Chemical Energy Chemical energy is released or absorbed from reactions in variou

Question

Chemical Energy

Chemical energy is released or absorbed from reactions in various forms. The most easily measurable form of energy comes in the form of heat, or enthalpy. The enthalpy of a reaction can be calculated from the heats of formation of the substances involved in the reaction:

Hrxn = npHf (products) nrHf (reactants)

Where n represents the stoichiometric coefficients.

Part A

Calculate the standard enthalpy change for the reaction

2A+B2C+2D

Use the following data:

Express your answer to three significant figures and include the appropriate units.

H rxn = [       ]

Explanation / Answer

HRxn = Hproducts - Hreactants

HRxn = 2C + 2D - (2A + B)

HRxn = 2*175 + 2*-479 - (2*-257 + -397)

HRxn = 303 kJ/mol

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