Chapter5_2 - Chemical Reactions When natural gas burns...

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Mary J. Bojan Chem 110 Chemical Reactions When natural gas burns: CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(l) Reaction produces heat and light Is energy conserved? What are the signs of q and w for the overall reaction? Where does the energy come from? Is energy stored or released when bonds are broken?
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Mary J. Bojan Chem 110 Relationship between Energy ( Δ E) and Enthalpy ( Δ H) Δ H = Enthalpy Energy transferred when P is constant (=q p ) = Δ H For many chemical processes, P Δ V is small and Δ E Δ H Like Δ E, Δ H is a state function Energy transferred when V is constant = Δ E Usually run chemical reactions at constant pressure (atmospheric). Δ E = q + w P Δ V = work done on system at constant P arises from expansion or contraction of the system: Δ V = volume change of system Δ E = q p P Δ V P = constant q p = Δ E + P Δ V Δ H
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Mary J. Bojan Chem 110 Thermochemical Equations A balanced chemical equation that also includes the energy change H 2 (g) + 1/2 O 2 (g) H 2 O(g) Δ H = 241.8kJ Δ H = enthalpy: heat given off or absorbed in the reaction 1. Enthalpy is an extensive property. 2. Δ H for a reaction is equal in magnitude and opposite in sign to Δ H for the reverse reaction. 3. Δ H for a reaction depends on the states of reactants and products (gas, liquid, solid). ENTHALPY
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Mary J. Bojan Chem 110 ENTHALPY OF REACTION Δ H = H(products) – H(reactants) If Δ H<0 ( ) exothermic (heat released) If Δ H>0 (+) endothermic (heat absorbed)
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Mary J. Bojan Chem 110 5.1 Is this reaction 1 exothermic 2 endothermic 5.2 How much heat is given off per mole of H 2 ?
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