Enthalpies of Reaction

# Enthalpies of Reaction - property The magnitude of ∆ H is...

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Enthalpies of Reaction Because the enthalpy change for a reaction is described by the final and initial enthalpies: H = H final - H initial we can also describe H for a reaction by comparing the enthalpies of the products and the reactants: H = H(products) - H(reactants) The enthalpy change that accompanies a reaction is called the enthalpy of reaction ( H rxn ). It is sometimes convenient to provide the value for H rxn along with the balanced chemical equation for a reaction (also known as a thermochemical equation ): 2H 2 (g) + O 2 (g) -> 2H 2 O(g) H = -483.6 kJ Note the following: H is negative, indicating that this reaction results in the release of heat (exothermic) The reaction gives of 483.6 kilo Joules of energy when 2 moles of H 2 combine with 1 mole of O 2 to produce 2 moles of H 2 O. The relative enthalpies of the reactants and products can also be shown on an energy diagram : Properties of enthalpy: 1. Enthalpy is an extensive

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Unformatted text preview: property. The magnitude of ∆ H is dependent upon the amounts of reactants consumed. Doubling the reactants, doubles the amount of enthalpy. 2. Reversing a chemical reaction results in the same magnitude of enthalpy but of the opposite sign . For example, splitting two moles of water to produce 2 moles of H 2 and 1 mole of O 2 gas requires the input of +483.6 kJ of energy. 3. The enthalpy change for a reaction depends upon the state of the reactants and products. The states (i.e . g, l, s or aq ) must be specified. CH 4 (g) + 2O 2 (g) -> CO 2 (g) + 2H 2 O(g) ∆ H = -802 kJ Given the above thermochemical equation for the combustion of methane, how much heat energy is released when 4.5 grams of methane is burned (in a constant pressure system)? The negative sign (exothermic) indicates that 225.5 kJ of energy are given off by the system into the surroundings....
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Enthalpies of Reaction - property The magnitude of ∆ H is...

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