Kinetics

Question 1

Define rate

Answer 1

The change in the concentration of a reactant or product per unit time (mol L^-1 s^-1)

Question 2

Which component do we usually base conversion on?

Answer 2

The rate limiting component (key component)

Question 3

What is a rate law?

Answer 3

An expression which relatres the rate of a reaction to the rate constant and the concentrations of the reactants.

Question 4

How can reaction order for a reaction be calculated?

Answer 4

By adding the exponents of the reactants in the rate law.

Question 5

Which equation shows that the rate constant depends on temperature? State this equation.

Answer 5

The Arrhenius equation

Question 6

What does E_a represent in the Arrhenuis equation?

Answer 6

The activation energy of the reaction

Question 7

What does A represent in the Arrhenius equaton and what is it related to?

Answer 7

A is the pre-exponential factor and is related to the frequency of collision between reaction molecules, so is a measure of the rate at which collisions occur, irrespective of their energy.

Question 8

What is the general formula for the units of k for a reaction of n^th order.

Answer 8

Question 9

Give the Arrhenius equation in it’s logarithmic form.

Answer 9

Question 10

With the Arrhenius equation in it’s logarithmic form, which variables would you plot on the y and x axis?

Answer 10

y = ln(k)

x = 1/T

Question 11

From a plot of the logarithmic Arrhenius equation, how would you find the pre-exponential factor and the activation energy?

Answer 11

lnA is given by the value of the y-intercept and -E_a/R is given by the gradient of the line.

Question 12

If given time instead of rate, what would you plot on the y-axis of a logarithmic Arrhenius plot?

Answer 12

ln(1/time), which is proportional to ln(k)

Question 13

What is the definition of activation energy?

Answer 13

The minimum kinetic energy that reactants must have in order to form products.

Question 14

What does the rate constant represent in terms of kinetics?

Answer 14

The rate of successful collisions between reactants.

Question 15

What does the exponential factor represent in the Arrhenius equation in terms of kinetics?

Answer 15

The fraction of collisions that have enough kinetic energy to lead to a reaction.

Question 16

Draw the graph of potential energy of a reaction against the progress of the reaction (exothermic).

Answer 16

Question 17

How does reaction rate change with temperature?

Answer 17

As temperature increases, rate will increase because the overall kinetic energy of the reactants will increase, resulting in a greater rate of successful collisions and hence products forming.

Question 18

What rule can be used to estimate how reaction rate will change with temperature?

Answer 18

Reaction rate will double each time the temperature is increased by 10 degrees.

Question 19

What is the limit on increasing the temperature of a reaction?

Answer 19

The temperature of a reaction may be increased until the temperature is a the point at which the reactants will degrade, at which the point the rate of the reaction will decrease.

Question 20

What equation can be used to compare the Arrhenius equation and two different temperatures?

Answer 20

Question 21

Define a catalyst.

Answer 21

A class of molecules that lower the activation energy of a reaction, but are not consumed in the reaction themselves. The assist in the progress of the reaction and therefore increase the rate.

Question 22

What is the most common type of catalyst?

Answer 22

An enzyme.

Question 23

How does an enzyme decrease the activation energy?

Answer 23

By ensuring that the reactants collide in perfect orientation, which increases the likelihood that the collision is successful.

Question 24

State what the 3 different rates (r, r’ and r’’) are based on.

Answer 24

r - a unit volume of fluid

r’ - the weight of the catalyst

r’’ - the surface area of the catalyst

Question 25

Write the rate equation based on the unit volume of a fluid.

Answer 25

Question 26

Write the rate equation based on the weight of a catalyst.

Answer 26

Question 27

Write the rate equation based on the surface area of a catalyst.

Answer 27

Question 28

For a homogeneous reaction, what is the rate based on?

Answer 28

A unit volume of the fluid.

Question 29

For a heterogeneous reaction, what is the rate based on?

Answer 29

The surface area of the species.

Question 30

What is an elementary reaction?

Answer 30

When the rate law of the reaction reflects the stoichiometry of the reaction.

Question 31

If the rate law or a reaction reflects the stoichiometry, is the reaction elementary?

Answer 31

It may be, or it may just be coincidence.

Question 32

What is a non-elementary reaction?

Answer 32

When the rate law does not reflect the stoichiometry of the reaction.

Question 33

State the three applications of rate laws.

Answer 33

1. Determining k to predtict the rate of reaction from a mixture of components.
2. To predict mixture compositions at different stages of the reaction
3. To determine the reaction mechanism

Question 34

Explain the isolation method to determine the rate law.

Answer 34

This is a way of determining the rate law by having one of the reactants present in a large excess so it will not have an effect on the reaction rate (it can be assumed to be constant). The affects of changing the concentration of the isolated component can then be observed.

Question 35

In the isolation method, how can the rate law be described when one reactant is present in a much greater concentration than the other?

Answer 35

The rate law can be said to be psuedo first-order.

Question 36

Explain the initial rate method to determine the rate law of a reaction.

Answer 36

The rate is measured at the start of the reaction for several initial concentration combinations. The initial rate law is compared for each combination to find the order of the reaction with respect to each reactant.

If there is only one reactant and one data set, the order of the reaction can be found by plotting the logarithmic curve of the initial rate equation.

Question 37

How is the integrated rate law found?

Answer 37

Separate the differential equation and integrate with respect to t and C_A.

Question 38

How would you find k from an integrated rate law for a first order reaction?

Answer 38

Plot ln(C_A/C_A0) against t. The gradient is -k.

Question 39

How do you find the equation for half-life of a substance?

Answer 39

Integrate the rate equation with the final concentration equalling half of the initial concentration.

Question 40

How can k be graphically determined from the equation for half life of a second order reaction?

Answer 40

Plotting 1/C_A against t, the slope will have a gradient of k.

Question 41

What is the equation for amount of material remaining using the half-life of the material?

Answer 41

starting material x 0.5ⁿ = ending material, where n is the number of half lives

Question 42

How can the rate equation for a bimolecular reaction (first order wrt A and B) be manipulated into an equation with one variable?

Answer 42

By letting m = C_A0-C_A = C_B0-C_B, therefore dC_A=dm. m = 0 when t = 0. The integration can then be carried out by separating the fraction, then replacing m back with the original variables.

Question 43

How do you derive the equation for the equilibrium constant?

Answer 43

For a reversible reaction A to B, integrate the rate equation for C_A. Equilibrium occurs when t tends to infinity. This can be used to find C_Aeq and C_Beq, and K is equal to the latter divided by the former.

Or it can be derived by equalling the dC_A/dt to 0, as there is no change in concentration at equilibrium. Then rearrange for C_Aeq and follow the same method.