics

Bond enthalpies formula

The bond enthalpies formula as it is used in A Level Chemistry, taken from the A Level Chemistry formula sheet.

The formula

\Delta H = \sum(\text{bonds broken}) - \sum(\text{bonds made})

Write the formula down before substituting — most mark schemes award a method mark for a correct, clearly quoted formula.
Check every quantity is in consistent units before you substitute, then substitute in one step so the working stays easy to follow.
Show the rearrangement rather than jumping to the answer, and give the final value to the accuracy the question asks for.

q = mc\,\Delta T

\Delta G = \Delta H - T\Delta S

\Delta S = \sum S_{\text{products}} - \sum S_{\text{reactants}}

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