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34 terms

LC Chem 6 Rates of Reaction

chemistry
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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