CHY2018 Physic+al Chemistry

TUTORIAL 1 (THERMOCHEMISTRY)1. A chemical reaction occurs in a cylinder container of cross-section 100 cm2 which is

sealed by a piston of negligible mass. When the reaction goes to completion the piston

is pushed out a distance of 10 cm.

a) Sketch the system before and after the reaction and calculate the work done in joules, assuming that the external pressure is 1 atmosphere.

b) How would the work done be different if the piston had moved inwards, a distance of 10cm as a result of the reaction?

2. A mole of calcium carbonate is heated to 700 °C in a vessel such as that described

above. It decomposes to produce CO2 and CaO (s).

a) Assuming the products are all at 700 °C immediately after the reaction and the

external pressure is 1 atmosphere, calculate the final volume of the system and the

work done.

b) Consider the case when the decomposition occurs in a vessel which is not sealed,

but open to the atmosphere. Is any work done? Explain your answer.

3. By first finding the volume of the product gas, calculate the work done when a mole

of lead azide solid, Pb(N3)2(s) , decomposes at room temperature to produce solid lead

and nitrogen gas. Assume that the external pressure is 1 atmosphere.

4. Show that the work done is given also by the expression - where -ngasRT is the

change in the number of moles of gas in the system, as a result of the reaction.

5. When a mole of methane, at room temperature is burnt and the products are collected

and brought to room temperature, the heat released is 890.4 kJ mol-1. Determine the

change in internal energy for this process, assuming pressure is constant throughout at

1atmosphere.

If it is true that energy is conserved in chemical reactions then the energy released

must have been present in the system before the reaction. Explain

6. When 0.394 g of benzene (C6H6) is burned in a bomb calorimeter, the surrounding

water bath rises from 25.2 oC to 40.3 oC. If the bath contains 250.0 g of water and the

calorimeter itself absorbs negligible amount of heat, calculate the combustion

enthalpies (including the sign!) for benzene in both kilojoules per gram (kJ/g) and

kilojoules per mole (kJ/mol).[NB: assume heat capacity of water to be 4.18 J/g. oC.

7. What is the maximum temperature that can be reached in a styrofoam cup (as a

calorimeter) containing 105 mL of water at 20.12 oC following the addition and

dissolving of some pellets of KOH (s) weighing 5.0 g? Assume the density of water

to be 1.00 g/mL.

KOH (s) KOH (aq) H = -57.6 kJ/mol

8. From the following enthalpies of reaction:

H2O

H2(g) + F2(g) 2 HF(g) H = -537 kJ

C(s) + 2 F2(g) CF4(g) H = -680 kJ

2 C(s) + 2 H2(g) C2H4(g) H = +52.3 kJ

Calculate H for the reaction of ethylene (C2H4) with F2:

C2H4(g) + 6 F2(g) 2 CF4(g) + 4 HF(g)

9. Use the bond dissociation enthalpies given below to make an estimate of the standard

enthalpy of formation of benzene vapour. The enthalpy of sublimation of graphite is

716.7 kJ mol-1 and the enthalpy of atomization of hydrogen is 436.0 kJ mol-1.

Compare your result with the experimentally determined value of +83 kJ mol-1 and

explain why they are very difficult.

C-C 348 kJ mol-1 C=C 612 kJ mol-1

C≡C 812 kJ mol-1 C-H 412 kJ mol-

10. A gas expands against a constant external pressure of 0.500 atm from an initial

volume of 1.500 L to a final volume of 6.25 L. The container in which this process is

taking place is well insulated, and you may assume no heat enters or leave the system.

(i) Determine the work w done by the system.

(ii) Calculate the energy change U for the system

11. The standard enthalpy of formation of ammonia gas, (Hf of ammonia,) is - 46.1 kJ

mol-1.

a) Calculate the Gibbs energy of the process, given the following standard molar

entropies;

S(NH3(g)) = 239.0 J K-1mol-1

S(H2(g)) = 130.7 J K-1mol-1

S(N2(g)) = 191.6 J K-1mol-1

and say whether or not the process is likely to be spontaneous at 25 C?

b) Sketch the variation of the Gibbs energy, G, of the system as it changes from

reactants only to product only, showing the likely position of equilibrium in this

case.

c) Calculate the equilibrium constant, K, for the process, and say whether or not the

value obtained is consistent with your finding on the spontaneity or non-

spontaneity of the process in part (a). Explain.

12. The reaction given below, in which nitrogen dioxide is formed from nitric oxide in an

oxygen atmosphere, effectively converts three gas molecules to two.

2NO(g) + O2(g) 2NO2(g)This represents a decrease in randomness, and therefore also a decrease in the entropy of the system.

a) On this basis and on the basis of the second law of thermodynamics, is the reaction spontaneous? Explain your answer. (10)

b) Given that H for the reaction is –114.1 kJ mol-1, and that S = -146.5 J K-1 at 25C, calculate the standard Gibbs energy for the reaction and say whether or not the reaction occurs spontaneously at 25C. (6)

(c) Determine a range of temperatures over which this reaction is likely to be

spontaneous. Say whether or not the reaction is likely to occur spontaneously in the cylinder of an internal combustion engine in which the temperature is 2500C. Point out any assumptions you make. (9)

13. Answer all parts.

(a) Explain what you understand by the internal energy, U, and enthalpy, H,

of substances. [4]

(b) Explain also what is means when it is said that these quantities are state

functions. [3]

(c) Say what you understand by the term “standard enthalpy of formation” of

substances. [4]

(d) Use the standard enthalpies given below to determine the standard enthalpy

of the reaction:

C6H6(g) + 3 H2(g) C6H12(g)

ΔfH of C6H6(g) = 82.9 kJ mol-1

ΔfH of C6H12(l) = -156.0 kJ mol-1

ΔvapH of C6H12(l) = 33.3 kJ mol-1 [6]

(e) Given that the constant pressure heat capacity of the gaseous cyclohexane

(C6H12(g)) is 156.5 J/ K mol and heat capacity of liquid cyclohexane (C6H12(l))

is 106 J/K mol, find the enthalpy of vaporization of (C6H12(l)) at its boiling

point, 80 oC. [8]

Selected questions from C10k Tutorial 1 (2001/2002) at University of the West Indies, Mona Campus and past examinations