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chemistry

Reactants

Substances that interact with each other

Products

New substances formed during a reaction

Law of conservation of Mass

During a chemical reaction, atoms are neither created nor destroyed, so the mass remains constant.

Rate of reaction

The change in concentration of any one reactant or product per unit time

Average Rate

= Total product produced / Total time to completion

Instantaneous rate

The rate at a particular point in time during the reaction = Slope of the Tangent to a curve

Hydrogen peroxide

Reactant in experiment to investigate the rate of production of oxygen

Manganese dioxide

Catalyst in experiment to investigate the rate of production of oxygen

Collision Theory

For a reaction to occur the particles must collide and the colliding particles must have enough activation energy to react

Activation Energy

The minimum energy which colliding particles must have for a reaction to occur

Reaction Profile Diagram

Diagram which shows how the energy content of a system changes during a reaction

Factors affecting Rates of Reaction

Nature of the reactants, Particle size (Surface area), Concentration, Temperature, Catalysts

Nature of the reactants

Covalent reactions are slower than ionic reactions

Particle Size

Finely divided particles react faster than large particles

Dust explosion

The fast combustion of dust particles suspended in the air in an enclosed location

Necessary conditions for dust explosion

A combustible dust, Ignition source, An oxidant, Dust suspended in the air at a high concentration, Dryness

Concentration

Increasing concentration increases the rate by increasing the number of collisions

Temperature

An increase in temperature causes an increase in the rate of the reaction.

Higher temperature

Causes a small increase in number of collisions per unit time, and a large increase in the proportion of the collisions to have the required activation energy

Sodium thiosulfate and HCl

Reactants used in experiment to investigate the effect of concentration or temperature on reaction rate

Yellow precipitate of sulphur

Product used to monitor the progress of the reaction in experiment to investigate the effect of concentration or temperature on reaction rate

Catalyst

A substance that alters the rate of a chemical reaction but is not consumed in the reaction.

Properties of Catalysts

1. Chemically unchanged at the end of reaction but can be physically changed.
2. Usually specific.
3. Usually only need to be present in very small amounts.
4. In reversible reactions, both directions are catalysed equally (doesn't affect the position of equilibrium).
5. Can be destroyed by catalytic poisons.

Enzymes

Biological catalysts

Homogeneous catalysis

Both reactants and catalyst are in the same phase (Iodine snake with KI + H2O2)

Heterogeneous catalysis

Reactants and catalyst are in different phases (H2O2 + MnO2)

Autocatalysis

One of the products acts as a catalyst (Mn2+ in reaction between MnO4- and Fe2+)

Intermediate Formation Theory

Occurs when one or more of the reactants combines with the catalyst to form an intermediate compound.

Absorption

Describes what happens when one substance moves into another

Adsorption

The accumulation of substances only at the surface of another substance

Surface Adsorption Theory

Occurs when reactant molecules settle on the surface of the catalyst forming temporary bonds between the molecules and the catalyst and causing an increase in concentration of the reactant molecules

Catalytic Converters

Used in a car to convert toxins such as CO, NO, NO2 and hydrocarbons to harmless gases

Platinum, Palladium, Rhodium

Catalysts in a Catalytic Converter

Poisons

Reason why catalytic converters need to be changed after 50,000 miles

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