Chemical Kinetics

Question 1

How do you calculate rate of reaction?

Answer 1

Concentration/ time

Question 2

Define rate of reaction?

Answer 2

The change of the concentration of a reactant or product with respect to time

Question 3

Define order of a reaction (with respect to a reactant)?

Answer 3

The power to which the concentration of a reactant is raised in the rate equation

Question 4

Define order of a reaction (overall) ?

Answer 4

The sum of the powers to which the concentration terms are raised in the rate equation

Question 5

Define rate constant?

Answer 5

The proportionality constant which links the rate of the reaction to the concentrations in the rate equations

Question 6

Define rate determining step?

Answer 6

The slowest step in the mechanism of a reaction

Question 7

How is the rate of formation and rate of removal obtained?

Answer 7

Multiplying the rate of reaction by the amount of moles of a substance in a chemical equation (front number)

Question 8

What is the initial rate and the average rate of reaction?

Answer 8

Initial rate is when the reaction begins t=0 used to compare rates of chemical reactions calculated by drawing tangent to curve at t=0 and using concentration/time

Average rate - average speed reactants are converted to products calculated from whole graphs concentration/ graphs time

Question 9

Measuring rates (Mr) gas volume?

Answer 9

Reaction has to use or produce a gas
A graduated gas syringe is used to monitor gas produced
The initial rate is obtained by plotting a graph of volume against time and calculating the slope of the tangent to the plot at t=0

Question 10

Mr change in mass?

Answer 10

Electronic balance used
Rate of reaction determined by drop in mass as gas is released

Question 11

Mr colorimetry?

Answer 11

Reaction between bromine and methanol acid
ROR determined by monitoring the amount of light absorbed by bromine
Amount of light absorbed is recorded at regular intervals and converted into a concentration using a calibration curve
The initial rate of the reaction can then be determined by plotting the concentration of bromine against time, and calculating the slope of the plot at time, t=0

Question 12

Mr titration

Answer 12

Progresses of the Reaction between propanone and iodine
Take sample from reaction and quench it by adding an excess of water /rapid cooling then titrate the H ions against a standard alkali

The amount of iodine present can be determined by quenching the sample of the reaction mixture and titrating the iodine present in sample against a standard solution of sodium thiosulfate Na2S2O3 using starch as an indicator.
The starch forms a blue-black complex with iodine which is replaced by a colourless solution at the end point of the titration

Question 13

Mr precipitation ?

Answer 13

Reaction between propanone and iodine
Take a sample and quench
Add excess acidificied silver nitrate to precipitate the iodine ions present as solid silver nitrate
Iodine ions present is identified by filtering the mixture and washing the precipitate with water then drying and weighing

Question 14

What units is rate and concentration in?

Answer 14

Rate = Moldm-3s-1
Concentration = Moldm-3

Question 15

Mr pH?

Answer 15

If H+ or OH- ions are present the change in pH can be measured using a pH meter
Graph plotted pH against time
Initial rate is determined by drawing a tangent at t=0 and calculating its gradient the gradient is the initial rate of reaction

Question 16

How do you find the rate constant k?

Answer 16

K is obtained by inserting values for the initial rate and corresponding concentrations into the rate equation -> rate = k(concentrations)

Question 17

How does temperature affect the rate constant?

Answer 17

Increasing the temperature increases the number of particles with at least the activation energy as a result, there is an increase in collisions involving particles with enough energy to react and the number of successful collisions per second increases. Therefore the rate of reaction increases

Question 18

How does adding a catalyst affect the rate constant?

Answer 18

Adding a catalyst lowers the activation energy resulting in a greater number of particles which have enough energy to react. Therefore the number of successful collisions per second increases. Therefore the rate of reaction increases and the rate constant for the reaction increases.

Question 19

What is a catalyst?

Answer 19

A substance that speeds up a reaction by providing an alternative pathway of lower activation energy and it is not consumed by the reaction (used up)

A catalyst is regenerated in its original form as the reaction proceeds

Question 20

Explain hydrolysis of primary halogenalkanes?

Answer 20

It is a Sn2 and contains a transition state
Both the primary halogenalkane the hydroxide ion are involved in the rate determining step!
- The hydroxide ion approaches the halogen alkane and uses its loan pair to begin, forming a coordinate bond with the electron deficient carbon in the carbon halogen bond
-Coordinate bond forms the carbon halogen bond begins to Break, and this process continues until the coordinate bond is fully formed, and the carbon halogen bond has undergone heterolytic fishing producing a halogen ion
-Transition state is not stable and cannot be isolated as a compound

Question 21

Explain the hydrolysis of a tertiary halogenalkane?

Answer 21

It is a Sn1 reaction and only contains the tertiary halogenalkane in the rate determining step
- The first step involves heterolytic fission of the carbon halogen bond to form a tertiary carbocation
-the hydroxide ion, then acts as a nucleophile by forming a coordinate bond with the electron deficient carbon in the carbon cation
Hydroxide ion is a much more effective nucleophile than water as it is more strongly attracted to the electron deficient carbon atom therefore the hydrolysis of halogenalkane is significantly faster when a small amount of alkaline is added to the reaction mixture

Question 22

What is the mechanism called for the hydrolysis of primary and tertiary halogenalkanes?

Answer 22

Nucleophilic substitution

Question 23

What is the molecuarity?

Answer 23

The number of species involved in the rate determining step
The overall order of the hydrolysis of primary halogen alkane is two so the molecurlarity is two
The overall order of the hydrolysis of tertiary halogen alkanes is one so the molecurlarity is one