AP Chemistry : Thermochemistry and Kinetics
Study concepts, example questions & explanations for AP Chemistry
All AP Chemistry Resources
Example Questions
Example Question #1 : Reaction Rate And Rate Law
You want to find the order of each reactant by manipulating the reactant concentrations in multiple trials. The table below shows the effect of altered reactant concentrations on initial reaction rate.
Using the above trials, write the rate law for the reaction.
Keep in mind that the chemical equation and its coefficients have NOTHING to do with the rate law. The order of each reactant must be determined by experiment.
To find the order of each reactant, compare the initial reaction rates of two trials in which only one of the three reactants' concentrations is altered. For example, trials 1 and 4 keep A and B equal, but C is doubled. When C is doubled, we see that the initial reaction rate is quadrupled. As a result, we determine that the order of reactant C is 2. When the reactant is altered, but the initial reaction rate is kept constant, as seen in trials 1 and 3 with respect to A, the order of that reactant is 0. Finally, when the reactant is multiplied by the same factor that the initial reaction rate is multiplied, as seen in trials 1 and 2 with respect to B, the order of the reactant is 1.
Putting the data together: A is zeroth order, B is first order, and C is second order. Our rate law can thus be written 
Example Question #1 : Reaction Rate And Rate Law
Given the following equation (2A+B --> 3C). Which of the following correctly displays the rate of the reaction?
I. -Δ[A]/2Δt
II. Δ[B]/Δt
III. Δ[C]/3Δt
I, II, and III
I only
II and III only
I and III only
II only
I and III only
The rate based on concentration is related to the coefficients in front of the compounds. Based on the reactants the rate should be negative (because change in concentration for the forward reaction will be negative) and based on the products should be positive. This means that II is incorrect. The rate for each compound in the reaction should be divided by it's coefficient to make it all related to 1M, showing that I and III are correct.
Example Question #1 : Reaction Rate And Rate Law
For the following reaction H2 + 2 ICl → 2 HCl + I2 describe the rate of consumption
of H2 in regards to the consumption of ICl and the production of I2
Half of ICl, half of I2
Half of ICl, same as I2
Same as ICl, same as I2
Same as ICl, half of I2
Half of ICl, same as I2
By the stoichiometry, since 1 mole of of H2 and 2 moles of ICl produces 2 moles of HCl
and 1 mole of I2 , we know that H2 is consume half as fast as ICl and produced at the same
rate as I2 .
Example Question #1 : Reaction Rate And Rate Law
Which of the following is a classic example of a first-order reaction?
Radioactive decay
A change in temperature
None of the other answers
A collision between 2 reactant molecules
Radioactive decay
First order reactions have rates that are directly proportional to only 1 reactant. In radioactive decay, the rate of decrease of a radioactive material is proportional only to the amount of the material.
Example Question #11 : Reaction Rate And Rate Law
Consider the Kinetic Experimental data obtained from reaction X+Y -> Z
Initial [X] Initial [Y} Inital Rate of Formation of Z
Experiment (mol/L) (mol/L) (mol/L*min)
1 0.20 0.10 1.0 * 10-5
2 0.20 0.20 2.0 * 10-5
3 0.40 0.10 2.0 * 10-5
Which of the following statements is true?
The order of the reaction with respect to reactant [X] is second order
The order of the reaction with respect to reactant [Y] is second order
The reaction is second order overall
The reaction is fourth order overall
The reaction is first order overall
The reaction is second order overall
The rules for determing the reaction order with respect to inividual reactants are as follows.
If the concentration of a reactant is doubled and the rate of product formation is doubled then the reaction is first order with respect to this individual reactant.
If the concentration of a reactant is doubed and the rate of product formation is quadrupled then the reaction is second order with respect to this indivudal reactant
If the concentration of a reactant is tripled and the rate of product formation increases by nine times then the reaction is third order with respect to this indivudal reactant
Example Question #11 : Reaction Rate And Rate Law
Consider the Kinetic Experimental data obtained from reaction X+Y -> Z
Initial [X] Initial [Y} Inital Rate of Formation of Z
Experiment (mol/L) (mol/L) (mol/L*min)
1 0.20 0.10 1.0 * 10-5
2 0.20 0.20 2.0 * 10-5
3 A 0.10 2.0 * 10-5
If the reaction is second order overall, what is the value of A?
0.20
None of the other answers
0.10
0.40
2.0
0.40
The rules for determing the reaction order with respect to individual reactants are as follows.
If the concentration of a reactant is doubled and the rate of product formation is doubled then the reaction is first order with respect to this individual reactant.
If the concentration of a reactant is doubled and the rate of product formation is quadrupled then the reaction is second order with respect to this indivudal reactant
If the concentration of a reactant is tripled and the rate of product formation increases by nine times then the reaction is third order with respect to this individual reactant
Example Question #11 : Reaction Rate And Rate Law
Given the reaction A + B → C. What is the rate law for the following experiment?
[A] [B] Rate
0.05 0.05 0.0125
0.05 0.1 0.0250
0.1 0.05 0.0125
rate = k[A]
rate = k[B]
rate = k[A][B]2
rate = k[A][B]
rate = k[A]2[B]
rate = k[B]
When the concentration of B doubles, the rate doubles. Making this reaction first order in regards to compound B. When the concentration of A doubles the rate is unaffected, making this reaction zero order in regards to compound A. This leaves a rate law of rate=k[B]
Example Question #22 : Thermochemistry And Kinetics
Which of the following factors can change the rate of a zero-order reaction?
Product concentration
Reactant concentration
Temperature
Pressure
Temperature
Zero-order reactions are independent of changes in the concentration of any of the reactants. The rate constant itself is dependent on temperature, so changing the temp will change the rate of a zero-order reaction.
Example Question #11 : Reaction Rate And Rate Law
Which of the following factors will increase the reaction rate of the following reaction, if it is an endothermic, zero order reaction?
A decrease in the temperature at which the reaction occurs
An increase in [B]
An increase in the temperature at which the reaction occurs
An increase in [A]
An increase in the temperature at which the reaction occurs
An increase in temperature leads to an increase in reaction rate for endothermic reactions, while a decrease in temperature will slow down their rate.
For zero order reactions, reaction rate is independent of reactant concentrations; therefore, changing [A] and [B] will have no effect on reaction rate.
The rate law for this reaction would simply be 
Example Question #13 : Reaction Rate And Rate Law
Chaning which of the following factors can alter the rate of a zero-order reaction?
Increasing the concentration of reactants
Temperature
Increasing the concentration of products
Decreasing the concentration of products
Temperature
A zero-order reaction has a rate of formation of product that is independent of changes in concentrations of any of the reactants; however, since the rate constant itself is dependent on temperture, changing the temperature can alter the rate.
Certified Tutor
All AP Chemistry Resources
