Kinetics 2

Question 1

List the main factors that affect the rate of a chemical reaction

Answer 1

• Temperature
• Concentration
• Pressure
• Surface area
• Catalysts

Question 2

Define rate of reaction

Answer 2

The change in concentration of any of the reactants or products per unit time.

Question 3

What does the rate expression tell us

Answer 3

How each of the species involved in a chemical reaction affects the reaction rate

Question 4

What is the rate equation for a reaction

Answer 4

• rate = k multiplied by the concentrations of each reaction raised to the power of the order of that reactant.

Question 5

What is k in the rate equation and how does it vary

Answer 5

• K is the rate constant for the reaction.
• it varies with temperature- so the temperature at which it is measured needs to be stated.

Question 6

How does the rate constant K vary with temperature

Answer 6

As temperature increases, the value of K increases

Question 7

What is the order of a reaction with respect to on of the species

Answer 7

The order of reaction with respect to one of the species is the power to which the concentration of that species is raised in the rate expression. It tells us how the rate depends on the concentration of that species.

Question 8

What is the overall order of a reaction

Answer 8

The overall order of the reaction is the sun of the orders of all of the species that appear in the rate expression.

Question 9

What does the rate expression tell us and what does it include

Answer 9

The rate expression tell you how the rate of a reaction depends on the concentration of the species involved. It only includes the species that affect the rate of the reaction.

Question 10

If the concentration of a species does not affect the rate of the reaction what is its order

Answer 10

Zero order

Question 11

If the concentration of a species is directly proportional to the rate of the reaction what is the order of that species

Answer 11

1st order

Question 12

If the rate of the reaction is directly proportional to the square of the concentration of a species, what is the order of that species

Answer 12

2nd order

Question 13

What are the two experimental methods by which you can determine the order of a reaction with respect to a particular species

Answer 13

1) Finding the order by using rate- concentration graphs
2) Using an initial rate method

Question 14

Describe how the order of reaction of a species can be determined by using rate-concentration graphs

Answer 14

• Plot a graph of rate against concentration
• If the graph is a horizontal straight line, the order of the species is 0
• If the graph is a straight line through the origin, then the order of the species is 1
• If the graph is a parabola/not a straight line then the order cannot be found directly- but is most likely two (at A-level). If the concentration of the species squared is plotted against rate, it should produce a straight line graph if the order is two.

Question 15

Describe how the order of reactants are determined by using the initial rate method

Answer 15

• A series of reactions is carried out at a constant temperature.
• Each experiment starts with a different combination of concentrations of reactants and catalysts and they are designed so that between any pair of experiments, the concentration of only one species varies.
• The initial rate of each of the experiments is determined and comparing the concentrations and initial rates allows us to work out the order of each reactant.

Question 16

What are two ways of measuring the initial rate of a reaction

Answer 16

1) Plot a concentration-time graph for the reactant and draw a tangent to the graph at time=0. The gradient of this tangent is the initial rate
2) Use 1/T. This works because the rate constant remains the same as long as temperature is kept constant.

Question 17

Why does the rate constant depend on temperature

Answer 17

• Temperature is a measure of the average kinetic energy of the particles
• As temperature increases, a higher proportion of the particles have the activation energy to react.
• So reaction rate increases with temperature which is what the rate constant represents.

Question 18

How is activation energy linked to the rate constant

Answer 18

By the Arrhenius equation

Question 19

What is the Arrhenius equation

Answer 19

k= Ae ^ -Ea/RT

Where:
- k= rate constant
- A= the pre-exponential factor which is related to the number of collision between reactant molecules.
- e ^ -Ea/RT = The fraction of collisions with enough energy to react.

Question 20

What is the logarithmic form of the Arrhenius equation

Answer 20

lnk= - Ea/RT + lnA

Question 21

Why is the logarithmic form of the Arrhenius equation useful- what does it help us to do

Answer 21

• The logarithmic form of the Arrhenius equation allows us to see that a graph of lnk against 1/T will be a straight line of the gradient -Ea/R
• This can be used to find a value for Ea experimentally by measuring the rate of reactions at different temperatures.

Question 22

Explain the meaning of reaction mechanism

Answer 22

• Most reactions take place in more than one step- these separate steps that lead to the formation of reactants and products are together called the reaction mechanism.
• The steps in between invoke very short-lived intermediates.
• These intermediate species, which give information about the mechanism of the reaction are usually difficult to isolate and identify which means other methods need to be used.
• This is the utility of the rate determining step

Question 23

What is the rate determining step

Answer 23

• The rate determining step is the slowest step in a reaction.
• It may form a ‘bottleneck’ and governs the rate of the whole process.

Question 24

Which stages of a reaction appear in the rate equation

Answer 24

• Only the rate determining step appears in the rate equation
• No stages after the rate determining step appear in the rate equation.
• Because the reactants for the rate determining step May depends on the stages before it, previous stages do occur in the rate equation.