3.1.5 Kinetics Knowledge

Question 1

what is collision theory?

Answer 1

Chemical reactions occur when particles of substances collide at energy greater than or equal to the activation energy of the reaction, with the correct particle orientation

Question 2

why are reaction conditions important in collision theory?

Answer 2

the conditions of a reaction impact the collisions of the particles, and can be changed to increase the likelihood of a collision occurring with sufficient energy to react in order to increase rate

Question 3

how can you increase the rate of a reaction, using collision theory?

Answer 3

• Decrease volume of container or increase concentration
• Increase number of particles
• Increase kinetic energy of particles by increasing temperature
• Break up clumps into individual particles
• Use a catalyst

Question 4

why does Breaking up clumps into individual particles increase the rate of a reaction?

Answer 4

Increases surface area of particles, to increase available areas to collide and react

Question 5

what is the Maxwell-Boltzmann Curve?

Answer 5

shows the distribution of molecular energies in a gas at a constant temperature

Question 6

draw an annotated diagram of a general maxwell-boltzmann curve

Answer 6

Question 7

what are the 3 main features of a maxwell-boltzmann curve?

Answer 7

• the area under the curve is equal to the total number of molecules in the sample
• the curve starts at the origin; there are no molecules in the system with 0 energy
• Only molecules with an energy greater than the activation energy can react

Question 8

what is the area under a maxwell-boltzmann curve equal to?

Answer 8

the total number of molecules in the sample

Question 9

why does the curve starts at the origin on a maxwell-boltzmann distribution?

Answer 9

there are no molecules in the system with 0 energy

Question 10

how does increasing temperature affect a maxwell-boltzmann curve?

Answer 10

the peak moves to the right and is lower, whilst the area under the curve remains the same

Question 11

how does increasing temperature affect the rate of a reaction?

Answer 11

• When a substance is heated, thermal energy is transferred to it; this energy is converted into kinetic energy
• The molecules move faster as they have more kinetic energy
• A greater proportion of the molecules have an energy that is greater than the activation energy
• Collisions will occur more often and with greater energy; more successful collisions occur in a given length of time
• the rate of reaction will increase

Question 12

draw a maxwell-boltzmann distribution cirve showing the effect of temperature

Answer 12

Question 13

how does increasing concentration/pressure affect a maxwell-boltzmann curve?

Answer 13

shifted to the right

Question 14

how does increasing concentration/pressure affect the rate of a reaction?

Answer 14

• More molecules of substance occur in the same volume, so they are packed closer together
• collisions between molecules become more likely
• the chances of a collision occurring with energy greater than the activation energy and the correct particle orientation increases
• the rate of reaction increases

Question 15

draw a maxwell-boltzmann distribution cirve showing the effect of concentration

Answer 15

Question 16

how does adding a catalyst affect a maxwell-boltzmann curve?

Answer 16

the addition of a catalyst does not change the distribution of the molecular energies

Question 17

how does adding a catalyst affect the rate of a reaction?

Answer 17

• Catalysts provide an alternative reaction route which requires a lower activation energy
• more molecules in the system have an energy in excess of the new, lower activation energy
• increases the likelihood that collisions occur at sufficient energy; more successful collisions occur in a given length of time
• the rate of reaction will increase

Question 18

draw a maxwell-boltzmann distribution cirve showing the effect of adding a catalyst

Answer 18

Question 19

what are the units of activation energy?

Answer 19

kJ mol^-1

Question 20

define collision frequency

Answer 20

Number of collisions between particles per unit of time

Question 21

define collision energy

Answer 21

Combined energy of colliding particles

Question 22

describe the movement of particles in a given system

Answer 22

Constant random motion

Question 23

define Emp

Answer 23

Most probable value for the energy of particles

Question 24

define Ek

Answer 24

kinetic energy of the particles

Question 25

define E≥Ea

Answer 25

Energy greater than or equal to activation energy

Question 26

If the temperature was increased, what happens to Ea?

Answer 26

stays the same

Question 27

If the temperature was increased what happens to Emp?

Answer 27

position moves to the right

Question 28

If the temperature is increased, what happens to the size of the area under the curve?

Answer 28

remains unchanged

Question 29

When the reaction is heated to a higher temperature, explains what happens to the number of particles having the most common amount of energy

Answer 29

fewer particles with Emp

Question 30

When the reaction is heated to a higher temperature, explains what happens to the number of particles with E≥Ea

Answer 30

more particles with E≥Ea

Question 31

define activation energy

Answer 31

the minimum energy needed for a reaction to occur/start

Question 32

define the rate of reaction

Answer 32

the rate of change in concentration per unit of time in mol dm^-3 s^-1

Question 33

what is a catalyst?

Answer 33

a substance that increases the rate of reaction but is not used up or changed, by providing an alternative reaction pathway that has a lower Ea

Question 34

what will happen if a small number of particles have energy greater than or equal to the activation energy?

Answer 34

the reaction will occur slowly

Question 35

why do most collisions not cause a reaction?

Answer 35

a small number of particles have energy greater than or equal to the activation energy

Question 36

If a reaction occurs slowly because only a small number of particles have energy greater than or equal to the activation energy, why will all of the reactants eventually gain enough energy to react?

Answer 36

molecules gain energy due to collisions

Question 37

why do some particles only have a very small amount of energy?

Answer 37

collisions cause some molecules to slow down or lose energy

Question 38

how does an increase in temperature affect the rate of reaction?

Answer 38

if temperature increases, more particles have E≥Ea, which increases the frequency of successful collisions

Question 39

how does an increase in concentration affect the rate of reaction?

Answer 39

if concentration increases, there is an increase in number of particles per unit volume, which increases the frequency of successful collisions

Question 40

how does an increase in pressure affect the rate of reaction?

Answer 40

if pressure increases, there is an increase in number of particles per unit volume, which increases the frequency of successful collisions

Question 41

how does an addition of catalysts affect the rate of reaction?

Answer 41

adding a catalyst lowers Ea, so more particles have E≥Ea, which increases the frequency of successful collisions

Question 42

how does an increase in surface area affect the rate of reaction?

Answer 42

if surface area is increased, there is an increased number of reactant particles made available, which increases the frequency of successful collisions

Question 43

what does the peak of the maxwell boltzmann curve show

Answer 43

the peak of the curve shows the most probable value for the energy of the particles (Emp)

Question 44

what does the area under the maxwell boltzmann curve to the right hand side of Ea represent

Answer 44

the particles with enough energy to react

Question 45

where is the Emp on a maxwell boltzmann curve

Answer 45

the mean energy of particles is a line where the area under the curve on either side of the line is equal

Question 46

how does increasing the number of particles affect the maxwell boltzmann curve

Answer 46

- The Emp and Ea would stay the same - the total area under the graph would increase

Question 47

why do Most chemical reactions begin with a fast rate of reaction and slow down before the reaction stops

Answer 47

- Initially there is a high concentration of reactants, so a large frequency of successful collisions - As the reactants start to get used up, successful collisions become less frequent - Once one or more reactants are used up, there can be 0 successful collisions

Question 48

how can you estimate the rate of a reaction from a graph

Answer 48

use the gradient