Chapter 4 - Thermochemistry Chapter 4 Enthalpy of Reaction...

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Unformatted text preview: Thermochemistry Chapter 4 Enthalpy of Reaction The enthalpy of reaction Hr is the heat withdrawn from the surroundings when a reaction occurs under conditions of constant T and P. The standard enthalpy of reaction Hr0 is the enthalpy of reaction per mole of reactant at standard temperature (298 K) and pressure (1 bar). Energy of Reaction The energy of reaction Ur is the heat withdrawn from the surroundings when a reaction occurs under conditions of constant T and V. Enthalpy of Formation The standard enthalpy of formation of a substance Hf0 is the standard reaction enthalpy for the formation of the compound from its elements in their standard states, i.e. their most stable state of aggregation under standard conditions. Examples: Standard state of oxygen is O2(g) Standard state of water is H2O (l ) Hf0 Examples Acetic Acid 2C(gr) + O2(g) + 2H2(g) CH3COOH(l) Ammonium Nitrate N2(g) + 2H2(g) + 3/2 O2(g) 43(s) Sodium Chloride Na(s) + 1/2 Cl2(g) NaCl(s) Hess' Law and Hf0 0 = 3f0(C) + f0(D) - 2f0(A) - f0(B) For the general reaction JJ = 0, 0 = if0(i) Given f0 values for all the reactants and products, 0 may be calculated for any reaction. For the reaction 2A + B 3C + D, Hess's Law products C -C = HA-B + HB-C reactants A A The enthalpy change for an overall reaction is the sum of the enthalpy changes of the individual reactions into which a reaction may be divided. BC AC T-Dependence of rH rH(T2) Reactants(T2) HR Reactants(T1) rH(T1) rH(T2) = HR + rH(T1) + HP Products(T2) HP Products(T1) Kirchhoff's Law H R = C p ,m ( R)dT T1 T2 r H (T2 ) = r H (T1 ) + r C p dT T1 T2 where r C p = J C p ,m (i ) i Calorimetry In a constant volume calorimeter, q=0 and w=0 Therefore U=0 Therefore Urxn + Uwater + Ucalorimeter = 0 0 nsample U rxn + nwater Cv ,m , water T + Ccalorimeter T = 0 Measure nsample, nwater, and T. Calculate Urxn Calorimetry In a constant pressure calorimeter, q=0 Therefore H=0 Therefore Hrxn + Hwater + Hcalorimeter = 0 0 nsample H rxn + nwater C P ,m , water T + Ccalorimeter T = 0 Measure nsample, nwater, and T. Calculate Hrxn ...
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