AQA Chemistry Unit 2: 8 Kinetics

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8.1 The Collision Theory

  • For a reaction to occur the particles must collide with enough energy to break bonds
  • To get a lot of collisions you need many particles in a small volume
  • Particles also need to be moving fast to breaks bonds

Most Collisions between molecules do not lead to reaction. They either do not have enough energy or are of the wrong orientation

Factors that Increase Rate of Reaction:

  • Temperature: Increase then increases the speed of molecules which increases their energy and number of collisions
  • Concentration of Solution: Increase means there are more particles in a given volume making collisions more likely. As the reactants are used up concentration falls reducing rate of reaction
  • Pressure: Increase in gas reaction has the same effect as concentration
  • Surface Area: Larger means more particles are available to collide with molecules meaning breaking a solid into smaller pieces increases rate of reaction as more sites for reaction
  • Catalyst: Changes the rate of a chemical reaction without being chemically changed itself
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The Collision Theory 2

  • For a collision to occur the molecule must have a certain minimum energy to start breaking the bonds
  • Activation energy is the minimum energy needed

Exothermic Reactions:

  • The Smaller the activation energy the faster the reaction
  • The curve at the top of an enthalpy diagram is the transition state/ activated complex
  • At the transition state bonds are being made and broken

Endothermic Reactions:

  • Products have more energy than the reactants
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8.2 The Maxwell-Boltzmann Distribution

  • The energy of a particle depends on its speed so particles ave a range of energies
  • The distribution graph shows: no particles have zero energy, most particles have energies around the peak of the curve, few have high energies and that the average energy isn't the same as the most probable

Activation Energy

  • When activation energy is drawn on the distribution graph the area to the right of the line represents the number of particles with enough energy to react

The Effect of Temperature

  • The distribution graph will have a lower peak at higher temperatures, it also moves to the right
  • The number of high energy particles increase
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8.3 Catalysts

  • It is cheaper to use a catalyst then t increase temperature and pressure

How they Work

  • Catalysts proved an alternate route for a reaction to take place at a lower activation energy
  • By lowering the activation energy the particles number with the activation energy shifts left on a distribution curve showing an increase in particles with the energy
  • Catalysts don't affect the enthalpy change of the reaction or affect the position of equilibrium in a reversible reaction

The Catagories of Catalysts:

  • Heterogeneous: In a different phase to the reactants e.g. solid catalyst, liquid reactants
  • Homogeneous: Reactants and catalyst in the same phase
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Catalytic Converters

  • Reduce the level of polluting gases
  • It is a honeycomb made of ceramic material coated in platinum and rhodium metals
  • The shape provides large surface area for the reaction to take place
  • As polluting gases pass over the catalyst they react with each other

Carbon Monoxide + Nitrogen Oxides -> Nitrogen + Carbon Dioxide

Hydrocarbons + Nitrogen Oxides -> Nitrogen + Carbon Dioxide + Water

The Reaction takes place in two steps:

  • Gases fist form weak bonds with the metal atoms on the catalyst this is adsorption. This holds the gases in the right position for them to react.
  • Products then break away from the metal atoms this is desorption. This frees up room on the catalyst surface for more reactions

The strength of the weak bonds holding the gases on the metal surface is critical. They must be strong enough to hold the gases for long enough to react but weak enough to release the products easily.

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