Exam 3 (Chemistry)

7. For the reaction, 2Cr₂⁺ + Cl₂(g) 2Cr₃⁺ + 2Cl⁻ E°cell is 1.78 V. Calculate E°cell for the related reaction Cr₃⁺ + Cl⁻ Cr₂⁺ + 1/2Cl₂(g)

-1.78 V *Coefficients don't change Ecell value. Just flip the sign since this is an electrolytic cell rxn.*

16. For the reaction, 2Cr²⁺ + Cl₂(g) 2Cr³⁺ + 2Cl⁻, E°cell is 1.78 V. Calculate E°cell for the related reaction: Cr³⁺ + Cl⁻ → Cr²⁺ + 1/2Cl₂(g)

-1.78 v

8. Consider the following Galvanic cell. U(s) │ U₃+(aq) ║Cl⁻(aq), Cl₂(g) │Pt If the standard cell emf is 3.16 V, what is the standard reduction potential for uranium?

-1.8V *(Don't forget to divide by the negative to solve for x)*

24. Consider the reversible vaporization of 1 mole of water in a large, isothermal water bath at 100°C and 1 atm pressure. The molar heat of vaporization is 40 kJ/mol. *Calculate the entropy change in the surrounding (∆Ssurr) as a result of vaporization.* Let the unit of your answer be J/Kmol.

-107 kj/mol

17. Consider the following Galvanic cell. U(s) │ U³⁺(aq) ║Cl⁻(aq), Cl₂(g) │Pt If the standard cell emf is 4.24 V, what is the standard reduction potential for U?

-2.88 v

21. Calculate the standard free-energy change for the following reaction at 25 °C: Cu²⁺(aq) + Zn(s) --> Cu(s) + Zn²⁺(aq) (F=96500 J/Vmol)

-212300 J

10. The rate of decay (disintegration) of radioactive elements can be affected by temperature or pressure. True or False?

. False, because, again, conditions like temperature or pressure cannot control the "nuclear" reaction rates. Removal of radioactive element from one location can only slow down the reaction rate at that location

22. Will the following reaction occur spontaneously at 25oC, given that [Co²⁺] = 0.1 M and [Ni²⁺] = 0.1 M? (R = 8.314 J/Kmol and F = 96500 J/Vmol) Ni²⁺(aq) + Co(s) → Ni(s) + Co²⁺(aq)

.03 V

16. The following shows a radioactive decay in series: ²²²Rn → ²¹⁸Po → ²¹⁰Pb. The half life of ²²²Rn is 20 days and the half life of ²¹⁸Po is 0.5 years. If the initial amount of ²²²Rn is 1.0 mol, what would be the amount of ²¹⁸Po after one year?

.25 mol

25. A galvanic cell is made of a Co electrode in a 1.0 M Co(NO₃)₂ solution and a Zn electrode in a 1.0 M Zn(NO₃)₂ solution at 25°C. What is the correct overall reaction for this galvanic cell? A. 3Co²⁺ + 2Zn → 3Co + 2Zn³⁺ B. 3Co + 2Zn³⁺ → 3Co²⁺ + 2Zn C. Zn²⁺ + Co → Co²⁺ + Zn D. 3Zn + 2Co³⁺ → 3Zn²⁺+ 2Co E. Co²⁺ + Zn → Co + Zn²⁺

.48 V

2. How many mass number does a positron have?

0

18. Calculate the equilibrium constant K for the reaction of SO₂(g) + NO₂(g) SO₃(g) + NO(g) at 298 K. Given the following standard free energies of formation: SO2(g) = -300.4 kJ/mol; SO3(g) = -370.4 kJ/mol; NO(g) = 86.7 kJ/mol; NO2(g) = 51.8 kJ/mol

1.42x10^6 J

19. Calculate the standard free energy change for the following reaction: 4Ag(s) + O₂(g) + 4H⁺(aq) → 4Ag⁺(aq) + 2H₂O(l)

1.7 x 10⁵

7. ⁴⁰K decays to ⁴⁰Ar with the half life of 1.3 x 10⁹ yr. A sample of moon rock was found to contain fifty-nine ⁴⁰Ar atoms for every forty-one ⁴⁰K atoms. ⁴⁰₁₉K → ⁴⁰₁₈Ar + ⁰⁺¹e The age of the rock is? (It is assumed that there was no ⁴⁰Ar at the time of the rockformation, i.e., there were only ⁴⁰K at the beginning.)

1.7 x 10⁹ years

If 12% of the original amount of a certain radioactive isotope is left after 5.2 years of its radioactive decay, what is the half-life of this isotope? (know the difference between this problem and #17)

1.7 years

20. Calculate the number of grams of aluminum produced in 2.00 hour by the electrolysis of molten AlCl₃ if the electrical current is 15.0 A. (1F = 96,500 C/mol e-)

10.1

19. For the reaction of 2C(graphite) + H2(g) C₂H₂(g), ∆G° = +209.2 kJ/mol at 25 oC. If P(H₂) = 100 atm, and P(C₂H₂) = 0.1 atm, calculate ∆G for the reaction

192.08 kj/mol *Don't forget to change Kj to mol initially, and also to take the Ln of .001* ∆G = ∆G° *+* RT(lnK) !!!Addition sign, not subtraction!!!!

21. At 1500 °C for the reaction of CO + 2H₂ = CH₃OH, the equilibrium constant is K = 1.4x10-7 Calculate ∆G° for this reaction at 1500 oC.

2.32 x 10^5

6. Calculate E°cell for a silver-aluminum cell in which the cell reaction is Al(s) + 3Ag⁺(aq) → Al₃⁺(aq) + 3Ag(s)

2.46 V

15. Calculate E°cell for a silver-aluminum cell in which the cell reaction is Al(s) + 3Ag⁺(aq) → Al³⁺(aq) + 3Ag(s)

2.46 V 3Ag⁺ + Al → 3Ag + Al³⁺

18. ⁴⁰K decays to ⁴⁰Ar with the half life of 1.3 x 10⁹ yr. A sample of moon rock was found to contain seventy-eight ⁴⁰Ar atoms for every twenty-two ⁴⁰K atoms. ⁴⁰₁₉K → ⁴⁰₁₈Ar + ⁰⁺₁e The age of the rock is:

2.8 x 10⁹

17. At 1500 °C for the reaction of CO(g) + 2H2(g) = CH3OH(g), the equilibrium constant K = 1.4x10^-7 Calculate ∆G for this reaction at 1500 oC.

233 kj/mol

15. How many nucleons does an atom with the symbol ³²₁₆S have?

32

14. Consider a Galvanic cell constructed from the following half cells, linked by a KCl salt bridge: An Al(s) electrode in 0.5 M Al2(SO4)3 solution A Pb(s) electrode in 1.0 M Pb(NO3)2 solution Write down the balanced overall (net) cell reaction (redox reaction).

3Pb²⁺ + 2Al → 3Pb + 2Al³⁺

12. How many coulombs of charge must pass through an electrolytic cell to reduce 0.25 mole of Cu²⁺ to Cu?

48250 C

17. If 12% of a certain radioactive isotope decays in 5.2 years, what is the half-life of this isotope?

5.2 years *If 12% decays, then 88% is left. Therefore, N° = .88%

11. Determine the equilibrium constant, K, for the reaction: 2Br⁻(aq) + I₂(s) = Br₂(l) + 2I⁻(aq)

5.4 x 10⁻¹⁹

22. Calculate the equilibrium constant (Kp) for the reaction at 25 °C , R = 8.314 J/molK: N₂g) + 3H₂(g) = 2NH₃(g) The standard free energy change (∆G°) of the reaction is -33.3 kJ/mol.

6.9 x 10^5 *don't forget that the negatives cancel out between G & RT*

5. Calculate the nuclear binding energy per nucleon, in joules, for ²⁵₁₂Mg proton mass = 1.007825 amu; neutron mass = 1.008665 amu; atomic mass of ²⁵₁₂Mg = 23.985839 amu; 1 amu =1.49x10⁻¹⁰ J ; and c = 3.00x10⁸ m/s.

7.3 x 10⁻¹² J/Nucleons

6. The half life of ²²²Rn is 27 days. What fraction of the atom in a sample of 222Rn would remain in a year, i.e. 365 days?

8.5 x 10⁻⁵ or 8.5 x 10⁻³%

11. Estimate the age of a bottled wine that has a tritium, ³H, content 60% that of freshly bottled wine. Tritium decays by beta decay and has the half life of 12.3 yr. ³H → ³He + ⁰B

9.1 years

14. In the ²³⁸₉₂U decay series there are eight radioisotopes that decay by alpha emission including ²³⁸₉₂ itself, and six radioisotopes that decay by beta emission. The final product of this series is a stable isotope. The symbol for this product is: A. ²⁰⁶₈₂Pb B. ²⁰⁶₇₀Yb C. ²²²₈₆Rn D.²⁰⁸₈₂Pb E. ²⁰⁸₇₈Pt

A: ²⁰⁶₈₂Pb

19. For the nuclear reaction: ¹⁴₆C → ¹⁴₇N + ⁰₋₁B A. C is radioactive. True or False B. the nucleus of C is not stable. True or False C. One neutron in the nucleus of C converts to a proton with the release of B particle.True or False D. N is stable. True or False

All true

2. Consider a Galvanic cell constructed from the following half cells, linked by a KCl salt bridge. . an Al(s) electrode in 0.5 M Al2(SO4)3 solution . a Pb(s) electrode in 1.0 M Pb(NO3)2 solution *Which electrode is the anode?*

Anode = Al Because this rxn takes place in a Galvanic Cell, Both Ecell values must be positive. Therefore, Al is the higher Ecell value = Anode

1. Given the following notation for a Galvanic cell diagram Pt(s) │ H2(g) │ H⁺(aq) ║ Ag⁺(aq) │ Ag(s) write down the balanced overall (net) cell reaction (redox reaction).

Anode = Pt Cathode = Ag Balanced eq: H₂ + 2Ag⁺ → 2H⁺ + 2Ag *Because Ag is reduced, we need 2Ag (2e⁻) to take 2H → H₂⁺

4. What would happen at the anode in Question #2 in terms of its mass?

Anode = decrease in mass

26. Select the correct statement. A. To calculate the standard entropy of reaction, ∆S°rxn the values of the standard entropy of formation of compounds involved should be provided. B. The standard free energy of formation of oxygen gas, ∆G°f(O₂) is zero. C. ∆Srxn for a reaction does not mean ∆Ssys for the reaction. D. ∆Srxn > 0 for a certain reaction means that the reaction is spontaneous. E. ∆Ssurr increases for an endothermic reaction.

B. The standard free energy of formation of oxygen gas, ∆G°f(O₂) is zero. *^ Law ^*

20. At 20 °C the reaction, 3O₂(g) --> 2O₃(g), is an endothermic reaction. Select the correct combination among ∆H, ∆S, ∆G, and spontaneity of this reaction. A. ∆H < 0, ∆S > 0, ∆G > 0, always nonspontaneous B. ∆H < 0, ∆S < 0, ∆G < 0, always spontaneous C. ∆H > 0, ∆S < 0, ∆G > 0, always nonspontaneous D. ∆H > 0, ∆S > 0, ∆G < 0, always spontaneous

C

23. Which redox reaction would have the smallest equilibrium constant, K? A. Co²⁺(aq) + Zn(s) → Co(s) + Zn²⁺(aq) B. Al(s) + 3Ag⁺(aq) → Al3⁺(aq) + 3Ag(s) C. Zn²⁺(aq) + Cu(s) → Zn(s) + Cu2⁺(aq)

C

20. Which statement is not correct? A. Radioactive decay of ²³⁵U follows the first order reaction kinetics. B. The decay constant of ²³⁵U must be constant. C. The rate of decay of ²³⁵U must be constant D. The larger the amount of ²³⁵U, the faster is the rate of its decay.

C. The rate of decay of ²³⁵U must be constant rate of decay depends on sample size

13. What is X for the nuclear reaction: ²¹²₈₄Po → ²⁰8₈₂Pb + X

C. ⁴₂a

5. What would happen at the cathode in Question #2 in terms of its mass?

Cathode = Increase in mass

24. For the overall redox reaction for a galvanic cell is as follows: Zn + Cu²⁺→ Zn²⁺ + Cu (a) Write down the half reaction at the cathode of this galvanic cell. (b) Write down the half reaction at the anode of this galvanic cell.

Cathode: Cu + 2e⁻ → Cu Anode: Zn → Zn²⁺ + 2e⁻

10. The strongest reducing agent listed above is A. Cr³⁺ B. Cr C. Mn²⁺ D. Co E. MnO4⁻ Cr₃⁺ + 3e⁻ → Cr Eo = -0.74 V Co₂⁺+ 2e⁻ → Co Eo= -0.28 V MnO4⁻ + 8H⁺+ 5e⁻ → Mn2⁺ + 4H₂O Eo = +1.51 V

Cr *Reducing agent = right side*

26. Which statement is false? A. Galvanic cell is a spontaneous electrochemical process. B. Electrolysis is a non-spontaneous electrochemical process. C. Batteries run through spontaneous electrochemical process and thus in essence are galvanic cells. D. No answer because all statements above are correct.

D

27. Select the correct statement. A. An exothermic reaction guarantees the spontaneity of that reaction. B. The larger the equilibrium constant K, the more positive the value of Grxn for a reaction. C. Increase in the entropy of a reaction guarantees spontaneity of that reaction D. The standard entropy of any element is always greater than zero at 25 °C. E. Increase in the enthalpy of a reaction guarantees the nonspontaneity of that reaction.

D. The standard entropy of any element is always greater than zero at 25 °C.

3. As a result of alpha decay, the product nucleus is... A. one atomic number lower than the original element. B. two atomic number higher than the original element. C. one atomic number higher than the original element. D. two atomic number lower than the original element. E. four atomic number lower than the original element.

D. two atomic number lower than the original element. *Alpha's are twice as hard*

4. In the ²³²₉₀Th decay series there are six radioisotopes that decay by alpha emission including Th-232 itself, and four radioisotopes that decay by beta emission. The final product of this series is a stable isotope. The symbol for this product is.... A. ²⁰⁴₇₄W B. ²⁰⁸₇₄W C. ²⁰⁴₈₂Pb D. ²⁰⁸₈₂Pb E.²⁰⁸₇₈Pt

D. ²⁰⁸₈₂Pb

At the anode, the mass...

Decreases

25. If ∆G for a reaction is positive, which one describes properly about this reaction? A. The reverse reaction is nonspontaneous. B. If ∆Hrxn is negative, ∆Srxn must be positive for this reaction. C. Entropy must increase as a result of the reaction. D. The reaction has a greater equilibrium constant than the reverse reaction. E. If ∆Hrxn is positive, ∆Srxn must be negative for this reaction at high temperature

E. If ∆Hrxn is positive, ∆Srxn must be negative for this reaction at high temperature

23. At 20 °C the reaction, 2H₂O₂(aq) --> 2H₂O(l) + O₂(g) is an exothermic reaction. Select the correct combination among ∆H, ∆S, ∆G, and spontaneity of this reaction A. ∆H < 0, ∆S > 0, ∆G > 0, always nonspontaneous B. ∆H < 0, ∆S < 0, ∆G < 0, always spontaneous C. ∆H > 0, ∆S < 0, ∆G > 0, always nonspontaneous D. ∆H > 0, ∆S > 0, ∆G < 0, always spontaneous E. ∆H < 0, ∆S > 0, ∆G < 0, always spontaneous

E. ∆H < 0, ∆S > 0, ∆G < 0, always spontaneous

Will the following reaction occur spontaneously at 25oC, given that [Fe2+] = 0.6 M and [Cd2+] = 0.010 M? Cd(s) + Fe2+(aq) --> Cd2+(aq) + Fe(s)

Ecell value Ecell = -0.0400 - (0.0591/2)log{0.01/0.75}

29. Every exothermic reaction is spontaneous. T/F?

False

Both enthalpy and entropy are involved in the 1st law of thermodynamics T/F?

False

9. The rate of decay (disintegration) of U-238 is constant regardless of its sample size because the rate is for the same element U-238. True or False?

False, because the decay law of radioactive elements, dN/dt = -N, shows that the rate is proportional to their amount.

The standard entropy of any element in its stable form is zero at 25 oC T/F?

False. *Anything greater than -273° or 0 Kelvin would have entropy change > 0*

An exothermic reaction guarantees the spontaneity of that reaction T/F?

False. In order to assess spontaneity, you use the gibbs free energy equation ∆G = ∆H - T(∆S) < 0

The case in question 4 guarantees the non-spontaneity of that reaction T/F?

False. ∆G = ∆H - T(∆S) < 0

The thermodynamic function entropy is involved in the 1st law of thermodynamics. T/F?

False. Enthlapy is involved in 1st law of thermodynamics. conservation of energy = 1st law

30. Entropy increases in every spontaneous reaction. True or False?

Fasle

At the cathode, the mass...

Increases

LEO GER

Loss of electrons = oxidized/ reducing agent Gain electrons = Oxidizing agent / reduced

9. The strongest oxidizing agent listed above is Cr₃⁺ + 3e⁻ → Cr Eo = -0.74 V Co₂⁺+ 2e⁻ → Co Eo= -0.28 V MnO4⁻ + 8H⁺+ 5e⁻ → Mn2⁺ + 4H₂O Eo = +1.51 V A. Cr₃⁺ B. Cr C. Mn₂⁺ D. Co E. MnO₄

MnO4

18. The strongest oxidizing agent listed below is A. Cr³⁺ B. Cr C. Mn²⁺ D. Co E. MnO₄⁻

MnO₄⁻ The higher the E°cell value, the stronger the oxidizing agent = most likely to be reduced

Synthesis of protein from amino acids is spontaneous or non-spontanteous process?

Non-Spontaneous

13. In an electrolytic cell, electrical energy is converted into chemical energy. T or F

T. Opposite for Galvanic cell (chem e → electrical e)

31. If lnK > 0, then the equilibrium constant K > 1. Thus, forward reaction is favored. True or False?

True

32. If lnK < 0, then the equilibrium constant K < 1. Thus, reverse reaction is favored. True or False?

True

If the free energy change of the system in a reaction is negative (ie, G < 0), the spontaneity of the reaction is guaranteed T/F?

True

The standard enthalpy of formation for any element in its stable form is zero T/F?

True

The standard free energy of formation for any element in its stable form is zero T/F?

True (i.e. ∆G° formation = 0)

8. No chemical treatments can make radioactive elements harmless. True or False?

True, because radioactive elements undergo "nuclear" reaction, not "chemical" reaction. This is why nuclear reactions are so dangerous.

If the entropy change of the system in a reaction is negative (i.e, Ssys < 0), it means the disorder becomes reduced as the result of that reaction T/F?

True. If ∆S < 0 , then the rxn is becoming more ordered

15. A reaction is endothermic and the entropy gets decreased as the result of the reaction. This reaction must be always nonspontaneous Yes or NO?

Yes

15. A reaction is endothermic and the entropy gets decreased as the result of the reaction. This reaction must be always nonspontaneous Yes or NO?

Yes *Endothermic rxn = ∆H > 0 T & ∆H are both +*

1. What is X for the nuclear reaction: ²³⁸₉₂U →²³⁴₉₀Th + X

⁴₂He or ⁴₂A