Chemistry SL

• Energy


  The ability to do work
• As energy is transferred from one form to another, some does useful work but some will always be lost as heat to the surroundings
• The energy that is lost to the surroundings will now be less available to do work and is said to have been degraded
• Energy is always transferred in the direction in which it goes from a more concentrated form to a less concentrated (more dispersed) form
• First law of thermodynamics

  Energy is conserved
• Burning coal in a coal-fired power station

  1. Chemical energy converted to heat energy

  2. Heat energy used to heat water to make steam

  3. Kinetic energy of steam transferred to turbine which turns generator

  4. Friction in mechanical components converts energy to low-level heat

  5. Steam condensed back to water, heat energy dissipated to surroundings
• Even the useful electrical energy obtained from the generator will eventually result in the production of heat which will spread out at a low level in the surroundings
• When some coal is burned, all the energy released is eventually dispersed in the surroundings and we cannot get this energy back
• Efficiency

  Useful energy out / Total energy in x 100
• Basic Oxide reaction for Mg


  MgO(s)+H2O->Mg(OH)2(aq)
• Energy transformations


  • Electric motor
  • Internal combustion engine
• Wind turbine increases kinetic energy of wind


  Electrical energy obtained
• Desirable characteristics of a useful energy source

  • Releases energy at a reasonable rate
  • Produces minimal pollution
• Energy sources

  • Renewable
  • Non-renewable
• Non-renewable energy sources


  • Will run out in the (near) future
  • Cannot be easily replaced
• Renewable energy sources

  • Naturally replenished
  • Will not run out
• Advantages and disadvantages of energy sources

  • Solar energy
  • Burning fossil fuels
  • Nuclear fission
  • Non-rechargeable batteries
  • Rechargeable batteries
  • Hydrogen fuel cells
  • Nuclear fusion
• Specific energy


  Energy released from fuel / Mass of fuel consumed
• Energy density

  Energy released from fuel / Volume of fuel consumed
• Hydrogen has a higher specific energy and energy density than octane
• 4.26


  163 cm3 or 0.163 dm3
• 0.703


  33 600 kJ dm−3
• Energy density is sometimes used to describe the energy released per unit mass, rather than per unit volume – it is important to check the units to make sure what the term is describing
• 1 MJ

  1 × 106 J
• We usually use the molar volume measured at STP (22.7 dm3 mol−1) but the problem here is that the standard enthalpy change of combustion is measured at 298 K
• We could have made the assumption that the enthalpy change of combustion does not vary much with temperature, in which case we could have calculated the energy density as 286/22.7 = 12.6 kJ dm−3
• 114.26 g


  Mass of one mole of octane
• A slightly different answer is obtained if more significant figures are carried through on the calculator
• When hydrogen burns it releases approximately three times more energy per gram than octane, but the energy released per unit volume is more than 2000 times higher for octane
• Basic Oxide reaction for Na


  Na2O(s)+H2O->2NaOH(aq)
• A typical family car could have an average fuel consumption of 7 litres (dm3) of gasoline per 100 km – so to travel the same distance more than 1500 litres (dm3) of hydrogen gas would be required (the size of a trunk/boot of a car is typically about 400 dm3)
• One solution to the problem of storing large volumes of hydrogen would be to store the hydrogen under pressure but this requires fuel tanks to be made of thicker material which is also heavier and more expensive
• Other methods of storage are being investigated such as cryo-compression (storing at low temperature under pressure) and various forms of chemical storage
• When considering the best fuel for a particular job, both the specific energy and the energy density must be considered
• When weight is a problem then specific energy is likely to be more important – for instance, the space shuttle used liquid hydrogen as a fuel
• When storage volume for the fuel is an issue then energy density becomes more important
• All science has to be funded – either by governments, international organisations or companies. Science should, however, not be biased or be influenced by the vested interests of large international corporations such as oil companies
• Scientists can sometimes be put in difficult positions when their findings do not fit with the interests of the people funding their research
• We can understand the non-existence of perpetual motion machines by describing energy as having both quantity and quality
• In an energy transfer the total amount of energy transferred is constant but the quality is degraded