CHM1311 - Lecture(8)

CHM1311 - Lecture(8) - Chemical Reactivity Chemical What...

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Chemical Reactivity Chemical Reactivity What drives chemical reactions? How do they occur? In general, reactions are product favored if they transfer energy to their surroundings ( U is negative). e.g. 2H 2 (g) + O 2 (g) 2H 2 O (l) + energy 1
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Most chemical reactions occur at constant pressure, P. U rxn = q P + w where q P is heat transferred at constant P The enthalpy enthalpy of the reaction is defined as the heat given off or absorbed by a reaction at constant pressure and is represented by the symbol H: H = q P heat transferred = final heat content – initial heat content 2
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Exothermic and Endothermic Exothermic and Endothermic q q system system > 0 > 0 Surroundings Surroundings System System heat heat q q system system < 0 < 0 Surroundings Surroundings System System heat heat 3 H > 0 H < 0 Heat flows into the system Heat flows out of the system
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Reaction Enthalpy or Heat of Reaction Reaction Enthalpy or Heat of Reaction 2 H 2 O (l) + 242 kJ/mole 242 kJ/mole 2H 2 (g) + O 2 (g) H rxn = + 242 kJ/mole 2H 2 (g) + O 2 (g) 2H 2 O (l) + 242 kJ/mol 242 kJ/mol EXOTHERMIC EXOTHERMIC ENDOTHERMIC ENDOTHERMIC H rxn = - 242 kJ/mole Reversing a chemical equation will reverse the sign of the enthalpy for the reaction. 4
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Determining Reaction Enthalpy Determining Reaction Enthalpy There are several ways to determine the enthalpy (or heat) of a reaction: 1. Experimentally (Calorimetry) 2. Use Hess’s Law to determine H for a reaction based on known H values of a series of reactions. 3. Calculate H from the enthalpies of formation of molecules and elements in the reaction by using the equation: H 0 rxn = Σ H 0 f (products) - Σ H 0 f (reactants) We will look at each of these in detail. 5
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Experimental Determination of Experimental Determination of Reaction Enthalpy: Reaction Enthalpy: Calorimetry Calorimetry 1. 1. Constant-Volume Calorimetry Constant-Volume Calorimetry 2. 2. Constant-Pressure Calorimetry Constant-Pressure Calorimetry 6
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sample is immersed in a water bath. 2) The sample is ignited 3) The heat produced by combustion of the sample is absorbed by the water and the bomb. 4) The heat of the combustion of
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CHM1311 - Lecture(8) - Chemical Reactivity Chemical What...

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