The sum of its internal energy and the product of its

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-the sum of its internal energy and the product of its pressure and volume H = E + PV Exothermic reaction: gives off heat to its surroundings (feels warm to touch) -example: the burning of gas Endothermic reaction: absorbs heat from its surroundings (feels cold to touch) -example: a chemical cold pack *the heat absorbed by the reaction is not used to increase temperature, but becomes potential energy to store in chemical bonds Calorimetry: a coffee cup is used to measure enthalpy changes for chemical reactions in solution Relationships involving 1. If a chemical equation is multiplied by some factor, then Hrxn is also multiplied by the same factor 2. If an equation is reversed, the Hrxn changes sign 3. If an equation can be expressed as the sum of steps, then Hrxn for the overall equation is the sum of the heats of reactions for each step Calculating the standard enthalpy change for a reaction Subtract the heats of formations of the reactants multiplied by their stoichiometric coefficients from the heats of formation of products multiplied by their stoichiometric coefficients
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