4u c5.05 Hess Law & Heat of Formation ws

4u c5.05 Hess Law - Hess Law Principle of Additivity of Reaction Heats(Nelson pg 313-328 Whenever a reaction can be expressed as an algebraic sum

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Hess’ Law - Principle of Additivity of Reaction Heats ( Nelson pg. 313-328) Whenever a reaction can be expressed as an algebraic sum of two or more other reactions, then the heat of reaction is the algebraic sum of the heats of these reactions. Note: (1) For a reaction: A + B C + D H = -20 kJ/mol Then, for: C + D A + B H = +20 kJ/mol (2) For an exothermic Reaction: A + B C + D + 20 kJ or A + B C + D H = -20 kJ/mol For an endothermic Reaction: A + B + 20 kJ C + D or A + B C + D H = +20 kJ/mol (3) For the reaction: 2A + 3B 4C + 5D + 120 kJ H = -60 kJ/mol of A or H = -40 kJ/mol of B or H = -30 kJ/mol of C or H = -24 kJ/mol of D Hess’ Law can be used to calculate the Heat of Reaction for a particular reaction without actually measuring the heat, provided data can be found for a number of other reactions, which, will then add up to give the desired reaction. Example: Reaction (1) C (s) + O 2 (g) CO 2 (g) H = -393 kJ/mol of C (s) Reaction (2) CO (g) + ½O 2 (g) CO 2 (g) H = -283 kJ/mol of CO
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This note was uploaded on 05/31/2011 for the course CHEM 4U taught by Professor Jargin during the Spring '11 term at York University.

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4u c5.05 Hess Law - Hess Law Principle of Additivity of Reaction Heats(Nelson pg 313-328 Whenever a reaction can be expressed as an algebraic sum

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