Unit 3 - Controlling the rate

Question 1

Why must chemists controll the rate of reaction?

Answer 1

Because if ROR is too low then manufactoring will not be economically viable but if ROR is too high there is a risk of explosion.

Question 2

For a chemical reaction to occur what must happen.

Answer 2

reactant molecules must collide

Question 3

Why must the energy required be at sufficient levels?

Answer 3

so that the bonds in the reactant molecules can be broken, and then new chemical bonds can form to create product molecules.

Question 4

What is the criteria for a successful collision?

Answer 4

• Particles must collide with sufficient energy.
• Particles must collide with correct alignment.

Question 5

Effect of Surface Area on ROR (for solids)

Answer 5

if the particle size is smaller then there is a larger number of particles on the surface of the solid that are able to take part in collisions.

Question 6

Effect of Pressure on ROR (for gases)

Answer 6

if the pressure is increased, then there will be more reacting particles in a given volume, which means that more collisions will take place between these reaction particles.

Question 7

Effect of concentration on ROR

Answer 7

at higher concentrations there are more reacting particles in a given volume which means more collisions will take place and therefore the reaction is faster.

Question 8

equation for relative rate of reaction

Answer 8

1/time

Question 9

Exothermic reaction

Answer 9

Causes heat to be released to the surroundings and increases temperature

Question 10

Endothermic reaction

Question 11

Enthalpy

Answer 11

energy content or ‘chemical potential energy’

Question 12

enthalpy units

Answer 12

Joules (J) or KiloJoules (kJ)

Question 13

molar enthalpy units

Answer 13

kilojoules per mole (kJ mol -1)

Question 14

How do you find the enthalpy change

Answer 14

by subtracting the enthalpy content of the reactants from that of the products

Question 15

What enthalpy change do exothermic reactions have

Answer 15

Negative because the products have less energy than the reactants

Question 16

What enthalpy change do endothermic reactions have

Answer 16

Positive because the products have more energy than the reactants

Question 17

Activation energy

Answer 17

the minimum energy required for a reaction to occur

Question 18

the activated complex

Answer 18

it is an unstable arrangement of atoms formed at the maximum of the potential energy diagram for a reaction

Question 19

how does the activated complex form

Answer 19

can form as a result of a successful collision between reactant molecules.

Question 20

What is happening on the Maxwell-Boltzmann diagram?

Answer 20

The number of particles that don’t have enough energy to react is represented by the unshaded large curve, and the shaded area represents the particles that have high-enough energies to react.

Question 21

How do you speed up the reaction?

Answer 21

you need to increase the number of the very energetic particles, those with energies equal to or greater than the activation energy.

Question 22

temperature

Answer 22

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

Question 23

Catalyst

Answer 23

a substance which alters the rate of reaction but is chemically unchanged at the end of a reaction.

Question 24

what does the catalyst do in the reaction

Answer 24

provides an alternitave route for the reaction which has a lower activation energy.

Question 25

Alterinative path - Adsorption

Answer 25

• Molecules bond to catalyst at active sites
• Weaking the intra-molecular bonds

Question 26

Alterinative path - Reaction

Answer 26

• Reaction takes place on the catalyst surface.
• More favourable collision geometry.
• Less activation energy needed to break weakened bonds

Question 27

Alterinative path - Desorption

Answer 27

• Product molecules leave active sites vacant allowing another reaction cycle.