unit 3 KA2 - controlling the rate

Question 1

what must be controlled in industrial processes

Answer 1

reaction rates

Question 2

what may happen if the reaction rate is too low in industrial processes

Answer 2

the process will not be economically viable

Question 3

what may happen if the reaction rate is too high in industrial processes

Answer 3

there will be risk of explosion

Question 4

what relationship may be used to perform calculations into reaction rate

Answer 4

the one between reaction time and relative rate with appropriate units

Question 5

collision theory can be used to explain the effects of factors on reaction rates. what are these factors?

Answer 5

• concentration
• pressure
• surface area (particle size)
• temperature
• collision geometry

Question 6

what can be used to show the energy pathway for a reaction

Answer 6

a potential energy diagram

Question 7

what is meant by the enthalpy change

Answer 7

this is the energy difference between the products and the reactants.

Question 8

when does the enthalpy value have a negative value

Answer 8

for exothermic reactions

Question 9

when does enthalpy change have a positive value

Answer 9

for endothermic reactions

Question 10

what is the activation energy

Answer 10

the minimum energy required by colliding particles to form an activated complex

Question 11

how can activation energy be calculated

Answer 11

from potential energy diagrams

Question 12

what is the activated complex

Answer 12

an unstable arrangement of atoms formed at the maximum of the potential energy barrier, during a reaction

Question 13

how do catalysts speed up chemical reactions

Answer 13

they provide an alternative reaction pathway with a lower activation energy

Question 14

what can be used to show the effect of a catalyst on activation energy

Answer 14

a potential energy diagram

Question 15

what is temperature

Answer 15

a measure of the average kinetic energy of the particles in a substance

Question 16

what is the activation energy IN TERMS OF TEMPERATURE

Answer 16

the MINIMUM kinetic energy required by colliding particles before a reaction may occur

Question 17

what can be used to explain the effect of changing temperature on the kinetic energy of particles and reaction rate

Answer 17

energy distribution diagrams

Question 18

how can the effects of temperature and adding a catalyst be explained

Answer 18

in terms of a change in the number of particles with energy greater than the activation energy

Question 19

why is the activation energy required to start a reaction

Answer 19

it breaks the bonds in the colliding particles

Question 20

what are the two conditions to allow a reaction to occur

Answer 20

• correct geometry

- activation energy

Question 21

why does an increase in particle size increase the reaction rate

Answer 21

as the surface area of a reactant increases, the area of contact between the reactants increases and this increases the rate of collisions between the reactant particles. there will therefore be more successful collisions and so the overall reaction rate will increase

Question 22

why does an increase in concentration of the reactants increase the reaction rate

Answer 22

as the concentration of a reactant increases the reactant particles are pushed closer together which increases the rate of collisions. this gives rise to more successful collisions and results in the reaction rate to increase

Question 23

why does an increase in temperature increase the reaction rate

Answer 23

temperature can be regarded as a measure of the average kinetic energy of particles in a substance. at higher temperatures particles have more kinetic energy and will therefore collide with greater force and more particles will have the required activation energy

Question 24

in terms of bond breaking, state why a reaction is exothermic

Answer 24

in an exothermic reaction, the total energy for the bond breaking steps is less than the energy released in the bond making steps

Question 25

in terms of bond breaking, state why a reaction is endothermic

Answer 25

in an endothermic reaction, the total energy for the bond breaking steps is greater than the energy released in the bond making steps

Question 26

do exothermic reactions have positive or negative enthalpy change

Answer 26

negative

Question 27

do endothermic reactions have positive or negative enthalpy change

Answer 27

positive

Question 28

do catalysts have any impact on the enthalpy change of a reaction

Answer 28

no