AP Chemistry Unit 4 Test Review by Il78OSS


									                                   AP Chemistry Unit 4 Test Review
Topics Covered:
Spontaneity, Entropy, & Free Energy
Kinetics (Extra Credit)

Section I: Multiple Choice (~50%)

  1. A cube of ice is added to some hot water in a rigid,         Energy                  Entrop.
  insulated container, which is then sealed. There is no    (A)   Remains constant        Remains constant
  heat exchange with the surroundings. What has
  happened to the total energy and the total Entrop.        (B)   Remains constant        Decreases
  when the system reaches equilibrium?                      (C)   Remains constant        Increases
                                                            (D)   Decreases               Increases
                                                            (E)   Increases               Decreases
  2. When solid ammonium chloride, NH4Cl(s) is added           ΔH         ΔS
  to water at 25 °C, it dissolves and the temperature of    A Postive     Positive
  the solution decreases. Which of the following is true
  for the values of ΔH and ΔS for the dissolving            B) Positive   Negative
  process?                                                  C) Positive   Equal to zero
                                                            D Negative    Positive
                                                            E) Negative   Negative

Questions 3-7: The set of lettered choices is a list of symbols used in thermodynamics and thermochemistry.
Select the one lettered choice that best fits each statement. A choice maybe used more than once or not at all.

       a. Ea
       b. ΔS
       c. Kc
       d. ΔG
       e. ΔH

3. Its value is negative for any exothermic reaction.
4. Its value is negative for any spontaneous reaction.
5. Its value changes when a catalyst is added to the system
6. Its value represents the change in randomness as the reaction proceeds.
7. Its value is determined by using concentrations at equilibrium in the mass action expression.

8. For which of the following is ΔH most nearly equal to ΔE?
        a. C2H5OH (l)  C2H5OH (g)
        b. N2O4 (g)  2NO2 (g)
        c. H2 (g) + ½ O2 (g)  H2O (g)
        d. H2 (g) + Cl2 (g)  2HCl (g)
        e. BaO2 (s)  2BaO2 (s) + ½ O2 (g)
9. Which applies to any reaction that proceeds spontaneously to form products from initial standard conditions?
                              I. Keq > 1 (the equilbrium constant here represents that products are favored)
                              II. ΔH < 0
                              III. ΔG < 0
       a. I only
       b. II only
       c. I and II only
       d. II and III only
       e. I, II, and III

10. Is a gas in its standard state at 298 K

       a. Lithium
       b. Fluorine
       c. Bromine
       d. Uranium
       e. Nickel

Section II: Free Response (~50%)

 ARRIVING AT YOUR ANSWERS. It is to your advantage to do this, because you may earn partial credit
 if you do and you will receive little or no credit if you do not. Attention should be paid to significant
 figures. Be sure to write all your answers to the questions on the lined pages following each question in this

1.                              C6H5OH(s) + 7 O2(g)  6 CO2(g) + 3 H2O(l)
When a 2.000-gram sample of pure phenol, C6H5OH(s), is completely burned according to the equation above,
64.98 kilojoules of heat is released. Use the information in the table below to answer the questions that follow.
                Standard Heat of Absolute Entropy,
               Formation, Hâ;         S, at 25C
  Substance      at 25C (kJ/mol)        (J/molòK)
  C(graphite)  0.00                 5.69
  CO2(g)       -393.5               213.6
  H2(g)        0.00                 130.6
  H2O(l)       -285.85              69.91
  O2(g)        0.00                 205.0
  C6H5OH(s)             ?           144.0

(a) Calculate the molar heat of combustion of phenol in kilojoules per mole at 25C.

(b) Calculate the standard heat of formation, Hâ, of phenol in kilojoules per mole at 25C.
(c) Calculate the value of the standard free-energy change, G, for the combustion of phenol at 25C.
(d) If the volume of the combustion container is 10.0 liters, calculate the final pressure in the container when
    the temperature is changed to 110.C. (Assume no oxygen remains unreacted and that all products are

2.                                  C2H2(g) + 2 H2(g)  C2H6(g)
Information about the substances involved in the reaction represented above is summarized in the following

         Substance      S (J/molK)   Hf (kJ/mol)
         C2H2(g)      200.9            226.7
         H2(g)        130.7            0
         C2H6(g)           ----        -84.7

         Bond        Bond Energy (kJ/mol)
          C-C                 347
         C=C                  611
          C-H                 414
          H-H                 436

(a) If the value of the standard entropy change, S, for the reaction is -232.7 joules per moleKelvin, calculate
    the standard molar entropy, S, of C2H6 gas.

(b) Calculate the value of the standard free-energy change, G, for the reaction. What does the sign of G
    indicate about the reaction above?

(d) Calculate the value of the CC bond energy in C2H2 in kilojoules per mole.

3. Propane, C3H8, is a hydrocarbon that is commonly used as fuel for cooking.
(a) Write a balanced equation for the complete combustion of propane gas, which yields CO2(g) and H2O(l).
(b) Calculate the volume of air at 30C and 1.00 atmosphere that is needed to burn completely 10.0 grams of
    propane. Assume that air is 21.0 percent O2 by volume.

(c) The heat of combustion of propane is -2,220.1 kJ/mol. Calculate the heat of formation, Hf, of propane
    given that Hf of H2O(l) = -285.3 kJ/mol and Hf of CO2(g) = -393.5 kJ/mol.

(d) Assuming that all of the heat evolved in burning 30.0 grams of propane is transferred to 8.00 kilograms of
    water (specific heat = 4.18 J/g.K), calculate the increase in temperature of water.

Part B: No calculator maybe used. For each of the following three reactions, in part (i) write a balanced
equation for the reaction and in part (ii) answer the question about the reaction. In part (i), coefficients should
be in terms of lowest whole numbers. Assume that solutions are aqueous unless otherwise indicated.
Represent substances in solutions as ions if the substances are extensively ionized. Omit formulas for any ions
or molecules that are unchanged by the reaction.

(a) A 0.1M solution of potassium iodide is mixed with a 0.1M solution of lead II nitrate.
    (i) Balanced equation:

    (ii) What evidence of a reaction will be observed? Explain.

(b) Sulfur dioxide is bubbled into water.
    (i) Balanced equation:

    (ii) Will the resulting product have a pH < 7, pH > 7, or pH = 7? Explain.

(c) The complete combustion of hydrogen.
     (i) Balanced equation:

    (ii) Is this a spontaneous reaction? Explain.
2.                     BCl3(g) + NH3(g)  Cl3BNH3(s)
The reaction represented above is a reversible reaction.
(a) Predict the sign of the entropy change, S, as the reaction proceeds to the right. Explain your prediction.

(b) If the reaction spontaneously proceeds to the right, predict the sign of the enthalpy change, H. Explain
    your prediction.

(c) The direction in which the reaction spontaneously proceeds changes as the temperature is increased above a
    specific temperature. Explain.

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