IB Chemistry Kinetics Exam Questions and Answers

28. Define half-life
29. Given the graph to the right, determine the half-life of the reaction:  A + B → AB

30. Define activated complex
31. Give two examples of possible activated complexes that could be formed in the following reaction.

CH3Br  + OH-1 → CH3OH + Br-1

32. Describe, in terms of bonds breaking and forming, two ways that the above reaction could occur.
33. Draw an energy diagram for the above reaction.  Assume the reaction is exothermic.
34. Show on your diagram, what happens when a catalyst is added.
35. The following reaction occurs by the following mechanism

CH3CH2Cl + OH-1 → CH3CH2OH + Cl-1

1) CH3CH2Cl + OH-1 → HO—(CH3)CH2—Cl

2) HO—(CH3)CH2—Br → CH3CH2OH + Cl-1

1. If the first step is the rate determining step, what is the rate law?
2. What is the overall order of the reaction?
3. What is the molecularity of the reaction?

36. The following reaction occurs by the following mechanism

(CH3)3CCl + OH-1 → (CH3)3COH + Cl-1

1) (CH3)3CCl → (CH3)3C+1 + Cl-1

2) OH-1 + (CH3)3C+1→ (CH3)3COH

1. If the first step is the rate determining step, what is the rate law?
2. What is the overall order of the reaction?
3. What is the molecularity of the reaction?

37. Define molecularity

38. A reaction occurs by the following mechanism

1. A + B → AB
2. AB + C → ABC
3. ABC + A → A2 + BC

1. What is the overall reaction?
2. If the rate law is first order with respect to A and first order with respect to B, what is the rate law?
3. What is the rate-determining step?
4. How do you know?

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Answers

1. Define rate of reaction 

Change in reactant/product concentration with time

2. What are the units of the rate of reaction?

Molarity/time or mol dm-3 s-1

3. Outline procedures by which you could obtain a value for the rate of reaction

Give a reaction, state what needs to be measured (mass/volume of gas/presence of color), record time, plot a graph

4. Draw a graph that shows the concentration of products with time as a reaction goes to completion.  Explain the shape of the graph.

At t=0, there is no product.  As time passes, the concentration of the product increases.  The rate is fast at first because the concentration of the reactants is high but begins to slow as the reactants get used up.  When the reactants get used up, their concentrations decrease which results in fewer collisions between reactants and hence, a slower rate of production of product.  At t = infinity, the most product that can be formed is formed and no more is made.

5. Draw a graph that shows the concentration of reactants with time as a reaction goes to completion.

At t=0, there is only reactant present so the concentration is high.  As time passes reactant molecules collide and form the product.  The concentration of reactant molecules decreases so that as time passes the rate that they react decreases since there are fewer collisions.  At t= infinity, the amount of reactant left goes to zero (unless the reverse reaction is significant, in which case, the concentration of the reactant reaches a constant, equilibrium value).

6. Outline the main features of collision theory

- reactants must collide wit the right orientation and with enough activation energy in order to react. Not all collisions result in a reaction.

7. Give two reasons why a collision would not result in a reaction

Not enough activation energy, don’t collide with the right geometry

8. The reaction between nitrogen and oxygen in the atmosphere under normal conditions is extremely slow. Which statement best explains this?

This is an IB exam question – narrow down your choices, then if two choices are almost the same thing, pick the 4th!

1. The concentration of oxygen is much lower than that of nitrogen

This may be true and concentration does affect reaction rate, but this isn’t the best answer for two reasons 1) the question says “under normal conditions” which implies that at higher temperatures the nitrogen and oxygen would react a lot more quickly at the same concentration.  2) This is nearly the same answer as C.

2. The molar mass of nitrogen is less than that of oxygen

This doesn’t really matter

3. The frequency of collisions between nitrogen and oxygen molecules is lower than that between nitrogen molecules themselves

This may be true but doesn’t explain why the reaction between nitrogen and oxygen is slow, only why its slower than a reaction between two nitrogen molecules.  Also, it’s pretty much the same answer as A.

4. Very few nitrogen and oxygen molecules have sufficient energy to react

This is the best explanation

9. List 4 factors that affect reaction rates and explain why using the principles of collision theory.

1. concentration: higher conc. means higher reaction rate because there are more chances for molecules to collide in a given amount of time. (on IB exam, they really want you to talk about the RATE of collisions, not just the number of collisions)
2. Surface area: the more surface area, the more chances for a collision in the same amount of time.
3. Temperature: the higher the temperature the faster the molecules are moving.  This does two thing which BOTH increase reaction rate.  So you get more out of increasing temperature than other changes.

1. More collisions per unit time
2. More molecules have the required activation energy so more of the collisions successfully result in a reaction.

4. Catalysts: increase reaction rate by make more collisions successful in a given amount of time.

1. Holding molecules in the right orientation for more successful collisions per unit time
2. Lowering activation energy so that more molecules, at a given temperature, have enough energy to have a successful collision

10. Define activation energy 

Minimum amount of energy required for a molecule to react

11. Sketch a Maxwell-Boltzmann Energy Distribution Curve for two different temperatures.  

x-axis: velocity/speed or energy

y-axis: number of molecules

Higher temperature has a lower peak, shifted to the right and distribution is spread out

12. Does activation energy change with temperature?

No.

13. How does the number of molecules with the required activation energy change at higher temperatures? Use energy distribution to explain.

At higher temperatures more molecules have the required activation energy for two reasons. 1) Distribution is shifted towards a higher energy, so that even if the distribution at a higher temperature was the same as at a lower temperature, the number of molecules with Ea would increase. 2) The wider distribution means even more molecules have required activation energy

14. Give two reasons why a temperature increase, increases reaction rate and identify the more important reason.

Does it seem like I keep asking the same thing?  Well, it’s important!

The higher the temperature the faster the molecules are moving.  This does two thing which BOTH increase reaction rate.  So you get more out of increasing temperature than other changes.

1. More collisions per unit time
2. More molecules have the required activation energy so more of the collisions successfully result in a reaction.  More molecules have the required Ea because the distribution is shifted to the right and because the distribution is wider. (THIS IS THE MORE IMPORTANT REASON.  It’s more important because it makes a bigger difference than the number of collisions per unit time)

15. Will the following changes increase the reaction rate of the following reaction when 50 cm3 of 1.0 mol dm-3 HCl is added to 1.0 g calcium carbonate?

CaCO3(s) + 2 HCl (aq) → CaCl2 + CO2(g) + H2O(l)

1. increasing volume of HCl

No.  Increasing the volume will not change the overall concentration.  If you add 100 cm3 of 1.0 mol dm-3, the rate will be same.

2. increasing mass of CaCO3

No. The rate will be the same

3. decreasing size of CaCO3 particles

Yes.  Smaller particles, means more chances for collisions

4. decreasing amount of CO2 present

No. The presence of CO2 gas does not affect, significantly, the chance for collision between HCl and CaCO3 molecules. The rate will be the same.

5. increasing temperature

Yes.  The rate will increase

6. adding water to the reaction

The rate will not increase BUT it will DECREASE because you are decreasing the concentration of HCl which means fewer collisions per unit time.

7. removing water from the reaction vessel

Yes, this will increase the concentration of HCl which will increase the chance for collisions

8. adding a catalyst

Yes, adding a catalyst increase reaction rate by lowering activation energy

16. Explain, using the Maxwell-Boltzmann Energy distribution, why adding a catalyst increases reaction rate.

If the activation energy is lower, than more molecules will have the required activation energy and more collisions will be successful.

17.  Determine the rate law for the following reaction

A + 2B →  AB2

        Rate = k[A]2[B]

18. Using the data from the previous problem, calculate the rate constant

40 = k (42)(4)= 40/64 = 0.625

19. What would the rate be if the [A] = 2.5 mol dm-3 and [B] = 3.0 mol dm-3?

Rate = (0.625)(2.52)(3.0) = 11.7 M/s

20. Determine the rate law for the following reaction

C2 + 2 D → 2 CD

Rate = k [C2]

21. Using the data from the previous problem, calculate the rate constant

10 = k (1.0)  → k = 10

22. What would the rate be if the [C] = 0.25 mol dm-3 and [D] = 0.30 mol dm-3?

Rate = 10 (0.25) = 2.5 M/s

23. What is the order of reaction for the following rate laws

24. Give the integrated rate law, the t1/2 equation, and the plot that would give a straight line for the following:

25. A plot of ln [NO] vs time gives a straight line.

1. Sketch the graph

**slope is negative!!

2. Write the rate law

Rate = k[NO]

3. What is the molecularity of this reaction? 1, unimolecular

26. A plot of 1/[N2] vs time gives a straight line.

1. Sketch the graph

**slope is positive!!

2. Write the rate law rate = k[N2]2
3. What is the molecularity of this reaction? 2, bimolecular

27. Given the graph below,

1. Give the rate law rate = k[A]
2. Calculate the rate constant k = slope = 2
3. Determine the half-life in the initial concentration of [A] is 2.0 mol dm-3  t½  = 0.693/k = 0.693/2 = 0.347 s
4. What is the molecularity of this reaction? 1, unimolecular

28. Define half-life time it takes for half of the initial concentration of a reactant to be used up.
29. Given the graph to the right, determine the half-life of the reaction:  A + B → AB

From the graph you can see that the initial concentration of A is 200 M so, when [A] = 100 M, a half-life will have passed.  This occurs at about 7 seconds.

30. Define activated complex

A high-energy, temporary transition state for molecule

31. Give two examples of possible activated complexes that could be formed in the following reaction.

CH3Br  + OH-1 → CH3OH + Br-1

        OH-CH3-Br   or         CH3+1

32. Describe, in terms of bonds breaking and forming, two ways that the above reaction could occur.

a) The OH group can attach to the carbon and start forming a bond as the bond between the Br and the carbon weaken. This results in formation of the activated complex.  Eventually the bond between the Br and the C break and the bond between the OH and the C strengthens.

33. Draw an energy diagram for the above reaction.  Assume the reaction is exothermic.

34. Show on your diagram, what happens when a catalyst is added.

35. The following reaction occurs by the following mechanism

CH3CH2Cl + OH-1 → CH3CH2OH + Cl-1

1) CH3CH2Cl + OH-1 → HO—(CH3)CH2—Cl

2) HO—(CH3)CH2—Br → CH3CH2OH + Cl-1

1. If the first step is the rate determining step, what is the rate law? Rate =k[CH3CH2Cl][OH-]
2. What is the overall order of the reaction? 2
3. What is the molecularity of the reaction? 2, bimolecular

36. The following reaction occurs by the following mechanism

(CH3)3CCl + OH-1 → (CH3)3COH + Cl-1

1) (CH3)3CCl → (CH3)3C+1 + Cl-1

2) OH-1 + (CH3)3C+1→ (CH3)3COH

1. If the first step is the rate determining step, what is the rate law?

Rate = k[(CH3)3CCl]

2. What is the overall order of the reaction? 1
3. What is the molecularity of the reaction? 1, unimolecular

37. Define molecularity

The number of molecules that need to collide in the rate determining step

38. A reaction occurs by the following mechanism

1. AC → A + C
2. A + B → AB
3. AB + C → ABC
4. ABC + A → A2 + BC

1. What is the overall reaction?

2AC + 2B +  → A2 + 2BC

2. If the rate law is first order with respect to A and first order with respect to B, what is the rate law? Rate = k[A][B]
3. What is the rate-determining step? The second step
4. How do you know? The rate determining step is always the slowest step in the reaction mechanism.  The rate law is a mathematical representation of the rate.  Since the slowest step is what determines the rate, than the rate law must be based on the slowest step. So the reactants that affect the rate must be involved in the rate-determining step.