Kinetics Review Answers
1. Define the following:
Activation energy
The minimum energy required in a successful collision
Mechanism
A series of steps that determines the overall equation
Activated Complex
kinetic energy
An unstable reaction intermediate with high potential and low
Successful collision
One that has favorable geometry and sufficient energy
Catalyst
Speeds up a chemical reaction by providing an alternate
mechanism with lower activation energy-it is not consumed
Reaction rate
time
The change in the amount of reactant or product per unit of
Enthalpy
Potential or bond energy
Intermediate
A species that is produced and then consumed in a reaction
mechanism
Homogeneous reaction
Reactants are in the same phase
Rate determining step
The slowest step or the one with highest activation energy in
a mechanism that determines the overall rate
Heterogenous reaction
Reactants have different phases
2. Na2CO3(aq) + 2HCl(aq) → CO2(g) + 2NaCl(aq) +H2O(l)
a) Give four ways to increase the rate of the reaction.
Increase the temperature
Increase[Na2CO3] or [HCl]
Add a catalyst)
b) Give three properties that you could measure in order to determine the rate of the rection. For
each one describe how they would change during the experiment.
The volume of CO2(g)
in a closed System Increase.
The pressure of CO2(g) in a closed System
Increase.
The mass of an open system where CO2(g) is allowed to escape
Decreases.
3. Zn(s) + 2HCl(aq) → H2(g) + ZnCl2(aq)
a) Give five specific ways to increase the rate.
Increase Zn(s) surface area
Increase[HCl]
Increase Temperature
Add a catalyst
Agitate.
4. In low light, H2 and Cl2 do not react at all. When exposed to UV light, they react explosively!
Explain using the collision theory. The activation energy is too high for successful collisions.
UV light provides the activation energy to begin the reaction and because it is
exothermic, it continues and explodes.
5. A mixture of KClO3 and C12H12O11 do not react at all at room temperature. A drop of H2SO4
starts the reaction, slowly at first, then it quickens into a flaming inferno. Explain using the
collision theory.
H2SO4 is a catalyst, which lowers the activation energy, allowing lower energy
collisions to be successful.
6. Water puts out a fire. Explain using the collision theory.
Water lowers the temperature of the reaction so that collisions no longer have the
required activation energy.
7. A glowing splint reignits in pure O2. Explain using the collision theory.
Pure O2 has a higher concentration and therefore more collision, which increases
the reaction rate.
8. Enzymes in the human body allow the oxidation of carbohydrates at 37 degrees celcius.
Enzymes are catalysts, which lower the activation energy allowing low temperature
collisions to be successful.
9. Draw an exothermic PE diagram. Include a catalyst. Label the change in enthalpy, the forward
and reverse activation energies and the activated complex.
10. Repeat the above for an endothermic reaction.
11.
A reaction generally starts fast because reactant concentrations are highest at the
start of a reaction. As the reaction proceeds the reactants concentrations decrease
and the rate will get slower and slower as time goes on.
12.
A + WY
→
AWY
FAST
AWY + HA
→
A2WY + H
A2WY + HA
→
A3 + WY + H FAST
SLOW
I. List the following:
a) the overall equation
2HA + A
b) a catalyst
WY
c) Intermediates
AWY and A2WY
d) Reactants
2HA + A
e) Products
2H + A3
→
2H + A3
II. Describe how each change affects the rate.
a) increasing the concentration of A
No change, A is not in rate the
determining step or the slowest
b) increasing the concentration of H
No change, H is not a reactant
c) increasing the concentration of HA
Increases, HA is a reactant in the rate
determining step
d) removing WY completely
Decreases, WY is a catalyst as it crosses
out from left to right
e) Decreasing the temperature
Decreases
13. Describe the KE and PE changes as two molecules:
a) approach to collide,
KE decreases and PE increases
b) form an activated complex, and
KE is a minimum and PE is a maximum
c) form products in an exothermic reaction.
KE increases to a maximum and PE
decreases to a minimum
14. Draw the PE diagram for a mechanism with three steps.
How many activated complexes are there?
Three
How many intermediates are there?
Two
15. Describe as endo or exothermic.
a) 2H2 + O2
→
b) NH4NO3(s) →
2H2O + 300 KJ
NH4NO3(aq)
Exothermic
ΔH = +150 KJ
Endothermic
16. Change each equation in 15. from standard to ΔH notation or vice-versa.
2H2 + O2
→
2H2O
NH4NO3(s)
+
150 kJ
ΔH = -300 KJ
→
NH4NO3(aq)
17. Which reaction at room temperature is faster and why?
a) Pb+2(aq)
+
2Cl-1(aq)
→
PbCl2
replacement- there are no bonds to break and low Ea
b) 2H2(g)
+ O2(g)
→
2H2O
reactants are slower than aqueous and higher Ea
18. List three commercial catalysts.
Pt
Os
Fast simple ionic or double
Slow as there are gaseous
H2SO4
Ni
19. Calculate the rate in mols/s.
Moles H2
10.0
15.0
21.0
24.0
Time (seconds)
200.0
300.0
400.0
500.0
Rate
=
(24.0 - 10.0) moles
=
0.0467 mol/ s
(500.0 – 200.0) s
20. Indicate how each change will effect the rate of the reaction and the PE diagram and explain
with the collision theory.
a) increasing the temperature
Increase the rate as the average collision will have more Ea and there will be more
collisions. PE diagram will not change.
b) increasing the concentration of a reactant
Increase the rate as there will be more collisions. PE diagram will not change.
c) increasing the concentration of a product
The rate will not increase. PE diagram will not change.
d) addition of a catalyst
Increase the rate as the the Ea will be lower and more low energy collisions will be
successful. The activated complex is lower.
21. X + Y
→
XY
slow
XY + Z
→
XYZ
fast
XYZ + W
→
XYW + Z
fast
a) Identify intermediates
XY and XYZ
Cross out from right to left
the catalyst
Z
Crosses out from left to right
and the rate determining step
1 which is the slow step
b) If the concentration of X was increased will the rate increase?
Yes
22. You can only increase the surface area of a substance if it is in certain physical state. What is
it?
Solid
23. Two solid reactants react. Is it a homogeneous or heterogeneous reaction?
Heterogeneous
24. What is the mathematical relationship between reaction rate and activation energy
Inverse
reaction rate and reactant concentration
Direct
reaction rate and temperature?
Direct
25. Draw a collision energy distribution diagram for a reaction where the y axis is fraction of
collisions and the x axis is collision energy. Draw the Ea line showing about 10% of the collisions
having sufficient energy. Draw the Ea line for the catalyzed reaction where 20% have sufficient
energy.
Fraction of collisions
Collision Energy
Ea
Ea (cat)
26.