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The enthalpy change for a reaction is equal to the heat absorbed at a constant pressure.

My notes say that the standard enthalpy change is the heat absorbed when the reactants (in their standard states) are completely converted to products (in their standard states). It uses this as an explanation as to why it is only possible to use heat measurements to determine the standard enthalpy of reaction for a reaction that goes to completion.

I am not sure why it is the case that the reaction must go to completion for heat flow to be equal to the standard enthalpy change.

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In this description, the reactants in the initial state and the products in the final state are all pure. Now, if you only let the system go partially to completion, and then separated all the remaining reactants and products into pure substances at the standard conditions (as part of your process), you could determine the heat of reaction from that also.

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