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AP Chem Problem Set # 4

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The First Law of Thermodynamics

1. (a) State the first law of thermodynamics. (b) What is meant by the internal energy of a system? (c) By what means can the internal energy of a system increase?

2. Calculate E, and determine whether the process is endothermic or exothermic for the following cases: (a) A system releases 113 kJ of heat to the surroundings and does 39 kJ of work on the surroundings; (b) q = 1.62 kJ and w = -874 kJ; (c) the system absorbs 77.5 kJ of heat while doing 63.5 kJ of work on the surroundings.

3. (a) What is meant by the term state function? (b) Give an example of a quantity that is a state function and one that is not. (c) Is temperature a state function? Why or why not?


4. (a) Why is the change in enthalpy a meaningful quantity for many chemical processes? (b) H is a state function, but q is not a state function. Explain. (c) For a given process at constant pressure, H is negative. Is the process endothermic or exothermic?

5. The complete combustion of acetic acid, HC2H3O2(l), to form H2O(l) and CO2(g) at constant pressure releases 871.7 kJ of heat per mole of HC2H3O2. (a) Write a balanced thermochemical equation for this reaction. (b) Draw an enthalpy diagram for the reaction.

6. Consider the following reaction, which occurs at room temperature and pressure: 2Cl(g) Cl2(g) H = -243.4 kJ

Which has the higher enthalpy under these conditions, 2Cl(g) or Cl2(g)?

7. Consider the following reaction:

    2Mg(s) + O2(g) 2MgO(s) H = -1204 kJ

(a) Is this reaction exothermic or endothermic? (b) Calculate the amount of heat transferred when 2.4 g of Mg(s) reacts at constant pressure. (c) How many grams of MgO are produced during an enthalpy change of 96.0 kJ? (d) How many kilojoules of heat are decomposed into Mg(s) and O2(g) at constant pressure?

8. When solutions containing silver ions and chloride ions are mixed, silver chloride precipitates:

    Ag+(aq) + Cl-(aq) AgCl(s) H = -65.5 kJ

(a) Calculate H for formation of 0.200 mol of AgCl by this reaction. (b) Calculate H for the formation of 2.50 g of AgCl. (c) Calculate H when 0.350 mmol of AgCl dissolves in water.

9. Consider the combustion of liquid methanol, CH3OH(l):

CH3OH(l) + 1.5O2(g) CO2 + 2H2O(l) H = -726.5 kJ

(a) What is the enthalpy change for the reverse reaction? (b) Balance the forward reaction with whole number coefficients. What is H for the reaction represented by this equation? (c) Which is more likely to be thermodynamically favored? (d) If the reaction were written to produce H2O(g) instead of H2O(l), would you expect the magnitude of H to increase, decrease, or stay the same? Explain.


10. (a) What is the specific heat of liquid water? (b) What is the heat capacity of 185 g of liquid water? (c) How many kilojoules of heat are needed to raise the temperature of 10.00 kg of liquid water from 24.6 C to 46.2 C?

11. The specific heat of copper metal is 0.385 J/g-K. How many joules of heat are needed to raise the temperature of 40.0 g of toluene from 10.4 C to 28.0 C?

12. A 2.200-g sample of quinone, C6H4O2, is burned in a bomb calorimeter whose total heat capacity is 7.854 kJ/ C. The temperature of the calorimeter increases from 23.44 C to 30.57 C. What is the heat of combustion per gram of quinone? Per mole of quinone?

13. Under constant-volume conditions the heat of combustion of glucose, C6H12O6, is 15.57 kJ/g. A 2.500-g sample of glucose is burned in a bomb calorimeter. The temperature of the calorimeter increased from 20.55 C to 23.25 C. (a) What is the total heat capacity of the calorimeter? (b) If the calorimeter contained 2.700 kg of water, what is the heat capacity of the dry calorimeter? (c) What temperature increase would be expected in this calorimeter if the glucose sample had been combusted when the calorimeter contained 2.000 kg of water?

Hesss Law

14. Consider the following hypothetical reactions:

                A B H = +30 kJ

                B C H = +60 kJ

(a) Use Hesss law to calculate the enthalpy change for the reaction A C. (b) Construct an enthalpy diagram for substances A, B, and C, and show how Hesss law applies.

15. Given the following enthalpies of reaction:

       P4(s) + 3O2(g) P4O6(s) H = -1640.1 kJ

      P4(s) + 5O2(g) P4O10(s) H = -2940.1 kJ

Calculate the enthalpy change for the reaction

            P4O6(s) + 2O2(g) P4O10(s)

16. From the following enthalpies of reaction:

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

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

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

calculate H for the reaction of ethylene with F2:

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

Enthalpies of Formation

17. (a) What is meant by the term standard conditions, with reference to enthalpy changes? (b) What is meant by the term enthalpy of formation? (c) What is meant by the term standard enthalpy of formation?

18. For each of the following compounds write a balanced thermochemical equation depicting the formation of 1 mol of the compound from its elements in their standard states and use Appendix C to obtain the value of Hfo: (a) NH3(g); (b) SO2(g); (c) RbClO3(s); (d) NH4NO3(s).

19. The following is known as the thermite reaction (Figure 5.7):

      2Al(s) + Fe2O3(s) Al2O3(s) + 2Fe(s)

This highly exothermic reaction is used for welding massive units, such as propellers for large ships. Using enthalpies of formation in Appendix C, calculate Ho for this reaction.

20. Using values from Appendix C, calculate the standard enthalpy change for each of the following reactions:

  1. 2SO2(g) + O2(g) 2SO3(g)

  2. Mg(OH)2(s) MgO(s) + H2O(l)

  3. 4FeO(s) + O2(g) 2Fe2O3(s)

  4. SiCl4(l) + 2H2O(l) SiO2(s) + 4HCl(g)

21. Complete combustion of 1 mol of acetone, C3H6O, results in the liberation of 1790 kJ:

C3H6O(l) + 4O2(g) 3CO2(g) + 3H2O(l) Ho = -1790 kJ

Using this information together with data from Appendix C, calculate the enthalpy of formation of acetone.

22. Calculate the standard enthalpy of formation of solid Mg(OH)2, given the following data:

    2Mg(s) + O2(g) 2MgO(s) Ho = -1203.6 kJ

Mg(OH)2(s) MgO(s) + H2O(l) Ho = +37.1 kJ

    2H2(g) + O2(g) 2H2O(l) Ho = -571.7 kJ