C7.2
- Created by: amy_mair
- Created on: 20-06-16 11:41
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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
- Energy Supplied
- 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
- Activation energy is the energy needed to start a reaction
- 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
- It is a reversible reaction
- 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
- Will the raw materials run out?
- Ammonia
- Energy Transfer
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