Chemical Kinetics (topic 6/16)

Question 1

Rate of Reaction

Answer 1

The change in concentration of reactants or products per unit time.

S.I. unit: mol dm^-3 s^-1

Question 2

Methods to measure the rate of a reaction

Answer 2

1. Change in volume method
2. Change in mass method
3. Change in transmission of light method
4. Change in concentration method
5. Change in conductivity method
6. Non-continuous methods

Question 3

Change in volume method (to measure rate of a reaction)

Answer 3

1. If the reaction whose rate you are measuring contains a gas that is evolved as a product, you can connect a gas syringe to the container where the reaction is taking place and measure the increase in volume per unit time.
2. This can be achieved through measuring the volume every 30 seconds or minute and then plotting a graph of volume against time.
3. This method can only work if a gas is formed as one of the products.
   Mg(s) + 2HCl(aq) -> MgCl2(g) + H2(g)
4. In this reaction, since both the products are gases, volume can be measured per unit time to obtain the rate of the reaction.

Question 4

Change in mass method (to measure rate of a reaction)

Answer 4

1. If there is change in mass of the reactants during the reaction, in cases where there is a gas that is evolved, the decrease in mass can be measured per unit time in order to obtain the rate of the reaction.
2. This can be achieved by placing the container in which the reaction is occuring on a digital balance and then measuring the mass change per unit time.

Question 5

Change in transmission of light method (to measure rate of a reaction)

Answer 5

1. This method can be used if one of the reactants or products is coloured. Coloured compounds absorb light of that is in the visible part of the electromagnetic spectrum.
2. Sometimes, an indicator can be added to the reaction if this method is to be used.
3. A colorimeter or photo spectrophotometer is used which works by passing light of a specific wavelength through the reaction container and measuring the intensity of light that is transmitted by the substances in the container.
4. As the concentration of the coloured compound increases, it absorbs more light and the intensity of the light transmitted from the substances decreases.
5. Likewise, as the concentration of a coloured compound decreases, it absorbs less light and the intensity of light that is transmitted from the substances increases.
6. The transmitted light can be reflected onto a charge couple device and the change in intensity of light can be measured per unit time.

Question 6

Change in concentration method (to measure rate of a reaction)

Answer 6

1. Titration can be used to measure the concentration of reactants or products per unit time.
2. By titrating a known volume and concentration of a substance against the product or reactant, we can calculate the exact amount of the concentration of the reactant or product at a given instant.
3. However, since the process of titration takes time, a method known as quenching is used where the reaction is stopped and a sample from the reaction can be obtained and titrated. This process can be repeated for several instances in time in order to understand how the concentration of the reactant or product changes per unit time.

Question 7

Change in conductivity method (to measure rate of a reaction)

Answer 7

Using electrolysis, we measure the total electrical conductivity of a solution. The total electrical conductivity is dependant on the ions and their charges. As reactant ions are converted into product compounds, the decrease in total electrical conductivity will tell us the change in the concentration of ions and therefore the change in the concentration of reactants per unit time, which tells us the rate of the reaction.

Question 8

Non-continuous methods (to measure rate of a reaction)

Answer 8

By measuring the time a reaction takes to reach a certain end point under different reaction conditions, we are able to determine the rate of the reaction. However, this rate of reaction is only an average rate and not a continuous rate.

Question 9

Temperature

Answer 9

The measure of average kinetic energy of the molecules. Therefore, the temperature of a substance (measure in Kelvin) is directly proportional to the kinetic energy of the particles.

Question 10

In order for a collision to be successful, there are two conditions that need to be met:

Answer 10

1. Energy of the particles (Ea)
2. Geometry of the collision

Question 11

Energy of the particles (Ea)

Answer 11

1. Activation energy is defined as the minimum amount of kinetic energy required for a collision to be successful.
2. It is the energy required to overcome repulsion between the molecules, and is denoted by Ea
3. Particles must have kinetic energy greater than the activation energy in order for the reaction to occur.

Question 12

Geometry of the collision

Answer 12

The collision geometry of the particles must also be correct in order for the collision to be successful.

Question 13

Factors affecting the rate of a reaction

Answer 13

1. Temperature
2. Concentration
3. Particle size
4. Pressure
5. Catalyst

Question 14

Temperature (Factors affecting the rate of a reaction)

Answer 14

If temperature increases, more particles gain kinetic energy greater than the activation energy, so there are more successful collisions, and therefore the rate of the reaction increases.

Question 15

Concentration (Factors affecting the rate of a reaction)

Answer 15

Increasing the concentration of the reactants increases the frequency of collisions, and so the frequency of successful collisions increases and therefore the rate of reaction increases.

Question 16

Particle size (Factors affecting the rate of a reaction)

Answer 16

If the particle size decreases, there is more surface area, leading to more frequent collisions, so the frequency of successful collisions increases and so the rate of reaction increases.

Question 17

Pressure (Factors affecting the rate of a reaction)

Answer 17

At higher pressures, a gas is compressed and therefore its concentration increases, so the frequency of collisions increases and the frequency of successful collisions also increases, and so rate of reaction increases.

Question 18

Catalyst (Factors affecting the rate of a reaction)

Answer 18

1. A catalyst is a substance that increases the rate of a reaction without being changed itself.
2. A catalyst speeds up the rate of the reaction by decreasing the activation energy for the reaction, so more particles have kinetic energy greater than the activation energy, therefore more successful collisions take place and the rate of reaction increases.

Question 19

Rate Expression

Answer 19

Also known as rate law for a reaction shows the relationship between the concentration of one or more reactants and the rate of the reaction.

Question 20

Zero order (rate expression)

Answer 20

• Changing concentration [R] has no effect on the rate of the reaction.
• Doubling concentration [R] has no effect on the rate of the reaction.

Question 21

First order (rate expression)

Answer 21

• Changing concentration [R] has an equal effect on the rate of the reaction.
• Doubling concentration [R] will double the rate of the reaction.

First order reactions have a constant half life (t1/2) - the time taken for concentration of a reactant to decrease by half is always the same.

Question 22

Second order (rate expression)

Answer 22

• Changing concentration [R] has a doubly proportional effect on the rate of the reaction.
• Doubling concentration [R] will quadruple the rate of the reaction.

Question 23

Orders are shown in the following way:

Answer 23

As exponents of the concentrations of the reactants.

1. zero order - [R]^0 (or not shown)
2. first order - [R]^1 (or simply [R])
3. second order - [R]^2

The overall order of the reaction is the sum of the individual orders of the reaction, which are the powers

Question 24

Units of the rate constant depend on the order of the reaction:

Answer 24

1. zero order : mol dm^-3 s^-1
2. first order : s^-1
3. second order : mol^-1 dm^3 s^-1
4. third order : mol^-2 dm^6 s^-1

Question 25

Rate constant

Answer 25

The proportionality of the rate and the concentrations of the reaction can be represented by a constant k.

The value of the rate constant k is temperature dependent.

Question 26

Reaction Mechanism

Answer 26

1. Many reactions do not occur in a single step. In these cases, the rate of the slowest step determines the rate of the overall reaction.
2. This step is therefore called the rate determining step (RDS), as the reaction can only proceed as fast as this step does.
3. The activation energy of the reaction is equal to the activation energy of the RDS.
4. Catalysts are involved in the RDS, by altering the reaction by introducing a step with lower activation energy, instead of the high activation energy of the RDS.
5. Any substance that is formed in an elementary step but is not present in the overall reaction equation is called an intermediate.
6. The molecularity of a step is the number of reactant particles taking part in that step.
7. Steps can be unimolecular or bimolecular (termolecular reactions are very rare as a result of the high improbability of three particles colliding with sufficient energy and with the right orientation).

Question 27

The Arrhenius equation

Answer 27

k = Ae^ (-Ea / RT)

Introducing the natural logarithm (ln) on both sides yields the equation
ln k = (-Ea / RT) + ln A

A is called the Arrhenius constant, frequency factor, or pre-exponential factor, and it takes into account the frequency of collisions with proper orientations.

Question 28

Arrhenius plot

Answer 28

Plotting a graph of 1/T on the x-axis against ln(k) on the y-axis will give a straight line with slope (-Ea / R) and y-intercept ln A.

The activation energy can be calculated from an Arrhenius plot by calculating the gradient and multiplying it by R (the gas constant)

As the temperature increases, the value of k increases

To calculate the value of activation energy when the value of k at two different temperatures is given, simultaneous equations are formed, which result in:

ln(k1 / k2) = Ea / R (1 / T2 - 1 / T1)