Up: Back to the old Chemistry 2720 index

Chemistry 2720 Spring 1998 Test 1

Aids allowed: calculator. Write your answers only in the booklets provided.

Answer all questions. The questions in this test have an aggregate value of 50 marks. Depending on your preferred style of questions, you may find it worthwhile to solve the problems out of order.

Following in Albert Einstein's footsteps, you are working as a clerk at the patent office. An inventor claims to have created a heat engine which produces power at a rate of 5kW at maximum thermodynamic efficiency, i.e. with an efficiency given exactly by the Carnot formula. Because the patent office will not award patents to machines that cannot possibly function as claimed, you are asked to decide whether the inventor's claim lies within the realm of the possible. What is your expert opinion? Support your decision using your knowledge of thermodynamics. [5 marks]
After quitting the patent office (the pay is lousy), you become a chemistry teacher (the pay is still lousy, but at least you feel that you're playing a socially useful role). You want your students to measure a heat capacity to help them develop some intuition about this quantity and its relationship to heat and temperature. You decide to have them measure the heat capacity of brass, a conveniently unreactive solid, and one whose heat capacity varies somewhat from one sample to another so that they can't just look up the value in a table. Give a brief procedure which your students should follow. Clearly indicate what they should measure and describe the data handling which will lead them from the raw data to a specific heat capacity. [8 marks]
Note and hint: Brass is an alloy (a mixture of metals). Because its composition varies somewhat from one sample to another, it doesn't make sense to talk about moles of brass. The heat capacity should therefore be expressed in or in some other similar units.
A pint of beer delivers approximately 196kcal of metabolizable energy. This figure assumes that the beer is served warm. Cold beer must also be warmed to body temperature ( ) and this requires an expenditure of metabolic energy. Calculate the metabolizable energy (in kcal) available from a pint of beer served at . Treat the beer as if its thermal properties were identical to those of water. The density of beer is typically just slightly higher than that of water (about 1g/mL) and a pint is 568mL. [6 marks]
All parts of this question refer to the reaction

at the standard temperature (298K).
1. Calculate the heat produced or absorbed per mole of HgS consumed if the reaction occurs at constant pressure. Indicate clearly whether you think the reaction absorbs or produces heat. [3 marks]
2. Calculate the heat produced or absorbed per mole of HgS consumed if the reaction occurs at constant volume. [5 marks]
  Hint: Assume that any gases involved in the reaction behave ideally.
3. Is the reaction spontaneous if the pressure of chlorine gas is a constant 2atm? [5 marks]
1. The enthalpy of combustion of liquid methanol (producing liquid water) at is . Calculate the enthalpy of combustion at in kJ/L. The molar mass of methanol is 32.0g/mol and the density is 791g/L. [8 marks]
2. An internal combustion engine burns methanol at and produces exhaust gases with a temperature of . If it consumes methanol at a rate of 2L/h, how much power can it produce? Assume that this engine is as efficient as permitted by the laws of thermodynamics. [5 marks]
3. If a device oxidizes methanol isothermally at 298K under atmospheric conditions ( pressure of 0.2atm and pressure of ) at the rate of 2L/h, how much power would it produce? Again, assume that the device operates without loss and at maximum thermodynamic efficiency. Assume also that power is produced in a form other than PV work. [5 marks]
  Hint: Oxidizing methanol would have the same overall reaction as combustion.