Kinetics Web 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 An unstable reaction intermediate with high potential and low kinetic energy
Successful collision One that has favourable 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 The change in the amount of reactant or product per unit of time
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
Add a catalyst
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 SLOW
A2WY + HA → A3 + WY + H FAST
I. List the following:
a) the overall equation 2HA + A → 2H + A3
b) a catalyst WY
c) Intermediates AWY and A2WY
d) Reactants 2HA + A
e) Products 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 → 2H2O + 300 KJ Exothermic
b) NH4NO3(s) → NH4NO3(aq) ΔH = +150 KJ Endothermic
16. Change each equation in 15. from standard to ΔH notation or vice-versa.
NH4NO3(s) + 150 kJ → NH4NO3(aq)
17. Which reaction at room temperature is faster and why?
a) Pb+2(aq) + 2Cl-1(aq) → PbCl2 Fast simple ionic or double replacement- there are no bonds to break and low Ea
b) 2H2(g) + O2(g) → 2H2O Slow as there are gaseous reactants are slower than aqueous and higher Ea
18. List three commercial catalysts. Pt Os H2SO4 Ni
19. Calculate the rate in mols/s.
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.
b) 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
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?
22. You can only increase the surface area of a substance if it is in certain physical state. What is it?
23. Two solid reactants react. Is it a homogeneous or heterogeneous reaction?
24. What is the mathematical relationship between reaction rate and activation energy
reaction rate and reactant concentration
reaction rate and temperature?
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
Fraction of collisions