Energy from fuels

combustion of metals

• general equation: metal + oxygen —> metal oxide

• reducing agent is the metal

combustion of non metals

• general equation: non-metal + oxygen —> non metal oxide

• reducing agent is the non-metal

complete combustion of organic compounds

• they don’t spontaneously combust because their reactions have high activation energy - this is useful

• general equation complete combustion: fuel + oxygen —> CO2 + water

• general equation hydrocarbon combustion: CxHy + (y/4)O2 —> xCO2 + (y/2)H2O

incomplete combustion

• limited supply of oxygen, produces water

• produces CO or C (if oxygen supply very limited)

  • from carbon present in the organic compound

  • the higher the carbon content in a fuel, the more likely it is that incomplete combustion will occur, as more oxygen is required for complete combustion

  • a sooty yellow flame is indicative of incomplete combustion

• CO is toxic gas that binds irreversibly to haemoglobin in blood, preventing it from carrying oxygen

coal

• advantages

  • relatively safe

  • abundant

  • long life span

• disadvantages

  • combustion produces lot of pollution

  • global warming and acid rain

  • not readily transported

oil

• advantages

  • easy to store and transport

  • impurities easily removed

• disadvantages

  • combustion produces lot of pollution

  • global warming and acid rain

  • limited lifespan

natural gas

• advantages

  • cheapest

  • easy to store and transport

  • large energy per unit mass

• disadvantages

  • combustion produces lot of pollution

  • expensive to store

  • global warming

  • greater risk of explosions

  • limited life span

burning fossil fuels

• specific energy is energy stored in a subtance

• as hydrocarbon chain increases

  • increased carbon content, production of more carbon dioxide/monoxide/carbon

  • stronger london forces, hydrocarbon is less volatile

  • releases less energy per unit mass of fuel (so methane makes the most)

specific energy of a fuel equation

energy released from fuel / mass of fuel consumed in kJ kg -1

carbon dioxide levels

• CO2 levels are rising due to human activities such as

  • combustion for electricity

  • deforestation

• CO2 and methane contribute carbon levels and raise temperature

• increased use of fossil fuels is main reason for increase

greenhouse effect

• GH gases absorb radiation emitted from earth and trap it so its not lost to space

• radiation from sun strikes earth, is absorbed, and re emitted from earth as IR radiation

• some IR radiation passes through the atmosphere, some is absorbed by GH gases

• this reduces thermal energy lost in space and traps it within earth

why GH gases absorb IR radiation

• molecules can vibrate as the bonds in them stretch and bend

• energy associated with bond vibrations is IR

• if stretching and bending involves a change in dipole, then vibrations are IR active

biofuels

• made from organic compounds which are produced by biological carbon fixation

• renewable, reduce pollution from combustion of fossil fuels

general photosynthesis equation

6CO2 + 6H2O —> C6H12OH + 6O2

bioethanol

• glucose made from photosynthesis converted into ethanol by fermentation

  • C6H12O6 —> 2C2H5OH+2CO2

• carbon neutral because CO2 absorbed during photosynthesis = CO2 produced by combustion of biofuel

• ethanol is then burned to produce energy

• ethanol can be combined with gasoline, gasohol is produced, used by cars

biofuels in combustion

• direct combustion of waste material from plants or animals

biogas

• fuel released when organic matter is broken down by microorganisms anaerobically

pros and cons of biofuels

• pros

  • carbon neutral, renewable, sustainable

  • reduce GH emissions

  • safer to produce

• cons

  • expensive

  • many developed countries don’t have space to produce enough plants for biofuels

  • lower specific energy than fossil fuels

fuel cells

• electrochemical cell where fuel gives electrons at one electrode and oxygen gains electrons at the other electrode

• as fuel enters cell it becomes oxidised, which sets up voltage within the cell

hydrogen oxygen fuel cell equations

methanol fuel cell equations

hydrogen oxygen fuel cell pros and cons

• pros

  • water is the only reaction product, so better for environment

  • no harmful oxides of nitrogen produced

• cons

  • hydrogen is highly flammable so dangerous to store

  • expensive

methanol fuel cell pros and cons

• pros

  • easier to store than hydrogen

  • has greater energy density than hydrogen

• cons

  • toxic gas

  • commonly made from non renewable fossil fuels

  • fuel cell produces lower voltage