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Rate of Reaction
Rate of reaction: Change in concentration per unit time
Factors Affecting Rates of Reaction
Concentration:
• Greater concentration of reactants, then the faster the reaction will be.
• The more particles the greater the chance of a collision.
Particle size:
• The smaller the particle size, the larger the surface area - the faster the rate of reaction.
Dust explosions:
• Conditions necessary for a dust explosion:
1. Combustible dust particles
2. Dry
3. Source of ignition
Temperature:
• Increased temperature increases the rate of reaction.
• Particles have increased kinetic energy with increased temperatures therefore effective
collisions more likely to occur.
Nature of reactants:
• Ionic bonds are fast as there is no bond breaking or forming.
• Covalent bonds are slower as bonds need to be broken before a reaction could take place.
Catalysts
• A catalyst is a chemical that alters the rate of a chemical reaction but is not used up in the
reaction.
• Catalysts increase the rate of reaction by lowering the activation energy
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Types of catalyst
1. Homogeneous catalysts
2. Heterogeneous catalysts
Homogeneous catalysis:
• The reactant(s) and catalyst are in the same phase i.e. the same boundary.
• Note: Solid catalysts and liquid reactants can be in the same phase as long as the solid
dissolves.
Heterogeneous catalysis:
• The reactants and catalyst are in different phases i.e. there is a boundary between the reactants
and the catalyst.
• e.g. Oxidation of methanol using a hot platinum catalyst.
Oxidation of methanol using a platinum catalyst
Observations:
- The wire glows (exothermic)
- Popping sounds (hydrogen gas)
- Organic smell (Methanal produced)
- Flask becomes hot (exothermic)
- Small flame (methanol ignited)
Type of catalysis in experiment:
Heterogenous or surface adsorption
Products of reaction:
Methanal, water and hydrogen gas
Term used to describe attachment of gas or liquid to a solid surface?
Adsorption
How does adsorption affect the rate of oxidation?
Lower activation energy due to the platinum catalyst present
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Mechanisms of Catalysts
1. Intermediate formation theory of catalysis
2. Surface adsorption theory
Note: Surface adsorption theory features in exams a lot more regularly.
Intermediate formation theory of catalysis
Oxidation of potassium sodium tartrate by hydrogen peroxide & catalysed by cobalt(II)
• Pink at the beginning of the reaction - colour of the catalyst (Co2+) solution.
• Bubbling and gas are produced as reaction mixture turns green due to formation of
intermediate complex.
• Pink at the end as the reaction finished and the catalyst (Co2+) is regenerated.
Surface adsorption of catalysis
• Most heterogeneous catalysis reactions are explained using the surface adsorption theory.
• Adsorption: Accumulation of substances only at the surface of another substance.
1. Adsorption
• Molecules settle onto the catalyst forming temporary weak bonds
2. Reaction on surface
• The molecule numbers build up on the surface.
• The higher concentration means more effective collisions.
• A reaction occurs and a product(s) is formed.
3. Desorption stage
• The product(s) leave the surface of the catalyst.
• Once product leaves, more reactants can now be adsorbed onto the surface.
• The cycle continues.
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Catalytic converters
• Catalytic converters are devices in the exhaust system of a vehicle which contains catalysts to
convert pollutants in the exhaust gases to less harmful substances.
• Note: It works by means of surface adsorption theory.
• Catalytic converter metals:
• Platinum
• Palladium
• Rhodium
What do catalytic converters convert?
1. Carbon monoxide to carbon dioxide
2. Hydrocarbons to water and carbon dioxide
3. Nitrogen monoxide to nitrogen gas
Catalyst poisons
• A catalyst poison is a substance that makes a catalyst inactive.
• Lead and sulphur compounds are permanently adsorbed onto the catalyst surface.
Collision theory and activation energy
Activation energy (EA) is the minimum amount of energy that colliding particles
must have for a reaction to occur.
• An effective collision is one that results in the formation of products.
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