Chem 121, section 03, Midterm 1 Page 1 CAMOSUN COLLEGE Chemistry 121 Section 03, Instructor: H. M. Cartwright Midterm 1 February 18 th 2015. Time 1 ½ hours Name ____________________________________ Student Number ____________________ Attempt as many questions as possible. Write your answers on this examination paper. If you need extra space, continue on the reverse of the paper, stating within your answer to the question that you have done so. The numbers in square brackets indicate the approximate number of marks available for each part of the question. Do not turn over the examination paper until you are instructed to do so.
Transcript
Chem 121, section 03, Midterm 1 Page 1
CAMOSUN COLLEGE
Chemistry 121 Section 03, Instructor: H. M. Cartwright
Midterm 1
February 18th 2015.
Time 1 ½ hours
Name ____________________________________
Student Number ____________________
Attempt as many questions as possible.
Write your answers on this examination paper. If you need extra space,
continue on the reverse of the paper, stating within your answer to the
question that you have done so.
The numbers in square brackets indicate the approximate number of marks
available for each part of the question.
Do not turn over the examination paper until you are instructed to do so.
Chem 121, section 03, Midterm 1 Page 2
Integrated rate laws:
0th order: [𝐴]𝑡 = −𝑘𝑡 + [𝐴]𝑜
1st order: ln [𝐴]𝑡 = −𝑘𝑡 + ln [𝐴]𝑜
2nd order: 1
[𝐴]𝑡= 𝑘𝑡 +
1
[𝐴]𝑜
Half-life of a 1st
order reaction: t ½ = ln 2/k
Chem 121, section 03, Midterm 1 Page 3
1. Circle the letter that corresponds to the one statement that is INCORRECT. A catalyst: A. Lowers the activation energy for a reaction. B. Can only increase reaction rate at elevated temperature. C. Does not change the position of equilibrium. D. Can be in the same phase or a different phase as the reactants. E. Always changes the rate of a reaction. [1] 2. For each of the following types of compound: (a) give the general formula; (b) draw the full structure of one example of this type of compound; (c) name the compound that you have drawn. [3 x 1.5]
(i) Alkene
(ii) Alcohol
(iii) Aromatic
Chem 121, section 03, Midterm 1 Page 4
3. Using suitable equations, explain the difference between a substitution reaction and
an addition reaction. [4]
4. At least one of the following statements is WRONG:
A. Increasing the temperature speeds up most reactions.
B. For a 1st order reaction the units of the rate constant k are time-1.
C. The rate of a chemical reaction in the forward direction is always greater than the
rate in the backward direction.
D. Catalysts provide an alternate pathway between reactants and products.
E. The magnitude of a rate constant depends on the concentration of reactants.
The INCORRECT statement(s) is/are:
(i) A and B (ii) E (iii) E and B (iv) C (v) C and E [1]
Chem 121, section 03, Midterm 1 Page 5
5. Of the following changes, which ONE will not affect the rate of most reactions? [1]
A. Increasing the concentration of reactants.
B. Raising the temperature of the reaction.
C. Adding a catalyst.
D. Removing products as they are formed.
E. Increasing the pressure of a gas-phase reaction.
6. (a) Explain the difference between geometric isomers and structural isomers. [3]
(b) Draw all the isomers of butene, C4H8. [3]
7. What is an electrophile? [3]
Chem 121, section 03, Midterm 1 Page 6
8. (a) What is a chiral molecule? [1]
(b) Is the molecule below chiral? If so, indicate any chiral atom(s) with a star. [2]
9. Which ONE of the following experimental techniques can be used to distinguish
between two enantiomers? [1]
A. Infrared spectroscopy
B. Nuclear Magnetic Resonance spectroscopy
C. Accurate analytical weighing
D. Optical rotation
E. Free radical reaction with an alkene
10. (a) What is a chain reaction? [2]
Chem 121, section 03, Midterm 1 Page 7
(b) Name the three main types of steps in a chain reaction. [1.5]
(c) Give an example of a chain reaction, showing each of the three steps you have
named in part (b). [3]
Chem 121, section 03, Midterm 1 Page 8
11. (a) Sketch the potential energy vs. progress of reaction (the reaction coordinate or
reaction profile diagram) for an endothermic reaction. Label the axes and show on your
diagram
(i) location of the reactants and products
(ii) position of any transition states (activated complexes).
(iii) activation energy for each step. [4]
(b) Based on your diagram, which of the forward or the reverse reaction would you predict is the slower? Justify your choice. [1] 12. The reaction between NO(g) and Br2(g) to form NOBr (g) was examined at 0 oC. The initial rate of formation of product was studied in the experiments tabulated below. Expt. [NO](mol L-1) [Br2] (mol L-1) Initial rate (mol L-1 min-1) 1 0.10 0.20 24 2 0.25 0.20 150 3 0.10 0.50 61 4 0.35 0.50 734 (a) What is the rate law for this reaction? [2]
Chem 121, section 03, Midterm 1 Page 9
(b) Calculate the average value of the rate constant, k. Include the correct units. [3] (c) Suppose an experimenter measured the change in the concentration of Br2 vs time for the reaction. Taking into account the rate law you have just derived, what function of the Br2 concentration should she plot against time to get a straight line? [1]
13. (a) Draw a circle around the carbocation(s) shown below which is/are the LEAST
stable, based on the position of the positive charge (there may be more than one correct
answer). Note that the charge actually resides on the carbon atom, not on a hydrogen
atom. [1]
(b) Justify your choice in part (a): [2]
Chem 121, section 03, Midterm 1 Page 10
14. (a) What is the difference between a conformer and an isomer? [1]
(b) Why is there "free" rotation around every bond in butane, C4H10, but not around
every bond in 2-butene, C4H8? [2]
15. (i) The value of the rate constant for a reaction depends upon temperature; why? [3]
(ii) What factors other than temperature influence the value of a rate constant? [2]
Chem 121, section 03, Midterm 1 Page 11
16. (a) What is meant by the half-life of a chemical reaction? [1]
(b) The rate constant for the first-order decomposition of N2O5 is 2.75 x 10-2 s-1.
Determine the half-life of the decomposition. [1]
(c) Sketch how the concentration of N2O5 will vary as a function of time, showing
clearly at least two half-lives on your diagram.
Chem 121, section 03, Midterm 1 Page 12
17. Hydrogen bromide gas, HBr, reacts with oxygen to form water in the gas phase and
bromine, also in the gas phase.
(a) Write down the equation for the reaction and balance it. [2]
(b) The concentration of which material changes most rapidly during the reaction?
[1]
(c) Using your balanced equation, write an expression for the rate at which the
concentration of bromine changes, expressed in terms of the rate at which the
concentration of HBr changes. (Note that the rate of change of concentration of HBr can be
written as t
HBr
][ ) [1]
18. The following sequence of steps is one of the main routes by which ozone is
destroyed in the Antarctic atmosphere:
Cl(g) + O3(g) → ClO(g) + O2(g)
ClO(g) + light → Cl(g) + O(g)
O(g) + O(g) → O2(g)
(a) Identify any reactive intermediate(s) and/or catalyst(s) in this mechanism. [2]
Chem 121, section 03, Midterm 1 Page 13
(b) Write down the rate law for each step in the mechanism. [3]
(c) The rate law for the overall reaction is Rate = k [Cl][O3]. Which of the three steps
given above is probably the slowest? Justify your choice. [2]
(d) Identify the probable origin of the chlorine atoms in the first step of the
mechanism. [1]
19. For the reaction A + 3B 2C the rate law is Rate = k[A][B]2. Circle the letter that
corresponds to the correct answer.
If the concentrations of A and B are both halved, then the rate will fall by a factor of:
A. 1/3
B. 1/4
C. 1/8
D. 1/9
E. 1/16 [1]
Chem 121, section 03, Midterm 1 Page 14
20. Give an example of a catalyst and describe briefly how it works. [3]
21. The reaction A → products has been found to be 1st order. In a typical experiment,
the concentration of A decreased by 4% in 8.00 minutes.
(a) Calculate the rate constant for this reaction, quoting the appropriate units. [3]
(b) If the initial concentration of A was 0.61 M, after how many seconds would the
