Chem Textbook Notes 4

lesson 5 . / - Hess 's Law & standard Enthalpies of Reaction Hess 's Law det s the principle that the enthalpy of reaction ( OH rxn ) for a process that is the sum of 2 or more reactions is equal to the sum of the OH rxn values of the constituent reactions ; aka Hess 's law of constant heat of summation in simpler terms : if a process is the result of multiple reactions , add the Otlrxn of each reaction to find total OH rxn add 0 Hrxn of intermittent steps sum for overall products and reactants will cancel out in these steps standard Enthalpy of Reaction ( OH r × n) definitions enthalpy change associated w/ a reaction that takes place under standard conditions , aka standard heat of reaction standard conditions a pressure of 1 barr flat m) and some specified temperature ( usually 25°C unless otherwise stated ) ; for solutions : concentration of 1 M standard state - most stable form of a substance under standard conditions
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