Rates Of Reaction

Question 1

Factors Affecting

Rate of Reaction

Answer 1

catalyst 
temperature 
surface area
nature of reactants 
concentration

Question 2

concentration

Answer 2

Doubling the concentration of one of the reactants doubles the rate of the reaction
The higher the concentration the more often the particles collide and so can react
Rate of reaction is directly proportional to the concentration of each reactant

Question 3

temperature

definition- activation energy

Answer 3

The higher the temperature the faster the rate

1. Particles are moving faster so collide more frequently in a given time
2. When they collide a higher proportion of them will have Activation Energy

Activation Energy- Minimum energy needed for colliding particles to react

General Rule:- A10oC rise in temperature doubles the rate of a reaction

Question 4

catalyst

Answer 4

A substance that speeds up a reaction but is not used up in the reaction. eg.iron (Fe)
Many are Transition Elements
Enzymes are protein biological catalysts
If the reaction is reversible they speed up the reaction in both directions equally

Question 5

Particle size [Surface Area]

Answer 5

The larger the surface area the more of the reactants can come into contact
Powder > Granules >Lumps
rate of reaction is directly proportional to the surface area of the reactants.

Question 6

Nature of Reactants

Answer 6

1. Ionic
Tend to have very fast reactions.  
Ions already dissociated
2. Covalent
Are slow to react in general
Bonds have to be broken first for reaction to continue

Question 7

enzymes

Answer 7

Biological Catalysts
Protein in nature
Temperature and pH-sensitive
Produced by living cells

Question 8

In order for a chemical reaction to occur

three conditions are required

Answer 8

The particles must collide
They must have enough energy (Activation Energy)
They must be in the correct orientation (position)

Question 9

Activation Energy

Answer 9

1. Higher the temperature the more energy each particle has
2. According to Maxwell-Boltzmann
3. The greater the proportion of particles that have an activation energy
4. The size of Activation Energy depends on the nature of the reactants
5. Covalent compounds need to break bonds before they can react so they tend to be slow
6. Ionic compounds have bonds broken if in solution so reactions tend to be very fast

Question 10

Catalysts work by lowering the activation energy

They can do this in three ways:

Answer 10

1. By providing an alternative pathway
2. By concentrating on one or more of the reactants
3. Changing the orientation of the particles

Question 11

catalytic converter pollution products

Answer 11

CO [poisonous]
NO [forms acid rain / poisonous]
NO2  [forms acid rain / poisonous] 
Unburned hydrocarbons
[poisonous / carcinogenic]

The catalytic converter changes these into harmless gasses [ CO2, H2O, and N2]

Question 12

Removal of Pollutants

Answer 12

CO reacts with oxides of nitrogen to form CO2 and N2
CO + NOx = CO2 + N2
Unburned hydrocarbons react with oxides of nitrogen to form CO2, N2 and H2O
C2H4 + NOx = CO2 + H2O + N2
Products are all less polluting

Question 13

Catalysts

Answer 13

Pt (Platinum),
Pd (Palladium),
Rh (Rhodium
Coated on a ceramic honeycomb to give large surface area
A good example of heterogeneous catalysis
Work best at high temperatures [300oC] - that’s why they are at the front near the engine

Reduce
Pollution
Acid rain
Photochemical smog

Question 14

Destruction of Converter

Answer 14

Catalysts Poisoned by Pb

Using leaded petrol destroys catalysts at once by blocking active site

Question 15

Theories of Catalysis

Answer 15

1. Surface Adsorption theory
   Reactants adsorbed onto the surface
   This increases the concentration of the reactants and so speeds up the reaction
   May also twist the molecules of reactants so that they find it easier to react
   Many enzymes work in this way
   Called Lock and Key Theory

Question 16

Theories of Catalysis (2)

Answer 16

2. Intermediate Compound Theory

Formation of ethene [C2H4] from alcohol [C2H5OH]

The catalyst reduces the activation energy
Method 1- C2H5OH = C2H4 + H2O
catalyst (Al2O3)

Method 2- C2H5OH + H2SO4 = C2H5HSO4 + H2O
(intermediate compound = C2H5HSO4)

C2H5HSO4 = C2H4 + H2SO4
catalyst ( 180oC)

Question 17

Autocatalysts

Answer 17

In some reactions one of the products of the reaction catalyses the reaction
This product is called an autocatalyst.
Mn2+ in MnO4- titrations

Question 18

Catalytic Poisons

Answer 18

Substances that stop catalysts from working are called catalytic poisons
don’t use leaded petrol if you have a catalytic converter
They block the active site of the catalyst and stop it from working

Question 19

Heterogeneous Catalysis

Answer 19

catalyst and the reactants are in different phases

Question 20

Homogeneous Catalysis

Answer 20

catalyst and the reactant are in the same phase