C7.2

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  • C7.2
    • Energy Transfer
      • Energy Supplied
        • 1. During a chemical reaction, old bonds are broken and new bonds are formed
        • 2. Energy must be supplied to break existing bonds
          • Endothermic Process
        • 3. Energy is released when new bonds are formed
          • Exothermic Process
        • In exothermic reactions the energy released by forming bonds is greater than the energy used to break them
        • In Endothermic reactions the energy used to break bonds is greater than the energy released by forming them
      • Bond Energy Calculations
        • 1. Every chemical bond has a particular bond energy associated with it. This bond energy varies slightly depending on the compound the bond occurs in
        • 2. You can use these known bond energies to calculate the overall energy change for a reaction
    • Catalysts and Reversible Reactions
      • Activation energy is the energy needed to start a reaction
        • 1. The activation energy is the minimum amount of energy needed for bonds to break and a reaction to start
        • 2. It is a bit like having to climb up one side of a hill before you can go down the other side
        • 3. If the energy input is less than the activation energy there will not be enough energy to start the reaction
      • Catalysts lower the activation energy
        • 1. A catalyst is a substance which changes the speed of a reaction, without being used up in the reaction
        • 2. Catalysts lower the activation energy needed for reactions to happen by providing alternative routes
        • 3. The effect of a catalyst is shown by the lower curve on the diagram
        • 4. The overall energy change for the reaction remains the same though
      • Reversible reactions
        • A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants
        • Reversible reactions reach a dynamic equalibrium
        • 1. If a reversible reaction takes place in a closed system then the equilibrium will always be reached
        • Equalibrium means that the relative quantities of reactants and products will reach a certain balance and stay there
    • The Haber Process
      • It is a reversible reaction
        • 1. The feedstocks for the haber process are nitrogen and hydrogen
        • 2. The nitrogen is obtained from the air, which is 78% nitrogen and 21% oxygen
        • 3. The hydrogen comes from the cracking of chemicals
        • 4. The reaction is reversible so not all the nitrogen and hydrogen will convert to ammonia.
        • 5. The N2 and H2 which do not react are recycled and passed througuh again and is not wasted
        • 6. Recycling N2 and H2 means that more ammonia will be produced using the same amount of reactant
      • Conditions
        • 200 atmospheric pressures
        • Temperature 450 degrees
        • Catalyst: Iron
      • Compromises to be made
        • 1. Higher pressures favour the forward reaction
        • 2.  So the pressure is set to as high as possible to get the best percentage yield
        • 3. The forward reaction is exothermic, the temperature will increase will move the equalibrium
        • 4. The trouble is, lower temperature mean a slower rate of reaction
        • 5. The 450 degrees is a comprimise between maximum yield and speed of reaction
        • 6. Remember the unused hydrogen and nitrogen are recycled
    • Nitrogen  Fixation
      • Ammonia
        • 1. Nitrogen fixation is the process of turning N2 from the air into useful nitrogen compounds like ammonia
        • 2. The Haber process is a non biological way of fixing nitrogen
        • 3. Most of the ammonia produced by the haber process is used to make fertilisers
        • 4. Fertilisers play a vital part in world food production as they increase crop yield so help to heed more people
        • 5 When used in large amount though, fertilisers can pollute water supplies and can cause eutropication
        • 6. Eutropication happens when fertilisers leach into lakes and rivers
        • 7. Ammonia is also very important in industry where it is used to manufacture plastics
      • Efficiency
        • 1. In the Haber process very high temperatures and pressures have to be used to turn nitrogen and hydrogen into ammonia
        • .2 Using an iron catalyst makes teh rate of reaction much faster
        • 3. Without the catalyst the temperature would be raised much further to get a quick enough reaction
        • 4. Some living organisms can fix bacteria at room temperature and pressure
      • Is it sustainable?
        • Will the raw materials run out?
          • Hydrogen comes from fossil fuels. They are non renuable and will run out
        • How good is the atom economy?
          • Excellent as it all makes ammonia
        • What do i do with my waste products?
          • There are no waste products as the chemicals are all recycled
        • What are the energy costs
          • Lots of energy is needed to keep the reaction at the same point
        • Will it damage the environment
          • Fertilisers can cause eutropication and water pollution
        • Are there any benefits or risks to society
          • making ammonia can help world food production

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