RATES OF REACTION - 1 A guide for A level students

COLLISION THEORY Collision theory states that... particles must COLLIDE before a reaction can take place not all collisions lead to a reaction reactants must possess at least a minimum amount of energy - ACTIVATION ENERGY Plus particles must approach each other in a certain relative way - the STERIC EFFECT

COLLISION THEORY According to collision theory, to increase the rate of reaction you therefore need... more frequent collisionsincrease particle speedor have more particles present more successful collisionsgive particles more energyor lower the activation energy

INCREASING THE RATE INCREASE THE SURFACE AREA OF SOLIDS INCREASE THE CONCENTRATION OF REACTANTS INCREASE TEMPERATURE others INCREASE THE SURFACE AREA OF SOLIDS INCREASE THE CONCENTRATION OF REACTANTS INCREASE TEMPERATURE others The following methods may be used to increase the rate of a chemical reaction

INCREASING SURFACE AREA Increasing surface area increases chances of a collision - more particles are exposed Powdered solids react quicker than larger lumps Catalysts (e.g. in catalytic converters) are in a finely divided form for this reason + In many organic reactions there are two liquid layers, one aqueous, the other non- aqueous. Shaking the mixture improves the reaction rate as an emulsion is often formed and the area of the boundary layers is increased giving more collisions. CUT THE SHAPE INTO SMALLER PIECES SURFACE AREA = 30 sq units SURFACE AREA 9 x ( ) = 54 sq units 1 1 1

Increasing concentration = more frequent collisions = increased rate of reaction INCREASING CONCENTRATION However, increasing the concentration of some reactants can have a greater effect than increasing others Low concentration = fewer collisionsHigher concentration = more collisions

Effectincreasing the temperature increases the rate of a reaction particles get more energy so they can overcome the energy barrier particle speeds also increase so collisions are more frequent INCREASING TEMPERATURE ENERGY CHANGES DURING A REACTION As a reaction takes place the enthalpy of the system rises to a maximum, then falls A minimum amount of energy is required to overcome the ACTIVATION ENERGY (E a ). Only those reactants with energy equal to, or greater than, this value will react. If more energy is given to the reactants then they are more likely to react. Typical energy profile diagram for an exothermic reaction

INCREASING TEMPERATURE MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

Increasing the temperature alters the distribution get a shift to higher energies/velocities area under the curve stays constant - it corresponds to the total number of particles T1T1 T2T2 TEMPERATURE T 2 > T 1 MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY INCREASING TEMPERATURE MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

EaEa ACTIVATION ENERGY - E a The Activation Energy is the minimum energy required for a reaction to take place The area under the curve beyond E a corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react. MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER INCREASING TEMPERATURE MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

Explanation increasing the temperature gives more particles an energy greater than E a more reactants are able to overcome the energy barrier and form products a small rise in temperature can lead to a large increase in rate T1T1 T2T2 TEMPERATURE T 2 > T 1 EaEa INCREASING TEMPERATURE MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY EXTRA MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER

The area under the curve beyond E a corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react. If a catalyst is added, the Activation Energy is lowered - E a will move to the left. MOLECULAR ENERGY EaEa NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER ADDING A CATALYST NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

The area under the curve beyond E a corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react. Lowering the Activation Energy, E a, results in a greater area under the curve after E a showing that more molecules have energies in excess of the Activation Energy EaEa MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY ADDING A CATALYST EXTRA MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

work by providing an alternative reaction pathway with a lower Activation Energy using catalysts avoids the need to supply extra heat - safer and cheaper catalysts remain chemically unchanged at the end of the reaction. Types Homogeneous Catalysts Heterogeneous Catalysts same phase as reactantsdifferent phase to reactants e.g. CFC’s and ozone e.g. Fe in Haber process Usesused in industry especially where an increase in temperature results in a lower yield due to a shift in equilibrium (Haber and Contact Processes) CATALYSTS - A REVIEW

work by providing an alternative reaction pathway with a lower Activation Energy using catalysts avoids the need to supply extra heat - safer and cheaper catalysts remain chemically unchanged at the end of the reaction. Types Homogeneous Catalysts Heterogeneous Catalysts same phase as reactantsdifferent phase to reactants e.g. CFC’s and ozone e.g. Fe in Haber process Usesused in industry especially where an increase in temperature results in a lower yield due to a shift in equilibrium (Haber and Contact Processes) CATALYSTS DO NOT AFFECT THE POSITION OF ANY EQUILIBRIUM but they do affect the rate at which equilibrium is attained a lot is spent on research into more effective catalysts - the savings can be dramatic catalysts need to be changed regularly as they get ‘poisoned’ by other chemicals catalysts are used in a finely divided state to increase the surface area CATALYSTS - A REVIEW

Reactions are fastest at the start and get slower as the reactants concentration drops. In a reaction such as A + 2B ——> C the concentrations might change as shown RATE CHANGE DURING A REACTION Reactants (A and B) Concentration decreases with time Product (C) Concentration increases with time the steeper the curve the faster the rate of the reaction reactions start off quickly because of the greater likelihood of collisions reactions slow down with time as there are fewer reactants to collide TIME CONCENTRATION B A C