Practice Short Answer Questions Exam 1Review
Chem 178 SI
Wednesday, September 19, 2007
PRACTICE SHORT ANSWER QUESTIONS – EXAM 1REVIEW
1. PH3 is known to decompose in a first-order process that has a rate constant of 1.3 x10–4 sec–1. If [PH3] = 0.75M at the start of a reaction, how long will it take until [PH3] = 0.010 M?
2. A particular first-order reaction has a rate constant of 1.35x102 s–1 at 25 C. What is the numerical value of k (including proper units) at 75 C if the activation energy is 85.6 kJ/mol?
3. The following experimental data were collected for the rate of disappearance of ClO2 in the reaction
2 ClO2 (aq) + 2 OH- (aq) ClO3- (aq) + ClO2- (aq) + H2O (l)
Exp. No.[ClO2] (M)[OH–] (M)Initial Rate (M/s)
10.0400.0300.028
20.0200.0300.014
30.0200.0900.014
Write the rate law for the reaction based on the above experimental data.
4. The half-life of a first-order reaction is 300. s. If the initial concentration of the reactant is 0.025 M, how long will it take for the concentration of the reactant to decrease to 0.010 M?
5. Consider the following reaction for the formation of phosphorus trichloride, PCl3(g):
P4(g) + 6Cl2(g) 4PCl3(g)
a) Calculate the standard molar entropy change, ΔS0.
b) Calculate the standard free-energy change, ΔGorxn .
c) Is this reaction spontaneous? Very briefly explain.
6. The normal boiling point of water (H2O) is 100C, and its molar enthalpy of vaporization is ΔHvap = 40.67 kJ/mol.
a) At what temperature range is the vaporization of water a spontaneous process at 1.0 atm pressure?
b) When water evaporates at its normal boiling point, does its entropy increase or decrease? Very briefly explain.
c) Calculate ΔS when 36.0 g of water vaporizes at 100C.
7. Write the rate law for the following elementary reaction.
2NO(g) + Br2(g) → 2NOBr(g)
What is the order of the reaction with respect to NO(g)?
What is the order of the overall reaction?
8. Consider the following reaction
2H2(g) + 2NO(g) → N2(g) + 2H2O(g)
The observed rate law is rate=k[H2][NO]2. The following mechanisms have been proposed for this reaction.
Mechanism A
H2(g) + NO(g) → H2O(g) + N(g) (slow)
N(g) + NO(g) → N2(g) + O(g) (fast)
O(g) + H2(g) → H2O(g) (fast)
Mechanism B
H2(g) + 2NO(g) → N2O(g) + H2O(g) (slow)
N2O(g) + H2(g) → N2(g) + H2O(g) (fast)
Mechanism C
2NO(g) N2O2(g) (fast, reversible)
N2O2(g) + H2(g) → N2O(g) + H2O(g) (slow)
N2O(g) + H2(g) → N2(g) + H2O(g) (fast)
a) Which of the mechanisms can be ruled out on the basis of the observed rate law? Why?
b) Which of the mechanisms is the least physically reasonable? Why?
9. Consider the following mechanism
(1) CH3CHO(g) + I2(g) → CH3I(g) + HI(g) + CO(g) (slow)
(2) CH3I(g) + HI(g) → CH4(g) + I2(g) (fast)
a) What is the overall equation for the reaction?
b) What is/are the catalyst(s) in the reaction if any?
c) What is/are the intermediate(s) in the reaction in any?
10. Consider the following reaction and kinetic data.
Chem 178 SI
Wednesday, September 19, 2007
2NOBr(g) → Br2(g) + 2NO(g)
Time (s) [NOBr] (mol/L)
0.00 0.0100
2.00 0.0071
4.00 0.0055
6.00 0.0045
8.00 0.0038
10.00 0.0033
Chem 178 SI
Wednesday, September 19, 2007
a) Write an expression for the rate of disappearance of NOBr in terms of concentration and time.
b) Calculate the average rate of the reaction between 2.00 sec and 6.00 sec using the expression that you wrote from part a. Be sure to include units in your answer.
11. For the simple decomposition reaction
AB(g) A(g) + B(g)
the rate = k [AB]2 and k = 0.20 M–1 s–1. How long will it take for [AB] to reach one-half of its initial concentration of 1.50 M? For full credit, show all work, use units, and report your answer to the correct number of significant figures.
12. Indicate whether the following statements are true or false:
a) ______The effect of a catalyst on a chemical reaction is to react with product, effectively removing it and shifting the equilibrium to the right.
b) ______The overall reaction order is always the sum of the stoichiometric coefficients of the balanced chemical reaction.