18.29

Consider the decomposition of calcium carbonate:

Calculate the pressure in atm of CO2 in an equilibrium process (a) at 25oC and (b) at 800oC. Assume that ΔHo = 177.8 kJ/mol and ΔSo = 160.5 J/K.mol for the temperature range.

CaCO3 (s) CaO (s) CO2 (g)

19.1 (d)

Br2 BrO3- Br- (in basic solution)

3Br2 + 6OH BrO3 + 5Br + 3H2O

Example 14.4

• What is the rate constant for a first order reaction that converts 74% of starting material to product in 33 minutes?

• What is the half-life of this reaction?

0.0408 min-1

17 min

Chemical Equilibria

• How can the yield of NH3 be maximized?

• How would the decomposition of NH3 be maximized?

• Is this reaction (as written) favored entropically?

• What is the effect of increasing temperature?

Increase [N2] or [H2], decrease [NH3]

Increase [NH3], decrease [N2] or [H2]

No, ΔS is negative since # moles of gas decrease

-TΔS would be positive since ΔS 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?

• Describe what is meant by equilibrium in terms of chemical change as well as reaction rates.

Kf = 1/Kr and Kr = 1/Kf

Rate of forward reaction equals rate of reverse reaction; no net chemical change

Chemical Equilibria

• Assume the Haber process proceeds with a ΔG 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 × 102 g of H2O. Kf of water is 1.86°C/m.

A. 3.59°C B. 6.68°C C. -13.4°C D. -6.68°C E. -3.59°C

The molality would be 2X (van’t 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 1.86°C/m.

• How would having 21.0 g of ethanol (CH3CH2OH) affect the calculation?

• How would the calculation be affected if the salt were KNO3?

The molality would be 1X, i = 1)

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)

cathode anode 0.00 V ( 0.76 V) cell 0.76 VE E E

2

2H

2

[Zn ]0.0257 Vln

[H ]

PE E

n

2

0.0257 V (0.45)(2.0)0.76 V ln

2 (1.8) 0.78 VE

Remember that the oxidant will be the reactant in the half reaction with the more positive reduction potential!