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CH225H

MT 2: 55 minutes KEY

You may use a calculator and a periodic table.

Part I. Multiple choice. [4 pts each]

|1. |Which of the following gives the molarity of a 17.0% by mass solution of sodium acetate, CH3COONa (molar mass = 82.0 g/mol) in |

| |water? The density of the solution is 1.09 g/mL. |

|A) |7.26 × 10 – 6 M |

|B) |0.207 M |

|C) |1.07 M |

|D) |2.26 M |

|E) |2.99 M |

|2. |Assuming complete dissociation of electrolytes, which of the following aqueous solutions has the lowest freezing point? |

|A) |0.18 m KCl |

|B) |0.15 m Na2SO4 |

|C) |0.12 m Ca(NO3)2 |

|D) |pure water |

|E) |0.20 m sucrose (a non-electrolyte) |

|3. |It takes 42.0 min for the concentration of a reactant in a first-order reaction to drop from 0.45 M to 0.32 M at 25°C. How |

| |long will it take for the reaction to be 90% complete? |

|A) |13.0 min |

|B) |86.0 min |

|C) |137 min |

|D) |222 min |

|E) |284 min |

|4. |Nitric oxide gas (NO) reacts with chlorine gas according to the equation |

| |NO + ½Cl2 → NOCl. |

| |The following initial rates of reaction have been measured for the given reagent concentrations. |

| |Expt. # |

| |Rate (M/hr) |

| |NO (M) |

| |Cl2 (M) |

| | |

| |1 |

| |1.19 |

| |0.50 |

| |0.50 |

| | |

| |2 |

| |4.79 |

| |1.00 |

| |0.50 |

| | |

| |3 |

| |9.59 |

| |1.00 |

| |1.00 |

| | |

| | |

| |Which of the following is the rate law (rate equation) for this reaction? |

|A) |rate = k[NO] |

|B) |rate = k[NO][Cl2]1/2 |

|C) |rate = k[NO][Cl2] |

|D) |rate = k[NO]2[Cl2] |

|E) |rate = k[NO]2[Cl2]2 |

|5. |The activation energy for the reaction CH3CO → CH3 + CO is 71 kJ/mol. How many times greater is the rate constant for this |

| |reaction at 170.0°C than at 150.0°C? |

| |(R = 8.31 J / Kmol) |

|A) |0.40 |

|B) |1.1 |

|C) |2.5 |

|D) |4.0 |

|E) |5.0 |

|6. |An increase in temperature causes an increase in the rate of reaction. The best explanation for this behavior is that as the |

| |temperature increases, |

|A) |the concentration of reactants increases. |

|B) |the activation energy decreases. |

|C) |the collision frequency increases. |

|D) |the fraction of collisions with total kinetic energy > Ea increases. |

|E) |the activation energy increases. |

|7. |The rate law for the reaction 2NO2 + O3 → N2O5 + O2 is rate = k[NO2][O3]. Which one of the following mechanisms is consistent |

| |with this rate law? |

|A) |NO2 + NO2 → N2O4 (fast) |

| |N2O4 + O3 → N2O5 + O2 (slow) |

|B) |NO2 + O3 → NO5 (fast) |

| |NO5 + NO5 → N2O5 + (5/2)O2 (slow) |

|C) |NO2 + O3 → NO3 + O2 (slow) |

| |NO3 + NO2 → N2O5 (fast) |

|D) |NO2 + NO2 → N2O2 + O2 (slow) |

| |N2O2 + O3 → N2O5 (fast) |

|8. |With respect to the figure below, which choice correctly identifies all the numbered positions? |

| |[pic] |

| |1. |

| |2. |

| |3. |

| |4. |

| | |

|A) |catalyst |

| |intermediate |

| |transition state |

| |product |

| | |

|B) |reactants |

| |transition state |

| |intermediate |

| |product |

| | |

|C) |reactants |

| |transition state |

| |catalyst |

| |product |

| | |

|D) |reactants |

| |intermediate |

| |transition state |

| |product |

| | |

|E) |reactants |

| |intermediate |

| |transition state |

| |catalyst |

| | |

|9. |Which is the correct equilibrium constant expression for the following reaction? |

| |Fe2O3(s) + 3H2(g) [pic] 2Fe(s) + 3H2O(g) |

|A) |Kc = [Fe2O3] [H2]3 / [Fe]2[H2O]3 |

|B) |Kc = [H2] / [H2O] |

|C) |Kc = [H2O]3 / [H2]3 |

|D) |Kc = [Fe]2[H2O]3 / [Fe2O3] [H2]3 |

|E) |Kc = [Fe] [H2O] / [Fe2O3] [H2] |

|10. |Which of the following is a true statement about chemical equilibria in general? |

|A) |At equilibrium the total concentration of products equals the total concentration of reactants, that is, [products] = |

| |[reactants]. |

|B) |Equilibrium is the result of the cessation of all chemical change. |

|C) |There is only one set of equilibrium concentrations that equals the Kc value. |

|D) |At equilibrium, the rate constant of the forward reaction is equal to the rate constant for the reverse reaction. |

|E) |At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction. |

|11. |Consider the following equilibria: |

| |2SO3(g) [pic] 2SO2(g) + O2(g) Kc = 2.3 × 10−7 |

| |2NO3(g) [pic] 2NO2(g) + O2(g) Kc = 1.4 × 10−3 |

| | |

| |Calculate the equilibrium constant for the reaction |

| |SO2(g) + NO3(g) [pic] SO3(g) + NO2(g). |

|A) |78 |

|B) |1.3 × 10−2 |

|C) |1.6 × 10−4 |

|D) |3.2 × 10−10 |

|E) |6.1 × 103 |

|12. |For the following reaction at equilibrium, which one of the changes below would cause the equilibrium to shift to the left? |

| | |

| |2NOBr(g) [pic] 2NO(g) + Br2(g), ΔHºrxn = +30 kJ/mol |

|A) |Increase the container volume. |

|B) |Remove some NO. |

|C) |Remove some Br2 . |

|D) |Add more NOBr. |

|E) |Decrease the temperature. |

|13. |The equilibrium constants (expressed in atm) for the chemical reaction |

| |N2(g) + O2(g) [pic] 2NO(g) are Kp = 1.1 × 10−3 and 3.6 × 10−3 at 2,200 K and 2,500 K, respectively. Which one of the following |

| |statements is true? |

|A) |The reaction is exothermic, ΔHº < 0. |

|B) |The partial pressure of NO(g) is less at 2,200 K than at 2,500 K. |

|C) |Kp is less than Kc by a factor of (RT). |

|D) |The total pressure at 2,200 K is the same as at 2,500 K. |

|E) |Higher total pressure shifts the equilibrium to the left. |

Part II. Short Essay

14. Concentrated nitric acid (HNO3, mw = 63 g / mol) is actually an aqueous solution containing 70.3 mass percent HNO3. The solution density is 1.43 g / mL. What is the molarity of this solution ? [10 pts]

molarity of HNO3 = mol HNO3 / 1 L soln

= (1.43 g soln/mL)(70.3 g HNO3/100 g soln) / (63 g HNO3/mol HNO3)(0.001 L/mL)

= 16.0 M

15. For A + B [pic] C

Kc = 4.2 x 103 at 20 °C. What is Kc at 20 °C for the equilibrium: [6 pts]

C [pic] B + A

K = 1 / Kc = 1 / 4.2E3 = 2.4 x 10-4

16. Calculate the freezing point of a solution made from 8.5 g of octane (a non-electrolyte with mw = 114 g / mol) in 300.0 g of benzene. Benzene freezes at 5.50°C and has Kf = 5.12 °C / m. [10 pts]

∆Tf = Tsolvent - Tsolution = Kf m

m = mol octane / kg solution = 8.5 g / (114 g/mol) (0.300 kg) = 0.25 m

5.50 °C - Tsolution = (5.12 °C / m) (0.25 m) = 1.3 °C

Tsolution = 4.2 °C

17. Give an example of a technologically significant reaction that requires a catalyst, indicate both the reaction and catalyst used. [10 pts]

Some possibilities are:

1. The Haber process (Fe+additives): N2 + 3 H2 = 2 NH3

2. Catalytic converter in car (Pt/Rh): 2 CO + O2 = 2 CO2

3. Fuel cell electrodes (Pt): H2 = 2e- + 2 H+

18. The equilibrium constant, Kc = 5.0 for the reaction:

SO2(g) + NO2(g) [pic] SO3(g) + NO(g)

at 1000°C. If 1.0 mole of SO2 and 1.0 moles of NO2 are placed in a 1.0 L container, what concentration of SO3 will be present at equilibrium at 1000 °C? [12 pts]

Compound Initial conc change equilibrium conc

SO2 1.0 - x 1.0 - x

NO2 1.0 - x 1.0 – x

SO3 0 + x x

NO 0 + x x

Kc = 5.0 = [SO3] [NO] / [SO2] [NO2] = x2 / (1.0 – x)2

Taking square roots of both sides:

2.236 = x / (1.0 - x)

2.236 – 2.236x = x

x = 2.236 / 3.236 = 0.69

[SO3]equ = 0.69

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