Reaction Kinetics

Question 1

Define rate of reaction.

Answer 1

The rate of reaction is defined as the change in concentration of a reactant or product per unit time.

Question 2

What is instantaneous rate?

Answer 2

Instantaneous rate is the rate at a particular time.

It is measured by measuring the gradient of the tangent draw to the curve at time t. The steeper the gradient the faster the reaction.

Question 3

What is average rate?

Answer 3

Average rate of a reaction during a specificed time interval is the change in concentration of a reactant or a product over that time interval.

Measured through measuring the gradient of the line drawn between two points on the curve that are related to the time interval.

Question 4

What is initial rate?

Answer 4

Initial rate is the instantaneous rate at time t=0.

Obtained by measuring the gradient of the tangent drawn to the curve at time t=0.

Question 4

State the rate equation.

Answer 4

rate = [A]^m[B]^n

Question 5

Define rate equation.

Answer 5

The rate equation is an experiementally determined mathematical equation between the reaction rate and the concentration of reactants at a particular temperature.

It can only be experimentally determined and cannot be deduced from the stoichiometric equation.

Question 6

Define order of reaction.

Answer 6

Order of reaction with respect to a particular reactant is the power to which the concentration of that reactant is raised in the experimentally determined rate equation.

Overall order of reaction is the sum of the individual orders of reaction.

Question 7

Define rate constant.

Answer 7

Rate constant, k, is a proportionality constant for a given reaction at a specified temperature.

Units of k depends on rate equation. Value of k can be affected by temperature and catalyst. The larger the value of k, the faster the rate of reaction.

Question 8

What is a zero order reaction?

Answer 8

rate = k It means that the rate is constant at a particular temperature. Thus, changing the concentration of the reactants will not affect the rate of reaction, (ie reaction is independent of the concentration of the reactant)

Question 9

What is a first order reaction?

Answer 9

rate = k[A] Its a reaction where the rate is directly proportional to the concentration of that reactant. Furthermore, half-life is constant and independent of te concentration of the reactant in a first order reaction.

Question 10

Define half life.

Answer 10

Half life of a reaction is the time taken for the concentration of a reactant to decrease to half its initial value.

Question 11

State the formula for half life.

Answer 11

t1/2 = ln2/k = 0.693/k
also
ct/c0 = (1/2)^n

Question 12

What is a second order reaction?

Answer 12

rate = k[A]^2 It is a reaction where the rate is directly proportional to the square of the concentration of that reactant.

Half life of a second order reaction increases over time.

Question 13

What is a pseudo first order reaction?

Answer 13

Suppose rate = [A][B’] If [B] is ten times more than [A], then [B] is effectively constant during the reaction and thus k[B] is also constant. Hence, rate = k’[A] where k’ = k[B] Thus it now follows first order kinetics. t1/2 = ln2/k’

Question 14

State when a reaction is a pseudo first order reaction.

Answer 14

1. Presence of a large excess of a reactant
2. When the solvent is a reactant: hence its concentration is essentially constant throughout the reaction
3. Presence of a catalyst: if a reactant is a catalyst, it only increases the rate of reaction but is not consumed by the reaction and is regenerated. Hence the concentration of the reactant is essentially constant.

Question 15

How do we determine the orders of reaction from initial rates data?

Answer 15

Comparing the initial reactions of two experiments where only the concentration of one reactant is varied and comparing the initial reactions. eg. When [A] is tripled, initial rate is tripled, hence the reaction is first order with respect to A.

Question 16

How do we use half lives to determine the order of reaction?

Answer 16

Plot a suitable graph of [A] against time. Find at least 2 consecutive half lives (or 3 for practical). If half life is constant at (Time)min, the reaction is first order with respect to A.

Question 17

State how a reaction may be quenched.

Answer 17

1. Adding a large volume of cold solvent to dilute the sample and slow down the reaction significantly
2. Adding a quenching agent which reacts with one of the reactants to stop the reaction
3. Adding an inhibitor or a negative catalyst to slow down the reaction

Question 18

For discontinuous method, why is the volume of water varied in each experiment?

Answer 18

The volume of water is varied in each experiment to keep the total volume of the reaction mixture constant for the different experiments. In this way, the initial concentration of each reactant in the reaction mixture is directly proportional to its volume used.

Question 19

What is the relationship between the rate of reaction and the time t taken?

Answer 19

Rate is directly proportional to 1/t as the inital rate of formation of the product is approximated by the average rate of formation of the product since the amount of product formed is small and t is measured from zero.

Question 20

Define rate determining step.

Answer 20

The rate determining step is the slowest step in a sequence of elementary reactions.

Question 21

What is an intermediate?

Answer 21

An intemediate is a species which is formed in the elementary reaction and used up in another. Hence they do not appear in the stoichiometric equation or rate equation.

Question 22

Define activation energy.

Answer 22

Activation energy is the minimum energy that the reactant particles must have to collide effectively to form products.

Question 23

Why are reactions with overall order 3 or higher tend to be slow?

Answer 23

Because it is difficult for 3 or more paricles to collide in the correct orientation with the necessary amount of energy in the rate determining step.

Question 24

How does surface area of reactants affect rate of reaction?

Answer 24

For a given mass of solid reactant, as particle size decreases, surface area per unit volume increases. Hence, the frequency of effective collisions increases and the rate of reaction increases.

Question 25

How does concentration of reactants affect rate of reaction?

Answer 25

As the concentration of a reactant increases, the reactant particles are closer together as the number of molecules per unit volume increases, Hence, frequency of effective collisions increases and the reaction rate increases.

Question 26

How does pressure of gaseous reactants affect rate of reaction?

Answer 26

Pressure changes in a gaseous system will affect the rate of reaction as the partial pressure of a gas is directly proportional to its concentration at a particular temperature. An increase in the partial pressure of a gaseous reactant leads to an increase in teh concentration of the reactant. Hence the frequency of effective collisions increases and the reaction rate increases.

Question 27

How does temperature affect rate of reaction?

Answer 27

When temperature increases, the average speed and hence average kinetic energy of the reacting particles increases, increasing the frequency of collisions. The number of reactant particles with energy greater than or equal to the activation energy increases. Hence, the freqeuncy of effective collisions increases and the reaction rate increases.

Question 28

How is the increase in rate of reaction due to an increase in temperature explained using a Maxwell-Boltzmann distribution curve?

Answer 28

The total area under the Maxwell-Boltxmann distribution curve is proportional to the total number of reactant particles present. The particles do not all have the same kinetic energy or speed. Only a small proportion of reactant particles possesses energy greater than or equal to the activation energy. Hence, only a small fraction of colliding reactant particles is able to lead to reaction. As temperature increases, the curve broadens and flattens out as the distribution of energies become more spread out. Since the total area under the curve must remain constant the peak shifts to the right and is lower than before.

Question 28

Define catalyst.

Answer 28

A catalyst is a substance which increases the rate of chemical reaction by providing an alternative reaction pathway with lower activation energy and remains chemically unchanged at the end of the reaction.

Question 29

Explain how a catalyst increases the rate of reaction.

Answer 29

When a catalyst is used, the reaction is able to proceed via a pathway of lower activation energy. The fraction of reacting particles having energy greater than or equal to the activation energy increases. The frequency of effective collisions increases and the reaction rate increases.

Question 30

Describe how a heterogenous catalysis works.

Answer 30

1. Reactant molecules are adsorbed (adhesion of a substance to the surface of another substance) onto the catalyst surface. The catalyst provides sites on its surface at which there can be formation of weak bonds between the reactant molecules and the surface catalyst atoms. 2. Adsorption weakens the covalent bonds within the reactant molecules, thereby reducing the activation energy for the reaction. 3. Adsorption also increases the concentration of reactant molecules at the catalyst surface and allows the reactant molecules to come into close contact and align in the correct orientation for successful reaction.

Question 31

What does the catalytic activity of a homogenous catalyst depend on?

Answer 31

It depends on its ability to exist in variable oxidation states.

Question 32

Define autocatalysis.

Answer 32

Autocatalysis is a process in which a product of a chemical reaction increases the rate of that reaction.

Question 33

Describe the characteristic of an autocatalysed reaction.

Answer 33

1. Reaction is initially slow due to lack of catalyst 2. Upon the formation of the product, the rate begins to increase more and more rapidly 3. The rate will reach a maximum as more of the product is formed. After this point, the rate slows down due to the decrease in concentration of reactants.

Question 34

Explain the relationship between enzymes, substrates and the rate of reaction.

Answer 34

For a fixed amount of enzymes, there is a limited number of active sites. Reaction is first order wrt the enzyme. At low substrate: There are avilable active sites for the substrate molecules to bind to, rate of reaction is directly proportional to [substrate]. (ie first order) As [substrate] increases, there are fewer active sites available for catalysis. Thus the increase in rate of reaction becomes smaller and reaction is no longer first order wrt the substrate At high substrate: Substrate molecules occupy all the active sites and the enzyme is saturated. Rate of reaction will be independent of [substrate] (ie zero order). The rate remains the same even though [substrate] increases.