8) Reaction kinetics

Question 1

Rate of reaction

Answer 1

• the speed at which a chemical reaction takes place and has the units mol dm⁻³ s⁻¹

Question 2

Collision frequency

Answer 2

number of collisions per unit time

Question 3

Catalyst

Answer 3

a substance that increases the rate of a reaction without taking part in the chemical reaction by providing the particles an alternative pathway with a lower activation energy

Question 4

Collision theory

Answer 4

• states that for a chemical reaction to take place the particles need to collide with each other in the correct orientation and with enough energy

Question 5

Table in notes

Question 6

Increase in reaction rate

Answer 6

• when more collisions per unit time take place, the number of particles with energy greater than the Ea increases and this causes an increase in rate of reaction
• a catalyst will increase the rate by providing the particles an alternative pathway with a lower activation energy

Question 7

Effect of concentration changes on the rate of reaction

Answer 7

• the more a concentrated a solution is, the greater the number of particles in a given volume of solvent
• an increase in concentration causes an increase in collision frequency and therefore an increase in rate of reaction

Question 8

Effect of pressure changes on the rate of reaction

Answer 8

• an increase in pressure in reactions that involve gases have the same effect as an increased concentration of solutions
• when the pressure is increased, the molecules have less space in which they can move which means the number of effective collisions increases due to an increased collision frequency
• an increase in pressure therefore increases the rate of reaction

Question 9

Rate of reaction calculation

Answer 9

change in amount of reactants or products (mol dm^-3) divided by time (s)
- draw tangents to figure this out and to make it even more accurate draw several of them at several points on the graph

Question 10

Activation energy (Ea)

Answer 10

minimum energy required for a collision to be effective

Question 11

Use Boltzmann distribution to explain significance in activation energy

Answer 11

• in a sample of a substance, a few particles will gave very low energy, a few particles will have very high energy and many particles will have energy in between
• a Boltzmann distribution curve shows that only a small proportion of molecules in the sample have enough energy for an effective collision and for a chemical reaction to take place

Question 12

Changes in temperature

Answer 12

• when the temperature of a reaction mixture is increases, the particles gain more kinetic energy which causes the particles to move around faster resulting in more frequent collisions
• the proportion of successful collisions increases, meaning a higher proportion of particles posses the Ea to cause a chemical reaction
• with higher temperatures the curve flattens and the peak shifts to the right

Question 13

An increase in temperature causes an increased rate of reaction due to:

Answer 13

• more effective collisions as the particles have more kinetic energy, making them move around faster
• a greater proportion of the molecules have kinetic energy greater than the Ea

Question 14

Catalysis

Answer 14

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

Question 15

Catalyst

Answer 15

• increases the rate of reaction by providing the reactants with an alternative reaction pathway which is lower in Ea than the uncatalysed reaction
• 2 types: homogeneous and heterogeneous

Question 16

Homogeneous
Heterogeneous

Answer 16

• the catalyst is in the same phase (state) as the reactants
• the catalyst is in a different phase (state) than the reactants

Question 17

Boltzmann distribution curve

Answer 17

• catalyst provides the reactants with an alternative pathway which has a lower Ea
• by lowering the Ea, a greater proportion of molecules in the reaction mixture have energy greater than the Ea
• the frequency of effective collisions increases
• the rate of the catalysed reaction increases compared to the uncatalysed reaction