chemistry 122 kinetics and equilibrium exam review

8/11/2019 Chemistry 122 Kinetics and Equilibrium Exam Review

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CHEMISTRY122

KINETICSANDEQUILIBRIUMEXAMREVIEW

1. NH4Cl(s)NH3(g)+ HCl(g)H= +42.1 kilocalories

Suppose the substances in the reaction above are at equilibrium at 600K in volume V and at pressureP. State whether the partial pressure of NH3(g)will have increased, decreased, or remained the samewhen equilibrium is reestablished after each of the following disturbances of the original system.Some solid NH4Cl remains in the flask at all times. Justify each answer with a oneortwo sentenceexplanation.

(a) A small quantity of NH4Cl is added.

(b) The temperature of the system is increased.

(c) The volume of the system is increased.

(d) A quantity of gaseous HCl is added.(e) A quantity of gaseous NH3is added.


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2. Ammonium hydrogen sulfide is a crystalline solid that decomposes as follows:

NH4HS(s)NH3(g)+ H2S(g)

(a)Some solid NH4HS is placed in an evacuated vessel at 25C. After equilibrium is attained, the total

pressure inside the vessel is found to be 0.659 atmosphere. Some solid NH 4HS remains in the vessel

at equilibrium. For this decomposition, write the expression for KPand calculate its numerical value

at 25C.

(b)Some extra NH3 gas is injected into the vessel containing the sample described in part (a). When

equilibrium is reestablished at 25C, the partial pressure of NH3 in the vessel is twice the partial

pressure of H2S. Calculate the numerical value of the partial pressure of NH3and the partial pressure

of H2S in the vessel after the NH3has been added and the equilibrium has been reestablished.

(c)In a different experiment, NH3gas and H2S gas are introduced into an empty 1.00 liter vessel at 25C.

The initial partial pressure of each gas is 0.500 atmospheres. Calculate the number of moles of solid

NH4HS that is present when equilibrium is established.


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3. Sulfuryl chloride, SO2Cl2, is a highly reactive gaseous compound. When heated, it decomposes as

follows: SO2Cl2(g)SO2(g)+ Cl2(g). This decomposition is endothermic. A sample of 3.509 grams ofSO2Cl2is placed in an evacuated 1.00 litre bulb and the temperature is raised to 375K.

(a) What would be the pressure in atmospheres in the bulb if no dissociation of the SO2Cl2(g)occurred?

(b) When the system has come to equilibrium at 375K, the total pressure in the bulb is found to be 1.43

atmospheres. Calculate the partial pressures of SO2, Cl2, and SO2Cl2at equilibrium at 375K.

(c) Give the expression for the equilibrium constant (either Kp or Kc) for the decomposition of SO2Cl2(g) at

375K. Calculate the value of the equilibrium constant you have given, and specify its units.

(d) If the temperature were raised to 500K, what effect would this have on the equilibrium constant?

Explain briefly.


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4.

CO2(g)+ H2(g)H2O(g)+ CO(g)

When H2(g)is mixed with CO2(g)at 2,000 K, equilibrium is achieved according to the equation above. In oneexperiment, the following equilibrium concentrations were measured.

[H2] = 0.20 mol/L[CO2] = 0.30 mol/L

[H2O] = [CO] = 0.55 mol/L

(a)What is the mole fraction of CO(g)in the equilibrium mixture?

(b)Using the equilibrium concentrations given above, calculate the value of Kc, the equilibrium constantfor the reaction.

(c)Determine Kpin terms of Kcfor this system.

(d)When the system is cooled from 2,000 K to a lower temperature, 30.0 percent of the CO (g) isconverted back to CO2(g). Calculate the value of Kcat this lower temperature

(e)

In a different experiment, 0.50 mole of H2(g)is mixed with 0.50 mole of CO2(g)in a 3.0liter reactionvessel at 2,000 K. Calculate the equilibrium concentration, in moles per liter, of CO(g) at this

temperature.


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5. In the equation below, the forward reaction is first order in both PCl3and Cl2and the reverse reactionis first order in PCl5.

PCl3(g)+ Cl2(g) PCl5(g)

(a) Suppose that 2 moles of PCl3 and 1 mole of Cl2 are mixed in a closed container at constant

temperature. Draw a graph that shows how the concentrations of PCl3, Cl2, and PCl5change with time

until after equilibrium has been firmly established.(b) Give the initial rate law for the forward reaction.

(c) Provide a molecular explanation for the dependence of the rate of the forward reaction on the

concentrations of the reactants.

(d) Provide a molecular explanation for the dependence of the rate of the forward reaction on

temperature.


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7. Ethyl iodide reacts with a solution of sodium hydroxide to give ethyl alcohol according to theequation.

CH3CH2I + OHCH3CH2OH + I

The reaction is first order with respect to both ethyl iodide and hydroxide ion, and the overallrateexpression for the reaction is as follows:

rate = k[CH3CH2I][OH]

What would you do in the laboratory to obtain data to confirm the order in the rate expression foreither of the reactants.


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8. Changing the temperature and no other conditions changes the rates of most chemical reactions. Twofactors are commonly cited as accounting for the increased rate of chemical reaction as thetemperature is increased. State briefly and discuss the two factors. Which of the two is moreimportant?


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9. For the reaction above, the rate constant at 380C for the forward reaction is 2.610 3liter2/mole2secand this reaction is first order in O2and second order in NO.

2 NO(g)+ O2(g)2 NO2(g)

The rate constant for the reverse reaction at 380C is 4.1 liter/molesec and this reaction is second

order in NO2.

(a) Write the equilibrium expression for the reaction as indicated by the equation above and calculate

the numerical value for the equilibrium constant at 380C.

(b) What is the rate of the production of NO2at 380C if the concentration of NO is 0.0060 mole/liter and

the concentration of O2is 0.29 mole/liter?

(c) The system above is studied at another temperature. A 0.20 mole sample of NO 2 is placed in a 5.0

liter container and allowed to come to equilibrium. When equilibrium is reached, 15% of the original

NO2 has decomposed to NO and O2. Calculate the value for the equilibrium constant at the second

temperature.


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10. Graphical methods are frequently used to analyze data and obtain desired quantities.

2 HI(g)H2(g)+ I2(g)

The following data give the value of the rate constant at various temperatures for the gas phase reaction

above.

T (K) k(litre/mol sec)

647 8.5810-5

666 2.1910-4

683 5.1110-4

700 1.1710-3

716 2.5010-3

(a) Describe, without doing any calculations, how a graphical method can be used to obtain the activation energyfor this reaction.

A(g)B(g)+ C(g)

The following data give the partial pressure of A as a function of time and were obtained at 100C for the

reaction above.

PA (mm Hg) t (sec)

348 0

247 600185 1200

105 2400

58 3600

Describe, without doing any calculations, how graphs can be used to determine whether this reaction is

first or second order in A and how these graphs are used to determine the rate constant.


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11. Consider the following general equation for a chemical reaction.

A(g)+ B(g)C(g)+ D(g) Hreaction = -10 kJ

(a) Describe the two factors that determine whether a collision between molecules of A and B results in a

reaction.

(b) How would a decrease in temperature affect the rate of the reaction shown above? Explain your

answer.(c) Write the rate law expression that would result if the reaction proceeded by the mechanism shown

below.

A + B[AB] (fast)

[AB] + B C + D (slow)

(d) Explain why a catalyst increases the rate of a reaction but does not change the value of the

equilibrium constant for that reaction.


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12. The following results were obtained when the reaction represented above was studied at 25C.

2 ClO2(g)+ F2(g)2 ClO2F(g)

Experiment Initial [ClO2], (mol.L-1) Initial [F2], (mol

.L-1)

Initial Rate of Increase of [ClO2F],

(mol.L-1.sec-1)

1 0.010 0.10 2.410-3

2 0.010 0.40 9.610-33 0.020 0.20 9.610-3

(a) Write the rate law expression for the reaction above.

(b) Calculate the numerical value of the rate constant and specify the units.

(c) In experiment 2, what is the initial rate of decrease of [F 2]?

(d) Which of the following reaction mechanisms is consistent with the rate law developed in (a). Justify

your choice.

I. ClO2+ F2ClO2F2 (fast)ClO2F2ClO2F + F (slow)

ClO2+ FClO2F (fast)

II. F2

2 F (slow)2 (ClO2+ FClO2F) (fast)

chemistry 122 kinetics and equilibrium exam review

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