kinetic

• collision theory

  reactions can only occur when collisions take place between particles having sufficient energy, the energy is usually needed to break the relevant bonds in one or either of the reactant molecules
• activation energy EA

  the minimum amount of energy required which particles need to collide for a reaction to occur
• Emp is the most probable energy is at the peak of the curve
• the area under the curve represents the total number of particles present
• beginning of curve shows particles with low energies as collisions cause some particles to slow down
• energy distribution curve should go through the origin because zero molecules have zero energy
• mean energy of particles is just after the peak of the curve
• most molecules have energies between the two extremes but the distribution is not symmetrical
• only a few particles have a greater energy than the EA
• energy distribution should never meet the x axis as there is no maximum energy for molecules
• maxwell boltzmann distribution shows spread of energies that molecules of a gas or liquid have at a particular temperature

  
• as the temperature increases the distribution shifts towards having molecules with higher energy

  Emp and mean shift to higher energy but number of molecules with those energies decrease

  same area underneath curve as particles are constant
• at higher temperature molecules have a wider range of energies than at lower temperatures
• the rate of reaction is defined as the change in concentration of a substance in unit time

  usual unit is mol dm-3 s-1
• when a graph of concentration of reactant is plotted vs time, the gradient of the curve is the rate of reaction
• initial rate is the rate at the start of the reaction where it is fastest
• reaction rates can be calculated from graphs of concentration of reactants or products, by drawing a tangent to the curve and calculating the gradient of the tangent
• In the experiment between sodium thiosulfate and hydrochloric acid we usually measure reaction rate as 1/time. The time is the time taken for a cross placed underneath the reaction mixture to disappear due to the cloudiness of the sulfur
• at higher concentrations and pressures there are more particles per unit volume so particles collide more frequently and successfully
• if concentration increases the shape of the energy distribution curves do not change, it should just be moved higher

  area under the curve will be greater because there are more particles
• at higher temperatures, the energy of particles increase

  more frequent successful collisions often with energy greater than the activation energy
• increasing surface area will cause successful collisions to occur more frequently between reactant particles and this increases the rate of reaction
• catalysts speed up rate of reaction without being used up
• catalysts work by providing an alternative reaction route with a lower activation energy
• if the activation energy is lower, more particles will have energy> Ea so there will be a higher frequency of successful collisions and a faster rate of reaction