# Entropy, Free Energy, and Equilibrium

18.29 Consider the decomposition of calcium carbonate: CaCO3 (s) CaO (s) CO2 (g) Calculate the pressure in atm of CO2 in an equilibrium process (a) at 25oC and (b) at 800oC. Assume that Ho = 177.8 kJ/mol and So = 160.5 J/K.mol for the temperature range. 19.1 (d) Br2

BrO3- Br- 3Br2 + 6OH BrO3 + 5Br + 3H2O (in basic solution) Example 14.4 What is the rate constant for a first order reaction that converts 74% of starting material to product in 33 minutes? 0.0408 min-1 What is the half-life of this reaction? 17 min

Chemical Equilibria How can the yield of NH3 be maximized? Increase [N2] or [H2], decrease [NH3] How would the decomposition of NH3 be maximized? Increase [NH3], decrease [N2] or [H2] Is this reaction (as written) favored entropically? No, S is negative since # moles of gas decrease What is the effect of increasing temperature? -TS would be positive since S is negative; reaction becomes less spontaneous with increasing T

Chemical Equilibria Write the equilibrium constant expression for the reaction in the forward direction. Write the equilibrium constant expression for the reaction in the reverse direction. How are the two equilibrium constants related? Kf = 1/Kr and Kr = 1/Kf Describe what is meant by equilibrium in terms of chemical change as well as reaction rates. Rate of forward reaction equals rate of reverse reaction; no net chemical change Chemical Equilibria

Assume the Haber process proceeds with a G of -33 kJ mol-1 at 298 K. Draw a reaction energy diagram showing the relative energies of reactants and products as well as the transition state. Define transition state. Define activation energy. Define free energy of the reaction.

What term relates to the equilibrium constant, K? What term relates to the rate constant, k? What is a catalyst and what parameter is affected by the presence of a catalyst? Colligative properties: freezing point depression Calculate the approximate freezing point of a solution made from 21.0 g NaCl and 1.00 10 2 g of H2O. Kf of water is 1.86C/m.

A. B. C. D. E. 3.59C 6.68C -13.4C -6.68C -3.59C The molality would be 2X (vant Hoff factor, i = 2) Colligative properties: freezing point depression Calculate the approximate freezing point of a solution made from 21.0 g NaCl and 1.00

102 g of H2O. Kf of water is 1.86C/m. How would having 21.0 g of ethanol (CH3CH2OH) affect the calculation? The molality would be 1X, i = 1) How would the calculation be affected if the salt were KNO3? The molality would be 2X, i = 2) Buffers How would you make a buffer of pH 5.0 from acetic acid and sodium acetate? The pKa of acetic acid is 1.74 x 10-5. Combine equal volumes of 1.74 M sodium acetate and 1.0 M acetic acid. Take acetic acid and titrate it with NaOH until a pH of

5.0 is reached. 19.31 Nernst equation Calculate the standard potential of the cell consisting of the Zn/Zn2+ half-cell and the SHE. What will the emf of the cell be if [Zn2+] = 0.45 M, PH2 = 2.0 atm, and [H+] = 1.8 M? The overall reaction is: Zn(s) + 2H+(aq) Zn2+(aq) + H2(g) Remember that the oxidant will be the reactant in the half reaction with the more positive reduction potential! Ecell Ecathode Eanode 0.00 V ( 0.76 V) 0.76 V

2 0.0257 V [Zn ]PH 2 E E ln n [H ]2 E 0.76 V 0.0257 V (0.45)(2.0) ln 0.78 V 2 2 (1.8)

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