How do I know which enthelpy change to calculate?

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Determining whether to find the enthalpy of combustion or the enthalpy of formation depends on the specific context of the question. The enthalpy of combustion refers to the energy change when one mole of a substance is completely burned in oxygen under standard conditions, while the enthalpy of formation is the energy change when one mole of a compound is formed from its elements in their standard states. Understanding the requirements of the question is crucial, as there is no universal approach; each scenario will dictate which enthalpy value is needed based on the substances involved and the reactions being considered.
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How do I know if I need to find the enthalpy of combustion or the enthalpy of formation? I know the equation to find the enthalpy change of each them but I don't know on a question if I have to find the enthalpy of combustion or the enthalpy of formation.
 
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It must depend on the question, there is no "one size fits all" answer.
 
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The standard enthalpy change of combustion of a compound is the enthalpy change which occurs when one mole of the compound is burned completely in oxygen under standard conditions, and with everything in its standard state.

https://www.chemguide.co.uk/physical/energetics/definitions.html
 

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