GENERAL CHEMISTRY II CHEM206 /2 52

Final Examination December 20, 2004 1900-2200

Dr. Cerrie ROGERS

x periodic table and formula sheet provided

x non-programmable calculators allowed

Chem 206 --- GENERAL CHEMISTRY II

LAST NAME: ____________________________ FIRST NAME: ____________________________

STUDENT NUMBER: _____________________ SIGNATURE: ____________________________

Instructions: PLEASE READ THIS PAGE WHILE WAITING TO START.

Make sure your exam has 13 pages plus a periodic table.. Write your student ID number on all pages. Write all answers in the spaces provided on this paper. No other paper is allowed.. Read ALL questions through quickly BEFORE starting the exam. Non-programmable calculators are allowed. You may detach the periodic table and “potentially useful information” pages.

PLEASE ASK TO SPEAK WITH THE PROFESSOR IF YOU NEED CLARIFICATION.

Mark breakdown:

Page 2. / 13 Page 7. / 7 Page 3. / 10 Page 8. / 10Page 4. / 5 Page 9. / 6Page 5. / 12 Page 10. / 12Page 6. / 10 Page 11. / 8

PAGECHEM 206 Fall 2004 Section 52 ID #:_______________________

TOTAL: / 90 (MAXIMUM MARK = 93)

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PAGECHEM 206 Fall 2004 Section 52 ID #:_______________________

# 1. (__/ 13 marks) Circle the word(s) that correctly completes each of the following statements.

a) When an ice cube is placed into a beaker of warm water, the sign of the heat flow according to the ice cube is ( POSITIVE / NEGATIVE ).

b) The first law of thermodynamics states that the ( ENERGY / ENTROPY ) of the universe is constant.

c) The Gibbs free energy change for a process is fundamentally defined as –TS, where S in this case refers to the entropy change of the ( SYSTEM / UNIVERSE ).

d) Because of entropic effects, gaseous solutes are ( MORE SOLUBLE / LESS SOLUBLE ) in warm water than in cold water.

e) A solution containing 0.500 M NaCl will freeze at a ( HIGHER / LOWER ) temperature than a solution containing 0.500 M CaCl2.

f) Polar substances dissolve in water because of favourable ( INTERMOLECULAR / INTRAMOLECULAR ) forces.

g) When a system is at equilibrium, the concentrations of all species involved are ( EQUAL / CONSTANT ).

h) A reaction with an equilibrium constant of ( 10-25 / 1025 ) would be described as product-favoured.

i) If the equation for a reaction is multiplied by a factor of two, the equilibrium constant for the reaction must be ( MULTIPLIED BY TWO / SQUARED ).

j) Based on the relative sizes of nitrogen and phosphorus atoms, we would predict ammonia, NH3, to be a ( STRONGER / WEAKER ) base than phosphine, PH3.

k) A solution of pH 12.0 would more likely contain ( 0.01 M NaOH / 0.01 M NH3 ).

l) In a saturated solution of a solid solute, the rates of ( EVAPORATION / DISSOLUTION ) of solid and precipitation of solid are equal.

m) Adding acid to a solution that contains a carbonate salt ( INCREASES / DECREASES ) the solubility of the salt.

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PAGECHEM 206 Fall 2004 Section 52 ID #:_______________________

# 2. (__/ 10 marks) Hydrogen cyanide (HCN) is produced industrially by the following reaction:

2NH3(g) + 3O2(g) + 2CH4(g) 2HCN(g) + 6H2O(g)

a) (2 marks) Is this reaction endothermic or exothermic? Show a calculation to support your choice.

b) (2 marks) Calculate the standard entropy change for this reaction (Sorxn). Is the reaction entropically

favoured or entropically disfavoured?

c) (3 marks) Determine whether the reaction is spontaneous or nonspontaneous in the forward direction at 298 K. Support your answer with calculations.

d) (3 marks) This reaction is actually performed industrially at 1000°C with a catalyst. Based on your calculations, is the high temperature needed for thermodynamic or for kinetic reasons? Explain.

Substance Hof

(kJ/mol)So

f (J/molK)

NH3(g) -46 193

O2(g) 0 205

CH4(g) -75 186

HCN(g) 135 202

H2O(g) -242 189

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PAGECHEM 206 Fall 2004 Section 52 ID #:_______________________

# 3. (__/ 5 marks) Under 1.00 atm of O2(g), the solubility of oxygen gas in water is 1.3810-3 M at 20°C. At sea level, the partial pressure of O2(g) in the air is 0.21 atm.

a) (2 marks) Calculate the concentration (in molarity) of oxygen gas in the surface water of a lake saturated with air at 20°C.

b) (3 marks) What is this concentration expressed in parts per million (grams of solute per 106 grams of

solution). Assume that the density of air-saturated water at 20°C is 0.998 g/mL.

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PAGECHEM 206 Fall 2004 Section 52 ID #:_______________________

# 4. (__/ 7 marks) Nitrogen oxides, a mixture of NO and NO2 described as NOx, are important atmospheric pollutants. During daylight hours, the NOx in the atmosphere breaks down slowly to N2 and O2 via a first-order reaction, with a half-life of approximately 3.9 h.

a) (3 marks) What is the rate constant k for this reaction?

b) (4 marks) How many hours of daylight must a sample be exposed to in order to decrease the NOx concentration in a sample from 2.010-5 M to 1.2510-6 M?

# 5. (__/ 5 marks) Indicate whether the following statements about chemical kinetics are true (T) or false (F).

T / F Bimolecular elementary steps always have second-order rate laws.T / F The activation energy of a chemical reaction decreases when the temperature increases. T / F A catalyst increases the activation energy of a reaction. T / F The rate determining step in a mechanism is the step with the largest activation energy. T / F When the rate of a reaction in one direction equals the rate of the reaction in the opposite

direction, the system is described as being at equilibrium.

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PAGECHEM 206 Fall 2004 Section 52 ID #:_______________________

# 6. (__/ 10 Marks) The reaction PCl3(g) + Cl2(g) PCl5(g) has Kp = 0.0870 at 300°C.

A reaction vessel is filled with 0.50 atm PCl3, 0.50 atm Cl2 and 0.20 atm PCl5 and heated to 300°C.

a) (2 marks) What is the initial value of the reaction quotient, Qp?

b) (3 marks) In which net direction will the reaction proceed while it progresses toward equilibrium? Explain.

c) (3 marks) Imagine that the reaction has reached equilibrium, but then the volume of the reaction vessel is increased. When the mixture has re-established equilibrium after this disturbance, will there be more PCl5 or less PCl5 present? Explain.

d) (2 marks) Equilibrium constants are routinely expressed in terms of concentrations of substances. Calculate the value of Kc for this equilibrium at 300°C.

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PAGECHEM 206 Fall 2004 Section 52 ID #:_______________________

# 7. (__/ 7 Marks) You are preparing a solution of ammonium fluoride, NH4F, in water.

a) (4 marks) Will the resulting solution be acidic, basic or neutral? Explain your answer, and write chemical reactions to support your conclusion.

The following data might be helpful: Kb of NH3 = 1.810-5 Ka of HF = 7.210-4

Kw = 110-14

b) (3 marks) Imagine you are dissolving the NH4F in a solution that already contains some dissolved calcium fluoride, CaF2. Will the solubility of NH4F be higher or lower in this solution than it is in pure water? Explain (include relevant chemical equations but do not perform calculations).

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PAGECHEM 206 Fall 2004 Section 52 ID #:_______________________

# 8. (__/ 10 Marks) Acetylsalicylic acid, HC9H7O4, is a monoprotic weak acid commonly known as aspirin. The Ka of acetylsalicylic acid is 3.27x10-4.

a) (6 marks) If you dissolve an aspirin tablet that contains 325 mg of acetylsalicylic acid in 250.0 mL of water, what will the pH of the solution be?

b) (2 marks) Imagine you now titrate this solution with a 0.110 M NaOH titrant solution. What volume of titrant is required to reach the equivalence point?

c) (2 marks) At the equivalence point, will the solution's pH be below 7, equal to 7, or above 7? Explain (without doing calculations).

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Titration curve for unknown acid with NaOH

0

2

4

6

8

10

12

14

0 20 40 60 80

Volume of NaOH solution added (mL)

pH

PAGECHEM 206 Fall 2004 Section 52 ID #:_______________________

# 9. (__/ 6 marks) The following graph of pH versus titrant volume was obtained during the titration of an unknown acid with NaOH.

a) (2 marks) Was the unknown acid a strong acid or a weak acid? How can you tell?

b) (1 mark) Based on the titration curve, what is the pH at the equivalence point of the titration?

c) (3 marks) Using the information in the table, choose an indicator that would minimize indicator error (i.e., end point very close to the equivalence point) for this titration. Explain your choice.

INDICATOR Colour of conjugate acid

pH range ofcolour change

Colour of conjugate base

Crystal violet Yellow-green 0.0 – 1.8 VioletBromphenol blue Green 3.0 – 4.5 Blue

Phenol red Yellow 6.8 – 8.2 RedThymolphthalein Colourless 9.5 - 10.5 Blue

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PAGECHEM 206 Fall 2004 Section 52 ID #:_______________________

# 10. (__/ 12 Marks) You are working in a biology lab and are asked to prepare a pH 7.40 buffer that mimics human blood. You will use KH2PO4 and Na2HPO4. The Ka of H2PO4

- is 6.310-8; the Ka of HPO42- is 4.210-13.

a) (3 marks) Briefly explain why HPO42- and H2PO4

- are a good pair of substances to use.

b) (3 marks) What should be the ratio of [HPO42-] / [H2PO4

-] in this buffer?

a) (6 marks) To mimic blood, the buffer must exert an osmotic pressure of = 8.00 atm at 37°C. Using this information, calculate the masses of KH2PO4 and Na2HPO4 you should use to prepare 1.0 L of buffer.

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PAGECHEM 206 Fall 2004 Section 52 ID #:_______________________

# 11. (__/ 8 Marks) People develop kidney stones when insoluble compounds like calcium phosphate, Ca3(PO4)2, precipitate out of their urine. The Ksp of calcium phosphate is 110-25.

a) (1.5 marks) Write a balanced chemical equation for the equilibrium that occurs in a saturated solution

of calcium phosphate.

b) (1.5 marks) Write an expression for the Ksp of Ca3(PO4)2.

c) (5 marks) Urine normally contains about 5.33 g/L of Ca2+ ions. What concentration of phosphate ions (in molarity) would cause calcium phosphate to begin precipitating from urine?

------------------------------------------------------ HAPPY HOLIDAYS ! ---------------------------------------------------

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PAGECHEM 206 Fall 2004 Section 52 ID #:_______________________

THIS PAGE IS BLANK (USE FOR ROUGH WORK)

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PAGECHEM 206 Fall 2004 Section 52 ID #:_______________________

POTENTIALLY USEFUL INFORMATION

R = 8.314 J•mol-1K-1 = 0.08206 L•atm•mol-1K-1

C(H2O) = 4.184 J•g-1K-1

Kw = 10-14

--------------------------------------------------------------------------------------------------------------------------------------

GO = HO - TSO

G = GO + RT lnQ

GO = -RT lnKeq

PV = nRT

C = k P

P = PO

T = K m

k = A e(-Ea/RT)

[A] = -k t + [A]O

ln[A] = -k t + ln[A]O

1/[A] = k t + 1/[A]O

x = -b ± √(b 2 -4ac) 2a

pH = -log[H3O+]

pH = pKa + log [A - ] .

[HA]

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