CHEM 116 Prince Mohammad Bin Fahd University Chemical Equilibrium Questions as last time we will be doing the same stuff i will include the matrail that we have and give more details later on. Test 3
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Chem 116
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Useful Information
R = 0.08206 atm L mol -1 K – 1 = 8.314 J mol -1 K -1
TK = TºC + 273.15
Kw = 1.0 x 10-14
∆ = ∆ − ∆
pH = pKa + log [A-]/[HA]
1. Consider the following reversible reaction.
HPO42- + HSO3- H2PO4- + SO32Which species is(are) acting as an acid?
a. HPO42- and SO32b. SO32- only
c. HPO42- and HSO3d. H2PO4- only
e. HSO3- and H2PO4-
2. Two species C and E react together to produce species G at 601 K,
C (g) + E (g) 2 G (g)
with Kp = 4.57. If the initial partial pressures of C , E and G are 2.63 atm, 2.50 atm and 3.60 atm
respectively, which statement best describes what happens after the reaction begins?
a. There is a decrease in the partial pressures of the products
b. There is an increase in the partial pressures of the products
c. There is an increase in the partial pressures of the reactants
d. There is no change in the partial pressure of any species
3. The water shift reaction
H2O (g) + CO (g) H2 (g) + CO2 (g)
is exothermic in the forward direction. If the system is in equilibrium in a closed reactor, which
of the below will result in an increase in the amount of CO2 in the system
a. Increasing the concentration of hydrogen
b. Decreasing the pressure by increasing the reactor volume
c. Adding a catalyst to the system
d. Decreasing the temperature
e. Decreasing the concentration of CO
4. A and B react together according to A + B 2 C. The initial concentrations of A and B are
both 0.672 M and initially [C] = 0 M. If Kc = 3.62 determine the equilibrium concentration of
species C
a. 0.903 M
b. 0.655 M
c. 0.557 M
d. 1.05 M
e. 1.10 M
5. A proposed mechanism for the reduction of NO by hydrogen is:
Step 1: H2 + 2NO –> N2O + H2O
Step 2: N2O + H2 –> N2 + H2O
Which statement is incorrect?
a. N2O is an intermediate species
b. Step 1 is trimolecular
c. NO is a catalyst
d. H2O is a product
6. A scientist is studying the decomposition of NO2 to produce NO and O2. If they measure the
rate constant to be 1.49 s-1 at temperature 617 K and 7.09 s-1 at temperature 720. K, what is the
value of the activation energy?
a. 140. kJ/mol
b. 27.9 kJ/mol
c. 142 kJ/mol
d. 55.9 kJ/mol
e. 39.8 kJ/mol
7. Species D and E react together to produce species F at 387 K according to
D (g) + E (g) 3F (g)
. If Kp is measured to be 67.6, determine the corresponding value of Kc.
a. 6.34
b. 3.86
c. 3.47
d. 2.13
e. 4.01
8. A rate constant obeys the Arrhenius equation. The activation energy is 50.7 kJ/mol and the
pre-exponential factor is 3.88 x 109 s-1. Calculate the value of the rate constant at 100 oC
a. 841 s-1
b. 800. s-1
c. 310. s-1
d. 263 s-1
e. 732 s-1
9. The rate constant for the first order decomposition of cyclobutane is 0.00589 day-1 at a
temperature of 506 K. If the initial concentration of the cyclobutane is 5.33 M, what will its
concentration be after 1.59 days?
a. 2.84 M
b. 5.28 M
c. 4.02 M
d. 4.38 M
e. 2.20 M
10. Calculate the pH of a 0.0610 M solution of a weak acid, that has Ka = 2.31 x 10-5
a. 1.65
b. 5.65
c. 5.93
d. 5.76
e. 2.93
11. Which of the following is not a characteristic of a zeroth order reaction?
a. The half-life is dependent on the initial concentration
b. The units of the rate constant are time-1
c. The overall order of the reaction is 0
d. A plot of reactant concentration against time is a straight line
e. The rate of reaction is constant
12. What is the conjugate base of HCO3-?
a. H2CO3
b. HCO3c. CO32d. CO3-
13. The rate of loss of NO2 to produce N2O4 is second order in NO2 with a rate constant of 8.12
M-1min-1. If the initial concentration of NO2 is 7.47 M, at what time will it have a concentration
equal to 1.87 M?
a. 0.0684 min
b. 0.108 min
c. 0.0997 min
d. 0.0140 min
e. 0.0494 min
14. Calculate the equilibrium constant Kc for the reaction
2A + 3F 2C + G
if at equilibrium [A] = 4.58 M, [F] = 0.643 M, [C]= 0.471 M and [G] = 4.87 M
a. 0.529
b. 0.0900
c. 0.562
d. 0.194
e. 0.164
15. Calculate the pH of a 0.158 M solution of a weak base that has Kb = 2.31 x 10-5
a. 8.17
b. 11.3
c. 9.79
d. 7.89
e. 10.1
16. If the equilibrium constant for the reaction A + B C is 1.14, what is the equilibrium
constant for the reaction 2A + 2B 2C
a. 1.30
b. 3.75
c. 2.47
d. 1.61
e. 1.14
17. A chemical reaction has a rate law with overall order 0. What are the units of the rate
constant?
a. Ms-1
b. M-1s-1
c. M-2s-1
d. s-1
18. A solution of a base is found to have [OH-] = 0.774 M. What is the pH of the solution?
a. 10.2
b. 12.3
c. 13.9
d. 7.74
e. 10.8
19. Calculate the percent ionization of the acid in a 0.375 M solution of a weak acid, HA, with
Ka = 4.11 x 10-5
a. 1.05 %
b. 6.31 %
c. 2.98 %
d. 0.945 %
e. 4.82 %
20. During a reversible chemical reaction which of the following statements is not true?
a. If the reaction quotient is larger than the equilibrium constant then the reaction moves
towards the reactants side
b. At equilibrium the concentrations of all species are equal
c. At equilibrium the rate of the forward reaction is equal to the rate of the reverse reaction
d. At equilibrium the concentrations of all of the species no longer change with time
key (number 0 throough19 rather than 1 through 20)
0. E
1. B
2. D
3. B
4. C
5. D
6. D
7. C
8. B
9. E
10. B
11. C
12. E
13. D
14. B
15. A
16. A
17. C
18. A
19. B
Chemistry 116: Exam 2
February 23, 2016
1. Which statement about the equilibrium constant is true? The value of Kc
a.
b.
c.
d.
e.
changes as product concentration changes
changes as reactant concentration changes
changes as temperature changes
never changes
there will be no further change in concentrations until the reaction is over
2. The decomposition of dinitrogen pentaoxide has an activation energy of 102 kJ/mol and
∆H°rxn= +55 kJ/mol. What is the activation energy for the reverse reaction?
a.
b.
c.
d.
27 kJ/mol
47 kJ/mol
55 kJ/mol
102 kJ/mol
3. The initial rate data for the reaction 2 NO(g) + Cl2(g) 2 NOCl(g) is shown in the following
table. Determine the numerical value of the rate constant for this reaction.
trial
1
2
a.
b.
c.
d.
[NO]
0.0300
0.0150
0.0150
[Cl2]
0.0100
0.0100
0.0400
Rate (M/s)
3.4 10–4
8.5 10–5
3.4 10–4
1.13
9.44
37.8
0.0265
4. A rate constant obeys the Arrhenius equation, the factor A = 2.2 1013 s–1 and the activation
energy being 150. kJ mol–1. What is the value of the rate constant at 227°C, in
a.
b.
c.
d.
6.710–22 s–1
2.11013 s–1
1.51011 s–1
4.710–3 s–1
5. The reaction: 2 HI H2 + I2, is second order and the rate constant at 800 K is 9.70×102 M1 s1.
How long will it take for an initial concentration of 8.00×102 mole/L of HI to decrease to 25% of
its initial concentration? Careful!
a.
b.
c.
d.
124 s
387 s
429 s
0.619 s
It’s not that I’m so smart, it’s just that I stay with problems longer.” ~ Albert Einstein
Page 1 of 5
Chemistry 116: Exam 2
February 23, 2016
6. , Which of the following is equivalent to increasing the value of Q?
a.
b.
c.
d.
addition of reactants
addition of products
removal of products
more than one of these
7. Given the following data, determine the rate law for the reaction shown below:
NH4+(aq) + NO2–(aq) N2(g) + H2O(l)
trial
1
2
3
4
a.
b.
c.
d.
e.
[NH4+]
0.250
0.300
0.500
0.250
[NO2–]
0.250
0.300
0.250
0.125
Rate (M/s)
1.25×103
1.80×103
2.50×103
6.25×104
rate = k [NH4+] [NO2–]
rate = k [NH4+]2 [NO2–]
rate = k [NH4+] [NO2–]2
rate = k [NH4+]½ [NO2–]
rate = k [NH4+] [NO2–]½
8. The radioactive isotope tritium decays with a first-order rate constant k of 0.056 year–1. What
fraction of the tritium initially in a sample is still present 30 years later?
a.
b.
c.
d.
0.19
0.60
0.15
2.8×10–38
9. The equilibrium constant for reaction A is is 54.9 at 699K. What is the equilibrium constant for
reaction B under the same conditions?
Reaction A:
Reaction B:
a.
b.
c.
d.
e.
2 HI (g)
H2 (g) + I2 (g)
2 H2 (g) + 2 I2 (g)
4 HI (g)
109.8
9.11×103
3.32×104
-109.8
0.0182
It’s not that I’m so smart, it’s just that I stay with problems longer.” ~ Albert Einstein
Page 2 of 5
Chemistry 116: Exam 2
February 23, 2016
10. Shown below is a concentration vs. time plot for the reaction A 2B. For this reaction the
value of the equilibrium constant is
a.
b.
c.
d.
Kc < 1.
Kc = 0
Kc = 1
Kc > 1
11. Consider the reaction below at 900 K. Calculate the value of Kp at the same temperature.
2 SO3 (g)
2 SO2 (g) + O2 (g)
Kc =13.0
3
a. 97.310
b. 0.176
c. 960
d. 0.00174
e. 0.077
12. A boiled egg can be cooked at 100.0 °C in exactly 5 minutes. At an altitude of around 2000 m
where the boiling point of water is 93.0 °C, it takes exactly 7.5 minutes to cook the egg to the same
amount. What is the activation energy for the reaction involved when an egg is boiled?
a.
b.
c.
d.
0.5 kJ/mol
4.5 kJ/mol
66 kJ/m
79 kJ/mole
13. The reaction shown below has the following rate law: Rate = k[CHCl3][Cl2]
CHCl3(g) + Cl2(g) CCl4(g) + HCl(g)
If the concentration of CHCl3 is increased by a factor of five while the concentration of Cl2 is kept
the same, the rate will
a.
b.
c.
d.
double
triple
stay the same
increase by a factor of five
14. Which of the following plots indicates the reaction is second order?
a.
b.
c.
It’s not that I’m so smart, it’s just that I stay with problems longer.” ~ Albert Einstein
d.
Page 3 of 5
Chemistry 116: Exam 2
February 23, 2016
15. The rate law for the reaction: A + B C is first order in each reactant. Of the three
representations below, which would have the fastest rate? The hollow circles represent molecules
of A and the solid circles represent molecules of B.
a.
b.
c.
16. Which of the following reaction profiles in the figure below represents the fastest reaction rate?
a.
b.
c.
d.
17. Nitric oxide (NO) can be removed from gas-fired power plant emissions by reaction with methane
as shown below. What is the rate at which H2O(g) is produced if the rate of disappearance of nitric
oxide is 3.6 M/s?
CH4(g) + 4 NO(g) 2 N2(g) + CO2(g) + 2 H2O(g)
a.
b.
c.
d.
e.
3.6 M/s
7.2 M/s
0.90 M/s
14.4 M/s
1.8 M/s
18. The reaction A2 + B2 2 AB has an equilibrium constant, Kc = 1.8. The following pictures
represent reaction mixtures that contain A2 molecules (shaded) and B2 molecules (unshaded), and
AB molecules.
Which represents a mixture is not at equilibrium and that will react in the forward direction to
reach equilibrium?
It’s not that I’m so smart, it’s just that I stay with problems longer.” ~ Albert Einstein
Page 4 of 5
Chemistry 116: Exam 2
February 23, 2016
19. Which statement is true for a reaction with Kc equal to 2.43×1012?
a.
b.
c.
d.
Increasing the temperature will not change the value of Kc.
There are appreciable concentrations of both reactants and products at equilibrium.
The reaction proceeds hardly at all towards completion.
The reaction proceeds nearly all the way to completion
20. For the reaction given below which of the following will cause an increase in the equilibrium
concentration of NO in a closed (constant volume) reaction chamber?
2 NO2(g)
2 NO(g) + O2(g)
a.
b.
c.
d.
e.
∆Hrxn = –118.9 kJ/mol
Adding an inert gas
Increasing the temperature of the system
Removing NO2 from the system
Decreasing the volume of the system
Both c and d
Key (version b, white)
1. C
3. C
5. B
2. B
4. D
6. B
7. A
8. A
9. C
10. A
11. B
12. C
13. D
14. C
15. A
16. D
17. E
18. C
19. C
20. B
Key (version a, green)
1. D
3. C
5. C
2. A
4. A
6. B
7. C
8. B
9. A
10. C
11. E
12. A
13. D
14. C
15. C
16. C
17. B
18. D
19. B
20. B
Key (version c, blue)
1. D
3. C
5. C
2. E
4. B
6. C
7. D
8. A
9. B
10. A
11. C
12. D
13. A
14. B
15. B
16. C
17. A
18. B
19. B
20. C
Key (version d, orange)
1. B
3. B
5. A
2. C
4. C
6. C
7. C
8. B
9. C
10. B
11. C
12. D
13. E
14. D
15. A
16. D
17. A
18. B
19. C
20. A
It’s not that I’m so smart, it’s just that I stay with problems longer.” ~ Albert Einstein
Page 5 of 5
Chem 116 topics for Exam 2
Week 3 concepts
1) Chemical equilibrium concept pictorially
2) Calculating Q from a given reaction condition and comparing that to K to determine if the
reaction shifts to the left or right.
3) What does a large and a small K mean in terms of amount of reactants and products at
equilibrium?
4) Kp and Kc interconversion question
5) Le Chatelier’s Principle and how the equilibrium shifts when adding or removing
reactant/products, when temperature changes for endo and exo reaction, when pressure changes, and
when catalyst is added.
6) Determining the solubility from Ksp
7) Equilibrium problems where you have to use the ICE table (Type A and B from the lecture
notes)
8) Bronsted-Lowry theory of acids and bases and how are they different from Arrhenius and
Lewis theory of acids and bases
9) Recognizing conjugate aid-base pair OR given a list of acids-finding their conjugate bases.
10) problem involving Ka and pKa (or thinking about pKa conceptually) eg, can give a list of pKa
values of some acids and then asked to rank the strongest to weakest acid based on the pKa numbers or
the Ka numbers.
11) Calculating the pH of a strong acid or basic solution.
12) % ionization problem also, be able to say what % ionization mean based on the formula.
13) What does Ka and Kb mean conceptually if formula is provided (or something in pictorial
form)
14) Polyprotic acids – what they mean and which is the strongest
15) calculating the pH of a weak acid titrated with a strong base
16) Buffer – conceptually what it means and what it does
17) Problem based on Henderson-Hasselbalch equation
Disclaimer: This list definitely will take you a long way in Exam 2. Having said that, this is not an
exhaustive list in any shape or form. Any concepts that was taught during the recorded video lecture
during Week 3 is a fair game.
Preview – Look through the PP slides BEFORE the class (spend roughly 15-25 minutes to have a general sense of what
concepts I will be covering)
Attend Class – OR make sure you watch the video that I post.
Review After Class – This will help you connect the dots, reinforce the concepts you learned or teach you the concepts you
DID not learn.
Study
i) Form study groups (if possible in the current situation)
ii) Question “Am I understanding the material?” “What am I not understanding”
iii) Have I memorized what I am supposed to?
iv) Solve problems without looking at an example.
v) DO NOT just highlight or re-read the material.
vi) Explain your material to your friend or someone else.
Assess
i) Can you solve random ALEKS’ questions?
ii) Can you solve Exam Practice problems?
iii) Can you rework the problems I worked out in class without looking at the answers?
Chapter 13: Chemical Equilibrium
Learning Objectives
By the end of this section, you will be able to:
▪ Describe the nature of equilibrium systems
▪ Explain the dynamic nature of a chemical equilibrium
▪ Derive reaction quotients from chemical equations representing
homogeneous and heterogeneous reactions
▪ Calculate values of reaction quotients and equilibrium constants,
using concentrations and pressures
▪ Describe the ways in which an equilibrium system can be stressed
▪ Predict the response of a stressed equilibrium using Le Châtelier’s
principle
▪ Calculate equilibrium concentrations or pressures and equilibrium
constants, using various algebraic approaches
15-2
Chemical Equilibrium
• The point at which the forward and reverse reaction rates become the same so that
the net composition of the system no longer changes with time
2NO2(g) ⇌
Brown
N2O4(g)
Colorless
• Chemical Equilibrium is the phenomenon observed in reversible reactions.
15-3
Concept of Equilibrium
A two-person juggling act illustrates the dynamic aspect of chemical
equilibria. Each person is throwing and catching clubs at the same rate,
and each holds a (approximately) constant number of clubs.
15-4
The Concept of Chemical Equilibrium
15-5
Chemical Equilibrium in Biological Systems
Transport of carbon dioxide in the body involves several reversible
chemical reactions, including hydrolysis and acid ionization (among
others).
15-6
The Reaction Quotient Qc
What is Q?
Q measures the relative amounts of products and reactants present during a reaction at a particular point in time.
Qc: The ratio of the product concentrations (multiplied together) to the reactant
concentrations (also multiplied together), with each concentrations raised to the power
equal to the coefficients in a balanced chemical equation.
For the reaction, 2 + 3 2 ⇌ 2 3 ( ) =
[ ]
[ ][ ]
15-7
The Reaction Quotient Qp
Qp: The ratio of the partial pressure of products (multiplied together)
to the partial pressure of reactants (also multiplied together), with each
partial pressures raised to the power equal to the coefficients in a
balanced chemical equation.
• For the reaction, 2 + 3 2 ⇌ 2 3 ( )
=
( )
( ) ( )
Knowledge Check 1
• Suppose you run this reaction in a chemistry lab:
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)
Write down the reaction quotient (Qp) for this reaction.
The Reaction Quotients Qc and Qp
The Equilibrium Constant K
15-11
ALEKS (Calculating an equilibrium constant from an
equilibrium composition)
Equilibrium Constant Practice (Q relation to K)
Consider the equation: 2NO2(g) ⇌ N2O4(g)
When 0.10 mol NO2 is added to a 1.0-L flask at
25oC, the concentration changes so that at
equilibrium, [NO2] = 0.016 M and [N2O4] =
0.042 M.
(a) What is the value of the reaction quotient
before any reaction occurs?
(b) What is the value of the equilibrium
constant for the reaction?
Think about these concepts
• What does a large and a small K mean in terms of amount of
reactants and products at equilibrium?
Knowledge Check 2
• For the question in the earlier slide, let us assume the concentration
changes so that at equilibrium, [NO2] = 0.042 M and [N2O4] = 0.016 M
(temperature and the volume of the flask are constant).
(a) What is the value of the reaction quotient before any reaction
occurs?
(b) What is the value of the equilibrium constant for the reaction?
Predicting the direction of reaction
Given below are the starting concentrations of reactants and products
for three experiments involving this reaction:
CO(g) + H2O(g) ⇌ CO2(g) + H2(g)
Kc = 0.64
Determine in which direction the reaction proceeds as it goes to
equilibrium in each of the three experiments shown.
Reactants/Products
Experiment 1
Experiment 2
Experiment 3
[CO]i
0.0203 M
0.011 M
0.0094 M
[H2O]i
0.0203 M
0.0011 M
0.0025 M
[CO2]i
0.0040 M
0.037 M
0.0015 M
[H2]i
0.0040 M
0.046 M
0.0076 M
15-16
Knowledge Check 3
• I worked through Experiment 1 in the earlier slide. Calculate the value
of the reaction quotient and then determine in which direction the
reaction proceeds as it goes to equilibrium in Experiment 3.
Homogeneous and Heterogeneous Equilibria
• Homogeneous equilibrium: An equilibrium in which the reactants and
products of an equilibrium reaction form a single phase, whether gas
or liquid
• C2H2(aq) + 2Br2(aq) ⇌ C2H2Br4(aq) =
• NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq)
[ 2 2 4 ]
[ 2 2 ][ 2 ]2
=
[ 4+ ][ − ]
[ 3 ]
• Heterogeneous equilibrium: An equilibrium in which the reactants of
an equilibrium reacti…
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