Chapter 13 129 Chapter 13 Chemical Kinetics Generally, the AP exam questions on kinetics are predictable. Students need to take data and determine the orders of reaction, write the rate law, solve for k, and write the proper units for k. Students should be able to choose from among proposed mechanisms, recognize intermediates and catalysts, and solve concentration-time equations. Students should be able to graph data and determine if a reaction is zero, first, or second order from graphing and determine an instantaneous rate from a graph. Animations for this chapter include activation energy, collision orientation, and catalysis. The media player section covers activation energy. Major Concepts to Know • Common mistakes are to use balanced equation coefficients as orders and to answer problems using casual inspection instead of mathematics. Students who know how to do a problem will often not get credit in the free response if they use inspection because they explain too little or they may make too hasty of a conclusion. This is not to say a student can’t use inspection, but the student must use great care when asked to give his or her reasoning. For instance, the student sees both rate and concentration double and concludes that it is first order, but when explaining their answer no reference is made to holding two concentrations constant or what experiments were used (which can be implied by the math). In this case, no credit would be earned. • Another common error is students forgetting that order is actually an exponent, so if they are asked by what factor the rate will change in a second order reaction where a concentration changes from 1 to 3, they often will say 3 2 = 6 times instead of 3 2 = 9 times. Another error is thinking a catalyst never takes part in a reaction, even in intermediate steps. • Students need to review graphical-equation relationships of different order reactions. Students should know the slope = k in first order reactions when plotted as the ln of the concentration versus time, and slope = k in a second order reaction when plotted as reciprocal concentration versus time.
Transcript
Chapter 13 129
Chapter 13 Chemical Kinetics
Generally, the AP exam questions on kinetics are predictable. Students need to take data and
determine the orders of reaction, write the rate law, solve for k, and write the proper units for k.
Students should be able to choose from among proposed mechanisms, recognize intermediates
and catalysts, and solve concentration-time equations. Students should be able to graph data and
determine if a reaction is zero, first, or second order from graphing and determine an
instantaneous rate from a graph. Animations for this chapter include activation energy, collision
orientation, and catalysis. The media player section covers activation energy.
Major Concepts to Know
• Common mistakes are to use balanced equation coefficients as orders and to answer
problems using casual inspection instead of mathematics. Students who know how to do a
problem will often not get credit in the free response if they use inspection because they
explain too little or they may make too hasty of a conclusion. This is not to say a student
can’t use inspection, but the student must use great care when asked to give his or her
reasoning. For instance, the student sees both rate and concentration double and concludes
that it is first order, but when explaining their answer no reference is made to holding two
concentrations constant or what experiments were used (which can be implied by the math).
In this case, no credit would be earned.
• Another common error is students forgetting that order is actually an exponent, so
if they are asked by what factor the rate will change in a second order reaction where a
concentration changes from 1 to 3, they often will say 3 2 = 6 times instead of
32 = 9 times. Another error is thinking a catalyst never takes part in a reaction, even in
intermediate steps.
• Students need to review graphical-equation relationships of different order reactions.
Students should know the slope = k in first order reactions when plotted as the ln of the
concentration versus time, and slope = k in a second order reaction when plotted as reciprocal
concentration versus time.
Chapter 13 130
Integrated Rate Laws and Reaction Orders
• Students must be able to plot data to determine the order of a reaction, as in the following
example.
Graphical Determination of the Reaction Order
• Another commonly missed problem is one where the student is given the rate of loss of one
compound and then asked to solve for the rate of loss or gain of a different compound in the
Chapter 13 131
equation. The student needs to use the ratio of the coefficients from the balanced equation to
determine the answer. This indicates students need practice in knowing when balanced
equation coefficients are important and when they are not.
• Students should practice working with half-lives for first order reactions. From data or a
graph, students should be able to determine a half-life. They should also be able to calculate
the concentration at any point in the reaction using the concentration-time equation. These
will both be used later in Chapter 23 as all nuclear reactions are
first order.
• Students often have trouble describing or calculating an instantaneous rate from a graph.
They need to describe taking the tangent through the point in question and solving for the
slope.
The Instantaneous Rates of Reaction
• Students need to create mind pictures of chemical reactions at the molecular level and thus
understand collision model theory and how it applies to chemical kinetics. One
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misconception students have is that if molecules collide, they always react. Students fail to
realize that a reaction must have the right energy in the collision as well as the right
orientation of the molecules. Students should be able to use their understanding of collision
theory to answer qualitative questions about factors that influence chemical reaction rates.
• Students should understand all the features of potential energy reaction coordinate diagrams.
Students should be able to look at a diagram and determine the activation energy of both the
forward and the reverse reaction and H of both forward and reverse reactions. Although not
tested often, students should be able to use the Arrhenius equation to calculate activation
energy.
Potential Energy Profiles
• Students should be able to look at these graphs and determine if they are exothermic or
endothermic.
• As mentioned above, students should be able to identify a possible mechanism that fits a rate
law by noting the rate-determining step. They need to know how to write and combine
elementary step equations to exclude intermediates from the final expression. However, as of
yet, AP has not expected students to propose a mechanism for a reaction.
• Students should develop a good qualitative understanding of what a catalyst does in a
chemical reaction but will not be expected to answer quantitative questions. Enzyme
catalysis has not so far been the subject of an AP Chemistry question.
Chapter 13 133
Vocabulary to Know
• Activated complex
• Activation energy
• Catalyst
• Elementary step
• Orders including zero order, first
order, and second order
• Half-life
• Intermediate
• Rate constant (k)
• Rate-determining step
• Rate law
• Reaction mechanism
• Reaction rate
Math Skills to Know
• Determine orders of a reaction
• Determine the rate law
• Solve for the rate constant k and its units
• Determine slope for finding k
• Make graphs to determine zero, first, and second order reactions
• Determine an instantaneous rate from a graph
• Compare rate of loss of A to rate of gain or loss of B using stoichiometry
• Determine how the rate constant changes with temperature using
ln (k1/k2) = Ea/R[(1/T2) – (1/T1)] and
ln (k1/k2) = Ea/R[(T1 – T2) / (T1T2)]
• From an energy diagram, determine activation energy and the energies released or absorbed
for exothermic and endothermic reactions
• Solve half life equations and concentration-time equations
Suggested Problems (* also electronic)
• Rate of a reaction: 1, 5, 6*–8*
• Rate law: 9–14, 15*, 16*, 17, 18*, 19, 20
• Relations between reactant concentration and time: 22, 24, 25*, 26*, 27*, 28–30
