Advanced Placement Chemistry

© Copyright 2004  Tom Stretton  (stretton@ripnet.com)
 

      Return

    Answers

     Course
     Outline

      Main
   Welcome
      Page
AP Chemistry -- Thermochemistry Unit Review
1.  What is meant by the term chemical energy?

2. What is meant by an energy source?

3. How many kilocalories are in 5225 J?

4. Which of these are exothermic reactions?

a) N2 + 3 H2 ----> 2 NH3 delta Horxn = -92.05 kJ

b) 2 HgO + 181.17 kJ ----> 2 Hg + O2

c) Fe2O3 + 3 CO ----> 2 Fe + 3 CO2 + 27.61 kJ

d) N2 + O2 ----> 2 NO - 180.75 kJ

5. How many kilojoules of heat energy are absorbed in raising the temperature of 10.0 grams of water from 5oC to 20oC?

6.  The specific heats of gold, silver and copper are 0.13, 0.24, and 0.39 J g-1 K-1, respectively. If one gram samples of each, at room temperature, are placed in the same beaker of hot water,

a) the temperature of all three samples will be different at the end.

b) the gold will gain the greatest quantity of heat.

c)   the copper will gain the greatest quantity of heat.

d) at the end the gold will be at the highest temperature.

e) initially, the temperature of the copper will rise the fastest.

7. Fat tissue is 85% fat and 15% water. The complete breakdown of the fat itself converts it to CO2 and H2O, and releases about 37.665 kJ/g (of fat in the fat tissue).

(a) How many kilojoules are released by a loss of 0.45 kg (1 lb.) of fat tissue in a weight-reduction program?

(b) A person running at 13 km/hr expends about 2.0 X 103 kJ/hr of extra energy. How far does a person have to run to "burn off" 0.45 kg of fat tissue by this means alone.

8. A well-nourished person adds about 0.23 kg of fat tissue for each 83.65 kJ of food energy taken in over and above that needed. Suppose that you decide to reduce your weight simply by omitting butter but keeping every other aspect of your diet and your activities the same. How many days would be needed to lose 4.3 kg of fat tissue by this strategy alone? The energy content of butter is 37.65 kJ/g. Suppose that you have been eating 1.1 g of butter a day.

9. What is the molar heat capacity of ethyl alcohol, C2H5OH, in units of J/moloC, if its specific heat is 0.586 cal/goC?

10.  The specific heat of helium is 5.188 J/goC and of nitrogen is 1.042 J/goC. How many joules can one mole of each gas absorb when its temperature increases 1.00oC?

11. We wish to determine how much heat paraffin gives off on burning. We use a candle flame to heat some water in a calorimeter. These data were obtained:
Mass of water in calorimeter 350 g
Initial mass of candle 150 g
Final mass of candle 112 g
Initial temperature of water 15oC
Final temperature of water 23oC

Calculate:
a) the temperature rise,
b) the joules absorbed by the water in the calorimeter,
c) the grams of paraffin burned,
d) the approximate value of heat of combustion of paraffin in J/g.
Neglect the energy absorbed by the calorimeter.

12. During the combustion of wax, 2.00 grams of wax were consumed. The heat was collected in 100.0 grams of a liquid (not water) and the temperature of the liquid increased by 20.0oC. If the heat of combustion of the wax is 450 kJ/kg, calculate the specific heat capacity of the unknown liquid.

13. Trimgristin, extracted from nutmegs, has a melting point of 56oC and a boiling point of 311OC. Sketch the heating curve of this substance from 50oC to 350oC. Identify the processes that occur at each portion of the curve.

14. 14. Aluminum and iron(III) oxide, Fe2O3, react and form aluminum oxide, Al2O3, and iron. For each mole of aluminum used, 426.9 kJ of energy is released under standard conditions. Write the thermochemical equation that shows the consumption of 4 mol of Al. (All substances are solids)

15. The following equation represents the dehydration of calcium hydroxide to make quicklime, CaO, a substance present in cement.
Ca(OH)2(s) -----> CaO(s) + H2O(l) delta Horxn = +65.3 kJ

One of the reactions when cement is mixed with water is the reverse of this reaction. Write the thermochemical equation for the reaction of 10 mol of quicklime with water under standard conditions.

16. A sample of 10.1 g of ammonium nitrate, NH4NO3, was dissolved in 125 g of water in a coffee cup calorimeter. The temperature changed from 24.5oC to 18.8oC Calculate the heat of solution of ammonium nitrate in kJ/mol. Assume that the energy exchange involves only the solution and that the specific heat of the solution is 4.18 J/goC.

17. Barium oxide, BaO, can be used to neutralize pure sulphuric acid, H2SO4. The equation is: 
                      BaO(s) + H2SO4(l) ----> BaSO4(s) + H2O(l)

What is the standard enthalpy change of this reaction? The following thermochemical equations can be used.
SO3(g) + H2O(l) > H2SO4(l)  delta Horxn= -78.2 kJ
BaO(s) + SO3(g) > BaSO4(s) delta Horxn = -213 kJ

18. What is the value for delta Horxn of neutralization of lithium hydroxide, LiOH(aq), by hydrochloric acid, HCl(aq), by the following equation?:    LiOH(aq) + HCl(aq) -------> LiCl(aq) + H2O(l)

The following thermochemical equations can be used in addition to any prepared equations, as needed from a suitable table.

Li(s) + ½ O2(g) + ½ H2(g) ----> LiOH(s) delta Horxn= -487.0 kJ
2 Li(s) + Cl2(g) -----> 2 LiCl(s) delta Horxn= -815.0 kJ
LiOH(s) ----> LiOH(aq) delta Horxn= -19.2 kJ
HCl(g) ----> HCl(aq) delta Horxn= -77.0 kJ
LiCl(s) ----> LiCl(aq) delta Horxn= -36.0 kJ

 19. Given these two reaction heat equations
S(s) + O2(g) ----> SO2(g) delta Horxn=-295 kJ/mol and
S(s) +  3/2 O2(g) -----> SO3(g) delta Horxn=-395 kJ/mol

what is the heat of reaction for

2 SO2(g) + O2(g) ----> 2 SO3(g)    under the same conditions?

20. Which one of the following processes is expected to involve the smallest energy change?
a. Condensing, at 25oC, one mole of water vapour
b. Nuclear fission of one mole of 235U
c. Reacting two moles of hydrogen atoms to make one mole of H2(g).
d. Fusion of one mole of hydrogen nuclei with one mole of helium nuclei to make lithium nuclei.
e. Mixing one mole of each of two reactants which liberate 200 kJ.

21. For the reaction, H2(g) + I2(g)  ------->  2HI(g), Ho=+52.0 kJ.  
Which statement is INCORRECT?
a. The heat of formation of one mole of HI(g) is +26.0 kJ.
b. H2 and I2 are energy poor compared to HI.
c. The temperature will decrease as the reaction proceeds.
d. Heating will cause the reaction to proceed to the left.

22. Arrange the steps below in the proper order to determine the molar heat of solution of a salt.
1. Add salt to water and stir well to dissolve the salt.
2. Calculate the heat of solution/mole of salt.
3. Determine the change in temperature.
4. Measure the temperature of water.
5. Record the temperature of the solution.
6. Weigh the salt to be dissolved and calculate number of moles of salt.

The correct order is 
a. 6, 4, 5, 3, 1, 2
b. 6, 4, 5, 1, 3, 2
c. 6, 4, 1, 5, 3, 2
d. 6, 1, 4, 5, 3, 2

23. Which of the following are endothermic?

(1) C3H8(g)  -------->  3C(s) + 4 H2(g) H = +84 kJ

(2) ½ N2(g) + 1½H2(g)  -------->  NH3(g) + 46.1 kJ

(3) H2O(l)  -------->  H2O(g)     H = ?

(4) N2O(g) + heat N2(g) + ½ O2(g)

a. 1 and 2
b. 1 and 3
c. 2 and 4
d. 2
e. 1, 3, 4

24. When 4.32 grams of HgO(s) forms from its elements how much heat is involved in kJ?
a. 1.81
b. 91.9
c. 397
d. 21.3
e. none of these

© Copyright 2004  Tom Stretton  (stretton@ripnet.com)