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Chem. 213 – Sample Final Exam WINONA STATE UNIVERSITY Summer 2002

Dr. Myoung Eun Lee

Name ______

Summary of Kinetics for Reactions

Order

Zero / First / Second

Rate law

/ Rate = k / Rate = k[A] / Rate = k[A]2
Integrated rate law / [A] = -kt + [A]o / ln[A] = -kt + ln[A] o / = kt +
Half-life (t1/2) / / /

Arrhenius equation

k = Ae-Ea/RT

lnk = - + lnA

Nernst equation

Ecell = Ecello -ln(Q)

or

Ecell = Ecello -log(Q) at 25oC

Henderson-Hasselbalch equation

pH = pKa + log ()

or

pH = pKa + log ()

Gas constant

R = 8.314 J/mol.K

Faraday constant

F = 96,485 C/mole of electrons

Part A: Multiple Choice

Instruction: For each of the following questions, write the letter of the best answer in the space provided.

  1. The rate constant for the beta decay of thorium-234 is 2.88 x 10-2 /day. What is the half-life of this nuclide? ______c______

a)53.1 days

b)1.22 days

c)24.1 days

d)101 days

  1. An unstable isotope of rhenium, Re-191, has a half-life of 9.8 minutes and is a beta producer. What is the other product of the reaction? ______c______

a)Os-190

b)W-190

c)Os-191

d)W-191

e)Pt-192

  1. Which of the following processes decreases the atomic number by 1? ______b______

a)Gamma ray production

b)Positron production

c)Alpha particle production

d)Beta particle production

  1. When the Pd-106 nuclide is struck with an alpha particle, a proton is produced along with a new element. What is this new element? ______a______

a)Ag-109

b)Cd-112

c)Cd-109

d)Ag-108

  1. How many electrons are transferred in the following reaction? ______c______

Fe + 2HCl FeCl2 + H2

a)0

b)1

c)2

d)3

e)4

  1. What is the oxidation state of Cr in Cr2O72-? ______b______

a)+7

b)+6

c)+14

d)-2

  1. The reaction quotient for a system is 7.2 x 102 and the equilibrium constant for the system is 36. What will happen to the system? ______b______

a)There will be a net gain in product.

b)There will be a net gain in reactant.

c)There will be a net gain in both product and reactant.

d)There will be no net gain in either product or reactant.

  1. Calculate the pH of a solution that has 0.020 M KOH ______c______

a)1.70

b)3.70

c)12.30

d)11.30

  1. Calculate the pH of 0.200 M solution of propionic acid, HC3H5O2. The acid dissociation constant, Ka, for propionic acid is 3.6 x 10-2. ______c______

a)1.35

b)1.44

c)1.07

d)0.70

  1. Which of the following will yield a linear plot for a second order reaction? ______d______

a)ln [A] vs. time

b)Rate vs. [A]

c)[A] vs. time

d)1/[A] vs. time

  1. The Ksp for BaF2 is 2.4 x 10-5. When 10 mL of 0.070 M NaF is mixed with 10 mL of 0.070 M BaNO3, will a precipitate form? ______a______

a)Yes, because Q is 4.3 x 10-5 and since it is greater than Ksp a precipitate will form.

b)No, because Q is 4.3 x 10-7 and since it is less than Ksp no precipitate will form.

c)Yes, because Q is 3.4 x 10-4 and since it is greater than Ksp a precipitate will form.

d)No, because Q is 1.3 x 10-5 and since it is less than Ksp no precipitate will form.

  1. In which of the following cases must Ecell be equal to zero? ______a______

a)In any cell at equilibrium.

b)In a concentration cell.

c)In a fuel cell.

d)Ecell can never be equal to zero.

  1. Copper is electroplated from Cu2+ solution. A constant current of 4.00 Amp is applied by an external power supply. How long will it take to deposit 100. g of copper? The molar mass of copper is 63.546 g/mol. ______b______

a)2.00 days

b)21.1 hours

c)11. 2 seconds

d)45.0 minutes

  1. Which energy conversion takes place in an electrolytic cell? ______a______

a)Electrical to chemical

b)Chemical to electrical

c)Mechanical to electrical

d)Electrical to mechanical

  1. For the following endothermic reaction,

CO2 (g) + 2H2O(g) CH4(g) + 2O2(g), Ho = + 803 kJ

Which of the following will shift the equilibrium position to the right? ______c______

a)Adding more methane, CH4

b)Increasing the volume of the reaction vessel

c)Increasing the temperature of system

d)Decreasing the temperature of system

e)Decreasing number of moles of carbon dioxide, CO2

  1. The reaction

2H2O(g) 2H2 (g) + O2(g)

has a positive value of Go. Which of the following statements must be true? ______d______

a)The reaction is spontaneous at all temperature.

b)The reaction is spontaneous at low temperatures, but not at high temperatures.

c)The reaction is exothermic.

d)The equilibrium lies far to the left.

  1. Calculate the pH of a buffer solution containing 0.80 M HCN and 1.40 M NaCN. The Ka of HCN is 6.2 x 10-10. ______b______

a)9.21

b)9.45

c)8.96

d)0.10

  1. Equilibrium is reached in chemical reactions when: ______a______

a)The rates of the forward and reverse reactions become equal.

b)The concentrations of reactants and products become equal.

c)The temperature shows a sharp rise.

d)All chemical reactions stop.

  1. Consider an exothermic reaction. Which of the following statements about its equilibrium is true? _a_

a)If the system is heated, the left side is favored.

b)If the system is heated, the right side is favored.

c)The temperature has no effect on the equilibrium constant.

d)The concentration of reactants is equal to the concentration of products.

  1. For which process is S of the system negative? ______c______

a)Evaporation of 1 mole of liquid water

b)Mixing 1 mole of water with 1 mole of ethanol

c)Compressing a mole of He at constant temperature from 2 atm to 0.5 atm

d)Raising the temperature of 1 mole of He from 273 K to 373 K

Part B: Short-Answer Questions

1.Explain why you agree or disagree with each the following statements.

a)In studying the rate law, the order of a reaction is determined by using the coefficients of the reactants and products in the balanced chemical equation.

False. The order of a reaction can be only determined experimentally.

b)Catalysts are an effective means of changing the position of an equilibrium.

False. Catalysts speed up a chemical reaction so the reaction can reach equilibrium faster but they do not affect the equilibrium position. Keq is unchanged in the presence of catalysts.

c)The change of entropy of the universe is always positive.

True. This is the second law of thermodynamics.

d)Chemical reactivity of an element is directly related to the nuclear stability of its nucleus.

False. Chemical reactions involve the valence electrons while nuclear stability is determined by the nucleons (protons and neutrons) in the nucleus. For example, all isotopes of oxygen have similar chemical reactivity.

Part C: Instructions: You must show all work for full credit.

  1. A galvanic cell is set up with silver and aluminum electrodes in contact with aqueous solutions of Ag+and Al3+, respectively, at 25oC. The standard reduction potentials are:

Ag+(aq) + e- Ag(s) Eo = 0.80 V

Al3+(aq) + 3e- Al(s) Eo = -1.66 V

a)Calculate the potential of the cell, Ecell, in volts at the standard state where Ag+and Al3+ are each 1M.

This is a standard state and therefore, Ecell= Eocell

Eocell = 0.80 V – (-1.66V) = 2.46 V

b)Write a balanced chemical equation for the overall reaction.

Al(s) will be oxidized because it has a lower Eo and Ag+ will be reduced.

Al(s) + 3Ag+ (aq) -> Al3+ (aq) + 3Ag(s)

c)Give a line notation for this galvanic cell.

Al(s) is at the anode and Ag is at the cathode.

Al(s) l Al3+(aq) ll Ag+ (aq) l Ag(s)

d)Calculate the potential of the cell, Ecell, in volts when 0.010 M Al3+ and 1.0 M Ag+ solutions are used instead. According to your overall reaction and Le Châtelier’s principle, do you predict the cell potential to be higher or lower than the standard cell potential calculated in 1-a)?

Ecell = Ecello -log(Q) where Q = [Al3+]/[Ag+]3 =0.010/(1.0)3 = 0.010

Ecell = Ecello -log(Q) = 2.46 V – (0.0592/3)log(0.010) = 2.46 V – (-0.039) = 2.50 V

The cell potential will be higher because you have lower product concentration than the standard state. The reaction becomes more spontaneous and the cell potential will be higher.

  1. A sample of wood from an Egyptian mummy case gives a C-14 count of 9.6 counts per minute per gram of carbon (cpm/g). How old is this wood? The initial decay rate of C-14 is 15.3 cpm/g, and its half-life is 5730 years.

lnN = -kt + lnN o

ln (9.6) = -t + ln (15.3)

t = (5730 yrs /0.693) {ln (15.3) – ln (9.6)} = 3850 years

  1. Explain the similarities and differences between fusion and fission.

They are both nuclear reactions and release large amounts of energy. The mass loss arises from both reactions, which is converted to energy.

Fusion involves two light nuclei such as hydrogen and helium coming together to form a larger nucleus. Fusion requires higher temperature to overcome the repulsion between two positively charged nuclei. Fission is splitting of a heavy nucleus (U-235 is a good example) with a neutron to form smaller nuclei.

Bonus Question

Many countries meet much of their energy needs via nuclear power. 70% of electricity in France and 50% in South Korea is generated by nuclear reactors while in the United States, it is about 10%. Discuss the advantages and disadvantages of using nuclear power and explain why certain countries use nuclear energy more readily than others.