1

CORK INSTITUTE OF TECHNOLOGY INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ

Semester 2 Examinations 2008/09

Module Title: Physical Chemistry 1

Module Code: CHEM 6004 School: Science Programme Title: Bachelor of Science in Analytical and Pharmaceutical Chemistry - Stage 1 Programme Code: SCHEM_7_Y1 External Examiner(s): Dr G Keaveney Internal Examiner(s): Dr M Sheahan Instructions: Answer FOUR questions. Duration: 2 Hours Sitting: Summer 2009 Requirements for this examination: Periodic Table Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination paper. If in doubt please contact an Invigilator.

2

Q1. (a) In the case of the following processes, is any work (w) done when the reaction is carried

out at constant pressure? Explain your answer. Where work is done, indicate if it is

done by the reacting system or on it? Hence or otherwise give the sign convention for

work (is it positive or negative)?

(i) Ba(OH)2 (aq) + 2HCl(aq) BaCl2 (aq) + 2H2O (aq)

2NO2(g) N2O4(g)(ii)

CaCO3(s) CaO(s) + CO2(g)(iii) (9 Marks)

(b) The standard enthalpy change (∆H°) for the formation of calcium carbide (CaC2) from

carbon and calcium oxide is 464.8kJ/mole

CaO(s) + 3C(s) CaC2(s) + CO(g)

How much heat is transferred when 233.0g of calcium oxide are used in the reaction?

(3 Marks)

(c) Butadiene (C4H6) reacts with itself to form a dimer with the formula C8H12. The

reaction is second order in butadiene.

(i) In the context of chemical reactions in general, explain the meaning of the term

reaction order. How might the order of a reaction be determined experimentally?

(4 Marks)

(ii) Write a rate law for the butadiene dimerization reaction. Hence state the effect on

the reaction rate if the concentration of C4H6 is doubled

(4 Marks)

(iii) If the rate constant at a particular temperature is 4.0 x 10-2 M-1 s-1 and the initial

concentration of C4H6 is 0.02M, how long (in hours) will it take the concentration

of C4H6 to reach a value of 0.002M?

(5 Marks)

3

Q2. (a) Distinguish clearly between the terms specific heat capacity and molar heat capacity.

Give units to each and show how one may be calculated from the other

(6 Marks)

(b) A 465g chunk of iron is removed from an oven and plunged into 375g water in an

insulated container. The temperature of the water increases from 26 to 87°C. If the

molar heat capacity of iron has a value of 25.1 (input units), calculate the amount of heat

(in kJ) released by the iron. Hence determine the change in temperature of the chunk of

iron? What is the significance of the insulated container?

(9 Marks)

(c) State Hess’s Law. Use the law and the information provided to determine the standard

enthalpy change (∆H°) for the hydrogenation of propadiene (C3H4)

C3H4(g) + 2H2(g) C3H8(g)

2H2(g) + O2(g) 2H2O(g) ∆HΟ = − 571 kJ

C3H4(g) + 4O2(g) 3CO2(g) + 2H2O(g) ∆HΟ = − 1937 kJ

C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(l) ∆HΟ = − 2219 kJ (8 Marks)

(d) Apart from calorimetry and the equations method, name two other methods that can be

used to get an estimate of the enthalpy change in a reaction.

(2 Marks)

4

Q3. Hydrogen peroxide oxidizes iodide ion in acidic solution to produce iodine (I2). The

reaction proceeds according to the equation.

H2O2 (aq) + 2I− (aq) + 2H+(aq) 2H2O(l) + I2(aq)

The data given in the table shows the variation in peroxide concentration with time.

Time (s) moles H2O2 remaining

28 4.375 x 10-4

62 3.750 x 10-4

104 3.135 x 10-4

148 2.500 x 10-4

206 1.875 x 10-4

293 1.250 x 10-4

461 0.625 x 10-4

(a) When the reaction was carried out in the laboratory, a large excess of both iodide (I-)

and acid were used. Why? (3 Marks)

(b) Briefly explain how the progress of reaction may be monitored.

(4 Marks)

(c) Establish if the reaction is first or second order by constructing the appropriate plots.

(8 Marks)

(d) Calculate a value for the rate constant k (assign units to k). What would be the effect on

k if the experiment was repeated at a different temperature?

(6 Marks)

(e) Will there be any of the peroxide remaining after one hour? Show all calculations.

(4 Marks)

5

Q4. One of the key reactions in the gasification of coal is the methanation reaction, in which

methane is produced from a mixture of carbon monoxide and hydrogen. The enthalpy of

formation of methane and the equilibrium constant (at 1000K) are given.

CO(g) + 3H2(g) CH4(g) + H2O(g) ∆Η = − 230kJ, Kc = 190 at 1000K

(a) Based on the information provided for this reaction, comment briefly on the

composition of the equilibrium mixture at 1000K. (2 Marks)

(b) List three strategies which may be used to increase the yield of this reaction. Discuss

each strategy by explaining why it should work. (9 Marks)

(c) An equilibrium mixture at 1000K was analyzed and found to contain 0.4g/L of H2 and

1.876g/L of CO. Calculate the equilibrium concentration of CH4 at this temperature.

Express your answer in mol/L. Assume equimolar amounts of water vapour and

methane are present at equilibrium.

(6 Marks)

(d) Derive an equation which shows the relationship between Kc and Kp. Hence or

otherwise calculate Kp for the methanation reaction at 1000K

(R = 0.08206 L atm mol-1 K-1). (8 Marks)

6

Q5. (a) What is the effect on the pH of a solution of the weak acid, nitrous acid (HNO2), when

the following substances are added to it? (i) NaNO2, (ii) NaCl, (iii) HCl and (iv)

Ba(OH)2. Explain your answer in each case. Which might have the greater effect and

why? (5 Marks)

(b) Answer the following questions pertaining to buffers:

(i) What is a buffer solution? (2 Marks)

(ii) What are the main components in a buffer solution? (2 Marks)

(iii) Explain how buffers work (2 Marks)

(iv) What do you understand by buffer capacity? (2 Marks)

(v) Derive the Henderson – Hasselbalch buffer equation (5 Marks)

(c) Calculate the mass of benzoic acid (C6H5COOH) that must be added to 1L of a 0.01M

aqueous solution of sodium benzoate (C6H5COONa) to give a buffer solution of pH 4.50

at 25°C (Ka for benzoic acid 6.5 x 10-5 at 25°C).

(7 Marks)

Q6. (a) What is the main difference between galvanic and electrolytic cells?

(4 Marks)

(b) Sketch the experimental set up for the cell that uses the spontaneous reaction between

Ag+ ions and nickel metal. Label the anode and the cathode, identify the sign of each

electrode, and indicate the direction of electron flow. (10 Marks)

2Ag+ (aq) + Ni (s) 2Ag(s) + Ni2+ (aq)

Write an equation for both half-cell reactions and clearly identify the oxidizing agent

and reducing agent. (6 Marks)

(c) Comment on the function of a salt bridge in galvanic cells. Give examples of two types

of salt bridge. (5 Marks)

7