CHM 123
EXAM 1 – Practice problems
It is your responsibility to study all topics that was discussed in the class and assigned from
Masteringchemistry.
Question I – Multiple choice
1.
Which is a net ionic equation for the neutralization of a weak acid with a strong base?
A) HCl(aq) + NaOH(aq) ⇌ H2O(l) + NaCl(aq)
B) H3O+(aq) + OH-(aq) ⇌ 2 H2O(l)
C) HF(aq) + NaOH(aq) ⇌ H2O(l) + NaF(aq)
D) HF(aq) + OH-(aq) ⇌ H2O(l) + F-(aq)
2.
When 50 mL of 0.10 M NH4Cl is added to 50 mL of 0.10 M NH3, relative to the pH of the 0.10 M NH3
solution the pH of the resulting solution will
A) become 7.
B) decrease.
C) increase.
D) remain the same
3.
Which is the best acid to use in the preparation of a buffer with pH = 3.3?
A) HOI (Ka = 2.0 × 10-11)
B) HNO2 (Ka = 4.5 × 10-4)
C) HNO3
D) HIO3 ((Ka = 1.7 × 10-1)
4.
Sodium hypochlorite, NaOCl, is the active ingredient in household bleach. What is the concentration of
hypochorite ion if 20.00 mL of bleach requires 28.30 mL of 0.500 M HCl to reach the equivalence point?
A) 0.208 M
B) 0.353 M
C) 0.708 M
D) 1.21 M
5.
The decomposition of ammonia to nitrogen and hydrogen on a tungsten filament at 800°C is independent
of the concentration of ammonia at high pressures of ammonia. What is the order of the reaction with respect to
ammonia?
A) zero
B) first
C) second
D) Third
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6.
Hydroquinone, HOC4H4OH, can be formed by the reaction with acetylene below:
3 HCCH + 3 CO + 3 H2O → 2 HOC4H4OH + CO2
How is the rate of disappearance of acetylene, HCCH, related to the appearance of hydroquinone (Hq)?
-
Δ[HCCH]
ΔT
A) +
C) +
=?
Δ[H ]
q
B) +
Δt
2 [H ]
q
D) +
3t
1[H ]
q
2 t
3[H ]
q
2 t
7.
Using the method of initial rates for the reaction A → B, if the initial concentration of A is doubled and the
rate of reaction quadruples, what is the order of reaction with respect to A?
A) zeroth
B) first
C) second
D) Fourth
8.
The elementary reaction representing the formation of ozone: O2(g) + O(g) + M(g) → O3(g) + M*(g) is an
example of a ________ reaction.
A) unimolecular
B) bimolecular
C) termolecular
D) Tetramolecular
9.
A three-step mechanism has been suggested for the formation of carbonyl chloride:
Step 1: Cl2 → 2 Cl
Step 2: Cl + CO → COCl
Step 3: COCl + Cl2 → COCl2 + Cl
Which species is an intermediate in the mechanism?
A) Cl
B) CO
C) COCl
D) COCl2
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10.
Which part of the Arrhenius equation contains a term which measures the number of molecules that have
the correct orientation for reaction?
A) activation energy
B) e-Ea/RT
C) frequency factor
D) none of these
Question II
1. Sketch the titration curve for a C5H5N versus HBr. Make sure to indicate the equivalence point (and whether it
is acidic, basic or neutral) and the buffer region.
a. Explain why the pH at the equivalence point is less than 7.00
2. (2pts) What is the difference between average reaction rate and instantaneous
3. Define half-life. Write the equation relating the half-life of zero-order reaction to the rate constant
4. How does a catalyst increase the rate of a reaction?
5.
Reactions involving two molecules colliding in order to react are said to be bimolecular. Reactions involving
three molecules colliding in order to react are called termolecular and are exceptionally rare. Why?
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Question II *Show all possible net-ionic neutralization reactions and hydrolysis reaction to receive Full Credit.
1.
What is the pH of the resulting solution if 45.0 mL of 0.432 M methylamine, CH3NH2, is added to 15.0
mL of 0.234 M HCl? Assume that the volumes of the solutions are additive.
Answer: 11.23
2.
Sodium hypochlorite, NaOCl, is the active ingredient in household bleach.
ạ
What is the concentration of hypochlorite ion if 20.00 mL of bleach requires 32.00 mL of 0.500 M HCl to
reach the equivalence point?
Answer: 0.800M
b.
What is the pH at the equivalence point?
Answer: 4.02
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Question IV
1. Hydrogen iodide decomposes at 800 K via a second-order process to produce hydrogen and iodine
according to the following chemical equation.
2 HI(g) → H2(g) + I2(g)
At 800 K it takes 142 seconds for the initial concentration of HI to decrease from 6.75 × 10-2 M to 3.50 × 10-2 M.
What is the rate constant for the reaction at this temperature?
Answer: 9.69 × 10-2 M-1s-1
2. The second-order reaction 2 Mn(CO)5 → Mn2(CO)10, has a rate constant equal to 3.0 × 109 M-1 s-1 at 25°C.
If the initial concentration of Mn(CO)5 is 2.0 × 10-5 M, how long will it take for 90.% of the reactant to
disappear?
Answer: 1.5 × 10-4 s
3. In the presence of excess thiocyanate ion, SCN–, the following reaction is first order in iron(III) ion, Fe3+;
the rate constant is 1.27 s–1.
3+
Fe (aq) + SCN–(aq) Fe(SCN)2+(aq)
a) What is the half-life in seconds?
0.546 s
b) How many seconds would be required for the initial concentration to drop to 6.25% remaining?
2.18 s.
4. A reaction of the form aA  Products is zero order with a rate constant of 4.68 x 10–3 Ms–1.
a) What is the half-life when the initial concentration of A is 0.813 M?
86.9 s
b) If [A] = 0.625 M, is the half-life the same, larger, or smaller than when [A] = 0.813 M
The half-life depends on the concentration (itʼs in the numerator). With a smaller value for concentration, the halflife will be less than in part a).
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5. The thermal decomposition of nitryl chloride, NO2Cl, is believed to occur by the following mechanism:
a. What is the overall reaction?
b. What is the molecularity of each of the elementary steps?
c. What is the intermediate?
d. What rate law is predicted by this mechanism ?
6. The first-order isomerization reaction: cyclopropane → propene, has a rate constant of 1.10 × 10–4 s–1 at
470°C and 5.70 × 10–4 s–1 at 500ºC. What is the activation energy, Ea, for the reaction?
260 kJ/mol
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