Chemical Kinetics I

Question 1

Reaction Kinetics is…

Answer 1

the study of rates of reaction and factors that influence rate

Question 2

Explain reaction kinetics;

Answer 2

Most chemical reactions are complex, involving many elementary reactions (steps)

Kinetics provides fundamental information regarding energy changes and molecular interactions

Kinetics can be use to deduce the mechanism of a reaction

Transition State: Highest energy on the energy curve – transient state whereby molecules can return to reactants or proceed to form products

Question 3

Kinetic Profile:

Answer 3

How the rate of a reaction varies as reaction proceeds

Question 4

Rate of a Reaction

Answer 4

Measure consumption of a reactant or formation of a product

At t=0, this is initial rate of reaction

Rate at a particular instant: draw tangent and calculate the gradient (rise over run)

Steepness of gradient is how fast the reaction is progressing

Question 5

Rate of a Reaction

Answer 5

Measure consumption of a reactant or formation of a product

At t=0, this is initial rate of reaction

Rate at a particular instant: draw tangent and calculate the gradient (rise over run)

Steepness of gradient is how fast the reaction is progressing

Question 6

What is a tangent?

Answer 6

A tangent is a straight line drawn so that it just touches the curve at the point of interest.

Question 7

What are Units for Rate?

Answer 7

moldm-3s-1 or Pas-1

Question 8

1 mol : 1 mol – At any time the rate of formation of product =

Answer 8

rate of consumption of reactant

Question 9

gradient is positive because the concentration _________ with time

Answer 9

increases

Question 10

List the 5 Factors Affecting Rate of Reaction =

Answer 10

1. Temperature of Reactants
2. Concentrations of Reactants
3. Chemical Nature of Reactants
4. State of Subdivision of the Reactants
5. Presence of a Catalyst

Question 11

Elementary Reactions

Answer 11

• single step reactions involving one or two molecules, ions or atoms
• Chemical equation represents the actual changes that take place at a molecular level
• The rate equation expresses the rate of the reaction under specific conditions Rate = k[H∙][Cl2]
• If the temperature is changed, the Rate of the Reaction changes and so k must change

Question 12

Order of a Reaction
[0, 1st,2nd,3rd]

Answer 12

The power to which the concentration of reactants must be raised to give a rate equation that describes the experimental data

Question 13

Differential Rate Equations

Answer 13

A rate equation where the rate of consumption of a reactant / formation of a product is written as a differential

Question 14

What is the different between rate equations and integrated rate equations?

Answer 14

Rate of reaction shows the rate of the reaction at a particular instant during the reaction while integrated shows how the concentration varies with time. (derived from differential rate equation). Equation - linking reactant conc at a particular time to conc at start of reaction.

Question 15

1st Order Elementary Reaction

Answer 15

A > products
If [A]0 and k are known, the concentration of the reactant / product, at any time can be calculated
If the concentrations during the reaction are measured, k can be calculated
Equation of a straight line: y = mx + c

Question 16

1st order reaction gradient;

Answer 16

x=y in proportion

Question 17

1st order Exponential Decay:

Answer 17

The concentration of the reactants decays as a logarithmic exponent with time
A constant fraction of the drug reacts per unit time

Question 18

2nd Order Elementary Reaction

Answer 18

A + A > products

Note: [A] approaches zero more slowly for 2nd order reactions

Question 19

2nd Order Elementary Reaction

Answer 19

where A + B > products

Question 20

Pseudo 1st Order Reaction

Answer 20

A + B > products

This is the method of isolating one reactant by having the other reactant in such a large excess.
If [A] ««< [B] then [B] hardly changed in the reaction, therefore [B] = [B]0

Question 21

1st Pseudo
RoR = k[A][B]

Answer 21

If k [Bo] then, RoR k’[A]

Question 22

What is the half life of a reactant?

Answer 22

the time taken for its concentration to fall to half of the initial value

Question 23

half life - first order

Answer 23

1st Order Reactions:
The half life of a reactant in a 1st order reaction is independent of its initial concentration, so t1/2 is constant through the course of the reaction

Question 24

half life - 2nd order

Answer 24

2nd Order Reactions:
The half life of the reactant in a second order reaction is inversely proportional to the initial concentration and to the rate constant, so t1/2 is “bigger” if [A]0 is lower in concentration. It is not constant