AP Chemistry › Kinetics and Energy
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(fast)
The mechanism for decomposition of ozone is shown. What is the intermediate of the process?
Intermediate is created and destroyed, and therefore does not appear on the net equation, which is . Thus, the intermediate is
. Note that when asked for an intermediate, the coefficient in front of it is not used, rather we are looking for the species that is a product of one reaction and a reactant in a subsequent step.
Which of the following is a classic example of a first-order reaction?
Radioactive decay
A collision between 2 reactant molecules
A change in temperature
None of the other answers
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.
Which of the following factors will increase the reaction rate of the following reaction, if it is an endothermic, zero order reaction?
An increase in the temperature at which the reaction occurs
An increase in \[A\]
An increase in \[B\]
A decrease 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 , where k is the rate constant at a given temperature.
Which of the following is a classic example of a first-order reaction?
Radioactive decay
A collision between 2 reactant molecules
A change in temperature
None of the other answers
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.
(slow)
(fast)
The mechanism for decomposition of ozone is shown. What is the intermediate of the process?
Intermediate is created and destroyed, and therefore does not appear on the net equation, which is . Thus, the intermediate is
. Note that when asked for an intermediate, the coefficient in front of it is not used, rather we are looking for the species that is a product of one reaction and a reactant in a subsequent step.
Chaning which of the following factors can alter the rate of a zero-order reaction?
Temperature
Increasing the concentration of reactants
Increasing the concentration of products
Decreasing the concentration of products
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.
The rate law of the reaction, , is
. Which of the following does not increase the rate of the reaction?
Increasing the concentration of
Adding the catalyst to the reaction
Increasing the temperature of the reaction
Increasing the concentration of
The reactant is not included in the rate law expression, and therefore altering its concentration does not affect the rate of the reaction. Catalysts always increase the rate of reactions by lowering its activation energy. Increasing temperature (average kinetic energy of the molecules) increases the frequency of collisions, and increases the proportion of collisions that have enough energy to overcome the activation energy and undergo a chemical reaction. Increasing the concentration of
will increase the rate of the reaction as indicated by the 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\[A\]\[B\]2
rate = k\[A\]2\[B\]
rate = k\[A\]\[B\]
rate = k\[A\]
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\[B\]
Which of the following changes to reaction conditions will always result in an increase in the reaction rate?
Increased temperature
Decreased temperature
Increased concentration of reactants
Decreased concentration of products
For this question, we're asked to identify something that will always increase the rate of a reaction. Notice that the question specifically states "always."
Let's go through each answer choice and see how it affects the reaction rate.
When the concentration of reactants is increased, this may increase the reaction rate, but not always. For example, if a reaction is zero-order with respect to its reactants, then changing the reactant concentration will have no effect on the rate.
Just like with reactants, decreasing the concentration of products may or may not change the reaction rate. If the reaction rate is zero-order with respect to the products, then a change in their concentration will have no effect on the reaction rate.
A change in temperature is the only thing that is guaranteed to change the reaction rate. This is because changing the temperature will directly change the rate constant of the reaction. Increasing the temperature will increase the rate constant, and hence the reaction rate.
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 only
II only
I and III only
II and III only
I, II, and III
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.