Reaction Kinetics

Question 1

What is the rate of reaction?

Answer 1

The change in concentration of a particular reactant or product per unit time.

Question 2

What is instantaneous rate?

Answer 2

Rate at a particular time.

Question 3

What is initial rate?

Answer 3

Instantaneous rate when t=0/when an infinitesimally small amount of the reactant has been used up.

Question 4

What is average rate?

Answer 4

The change in concentration of a reactant or a product over that time interval.

Question 5

When can initial rate be approximated to average rate?

Answer 5

When the time interval is small enough and the time interval starts from 0.

Question 6

What is the relationship for the different rates of reaction?

Answer 6

aA + bB –> cC + dD

rate = (-1/a)(d[A]/dt) = (-1/b)(d[B]/dt) = (1/c)(d[C]/dt) = (1/d)(d[D]/dt)

Question 7

What is the continuous method?

Answer 7

Method to monitor the concentration of a reactant or product continuously over time.

Question 8

What is the clock reaction?

Answer 8

Measuring the time taken for a stated reaction to occur.

Question 9

What are the 6 continuous methods?

Answer 9

1. Sampling and titration
2. Measuring colour intensity
3. Measuring electrical conductivity
4. Measuring the volume of gas
5. Measuring the mass of the reaction mixture
6. Measuring the pressure

Question 10

Sampling and titration:

Answer 10

Used when reactant/product is acidic/basic or oxidising/reducing

1. Start the reaction by mixing the reactants and starting the stopwatch
2. Sampling: take out an aliquot at a suitable time
3. Quenching: at a specified time, stop the reaction by adding a large volume of ice water or a quenching agent
4. Titration to find the concentration
5. Repeat at regular time intervals
6. Plot a graph of volume of titrant used against time

Question 11

Measuring the colour intensity:

Answer 11

Used when there reactant or product is coloured
Concentration is directly proportional to colour intensity
1. Prepare a calibration curve
2. Measure the colour intensity at regular time intervals with a colorimeter
3. The rate of reaction can be determined from the change in colour intensity
4. Use the calibration curve to determine the concentrations

Question 12

Measuring electrical conductivity:

Answer 12

Used when there is a change in the total number of ions

1. Mix reactants
2. Monitor electrical conductivity with conductivity meter and two inert electrodes
3. A graph of electrical conductivity against time can be plotted

Question 13

Measuring the volume of gas:

Answer 13

Used when the reaction produces a gas

1. Use a graduated syringe to collect the gas and measure the volume at regular time intervals
2. Plot a graph of volume against time

Question 14

Measuring the mass of the reaction mixture:

Answer 14

Used when a gas is produced and allowed to escape
1. Allow the gas to escape
2. Measure the mass at regular time intervals
3. Plot a graph of mass loss against time
Weighing scale must be able to detect very small changes

Question 15

Procedure for reaction between thiosulfate ions and hydrogen ions:

Answer 15

1. Add known volumes of Na2S2O3 and HCl into a beaker
2. Start a stopwatch
3. Measure the time taken for a fixed amount of sulfur to form (when the cross is obscured)
4. Repeat with different volumes of the two solutions and water.

Question 16

Procedure for reaction between hydrogen peroxide and iodide ions in acidic medium:

Answer 16

1. Add a small amount of Na2S2O3 and known volumes of H2O2, I- and H+ to a beaker
2. Start the stopwatch
3. Measure the time taken for the blue-black to appear
4. Repeat with different volumes of H2O2, I- and H+

Question 17

What is the relationship between time t and initial rate of reactions in clock reactions?

Answer 17

Initial rate of reaction ∝ 1/t
Initial rate of reaction = initial rate of formation = average rate of formation (since t is small and measured from 0) = x/t
Since x is constant, initial rate ∝ 1/t

Question 18

What are the factors affecting rate of reaction?

Answer 18

1. Physical states of reactants
2. Concentration of reactants
3. Temperature
4. Catalyst

Question 19

How do physical states affect rate of reaction?

Answer 19

Solution vs solid: Solids react slower while solutions react faster
Molecules or ions: Activation energy is high when covalent bonds of molecules have to be broken while activation energy of ionic compounds is low as no covalent bonds need to be broken and ionic compounds dissociate, and oppositely charged ions have a natural tendency to attract each other.
Solid vs finely divided: When a solid is finely divided, SA to volume ratio is larger, the more contact it makes with the other reactant, the faster the reaction.

Question 20

How does concentration of reactants affect rate of reaction?

Answer 20

As concentration increases, particles come closer together, frequency of collisions increases, probability of a collision having the correct geometry and sufficient energy to occur increases, frequency of effective collisions increases, reaction rate increases.

Question 21

How does temperature affect rate of reaction?

Answer 21

Increasing temperature increases rate of reaction.

Question 22

How do catalysts affect rate of reaction?

Answer 22

It increases rate of reaction by providing an alternative path of reaction with a lower activation energy.