Name:____________
Chemistry 114
Third Hour Exam
1. (13 points) From the following initial rate data, determine the order parameters and rate constant
for the reaction A + B 6C
[A]
[B]
Rate of reaction (M/minute)
.06
.06
2.18
.06
.1
5.33
.1
.1
8.89
2. (12 points) Fill in the following table:
Reaction type
Integrated Equation
Linear Plot
t1/2 Equation
Zero Order
[A] = -kt +[A]o
[A] vs t
t1/2=[A]o/2k
First Order
ln[A] = -kt +ln[A]o
ln[A] vs t
t1/2=.693/k
Second Order
1/[A] = +kt +1/[A]o
1/[A] vs t
t1/2=1/(k[A]o)
Hint: Here are some example answers that you might use for some of the table entries:
Integrated equation
Linear plot
t1/2 equation
[A] =-kt +[A]o
ln[A] vs t
t1/2=1/(k[A]o)
2
3. (13 points) The reaction of methane and diatomic sulfur (shown below) has an activation energy
of 140 kJ/mol. If the reaction has a k of 1.1 L/mol"s at 550oC, what is the rate of this reaction at
625oC?
Reaction : CH4(g) + 2S2(g) 6 CS2(g) + 2H2S(g)
4. (12 points) Define the following terms:
Homogeneous catalysis
A catalyst that is in the same phase as the reactants and products.
Elementary step
A step in a kinetic mechanism whose rate law can be written from its molecularity
Arrhenius factor
The A in the equation rate = Ae-Ea/RT. It includes both the number of collisions and the steric
factor.
Rate determining step
The slow step in a reaction mechanism.
Reaction Quotient
Q=[Products]/[Reactants]. It looks like on equilibrium expression except that you plug your
current concentrations in instead of the equilibrium concentrations.
Heterogeneous equilibrium
An equilibrium that is achieved between chemicals in different phases.
3
5. (12 points) Given that the reaction H2(g) + Br2(g) W 2HBr(g) has a Kp = 3.5x104 at 25oC.
A. What is the Kc for this reaction?
Kp = Kc(RT))n; )n = 2-2 = 0 ; (RT)0 =1
Kp=Kc = 3.5x104
B. What is Kp for the reaction HBr(g) W½ Br2 (g) + ½ H2(g)
This equation is ½ times (the reverse of the original) , so Knew = (1/Kold)1/2
K = (1/3.5x104)1/2
K = 5.35x10-3
C. What is Kp for the reaction 2HBr(g) WBr2(g) + H2(g)
This equation is just the reverse of the original , so Knew = 1/Kold
K = 1/3.5x104
K = 2.86x10-3
D. What is Kc for the reaction 2H2(g) + 2 Br2(g) W4HBr(g)
This equation is just two times the original , so Knew = (Kold)2
K = (3.5x104)2
K = 1.225x109
6. (13 points) The reaction Pb(IO3)2(s) WPb+2(aq) + 2IO3-(aq) has the equilibrium constant 1.2x10-13
A. Write the equilibrium expression for this reaction:
K=[Pb+2][IO3-]2
B. If I mix up solutions under the following conditions, will the reaction move forward to produce
products (aqueous ions), or move backwards to produce reactants (solid in precipitating in the
mixture), or is it at equilibrium already? (Circle correct answer)
[Pb+2 ]
[IO3-]
1x10-8
1x10-3
Forward
Backward
At equilibrium
Q=[Pb+2][IO3-]2 = 1x10-8(1x10-3)2 = 1x10-14; Q<K Reaction will go forward
1x10-7
1x10-3
Forward
Backward
At equilibrium
Q=[Pb+2][IO3-]2 = 1x10-7(1x10-3)2 = 1x10-13; Q<K Reaction will go forward
This is so close to K that I also accepted at equilibrium
1x10-8
1x10-2
Forward
Backward
At equilibrium
Q=[Pb+2][IO3-]2 = 1x10-8(1x10-2)2 = 1x10-12; Q>K Reaction will go backwards
4
7. (20 points) Suppose you were studying the reaction 2PBr3(g) + 3Cl2(g) W2PCl3(g) + 3Br2(g)
A. What is the equilibrium expression for this reaction?
B. Suppose the concentrations of all reactants and products is 2.5 mole/liter, what is Q for
the reaction?
Q=[(2.5)2(2.5)3]/[(2.5)2(2.5)3] = 1
C. If K for the reaction is 12.5, does the reaction move toward products or reactants?
Q<K, reaction goes forward toward products
D. Make X the change in concentration of Br2 as the system comes to equilibrium, so that
at equilibrium the concentration of Br2 is 2.5 + X
What is the concentration of PCl3 in terms of X?
In this reaction 2 moles of PCl3 are made for 3mole of Br2 made
2.5 + 2/3X
What is the concentration of Cl2 in terms of X?
In this reaction 3 moles of PCl3 are used up for 3mole of Br2 made
2.5 - X
What is the concentration of PBr3 in terms of X?
In this reaction 2 moles of PBr3 are used up for 3 mole of Br2 made
2.5 - 2/3X
E. What is the final equilibrium expression you would have to solve to find X? (Don’t solve
it, just show me the equation.)
8. (5 points) Hydrogen for use in ammonia production is made using the reaction
CH4(g) + H2O(g) W CO(g) + 3H2(g)
What will happen to the equilibrium mixture when....
(Answer is either ‘Nothing’, ‘Make more products’, or ‘Make more reactants’)
A. H2O(g) is removed More Reactants
B. The temperature is increased? (The reaction is exothermic)
More Reactants
C. Ar gas is added?
Nothing
D. CO(g) is removed?
More Products
E. The volume of the container is tripled?
More Products