November 5, 2003
Review and Problem set for Exam 2
I. Kinetics. The gas-phase decomposition of NO2, NO2(g) NO(g) + ½ O2 (g), is studied at 383oC, giving the
following data:
0
ln[NO2]
-1
0
3
6
9
12
15
18
21
-2
-3
-4
-5
-6
time (s)
250
1/[NO2]
200
150
100
50
0
0
2
4
6
8
10
12
14
16
18
20
time (s)
a) Is the reaction first or second order with respect to the concentration of NO2?
Second order: plot 1/[NO2] vs. time is linear: 1/[NO2]= 1/[NO2]0 + kt
b) What is the value and the units of the rate constant?
(150-50)/(14-4)=10 (M-1 x s-1)
c) Determine the half-life of NO2 assuming that its initial concentration is 3x10-6 M
For second-order reaction, t1/2=1/(k x [NO2]0). Hence, t1/2=1/(10 x 3 x 10-6) = 33333 sec = 9.26 hours
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II Chemical Equilibria. A mixture of H2, S, and H2S is held in a 1.0 L vessel at 90 oC until the following
equilibrium is achieved:
H2 (g) + S (s)
H2S (g)
At equilibrium the mixture contains 0.46 g of H2S and 0.40 g of H2. The reaction is exothermic.
a) Write the equilibrium constant expression for this reaction
Kc =
[H 2 S (g )]
[H 2 (g )]
b) What is the value of Kc for the reaction at this temperature?
Calculate the molarity of H2S and H2:
[H2S] = 0.46 g H2S x (1 mol H2S/34.1 g H2S) x (1/1.00 L) = 0.0135 M
[H2] = 0.40 g H x (1 mol H2S/2.02 g H2S) x (1/1.00 L) = 0.198 M
Kc = (0.0135)/(0.198) = 0.068
c) Which direction will the equilibrium shift (i.e., tend to shift toward the formation of reactants, toward the formation of
products, or have no effect) if the system is subjected to each of the following changes:
Stress 1 H2 is added
products
Stress 2 The amount of S is increased
no change
Stress 3 The temperature is increased
reactants since reaction is exothermic
Stress 4 The amount of H2S is decreased
products
Stress 5 The partial pressure of H2S is decreased
no change
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III. Acids and Bases. Predict the products of the following acid-base reactions, identify the conjugate acid-base
pairs, and also predict whether the equilibrium lies to the left or to the right of the equation:
Acid-base equilibria favor formation of the weaker acid and base. Compare the substances acting as acids on
opposite sides of the equation. (Bases can also be compared; the conclusion should be the same)
a) H3O+ + OH-
2 H2O
a) HCO3- (aq) + F- (aq)
b) HC2H3O2 (aq) + HS- (aq)
c) HClO4 (aq) + Cl- (aq)
Strongest Acids
right (strong acid + strong base = weak acid + weak base)
HF (aq) + CO32- (aq)
left (HF is a stronger acid than HCO3-)
C2H3O2- (aq) + H2S (aq) right (H2S is a weaker acid than HC2H3O2)
ClO4- (aq) + HCl (aq)
Acid
HClO4
H2SO4
HI
HBr
HCl
HNO3
H3O+
HSO4H2SO3
H3PO4
HNO2
HF
HC2H3O2
H2CO3
H2S
HClO
HBrO
NH4+
HCN
HCO3H2O2
HSH2O
right (HCl is a weaker acid than HClO4)
Base
ClO4HSO4IBrClNO3H2O
SO42HSO3H2PO4NO2FC2H3O2HCO3HSClOBrONH3
CNCO32HO2S2OH-
Weakest Bases
Strongest Bases
Weakest Acids
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IV. Definitions. Define each of the following:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
Chemical kinetics
Reaction rate
Rate law
Rate constant
First order reaction
Expression for the first order reaction rate
Second order reaction
Expression for the second order reaction rate
Zero-order reaction
Expression for the zero-order reaction rate
Half-life of a reaction
Expression for half-life of the first-order reaction
Expression for half-life of the second-order reaction
Collision theory
Activation energy
Activated complex
Arrhenius equation
Reaction mechanism
Elementary step
Reactants
Products
Intermediates
Catalysts
Molecularity of a reaction
Unimolecular reaction
Bimolecular reaction
Termolecular reaction
Rate-determining (rate-limiting) step
Homogeneous catalysis
Heterogeneous catalysis
Chemical equilibrium
Equilibrium constant: definition, expression for a given chemical reaction
Homogeneous equilibrium
Heterogeneous equilibrium
Reaction quotient
Equilibrium position
Le Châtelier’s principle
Bronsted acid, Bronsted base
Conjugate acid-base pairs
Strong acid, strong base
Weak acid, weak base
Acid ionization constant, base ionization constant
Ion product of water
pH, pOH
Neutral, acidic, basic solutions
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