The rate and extent of chemical change

• The rate of a chemical reaction tells us how quickly reactants are converted into products. In industry, reaction rates are very important, however, faster reaction rates may not always be better
• Drawbacks of faster reactions

  • It may be expensive to generate the conditions for very fast reactions
  • There can be safety concerns associated with very fast reactions
  • Therefore reaction rates used in industry are the result of a trade-off between speed, cost and safety
• • Maximum profit - in industry, making the required amount of product as cheaply as possible maximises all-important profit
  • Faster reactions - are often better as they yield more product in a given amount of time
• Particle collisions 


  Particles must collide for chemical reactions to happen

  Importantly, these collisions must happen with enough energy. This amount of energy is called the activation energy
• Activation energy


  The activation energy is the minimum energy with which particles must collide in order to cause a chemical reaction
• The main factors affecting the rate of chemical reactions are:

  • Concentration of dissolved reactants
  • Pressure of gas reactants
  • Temperature
  • Surface area of solid reactants
  • Catalysts
• rate of reaction: Concentration of dissolved reactants

  Increasing the concentration increases the frequency of collisions. This increases the rate of reaction
• Rate of reaction: Pressure of gas reactants

  Increasing pressure is like increasing the concentration

  It increases the frequency of the collisions and therefore making the rate of reaction increase
• Rate of reaction: Temperature 


  Increasing the temperature increases the frequency of collisions

  Increasing the temperature increases the energy of reactant particles. This means that a greater proportion of the particles will have more energy than the activation energy that is needed.

  Together, these lead to more successful collisions. This increases the rate of reaction
• Rate of reaction: Surface area of solid reactants

  Increasing the surface area increases the frequency of collisions. This increases the rate of reaction

  This is often done by breaking up solids into smaller lumps

  This increases the SA:V ratio making sure that more particles are exposed to attack
• Catalysts 


  Catalysts are substances that increase the rate of a chemical reaction without being used up in the process as they lower the activation energy by providing a different reaction pathway
• Catalysts are: Not used up 


  Catalysts are not used up during chemical reactions and this means that:

  They can be reused indefinitely and are not found in chemical equations

  However, catalysts will often need cleaning or regenerating, which has knock-on effects for energy expenditure and environmental impact
• Catalysts are: Different 


  Different reactions need different catalysts:

  • Iron is used in the process that makes ammonia
  • Platinum and palladium are used in catalytic converters in cars
  • Enzymes catalyse reactions in biological systems
• Catalysts are: Powders


  Catalysts often come as powders, pellets or fine gauzes because these types of substances have particularly high surface areas
• Catalyst advantage: Cost-effective 


  Despite the fact that some catalysts are expensive precious metals, they are cost-effective

  This is because a small quantity can be used to speed up a reaction by a lot. It may be cheaper to pay for the catalyst at the start and increase the rate of reaction after that

  Paying for a catalyst may be cheaper than paying for the energy needed to increase either temperature or pressure
• Catalyst advantage: Reduced burning of fossil fuels


  By reducing the necessary temperatures/pressure, fewer fossil fuels need to be burned thus reducing negative environmental impact
• Catalyst disadvantage: Toxicity


  Many catalysts are toxic (Eg transition metals) and these can escape into the environment and contaminate ecosystems
• Some chemical reactions are reversible which means that they can proceed both forwards and backwards
• if the forward reaction is exothermic the backwards reaction will be endothermic
• • Energy is conserved during chemical reactions
  • The energy released/absorbed by the forward reaction will be exactly equal to the energy absorbed/released by the backwards reaction
• If a reversible reaction happens in a closed system, a dynamic equilibrium will eventually be reached
• Dynamic equilibrium 


  At dynamic equilibrium, the rate of the forward reaction equals the rate of the backward reaction

  The equilibrium of the reaction is dynamic because both the forward and backward reactions are still taking place
• conditions and equilibrium: changing conditions 


  The relative amounts of the substances in a reaction at equilibrium are determined by the conditions
• Conditions and equilibrium: Le Chatelier's principle

  Le Chatelier's principle says that if any of the conditions of a reversible reaction at equilibrium are changed, the system will change to counteract this change

  This principle is used to predict the outcome of any change imposed on a system at equilibrium
• The effect of temperature on the position of equilibrium

  Decrease temperature - The position of equilibrium will shift in the exothermic reaction resulting in the products o an exothermic reaction increasing and decreasing the products of the endothermic reaction

  An increase in temperature - will do the opposite. Shift in endothermic reaction. More products on the endothermic side less on the exothermic
• The effect of pressure on the position of Equilibrium

  Increase pressure - The position of equilibrium will shift to favour the reaction that produces fewer gas molecules

  Decrease pressure - The position of equilibrium will shift towards the side of the reaction that produces the most gas molecules
• The effect of concentration and the position of equilibrium

  increased concentration of reactants - This will shift the position of equilibrium towards the products

  Increase the concentration of products - this will shift the position of the equilibrium towards the reactants