U1.A: Rates of Reaction

Question 1

Collision theory

Answer 1

For a chemical reaction to occur, reactant particles must…
1. collide with enough energy to react (ACTIVATION ENERGY)
2. must have the correct collision geometry (correct orientation in space)

Question 2

Factors affecting rate of reaction:

Answer 2

1. Concentration
2. Particle size
3. Pressure
4. Temperature
5. Catalyst

Question 3

Concentration

Answer 3

Increasing, increases number of particles in a given volume. This results in more collisions, therefore, increasing rate of reaction.

Question 4

Pressure

Answer 4

Increasing the pressure in a reaction involving gases, reduces the volume for particles to move around in. This results in more collisions, therefore increases reaction rate.

Question 5

Surface area

Answer 5

Reactants of solids can only take place at the surface of the solid. Smaller particles have a larger surface area. This results in more collisions, therefore increases reaction rate.

Question 6

Temperature

Answer 6

Particles must collide with a minimum energy to react, this is called the activation energy. Heating particles given them more kinetic energy, causing an increase in the number of particles with energy greater than EA. This results in a greater number of successful collisions, therefore increases the reaction rate.

Question 7

Rate of reaction calculation

Answer 7

Rate = 1/tim

unit: (s-1)

Question 8

Activation energy

Answer 8

Minimum kinetic energy required by colliding particles before a reaction can occur. (note: does not guarantee successful collision particles also need correct geometry)

Question 9

Distribution Energy Graph

Answer 9

No of molecules with energy against kinetic energy (KJ)

2 bell graphs one more right than other, EA line drawn constant and E>EA shaded in

Faster the reaction the more left EA will be

Question 10

Catalyst

Answer 10

A catalyst alters the rate of a reaction, allowing it to be done at a lower temperature. Catalysts are therefore used in the chemical industry to make manufacturing processes more economical.

Question 11

Catalyst example in industry

Answer 11

Iron – used to make ammonia by the Haber Process

Platinum – used in manufacture of nitric acid (Ostwald process)

Rhodium and Platinum - in catalytic converters

Nickel – to make margarine by hardening vegetable oil

Vanadium (V) Oxide – in the contact process, to make sulphuric acid

Question 12

rate against concentration diagram

Answer 12

straight line through origin, rate and concentration are directly proportional

Question 13

exothermic

Answer 13

release of energy, usually in the form of heat energy, but also sometimes light and sound energies.

reactants - products + ENERGY

Question 14

endothermic

Answer 14

a smaller number of changes involve absorption of energy from the surrounding from the surroundings

reactants + ENERGY - products

Question 15

overall energy change in going from a set of reactant to products is called…

Answer 15

enthalpy change (deltaH)

Question 16

Delta H

Answer 16

H (products) - H (reactants)

Question 17

exothermic reactions where…

Answer 17

H Reactants > H products

negative delta H values

Question 18

Endothermic reactions where…

Answer 18

Reactants < products

have positive delta H values

Question 19

Activation energy forward exothermic reaction

Answer 19

First line to peak (C-B)

Question 20

Enthalpy change forwards for exothermic reaction

Answer 20

Bottom of second line to first line (A-B)

Question 21

EA reverse reaction for exothermic

Answer 21

C-A peak take away bottom last line

Question 22

enthalpy change of reverse reaction

Answer 22

B-A first line take away last line

Question 23

Endothermic forward reaction

Answer 23

C-A peak take away first bottom line

Question 24

Endothermic forward enthalpy change

Answer 24

B-A last top line take away first bottom line

Question 25

EA reverse reaction endothermic reaction

Answer 25

C-B peak take away last top line

Question 26

endothermic reverse enthalpy change

Answer 26

A-B bottom first line take away top last line

Question 27

The peak in the energy profile diagram denotes a point of high energy, which must correspond to the formation of a species of

Answer 27

low stability

Question 28

transition or activated complex: our theory copes with this by proposing that…

Answer 28

Reactions go through a transition stage, following collision, during which a highly unstable ‘activated’ complex forms.

Question 29

Effectiveness of a surface catalyst

Answer 29

Absorption at surface increases concentration of reactants. Absorption weakens bonds within reacting molecules so that EA is lower for the formation of a complex which involves that catalyst.

Question 30

reaction where catalysts are of different physical state from reactants

Answer 30

HETEROGENOUS

Question 31

Reactions where reactant are same state as catalyst

Answer 31

HOMOGENOUS

Question 32

Negative catalyst

Answer 32

Some substances act as negative catalyst, called inhibitors, which provide alternative reaction pathway of higher EA.

Question 33

What may affect active site of enzyme

Answer 33

pH and temperature, deactivating enzyme

Question 34

Ways of overcoming ‘Energy barrier’

Answer 34

1. Mechanical Energy (shock)
2. Light Energy (photosynthesis/black and white photography)