Topic 6 : Chemical Kinetics

Question 1

kinetic theory of matter

Answer 1

The kinetic theory of matter accounts for the properties of solids, liquids, and gases in terms of the interactions of particles and their relative energies

Question 2

Kinetic Energy

Answer 2

1. KE = 1/2mv^2
2. The average kinetic energy of the particles is directly proportional to the temperature of the system in Kelvin
3. Kinetic energy refers to the energy associated with movement or motion.
   4.Substances with a lower mass diffuse more quickly than those with greater mass at the same temperature

Question 3

Collision Theory

Answer 3

1. If a chemical reaction is to take place between two particles, they must first collide
2. The number of collisions between particles per unit time in a system is known as the collision frequency
3. The collision frequency of a given system can be altered by changing the concentration of the reactants, by changing the total pressure, by changing the temperature or by changing the size of the reacting particles

Question 4

Collision Energy

Answer 4

1. Not all collisions will create a reaction as the particles may just bounce off each other. This is known as unsuccessful collision
2. Unsuccessful collisions happen when the colliding species do not have enough energy to break the necessary bonds
3. If they do collide, the combined energy of the colliding particles is known as the collision energy

Question 5

Activation Energy

Answer 5

1. The minimum energy the colliding particles need in order to react is known as the activation energy
2. If the collision energy of the colliding particles is less than the activation energy, the collision will be unsuccessful
3. If the collision energy is equal to or greater than the activation energy, the collision will be successful, and a reaction will take place
4. The activation energy can be changed by the addition of a catalyst

Question 6

Collision Geometry

Answer 6

1. Particles have to have the right orientation when they collide for the reaction to be successful
2. Most collisions do not result in reaction because they do not reach the activation energy rather than not having the correct collision geometry

Question 7

Reaction rate

Answer 7

1. As a chemical reaction proceeds, the concentration of the reactants decreases and the concentration of the products increases
2. The rate of a reaction is the speed at which a chemical reaction takes place and has units mol dm-3 s-1
3. Rate of reaction = Change in amount of reactants or products/time
4. Reactants curve downwards, products curve upwards

Question 8

Ways to measure rates of reaction

Answer 8

1. Mass loss
2. Gas production
3. Colourimetry

Question 9

Changes in colour

Answer 9

1. A colorimeter measures the amount of light that passes through a solution
2. If a solution changes colour during a reaction this can be used to measure the rate
3. The intensity of light reaching the detector is measured every few seconds and the data is plotted to show how the concentration of the reactants or products changes with time
4. The light intensity is related to the concentration, so the graph represents a graph of concentration of products or reactants against time

Question 10

Change in mass

Answer 10

1. When a gas is produced in a reaction it usually escapes from the reaction vessel, so the mass decreases
2. For example, the reaction of calcium carbonate with hydrochloric acid produces CO2. The mass is measured every few seconds and change in mass over time is plotted as the CO2 escapes
3. However, one limitation of this method is the gas must be sufficiently dense or the change in mass is too small to measure on a 2 or 3 dp balance

Question 11

Volume of gases

Answer 11

1. When a gas is produced in a reaction, it can be trapped and its volume measured over time
2. For example, the reaction of magnesium with hydrochloric acid produces hydrogen
3. An alternative gas collection set up involves collecting a gas through water using an inverted measuring cylinder ( as long as the gas is not water soluble)
4. The volume can be measured every few seconds and plotted to show how the volume of gas varies with time

Question 12

Measuring concentration changes

Answer 12

1. Measuring concentration changes during a reaction is not easy; the act of taking a sample and analysing it by titration can affect the rate of reaction (unless the reaction is deliberately stopped- this is called quenching).
2. Often it is more convenient to ‘stop the clock’ when a specific (visible) point in the reaction is reached. For example when a piece of magnesium dissolves completely in hydrochloric acid
3. The main limitation here is that often it only generates one piece of data for analysis

Question 13

Activation Energy in exothermic and endothermic reactions

Answer 13

1. In exothermic reactions the reactants are higher in energy than the products
2. In endothermic reactions the reactants are lower in energy than the products
3. Therefore, the activation energy in endothermic reactions is relatively larger than in exothermic reaction

Question 14

Catalysts

Answer 14

1. A catalyst increases the rate of a reaction by providing the reactants with an alternative reaction pathway which is lower in activation energy than the uncatalysed reaction
2. By decreasing Ea, a catalyst increases the rate of a chemical reaction, without itself being permanently chemically changed.
3. More particles are able to collide with sufficient energy to react under the lower activation energy
4. More frequent, successful collisions lead to a faster rate of reaction

Question 15

Concentration affecting rate of reaction

Answer 15

1. The more concentrated a solution is, the greater the number of particles in a given volume of solvent
2. An increase in concentration causes in an increased collision frequency and therefore an increased rate of reaction

Question 16

Pressure affecting rate of reaction

Answer 16

1. An increase in pressure in reactions that involve gases has the same effect as an increased concentration of solutions
2. When the pressure is increased, the molecules have less space in which they can move
3. This means that the number of effective collisions increases due to an increased collision frequency
4. An increase in pressure therefore increases the rate of reaction

Question 17

Temperature affecting rate of reaction

Answer 17

1. Increasing the temperature of the reaction mixture increases the rate of reaction in two ways
2. At higher temperatures, the particles are moving faster, so collide more frequently. A higher number of collisions in total mean a higher number of successful collisions
3. At higher temperatures, a higher proportion of the molecules have the activation energy or more. This means that a higher proportion of collisions are successful

Question 18

Particle Size affecting rate of reaction

Answer 18

1. Only the particles on the surface of a solid will collide with particles of the other reactant
2. If the surface area is increased, more particles will be on the surface and able to collide with particles of the other reactant
3. This means that there will be more collisions in total and therefore more successful collisions.

Question 19

Boltzmann distribution curve

Answer 19

1. The Boltzmann distribution curve is a graph that shows the distribution of energies at a certain temperature
2. In a sample of a substance, a few particles will have very low energy, a few particles will have very high energy, and many particles will have energy in between
3. The graph shows that only a small proportion of molecules in the sample have enough energy for an effective or successful collision and for a chemical reaction to take place

Question 20

Changes in temperature affecting Boltzmann distribution curve

Answer 20

1. When the temperature of a reaction mixture is increased, the particles gain more kinetic energy
2. This causes the particles to move around faster resulting in more frequent collisions
3. Furthermore, the proportion of successful collisions increases, meaning a higher proportion of the particles possess the minimum amount of energy (activation energy) to cause a chemical reaction
4. With higher temperatures, the Boltzmann distribution curve flattens and the peak shifts to the right

Question 21

Effect of a catalyst on the Maxwell-Boltzmann distribution curve

Answer 21

1. A catalyst provides the reactants another pathway which has a lower activation energy
2. By lowering Ea, a greater proportion of molecules in the reaction mixture have sufficient energy for a successful collision
3. As a result of this, the rate of the catalysed reaction is increased compared to the uncatalyzed reaction

Question 22

Catalysis

Answer 22

Catalysis is the process in which the rate of a chemical reaction is increased, by adding a substance called a catalyst

Question 23

Types of catalysts

Answer 23

1. Homogeneous catalysts
2. Heterogeneous catalysts

Question 24

Homogeneous catalysts

Answer 24

1. Homogeneous means that the catalyst is in the same phase as the reactants
2. For example, the reactants and the catalysts are all liquids

Question 25

Heterogeneous catalysts

Answer 25

1. Heterogeneous means that the catalyst is in a different phase to the reactants
2. For example, the reactants are gases but the catalyst used is a solid