5. Kinetics

• Collision theory


  For a reaction to take place between two particles, they must collide with enough energy to break bonds.The collision must also take place between the parts of the molecule that will react together, so orientation has a part to play. For the particles to have enough energy to break bonds, they need to be moving fast. So, for a fat reaction rate you need plenty of rapidly moving particles in a small volume.
• Activation energy

  Reactions can only occur when collisions take place between particles having the certain minimum energy to start breaking bonds.
• Why don't most collisions lead to a reaction?

  They either do not have enough energy or are in the wrong orientation
• Factors that affect the rate of chemical reactions

  - Increasing the temperature- Increasing the concentration of a solution- Increasing the pressure of a gas reaction- Increasing the surface area of solid reactants- Using a catalyst
• Effect of temperature on Rr


  Increases the kinetic energy of the molecules therefore the speed increases. This therefore increases both their energy and the number of collisions.
• What is rate of reaction


  The change in the amount or concentration of a reactant OR product per unit time.
• Increasing the concentration of a solution


  If there are more particles present in a given volume then the collisions are more likely so more successful collisions and the reaction rate would be faster. However, as a reaction proceeds, the reactants are used up and their concentration falls. So in most reactions the rate of reaction drops as the reaction goes on.
• Increasing the pressure of a gas reaction


  This has the same effect as increasing the concentration of a solution - there are more molecules or atoms in a given volume so collisions are more likely
• Increasing the surface area of solid reactants


  The greater the total surface area of the solid, the more of its particles are available to collide with molecules in a gas or a liquid. This means that breaking a solid lump into smaller pieces increases the rate of its reaction because there are more sites of reaction
• What is a catalyst


  A catalyst is a substance that increases the rate of a chemical reaction without being changed in chemical composition or amount.
• Transition state or activated complex on an enthalpy diagram


  Some bonds are in the process of being made and some bonds are in the process of being broken. Like a ball at the very top of the hill, it has extra energy and is unstable
• Exothermic reaction profile

  
• Endothermic reaction profile

  
• What does the Maxwell-Boltzmann distribution tell us


  It tells us about the distribution of energy amongst the particles
• Why does the graph start at (0,0)

  No particles have zero energy
• Why doesn't the graph touch the x-axis?

  Very few particles have high energies.In fact there is no upper limit.
• Where is the most probable energy in the graph

  At the peak
• Where is the average energy in the graph


  To the right of the peak
• Why are fuels safe at room temperature?


  Activation energy!They need a higher minimum amount of energy to break the bonds, so there isn't enough activation energy for the reaction to take place.However, a small spark is enough to start a combustion reaction. The heat given out by the initial reaction is enough to supply the activation energy for further reactions.
• Why are catalysts used in industry?


  It is cheaper to speed up a reaction by using a catalyst than using high temperatures and pressures. This is true, even if the catalyst is expensive, because it is not used up.
• How do catalysts work


  They provide a different pathway for the reaction, one with a lower activation energy. Therefore, they reduce the activation energy of the reaction (the minimum amount of energy that is needed to start the reaction)Catalysts work by providing an alternative reaction route of lower activation energy.
• M-B temperature + explain

  At higher temperatures, the Maxwell Boltzmann distribution shifts to higher speeds, meaning that a greater proportion of molecules have higher speeds. This is because higher temperatures cause the molecules to move more rapidly and with greater kinetic energy.Therefore, more particles have energy equal to or greater than the activation energy. Therefore, the rate of reaction increases.The graph is also lower because the total amount of particles is the same.

  
• M-B concentration + explain


  By increasing the concentration of molecules within a system, the actual energy of the molecules doesn't change. The proportion of molecules with the required activation energy is the same, there are just more of them! Because of this, more particles in the system have the required activation energy and react.Also, the peak increases because the there is a larger amount of molecules.

  
• M-B catalyst + explain


  Catalysts do not affect the enthalpy change of the reactions or the position of equilibrium therefore the graph is mostly the same however the line for activation energy shifts to the left which shows that more particles have energy equal to or greater than the activation energy needed to start the reaction.

  
• Reaction: Haber process


  Catalyst: IronPurpose: Making fertilisers
• Reaction: Ostwald process to make nitric acid


  Catalyst: Platinum and rhodiumPurpose: Making fertilisers and explosives
• Reaction: Hardening of fats with hydrogen


  Catalyst: NickelPurpose: Making margarine
• Reaction: Cracking hydrocarbon chains from crude oil


  Catalyst: Aluminium oxide and silicon dioxide zeolitePurpose: Making petrol
• Reaction: Catalytic converter reactions in car exhausts


  Catalyst: Platinum and rhodiumPurpose: Removing polluting gases
• Reaction: Hydration of ethene to produce ethanol


  Catalyst: H+ absorbed on solid silica phosphoric acidPurpose: Making ethanol (a fuel additive, solvent and chemical feedstock)
• Reaction: Esterification


  Catalyst: H+Purpose: Making solvents
• Catalytic converter purpose


  Reduce the levels of a number of polluting gasesAs the polluting gases pass over the catalyst, the polluting gases react with each other to form less harmful products by the following reactionscarbon monoxide + nitrogen oxide --> nitrogen + carbon dioxidehydrocarbons + nitrogen oxides --> nitrogen + carbon dioxide + water
• Why is the catalytic converter shapes like a honeycomb


  It is a honeycomb made of ceramic metal coated with platinum and rhodium (the catalysts).It provides a large surface area on which the reactions take place so a little of these expensive metals goes a long way.