Clayton State University

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Examination III

Principles of Chemistry (CHEM 1211)

Spring 2010

April 27, 2010

The following exam is worth 100 points

Each question is worth 10 points

Please…………………………..

Make sure there are seven pages in your exam copy

Use non-erasable PEN

Write your name on all exam sheets

Sign the upper right-hand corner of the first sheet

(signature is taken as a statement of honest and independent work)

State appropriate units with each numerical answer

Show all work and clearly indicate the final answers using boxes

Question 1

(a) Calculate the standard enthalpy change (ΔHorxn) of the following reactions from their standard enthalpies of formation:

(ΔHfo of C2H2(g) = 226.73 kJ/mol, ΔHfo of H2S(g) = -20.63 kJ/mol, ΔHfo of CO2(g) = -393.51 kJ/mol, ΔHfo of H2O(l) = -285.83 kJ/mol, ΔHfo of SO2(g) = -296.83 kJ/mol)

(i) 2C2H2(g) + 5O2(g) → 4CO2(g) + 2H2O(l)

ΔHorxn = 4(-393.51) + 2(-285.83) – 2(226.73) = -2599.2 kJ

(ii) 2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g)

ΔHorxn = 2(-285.83) + 2(-296.83) – 2(-20.63) = -1124.06 kJ

(b) How much heat (in kJ) is given off when 1.26 x 104 g of ammonia is produced according to the equation below?

N2(g) + 3H2(g) → 2NH3(g) ΔHorxn = -92.6 kJ

Assume that the reaction takes place at standard state conditions.

Question 2

(a) Stoichiometry is based on the law of conservation of mass. On what law is thermochemistry based? Law of conservation of energy

(b) Calculate the work done (in kJ) when 1.0 mol of water is frozen at 0 oC and 1.0 atm. The volumes of 1.0 mol of water and ice at 0 oC are 0.0180 L and 0.0196 L, respectively.

w = -PΔV = - (1.0 atm)(0.0196 L – 0.0180 L) = - 0.0016 L∙atm

(c) A sheet of gold weighing 10.0 g and at a temperature of 18.0 oC is placed flat on a sheet of iron weighing 20.0 g and at a temperature of 55.6 oC. What is the final temperature of the combined metals? Assume that no heat is lost to the surroundings (the specific heat capacities of gold and iron are 0.129 J/g·K and 0.449 J/g·K, respectively).

- (10.0)(0.129)(Tf – 18.0) = (20.0)(0.449)(Tf – 55.6)

Tf = 50.9 oC

Question 3

(a) Calculate the standard enthalpy change for the reaction

2Al(s) + Fe2O3(s) → 2Fe(s) + Al2O3(s) ΔHorxn = ?

given that

ΔHorxn = -1601 kJ

ΔHorxn = -821 kJ

Al is needed on reactant side, first equation remains as is

ΔHorxn = -1601 kJ

Fe is needed on product side, reverse second equation

ΔHorxn = +821 kJ

Combine equations

2Al(s) + Fe2O3(s) → 2Fe(s) + Al2O3(s) ΔHorxn = - 780 kJ

(b) A piece of silver of mass 362 g has a heat capacity of 85.7 J/oC. What is the specific heat of silver in cal/oC?

Question 4

(a) The atmospheric pressure at the summit of Mt. McKinley is 606 mm Hg on a certain day. What is the pressure in psi, atm, and kPa?

(b) A gas-filled balloon having a volume of 2.50 L at 1.2 atm and 25 oC is allowed to rise to the stratosphere (about 30 km above the surface of the earth), where the temperature and pressure are -23 oC and 3.00 x 10-3 atm, respectively. Calculate the final volume of the balloon.

T1 = 25 oC = 298 K, P1 = 1.2 atm, V1 = 2.50 L,

T2 = -23 oC = 250 K, P2 = 3.00 x 10-3 atm, V2 = ?

Question 5

(a) Calculate the volume occupied by 0.678 g of N2 gas at STP.

PV = nRT

(b) A volume of 4.9 L of H2 gas measured at STP is reacted with an excess of Cl2 gas. Calculate the mass (in grams) of HCl produced. The reaction equation is:

H2(g) + Cl2(g) → 2HCl(g)

Question 6

(a) A mixture of gases contains CH4, C2H6, and C3H8. If the total pressure is 1.50 atm and the numbers of moles of the gases present are 0.31 mol for CH4, 0.25 mol for C2H6, and 0.29 mol for C3H8, calculate the partial pressures of the gases.

Total moles = 0.31 + 0.25 + 0.29 = 0.85 mol

Let P1, P2 and P3 equal partial pressures for CH4, C2H6, and C3H8, respectively

(b) A compound of empirical formula C2H5 effused through a porous barrier in 1.62 min. It took an equal volume of xenon 2.43 min to effuse under the same conditions of temperature and pressure. What is the molecular formula of the compound?

Empirical formula molar mass = 2(12.01) + 5(1.01) = 29.07 g/mol

Multiple = 58.3/29.07 = 2

Molecular formula = C4H10

Question 7

(a) What is the frequency of light of wavelength 456 nm?

(b) What is the wavelength (in nm) of radiation of frequency 2.20 x 109 Hz?

Sr2+

Ga: 1s22s22p63s23p64s23d104p6

Question 8

(a) The ground state electron configuration of a neutral atom is 1s22s22p2. Write a complete set of quantum numbers for each of the electrons. How many valence and core electrons are present in the atom?

n = 1, l = 0, ml = 0, ms = +1/2 n = 1, l = 0, ml = 0, ms = -1/2

n = 2, l = 0, ml = 0, ms = +1/2 n = 2, l = 0, ml = 0, ms = -1/2

n = 2, l = 1, ml = -1, ms = +1/2 n = 2, l = 1, ml = 0, ms = +1/2

b) Which of the following sets of quantum numbers are invalid? Explain your reasoning.

n = 3, l = 2, ml = +1, ms = +1 invalid, ms = +1 does not exist

n = 4, l = 3, ml = -4, ms = +1/2 invalid, ml = -4 does not exist in l = 3

Question 9

(a) For each of the following pairs of orbitals, indicate which has lower energy

1s and 3s 1s

2p and 5s 2p

5d and 4f 4f

(b) How does the size change when an atom is converted to

(i) an anion? larger

(ii) a cation? smaller

(c) On the basis of their positions in the periodic table, select the atom with the largest atomic radius in each of the following sets.

(i) Mg, Ba, and Sr Ba

(ii) C, F, and Li Li

(iii) As, Bi, and N Bi

Question 10

(i) Sketch the outline of the periodic table and show the general group and period trends in the first ionization energy of the elements.

(ii) Two atoms have the electron configurations 1s22s22p6 and 1s22s22p63s1. The first ionization energy of one is 2080 kJ/mol, and that of the other is 496 kJ/mol. Pair up each ionization energy with one of the given electron configurations. Justify your choice.

496 kJ/mol for 1s22s22p63s1

2080 kJ/mol for 1s22s22p6

Higher principal quantum number (shells) implies lower IE

Complete octet in 1s22s22p6 implies higher IE

(iii) Arrange the following species in isoelectronic pairs: Ne, Zn, O+, N, As3+ and N3-.

Ne & N3- Zn & As3+ O+ & N

(iv) State which member of the following elements has the smallest electron affinity (least negative value): Mg, S, Al, P

Bonus

(i) An electron in the hydrogen atom can return to the ground state in two different ways: (a) via a direct transition in which a photon of wavelength λ1 is emitted and (b) via an intermediate excited state reached by the emission of a photon of wavelength λ2. This intermediate excited state then decays to the ground state by emitting another photon of wavelength λ3. Derive an equation that relates λ1, λ2, and λ3.

λ1 = λ2 + λ3

(ii) You pick up a 6-pack of soft drinks from the floor, but it slips from your hand and smashes into your foot. What forms of energy are involved; at what stages of the process?

Pick up: kinetic and potential energies

Slips: kinetic and potential energies

Smashes: sound energy and may be heat energy