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B18PA1/B18AP1Chemical Thermodynamics

Lecture 15 & 16

Dr N Hendrik Nahler

3rd December 2012

Introduction

During this lecture the following examples will be discussed in reference to previouslectures. These are worked examples to support your understanding of Chemical Ther-modynamics. This is not a revision class and the examples are not purposely linked topast or future exam questions. Please, as always take notes during this lecture. Pro-blems for which you see the solution almost always appear logical and easy. You willonly understand the subject properly by attempting additional problems yourself e.g.from Elements of Physical Chemistry, Atkins & de Paula, without the answer at hand.

Lecture 1 & 21. 1.95mol of an ideal gas (CV,m = 3/2R) undergo the following changes starting

from T= 290K and p = 1.00 bar. Calculateq, w, U, H, and S.

a) The gas is heated to T= 520 K at constant pressure.

b) The gas is heated to T= 520 K at constant volume.

c) The gas undergoes a reversible isothermal expansion at T = 290 K until thepressure is half of its initial value.

2. Calculate rH and rU

at T = 298.15 K for the following reactions using thedata provided below and assuming ideal gas behaviour.

a) OH(g) H(g) + O(g)

b) H2O(g) 2 H(g) + O(g)

c) H2O H(g) + OH(g)

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fH/kJmol1

12

H2(g) + 12

O2(g) OH(g) 38.95H2g +

12

O2(g) H2O(g) 241.814H2(g) 2 H(g) 435.994

O2(g) 2 O(g) 498.34

3. Consider the photosynthetic process 6 CO2(g)+6H2O(l) C6H12O6(s)+6O2(g)with the data given at T= 298K.

fH/kJmol1 S/JK1mol1 Cp,m/JK

1mol1

CO2(g) 393.5 213.8 37.1H2O(l) 285.8 70.0 75.3C6H12O6(s) 1273.1 209.2 219.2O2(g) 0.0 205.2 29.4

Calculate the following, both atT1= 298K and T2 = 330 K

a) Hb) Ssys

c) Ssurr

d) Stot

Lecture 3 & 4

4. An ice cube with a mass of 17.5 g at a temperature ofTice = 273K is added to125 g of liquid water atTwater= 325 K at constant pressure.

a) Is the final state of the system ice or liquid water?

b) Calculate S for the process.

c) Is the process spontaneous?

5. Calculate Gfor the isothermal expansion in 1c.

Lecture 5 & 6

6. Carbon tetrachloride melts at Tfus= 250 K.

Phase Temperature / K Vapour pressure / Pascal

Solid 232 270Solid 250 1092Liquid 290 10 539Liquid 340 74 518

a) Calculate vapHm and subHm.

b) Calculate fusHm.

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c) Calculate the normal boiling point and vapSm at the boiling point.

d) Calculate the triple point temperature and pressure.

7. Use the following vapour pressures of 1-butene to calculate the enthalpy of vapo-

risation using a graphical method of best fit.Temperature / K Pressure / bar

273.15 1.252275.21 1.349277.60 1.471280.11 1.607283.15 1.787

8. Consider the phase transition between two forms of solid tin,Sn(s, grey) Sn(s,white). The two phases are in equilibrium at 1 bar and 18 C.The densities for grey and white tin are 5750 kgm3 and 7280kgm3 respec-

tively. The molar entropies for grey and white tin are 44.14 JK1

mol1

and51.18 JK1mol1, respectively. Calculate the temperature at which the twophases are in equilibrium at a pressure of 200 bar.

Lecture 7 & 8

9. Consider the equilibrium CO(g) + H2O(g) CO2(g) + H2(g) atT1= 1000 K.

fH/kJmol1 Mole fraction

CO2(g) 393.5 0.271H2(g) 0.271

CO(g) 110.5 0.229H2O(g) 241.8 0.229

a) CalculateKp and rG at 1000 K.

b) Use your answer from part (a) to calculate rG at 298 K.

10. Ca(HCO3)2(s) decomposes when heated following this equation:Ca(HCO3)2(s) CaCO3(s) + H2O(g) + CO2(g).

a) Pure Ca(HCO3)2(s) is put into a reaction vessel, the vessel is then evacua-ted and its contents heated, the total pressure is 0.235 bar. Calculate theequilibrium constantKp.

b) Pure Ca(HCO3)2(s) is put into a reaction vessel, the vessel is then evacuated.0.105 bar of water vapour is added before the vessel is heated to the sametemperature as in (a). What is the partial pressure of CO2(g) at equilibrium?

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Lecture 9 & 10

11. At high altitudes the absorption of oxygen into the bloodstream is reduced makingexercising a lot harder and may lead to altitude sickness. At a pressure of 1 .00bar

at sea level, blood is typically 95% saturated with O2 while at 0.50bar at 5400m,blood is typically 71% saturated with O2. Assuming that the Henrys law constant

for blood is the same as for water (kO2

H = 4.95 104 bar) calculate the amount of

O2 dissolved in 1.0 l of blood at 1.00bar and 0.50 bar. Air contains 20.99% O2 byvolume and the density of blood is 998 kgm3.

12. At 10 C the vapour pressure of ethyl bromide is 90.0torr and that of ethylchloride is 302.5 torr. A solution of the two solvents is prepared which is consideredideal and only a trace of liquid is present. The mole fraction of ethyl chloride inthe vapour is 0.82.

a) What is the total pressure and the mole fraction of ethyl chloride in the liquid?

b) If there are 6.50mol of liquid and 3.80 mol of vapour present at the samepressure as in part (a), what is the overall composition of the system?

13. At 39.9 C a solution of ethanol (xe = 0.9006, p

e = 130.4 torr) and iso-octane(pi = 43.9 torr) forms a vapour phase with ye = 06667 at a total pressure of185.9 torr.

a) Calculate the activity and activity coefficient of each component.

b) Calculate the total pressure that the solution would have if it were ideal.

Lecture 11 & 12 and Lecture 13 & 14

Examples have been given for lectures 11 & 12 and 13 & 14 at the time it was given. Theworked examples for lecture 13 & 14 are available on VISION due to the interruptionwe had caused by the fire alarm.

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