topic 5 - energy changes

• Energy is conserved in chemical reactions, so the total amount of energy in the universe at the start of a reaction is equal to the total energy in the universe at the end of the reaction
• When a chemical reaction happens, energy is transferred to or from the surroundings
• When energy is transferred to the surroundings, it is an Exothermic reaction. For example:

  • combustion reactions
  • (many) oxidisation reactions
  • (many) neutralisation reactions
• Everyday uses of Exothermic reactions include self heating cans and hand warmers
• When energy is taken in from the surroundings, it is an Endothermic reaction. For example:

  • thermal decomposition reactions
  • reaction of citric acid and sodium hydrocarbonate
• Everyday uses of endothermic reactions include instant ice packs
• An energy level diagram shows whether a reaction is exothermic or endothermic. It shows the energy in the reactants and products, and the difference in energy between them
• The energy level decreases in an exothermic reaction. This is because energy is given out to the surroundings

  
• The energy level increases in an endothermic reaction. This is because energy is taken in from the surroundings

  
• It is usually more helpful to describe how the energy of the chemicals changes during the reaction, so a reaction profile is more useful than an energy level diagram
• This is the reaction profile for an exothermic reaction:

  
• This is the reaction profile for an endothermic reaction:

  
• Energy is transferred when bonds are broken or formed. During a chemical reaction:

  • bonds in the reactants are broken
  • new bonds are made in the products
• The difference between the energy needed to break bonds and the energy released when new bonds are made determines the type of reaction.

  A reaction is:

  • exothermic if more heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants
  • endothermic if less heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants
• The energy change in a reaction can be calculated using bond energies. A bond energy is the amount of energy needed to break one mole of a particular covalent bond
• To calculate an energy change for a reaction:

  • add together the bond energies for all the bonds in the reactants - this is the 'energy in'
  • add together the bond energies for all the bonds in the products - this is the 'energy out'
  • energy change = energy in - energy out
• Chemical cells use chemical reactions to transfer energy by electricity. The voltage of a cell depends on many factors like what the electrodes are made from and the substance used as the electrolyte
• A simple cell can be made by connecting two different metals in contact with an electrolyte. A number of cells can be connected in series to make a battery, which has a higher voltage than a single cell
• In non-rechargeable cells, eg alkaline cells, a voltage is produced until one of the reactants is used up. When this happens, we say the battery ‘goes flat’
• In rechargeable cells and batteries, like the one used to power your mobile phone, the chemical reactions can be reversed when an external circuit is supplied
• Using different combinations of metals to make cells will change the voltage of the cell
• Fuel cells produce a voltage continuously, as long as they are supplied with:

  • a constant supply of a suitable fuel
  • oxygen, eg from the air
• The fuel is oxidised electrochemically, rather than being burned, so the reaction takes place at a lower temperature than if it was to be burned. Energy is released as electrical energy, not thermal energy
• Hydrogen-oxygen fuel cells are an alternative to rechargeable cells and batteries. In a hydrogen-oxygen fuel cell, hydrogen and oxygen are used to produce a voltage. Water is the only product. The overall reaction in a hydrogen-oxygen fuel cell is:

  hydrogen + oxygen → water

  2H2(g) + O2(g) → 2H2O(l)
• Hydrogen-oxygen fuel cells:

  At the negative electrode: 2H2 + 4OH- → 4H2O + 4e-

  At the positive electrode: O2 + 2H2O + 4e- → 4OH-
• Hydrogen-oxygen fuel cells are used in spacecraft. The water they produce is useful as drinking water for astronauts.
• Pros and cons of hydrogen fuel cells:

  Pros:

  • easy to maintain, small size, only produce water

  Cons:

  • expensive to manufacture, need constant supply of hydrogen fuel which is flammable
• Pros and cons of rechargeable cells:

  Pros:

  • can be recharged many times, reduces use of resources

  Cons:

  • cost more to manufacture
• Pros and cons of alkaline cells:

  Pros:

  • cheaper to manufacture

  Cons:

  • may end up in landfill, expensive to recycle