vanier college faculty of science and general...

GENERAL CHEMISTRY II CHEM206 W2004 /4 02

SUPPLEMENTAL

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 16 pages including this cover page. Write your student ID number on all pages. Write all answers in the space provided (use examination booklets for rough work only). The final page contains optional bonus questions (MAXIMUM GRADE = 107/100). Read ALL questions carefully BEFORE starting the exam. Non-programmable calculators are allowed. You may detach the periodic table and “potentially useful information” pages.

Mark breakdown:

Page 2. / 15 Page 9. / 8 Page 3. / 10 0. / 3Page 4. / 8 Page 11. / 8Page 5. / 10 Page 12. / 9Page 6. / 5 Page 13. / 8Page 7. / 8 Page 14. / 6 BONUS Page 8. / 9

TOTAL: / 100 (MAXIMUM MARK = 107)

PAGECHEM 206 W2004 Section02 SUPPLEMENTAL EXAM ID #:__________________________

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# 1. (__/ 15 marks) Circle the word(s) that correctly completes each of the following statements.

a) When a hot rock is placed into a bucket of cold water, the sign of the heat flow according to the water is ( POSITIVE / NEGATIVE ).

b) The second law of thermodynamics states that the ( ENTHALPY / ENTROPY) of the universe increases when a spontaneous process occurs.

c) Osmotic pressure is exerted by a solution in contact with a ( PERMEABLE / SEMIPERMEABLE ) membrane because only the ( SOLUTE / SOLVENT ) molecules can pass through the membrane.

d) A reaction that displays overall ( FIRST / SECOND ) order kinetics cannot be conveniently described using the term “half life” because the ( RATE / HALF-LIFE ) is dependent on the initial concentration.

e) A catalyst increases the ( RATE / EQUILIBRIUM ) constant of a chemical reaction because it decreases the reaction’s ( GIBBS FREE ENERGY / ACTIVATION ENERGY ).

f) When a system is not at equilibrium, the reaction’s Gibbs free energy is ( ZERO / NON-ZERO ) and the reaction is ( NON-SPONTANEOUS / SPONTANEOUS ) in one direction.

g) Le Châtelier’s principle is a short-cut based on understanding the ( KINETICS / THERMODYNAMICS ) involved in chemical equilibria.

h) The base dissociation constant or Kb for a base is the ( EQUILIBRIUM / RATE ) constant for the reaction of the base with ( ACID / WATER ).

i) The percent dissociation of an acid is ( LARGER / SMALLER ) in dilute solutions than it is in concentrated solutions.

j) The conjugate acid of water is called ( HYDRONIUM ION / HYDROXIDE ION ).

k) The equilibrium constant for the reaction of OH- with H3O+ is ( 10-14 / 1014 ).

l) A weak acid whose conjugate base is a different colour is useful as a(n) ( BUFFER / INDICATOR ).

m) A buffer’s buffering capacity is higher when the members of the conjugate weak acid - base pair are present in ( LOW / HIGH ) concentrations.

n) The ( SOLUBILITY / SOLUBILITY PRODUCT ) of a substance is the concentration of that substance in a solution that is in equilibrium with undissolved solute.

o) Adding a ( BASIC / COMMON ) ion to a salt solution ( DECREASES / INCREASES ) the solubility of the salt.

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# 2. (__/ 10 marks) Ammonium nitrate is dangerous because it decomposes, sometimes explosively, when heated:

NH4NO3(s) N2O(g) + 2H2O(g)

a) Using logic only, explain whether the entropy of the system increases or decreases during this reaction.

b) Using the thermodynamic data provided, show that this reaction is spontaneous at 25°C.

c) Does the driving force for the reaction increase or decrease when the temperature is raised?

d) Calculate the equilibrium constant at 25°C. [NOTE: Since both products are gases, you will find Kp directly, and won’t have to convert between Kc and Kp.]

SUBSTANCE Hof

(kJ/mol)So

f (J/molK)

NH4NO3 (s) -365.6 151.1

N2O(g) 82.0 219.7

H2O(g) -241.8 188.7

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# 3. (__/ 8 marks) To make homemade ice cream, you cool milk and cream by immersing the container in a mixture of ice and a concentrated solution of rock salt (NaCl).

a) If you want the water-salt solution to freeze at -12°C, what mass of NaCl must you add to 3.0 kg of ice-water? [Use van’t Hoff factor i for NaCl = 1.85. For H2O: freezing point = 0.00°C; Kf = 1.86°Ckgmol-1].

b) Briefly explain why the van’t Hoff factor for sodium chloride is greater than one.

c) Provide a reason why the real van’t Hoff factor for NaCl is less than the ideal value.

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# 4. (__/ 10 marks) The following mechanism has been proposed for the reaction of nitric oxide with hydrogen to form nitrous oxide and water:

STEP 1: NO(g) + NO (g) N2O2(g)STEP 2: N2O2(g) + H2(g) N2O(g) + H2O(g)

a) Show that the elementary steps of the proposed mechanism add to provide a balanced chemical equation for the reaction.

b) Write the rate law for each elementary step. STEP 1: RATE =STEP 2: RATE =

c) Identify any intermediates in the mechanism:

d) By experiment, the reaction was found to be second order with respect to NO and first order with respect to H2. Assuming the proposed mechanism is correct, which step is rate limiting?

e) Draw an energy diagram illustrating this mechanism. Assume that the reaction is exothermic overall, and label the reactants and products, activation energy(ies), transition state(s), intermediate(s).

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# 5. (__/ 5 marks) We learned about chemical equilibria AFTER studying kinetics for good reasons.

a) Use KINETICS arguments (i.e., a discussion of reaction rates) to explain the dynamic nature of equilibrium.

b) Next, using KINETICS ARGUMENTS, explain how a reaction mixture that is NOT initially at equilibrium

will eventually reach a state of equilibrium. [Hint: reaction rates depend on reactant concentrations….]

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# 6. (__/ 8 marks) During volcanic eruptions, gases rich in sulfur dioxide and as hot as 700°C are released into the atmosphere. As air mixes with these hot gases, the following reaction occurs:

2SO2(g) + O2(g) 2SO3(g) Kc = 359 at 700°C

a) Determine whether or not the system is at equilibrium when the gas concentrations are: [SO2] = 8.110-3 M, [O2] = 8.410-3 M, [SO3] = 2.410-4 M. If it is not at equilibrium, in which direction will the reaction proceed until it does reach equilibrium?

b) Assume that the gases from part a) reach equilibrium at 700°C but then begin to cool down. Will the concentration of sulfur trioxide be higher or lower when the system has cooled down? [Assume that nothing is added or removed, & the only chemistry we have to think about is the reaction shown above.]

You must justify your answer using calculations involving the thermodynamic data at the right, and explain your conclusion using Le Châtelier’s principle.

SUBSTANCE Hof

(kJ/mol)

SO2(g) -296.8

SO3(g) -395.7

O2(g) 0

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# 7. (__/ 9 marks) You are asked to prepare a 250.0 mL sample of a solution with a pH of 9.50 by dissolving the appropriate amount of a solute in pure water (pH 7.00). Which ONE of the following solutes would you use, and in what quantity? Explain your choice, and show all relevant calculations.

CIRCLE YOUR CHOICE:

a) 10 M NaOH(aq) b) 12 M HNO3(aq)c) NH4Cl(s) d) Pure pyridine, C5H5N

SUBSTANCE Ka

HNO3 very large

C5H5NH+ 6.710-6

NH4+ 5.610-10

H2O Kw = 110-14

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# 8. (__/ 8 Marks) You are doing a titration to quantify the amount of tartaric acid (HC4H5O6, Ka = 1.010-3) in a 25.00 mL sample of a very old wine. The titrant solution you are using is 8.0010-2 M NaOH.

a) If you require 22.65 mL of NaOH titrant to reach the equivalence point, what was the original concentration of lactic acid in your sample?

b) At the equivalence point, the sample will essentially be a solution of sodium tartrate. What is the pH?

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# 8. (__/ 3 Marks) CONTINUED FROM PREVIOUS PAGE…

c) Which of the indicators in the table would minimize indicator error for titrations of other samples like the one from parts a & b? Briefly explain what indicator error is and the reason for your choice of indicator.

INDICATOR Colour of conjugate acid

pH range of colour change

Colour of conjugate base

Methyl orange Orange 3.3 – 4.5 Yellow

Bromthymol blue Green 6.0 - 8.0 Blue

Phenolphthalein Colourless 8.0 - 9.0 Pink

Alizarin yellow Yellow 10.0 - 11.8 Red

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# 9. (__/ 8 marks) A buffer consisting of H2PO4- and HPO4

2- helps control the pH of physiological fluids (e.g., inside cells). Many carbonated soft drinks also use this buffer system. The Ka of H2PO4

- is 6.210-8.

a) What is the pH of a soft drink in which the major buffer ingredients are 6.5 g of NaH 2PO4 and 8.0 g of Na2HPO4 per 355 mL of solution? [Ignore the dissolved CO2 in the drink.]

b) What would the pH of this soft drink be if you dissolved a pellet of solid NaOH (mass of 0.050 g) in it?

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# 10. (__/ 9 Marks) The ancient Romans added calcium sulfate to wine to clarify it ( i.e., to remove cloudiness). They didn’t know it at the time, but this treatment also removed any dissolved lead in the wine. [Note: Roman water pipes were made of lead, and wealthy people drank from lead cups….]

a) What is the maximum concentration of dissolved lead (II) ions, in molarity, that might be present in wine to which excess calcium sulfate has been added? [Ksp PbSO4 = 1.6x10-8; Ksp CaSO4 = 6.1x10-5 ]

b) Chronic exposure to lead is dangerous, particularly to children, because lead is a poison that builds up in the bloodstream. Even very low concentrations of lead in the blood (50 parts per billion, = 0.050 ppm) causes increased blood pressure; above 100 ppb, intelligence is affected, and coma or death can result above 800 ppb. Convert the concentration of Pb2+ ions in the wine from part (a) to parts per billion, ppb, and comment on whether or not you think the fall of the Roman Empire might have been related to lead poisoning.

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# 11. (__/ 8 Marks) The main compound in marble, which has been widely used for statues and ornamental work on buildings, is CaCO3 (Ksp = 2.810-9). Unfortunately, marble is readily attacked by acids because of the basicity of the carbonate ion.

Determine the solubility of marble:

a) In normal rainwater, which has pH = 5.60

b) In acid rainwater, which has pH = 4.20

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BONUS QUESTIONS: (6 marks)

B1. (___/ 1 mark) The reaction of nitrogen and oxygen to form nitric oxide plays an important role in the planet’s nitrogen cycle, by converting unreactive N2 molecules from the air into highly reactive NO molecules:

N2(g) + O2(g) 2NO(g)

The reaction does not occur to a measurable extent at 298 K (Kp = 4.710-31), but does proceed at elevated temperatures (Kp = 1.310-4 at 1800 K). Ttemperatures as high as 1800 K are not exactly common on earth. Can you think of a naturally occurring phenomenon that would expose air to such a high temperature?

B2. (___/ 5 marks) Cyanic acid, HOCN, and hydrocyanic acid, HCN, are both weak monoprotic acids in aqueous solution. [HOCN: Ka = 3.510-4; HCN Ka = 6.210-10.]

a) Draw Lewis structures for both HOCN and HCN.

b) Cyanic acid is a much stronger acid than HCN. Based on their structures, can you explain why?

c) Hydrogen cyanide, HCN(g), is a lethally poisonous gas. If you need to work with cyanide salts (e.g., NaCN) in the lab, what conditions must you carefully control so you don’t die? Please be specific.

---------------------------------------------------- HAVE A GOOD SUMMER ! ------------------------------------------------------

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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

G0 = H0 - TS0

G = G0 + RT lnQ

G0 = -RT lnKeq

PV = nRT

C = k P

P = P0

T = K m

k = A e(-Ea/RT)

[A] = -k t + [A]0

ln[A] = -k t + ln[A]0

(1/[A]) = k t + (1/[A]0)

x = -b ± √(b 2 -4ac) 2a

pH = -log[H3O+]

pH = pKa + log [A - ] .

[HA]

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