Deck 5: Chemical Change

Question 1

The standard Gibbs energy of reaction for the equilibrium
2 NO₂(g) ∏ N₂O₄(g)
Is -4.73 kJ mol^-1 at 298 K. Calculate the value of the equilibrium constant at this temperature.

A) 1.00
B) 6.75
C) 0.15
D) 81

Answer 1

B

Question 2

Calculate the standard reaction Gibbs energy for the reaction
2 N₂O₅(g) 4 NO₂(g) + O₂(g) at a temperature of 298 K. The standard Gibbs energies of formation of the components at this temperature are given in the table below.
$\begin{array}{lc}& \Delta \mathrm{f} G^{e}(298 \mathrm{~K}) / \mathrm{kJ} \mathrm{mol}^{-1} \\\mathrm{N}_{2} \mathrm{O}_{5} \\& +118.0 \\\mathrm{NO}_{2} \\& +51.8\\\mathrm{O}_{2}\\&0\\\end{array}$

A) -66.2 kJ mol^-1
B) -89.2 kJ mol^-1
C) -28.8 kJ mol^-1
D) -169.8 kJ mol^-1

Answer 2

-28.8 kJ mol^-1

Question 3

The equilibrium constant for the reaction
H₂(g) + Br₂(g) ∏ 2 HBr(g)
Is 2.1  10⁹ at 298 K. What is the value of the equilibrium constant for the reaction
½ H₂(g) + ½ Br₂(g) ∏ HBr(g) at this temperature?

A) 2.1  10⁹
B) 4.4  10¹⁸
C) 1.0  10⁹
D) 46  10³

Answer 3

D

Question 4

The value of the equilibrium constant for the electrochemical reaction
MnO₄^-(aq) + 2 H⁺(aq) + ½ Cl₂(g) ∏ Mn²⁺(aq) + ClO₃^-(aq) + H₂O(aq)
Is 5.88  10^-5. What is the value of the equilibrium constant for the reverse reaction?

A) 5.88  10^-5
B) -5.88  10^-5
C) 17.0  10³
D) -1.70  10³

Question 5

The standard Gibbs energy of reaction for the thermal decomposition of ammonium chloride, NH₄Cl
NH₄Cl(s) ∏ NH₃(g) + HCl(g)
Is +35.0 kJ mol^-1 at 225 °C. Calculate the partial pressure of ammonia, NH₃, gas produced when ammonium chloride is allowed to decompose at a pressure of 1 bar at this temperature.

A) 1460 Pa
B) 21.2 Pa
C) 8.66 Pa
D) 21.3 kPa

Question 6

When phosphorus pentachloride, PCl₅, is heated at a constant pressure of exactly 1 bar to a temperature of 1400 K, the amount of phosphorus pentachloride decreases by 20.7% because of dissociation to phosphorus trichloride, PCl₃, and chlorine, Cl₂. Determine the equilibrium constant at this temperature.

A) 42.8  10^-3
B) 87.7
C) 0.207
D) 0.045

Question 7

When lead (II) oxide, PbO, is allowed to react with oxygen, O₂, at a temperature of 823 K and standard pressure through the reaction
PbO(s) + ½O₂(g) ∏ PbO₂(s)
The partial pressure of oxygen at equilibrium is 11.6 Pa. Determine the standard Gibbs energy of reaction at this temperature.

A) -31.0 kJ mol^-1
B) 62.0 kJ mol^-1
C) -20.8 kJ mol^-1
D) -41.4 kJ mol^-1

Question 8

The value of the equilibrium constant for the gas-phase reaction
2 NOCl(g) ∏ 2 NO(g) + Cl₂(g)
Is K = 7.65  10^-8 at 298.15 K. Determine the value of the equilibrium constant expressed in terms of concentrations, K_c, at this temperature.

A) 7.65  10^-8
B) 3.10  10^-9
C) 1.90  10^-6
D) 1.25  10^-10

Question 9

For sulfur hexafluoride, SF₆, the standard Gibbs energy of formation of is -992 kJ mol^-1 and the standard entropy of formation is -350 J K^-1 mol^-1 at 298 K. Assuming that the values of the enthalpy and entropy of formation vary little with temperature, estimate the standard Gibbs energy of formation of sulfur hexafluoride at 348 K.

A) -992 kJ mol^-1
B) -18 kJ mol^-1
C) -975 kJ mol^-1
D) -1010 kJ mol^-1

Question 10

The value of the equilibrium constant for the reaction
C₂H₄(g) + H₂O(g) ∏ C₂H₅OH(g)
Is 9.920 at 423 K and 6.397 at 523 K. Determine the average value of the standard enthalpy of reaction over this temperature range.

A) -8.1 kJ mol^-1
B) -36.5 kJ mol^-1
C) -64.8 kJ mol^-1
D) -3.50 kJ mol^-1

Question 11

Calculate the concentration of hydronium, H₃O⁺, ions in a solution of pH = 11.2.

A) 11.2  10^-3 mol dm^-3
B) 1.36  10^-5 mol dm^-3
C) 1.58  10¹⁰ mol dm^-3
D) 6.3  10^-12 mol dm^-3

Question 12

The pK_a of 2-chloropropanoic acid, CH₃CHClCO₂H, is 2.83. Calculate the Gibbs energy for the dissociation
CH₃CHClCO₂H(aq) + H₂O(aq) ⇌ CH₃CHClCO₂^-(aq) + H₃O⁺(aq)
At a temperature of 298 K.

A) +16.1 kJ mol^-1
B) +7.01 kJ mol^-1
C) +3.05 kJ mol^-1
D) +496 J mol^-1

Question 13

The concentration of OH^- ions in a solution is 5.62 mmol dm^-3. Determine the pH of the solution.

A) 11.7
B) 2.2
C) 14.7
D) 13.3

Question 14

The pK_b of the base cyclohexamine, C₆H₁₁NH₂, is 3.36. What is the pK_a of the conjugate acid, C₆H₁₁NH₃⁺?

A) 3.36
B) 14.00
C) 10.64
D) 7.64

Question 15

Determine the pH of an aqueous solution of benzoic acid, C₆H₅COOH, of concentration 0.064 mol dm^-3. The pK_a of benzoic acid is 4.19.

A) 4.19
B) 1.50
C) 2.69
D) 2.10

Question 16

Calculate the pH of an aqueous solution of sodium hydrogenphosphite, NaHPO₃. For phosphorous acid, H₂PO₃, pK_a1 = 2.00 and pK_a2 = 6.59.

A) 4.30
B) 2.00
C) 6.59
D) 2.29

Question 17

Calculate the pH of a solution produced by mixing 4.3 cm³ of 0.100 mol dm^-3 potassium hydroxide, KOH, solution to 50.0 cm³ of 0.136 mol dm^-3 propanoic acid, C₂H₅COOH, solution. The pK_a of propanoic acid is 4.87.

A) 1.93
B) 2.44
C) 3.67
D) 2.16

Question 18

When an excess of copper (II) iodate, Cu(IO₃)₂, is added to water at 298 K, the equilibrium concentration of copper ions is 3.27  10^-3 mol dm^-3. Determine the solubility constant at this temperature.

A) 3.27  10^-3
B) 1.40  10^-7
C) 1.07  10^-5
D) 3.50  10^-8

Question 19

Estimate the solubility of copper (I) chloride, CuCl, in a 0.10 mol dm^-3 solution of sodium chloride, NaCl, at a temperature of 298 K. The solubility of copper (I) chloride in water is 1.0  10^-3 mol dm^-3 at this temperature.

A) 1.0  10^-5 mol dm^-3
B) 1.0  10^-3 mol dm^-3
C) 1.0  10^-4 mol dm^-3
D) 1.0  10^-5 mol dm^-6

Question 20

One drop, 0.200 cm³, of 1.00 mol dm^-3 of aqueous sodium hydroxide, NaOH, solution, is added to 25.0 cm³ of a phosphate buffer that is 0.040 mol dm^-3 in KH₂PO₄(aq) and 0.020 mol dm⁻³ in K₂HPO₄(aq). Calculate the resulting pH of the solution given that the second acid dissociation constant, pK_a2, for phosphoric acid is 7.21.

A) 7.21
B) 6.91
C) 7.16
D) 7.27

Question 21

Use the Debye^-Hückel limiting law to determine the mean activity coefficient for the Ca²⁺ and NO₃^- ions in a 0.005 mol dm^-3 aqueous solution of calcium nitrate, Ca(NO₃)₂.

A) 0.94
B) 0.97
C) 0.75
D) 0.12

Question 22

Calculate the limiting molar conductivity of sodium chloride, NaCl, from the following measurements of molar conductivity at various concentrations. The data refer to a temperature of 298 K.
$\begin{array}{lcccc}c / \mathrm{mol} \mathrm{dm}^{-3} & 0.010 & 0.020 & 0.050 & 0.100 \\\Lambda_{\mathrm{m}} / \mathrm{S} \mathrm{m}{ }^{-1}\left(\mathrm{~mol} \mathrm{dm}^{-3}\right)^{-1} & 117.45 & 115.70 & 111.01 & 106.69\end{array}$

A) 122.6 S m^-1 (mol dm^-3)^-1
B) 106.9 S m^-1 (mol dm^-3)
C) 119.3 S m^-1 (mol dm^-3)^-1
D) 96.3 S m^-1 (mol dm^-3)^-1

Question 23

Calculate the limiting molar conductivity of an aqueous solution of sodium phosphate, Na₃PO₄, given that the ionic conductivities of the sodium, Na⁺, and phosphate, PO₄^3-, ions are 5.01 and 24.0 mS m² mol^-1 respectively.

A) 29.0 mS m² mol^-1
B) 77.0 mS m² mol^-1
C) 39.0 mS m² mol^-1
D) 19.0 mS m² mol^-1

Question 24

The molar conductivity of a 0.010 mol dm^-3 solution of propanoic acid, C₂H₅COOH, has been measured to be 2.25 mS m² mol^-1 at 298 K. The ionic conductivity of hydronium, H₃O⁺, ions is 34.96 mS m² mol^-1 and of propanoate, C₂H₅COO^-, ions is 26.3 mS m² mol^-1 at this temperature. Determine the acid dissociation constant of propanoic acid at 298 K.

A) 2.88  10^-8
B) 3.67  10^-4
C) 1.91  10^-3
D) 1.35  10^-5

Question 25

The mobility of a tetramethylammonium, [N(CH₃)₄]⁺, ion in dilute aqueous solution is 4.65  10⁻⁸ m² s⁻¹ V⁻¹ at a temperature of 298 K. Estimate the radius of the ion, given that the viscosity of water at this temperature is 1.00 mN s m^-2.

A) 1.2 Å
B) 2.4 Å
C) 1.6 Å
D) 1.8 Å

Question 26

The standard cell potential for the reaction
Sn²⁺(aq) + 2 Cr³⁺(aq) ⇌ Sn⁴⁺(aq) + 2 Cr²⁺(aq)
Is -0.260 V at 298 K. Calculate the standard Gibbs energy of reaction at this temperature.

A) +25.1 kJ mol^-1
B) +50.2 kJ mol^-1
C) -50.1 kJ mol^-1
D) -25.1 kJ mol^-1

Question 27

The standard reaction Gibbs energy for the redox process
MnO₄^-(aq) + NO(g) ⇌ MnO₂(s) + NO₃^-(aq)
Is -206 kJ mol^-1. Calculate the standard cell potential.

A) +2.135 V
B) +1.068 V
C) +0.712 V
D) +0.427 V

Question 28

The standard cell potential for the Daniell cell
Zn(s)|ZnSO₄(aq)||CuSO₄(aq)|Cu(s)
Is +1.102 V. Determine the cell potential at a temperature of 25°C if the concentration of zinc sulfate, ZnSO₄, is 0.050 mol dm^-3 and the concentration of copper sulfate, CuSO₄, is 0.200 mol dm^-3.

A) +1.066 V
B) +1.102 V
C) +1.087 V
D) +1.138 V

Question 29

The standard potential for the cell
Cl₂(g) + Cu(s) ⇌ CuCl₂(aq)
Is +1.02 V. Determine the potential of the cell at a temperature of 298 K when the partial pressure of chlorine, Cl₂, gas is 0.100 bar and the concentration of copper (II) chloride, CuCl₂, is 0.017 mol dm^-3.

A) +1.02 V
B) +1.13 V
C) +0.91 V
D) +1.23 V

Question 30

The standard cell potential for the fuel cell
2 H₂(g) + O₂(g) ⇌ 2 H₂O(l)
Is E^o_cell = +1.23 V at a temperature of 298 K and +1.18 V at 358 K. Estimate the standard entropy of the cell reaction.

A) -320 J K^-1 mol^-1
B) -80 J K^-1 mol^-1
C) -160 J K^-1 mol^-1
D) -107 J K^-1 mol^-1