Rates of Reaction
Reaction Rates
RATE OF REACTION: is the change in concentration per unit time of any one reactant or product
INSTANTANEOUS RATE OF A REACTION: is the change in concentration per unit time of any one reactant or product at a given moment in time
INSTANTANEOUS RATE OF A REACTION: is the change in concentration per unit time of any one reactant or product at a given moment in time
Factors That Affect The Rate of a Reaction
a) Nature of Reactants
Ionic reactions usually faster than covalent ones as in ionic reactions there is no need to break bonds- ions simply come together. Covalent reactions require the bonds to be firstly broken
b) Particle Size
A solid divided into small pieces will offer a much greater surface area for a reaction to occur so finely divided particles react faster than large particles
c) Concentration
By experiment it is found that increasing the concentration of the reactants causes increase in rate of reaction
d) Temperature
In general, temperature increase brings about increase in rate of reaction
e) Catalyst
Either increase or decrease rate of reaction
Ionic reactions usually faster than covalent ones as in ionic reactions there is no need to break bonds- ions simply come together. Covalent reactions require the bonds to be firstly broken
b) Particle Size
A solid divided into small pieces will offer a much greater surface area for a reaction to occur so finely divided particles react faster than large particles
c) Concentration
By experiment it is found that increasing the concentration of the reactants causes increase in rate of reaction
d) Temperature
In general, temperature increase brings about increase in rate of reaction
e) Catalyst
Either increase or decrease rate of reaction
Catalysts
CATALYST: is a substance that alters the rate of a reaction but is not consumed in the reaction
General Properties of Catalyst
Types of Catalysis:
a) Homogeneous Catalysis
Both catalyst and reactants in same phase i.e. no boundary between reactants and catalysts. In reaction between permanganate ions and Fe2+, which is catalysed by Mn2+ ions, all species are in solution i.e. are in liquid phase
b) Heterogeneous Catalysis
Catalyst is in a different phase to the reactants i.e. distinct boundary between them. In reaction between methanol and oxygen (both in gaseous phase) using a platinum catalyst (solid)
c) Autocatalysis
Where one of the products of a reaction also act as catalyst for the reaction. Means rate of reaction increases as the reaction proceeds. For example, reaction between potassium permanganate and Fe2+ ions, Mn2+ ions formed by the reaction behave as an autocatalyst
Mechanisms of Catalysis:
a) The Intermediate Formation Theory
In the general reaction W + X -> Y + Z (slow reaction), W may combine with the catalyst C in a fast reaction to form a short-lived intermediate compound WC. This rapidly combines with X to form the products and regenerate the catalyst
a) W + C -> WC (fast reaction)
b) WC + X -> Y + Z + C (fast reaction)
b) The Surface Adsorption Theory
Model describes process of catalysis in terms of molecules of a substance being drawn onto (adsorbed onto) the surface of a solid catalyst. Brings molecules in contact with each other and allows them to react.
Catalytic Convertors:
General Properties of Catalyst
- Chemically unchanged at end of reaction
- Are specific. For example, enzymes will act on only one particular reaction. Catalase - breaks down hydrogen peroxide
- Catalysts required in small amounts
- Catalysts act on both forward and backward reaction to the same extent
- A catalyst poison is substance that destroys activity of a catalyst e.g. lead in petrol causes catalyst in the catalytic convertor of a car to become inactive
Types of Catalysis:
a) Homogeneous Catalysis
Both catalyst and reactants in same phase i.e. no boundary between reactants and catalysts. In reaction between permanganate ions and Fe2+, which is catalysed by Mn2+ ions, all species are in solution i.e. are in liquid phase
b) Heterogeneous Catalysis
Catalyst is in a different phase to the reactants i.e. distinct boundary between them. In reaction between methanol and oxygen (both in gaseous phase) using a platinum catalyst (solid)
c) Autocatalysis
Where one of the products of a reaction also act as catalyst for the reaction. Means rate of reaction increases as the reaction proceeds. For example, reaction between potassium permanganate and Fe2+ ions, Mn2+ ions formed by the reaction behave as an autocatalyst
Mechanisms of Catalysis:
a) The Intermediate Formation Theory
In the general reaction W + X -> Y + Z (slow reaction), W may combine with the catalyst C in a fast reaction to form a short-lived intermediate compound WC. This rapidly combines with X to form the products and regenerate the catalyst
a) W + C -> WC (fast reaction)
b) WC + X -> Y + Z + C (fast reaction)
b) The Surface Adsorption Theory
Model describes process of catalysis in terms of molecules of a substance being drawn onto (adsorbed onto) the surface of a solid catalyst. Brings molecules in contact with each other and allows them to react.
Catalytic Convertors:
Gas:
Carbon Monoxide (CO) Nitrogen Monoxide (NO) and Nitrogen Dioxide (NO2) Unburned Hydrocarbons |
Source:
Incomplete combustion of Hydrocarbons Atmospheric nitrogen reacting with oxygen at high temperature of spark plug Fuel |
Harmful Effects:
Highly toxic in the bloodstream Cause acid rain Cause smog |
The converter consists of ceramic honeycomb which offers a large surface area for reactions. Honeycomb is lined with platinum, palladium and rhodium catalysts. The catalysts cause harmful gases to combine with each other
2CO + 2NO -> 2CO2 +N2
Also unburned hydrocarbons react with the oxides to form CO2, H2O and N2. Thus harmful gases are converted into safer ones
2CO + 2NO -> 2CO2 +N2
Also unburned hydrocarbons react with the oxides to form CO2, H2O and N2. Thus harmful gases are converted into safer ones
Collision Theory and Activation Energy
Molecules must collide for a reaction to take place. If particles do not have enough kinetic energy when they collide, they will merely bounce off each other without reacting. A collision that does have the minimum amount of energy required results in a reaction and this is said to be an effective collision.
ACTIVATION ENERGY: minimum energy which colliding particles must have for a reaction to occur
A reaction with high activation energy is unlikely to occur spontaneously. On the other hand a reaction with low activation energy will occur more readily
a) Nature of Reactants
Reactions involving covalent bonds tend to have a slow rate of reaction because energy is required to break bonds
b) Particle Size
Since small particles have larger surface area than large particles, a greater number of collisions take place in the case of smaller particles. Hence, greater probability that number of effective collisions will be increased
c) Concentration
When concentration of reactants is increased number of collisions is increased. Hence there is a greater probability that the number of effective collisions will be increased
d) Temperature
If temperature increased, the moving particles have greater energy and there is a higher probability that there's sufficient energy to overcome activation energy
e) Catalysts
A catalyst provides an alternative pathway with a lower activation energy. Therefore, more particles possess the energy needed for effective collisions to take place
ACTIVATION ENERGY: minimum energy which colliding particles must have for a reaction to occur
A reaction with high activation energy is unlikely to occur spontaneously. On the other hand a reaction with low activation energy will occur more readily
a) Nature of Reactants
Reactions involving covalent bonds tend to have a slow rate of reaction because energy is required to break bonds
b) Particle Size
Since small particles have larger surface area than large particles, a greater number of collisions take place in the case of smaller particles. Hence, greater probability that number of effective collisions will be increased
c) Concentration
When concentration of reactants is increased number of collisions is increased. Hence there is a greater probability that the number of effective collisions will be increased
d) Temperature
If temperature increased, the moving particles have greater energy and there is a higher probability that there's sufficient energy to overcome activation energy
e) Catalysts
A catalyst provides an alternative pathway with a lower activation energy. Therefore, more particles possess the energy needed for effective collisions to take place