Lecture 22

1. Fuels. Reaction Rate. Electrolysis. Liquid, Solid, and Gaseous Fuels Reaction Rates Oxidation and Reduction Lecture 22 Chapter 11.6  11.15

2. Fuels Fuel is a substance whose oxidation reaction is strongly exothermic. Other requirements for good fuels: low price, abundance, easy transportation, non-toxic combustion products. Carbon and some of its compounds fulfill these requirements. These are: petroleum, natural gas, coal. Their combustion products are CO2 and H2O. Petroleum products give off 4449 kJ/g of heat. Each gram of gasoline requires 15 g of air for complete combustion.

3. Fuel Combustion Complete combustion reaction: 2C8H18 + 25O2 16 CO2 + 18H2O Incomplete combustion reaction: 2 C8H18 + 20 O2 11 CO2 + 15 H2O + 3 CO + C2H6  Ethane  Octane CO combines with the hemoglobin, replacing O2, and is lethal even in small concentrations (~1%).

4. Pollution Problem Incomplete combustion occurs due to a non-perfect burning process and causes air pollution. In 1920s it was found that adding a lead compound (tetraethyllead) to gasoline improves burning in engines. This improves engine performance, but adds lead compounds to engine emission. One way to reduce pollutions is to add a catalytic converter to the exhaust system (platinum or rhodium) to change polluting gases into harmless. Another method is to change gasoline (add ethanol or produce a different mixture of hydrocarbons).

5. Gaseous and solid fuel produce mostly the same substances as a result of combustion (CO2 and H2O). Gaseous Fuels The most common gaseous fuel is natural gas (CH4). A gas fuel can be produced synthetically by passing very hot steam over coal: C + H20  CO + H2, C + 2H2  CH4 Hydrogen is a very good fuel as it gives off 143 kJ/g. However, it is still too expensive.

6. Solid Fuels The main solid fuels in use are coal (32 kJ/g) and wood (19 kJ/g). Natural coal contains additional substances (e.g., sulfur). Additionally, oxidation of nitrogen produces acid rains. pH of acid rains can be as low as 23 (pure water pH=7). Poisonous products can be converted into less harmful. 2SO2 + O2 + 2CaCO3 2CaSO4 + 2CO2

7. Reaction Rates Some reactions, especially with ionic substances, are very fast. The most important factors that affect reaction rates are: Temperature: the higher it is, the faster the reaction Concentration: the same as above Surface area: the greater it is, the faster the reaction Catalysts: can increase or decrease the reaction rate Catalysts are substances that either speed up or slow down a reaction being unaffected.

8. Oxidation and Reduction Oxidation is the chemical combination of a substance with oxygen. Reduction is the removal of oxygen from a compound. A more general definition: Oxidation is a loss of electrons by the atoms of an element. Reduction is a gain of electrons. The two processes must take place together.

9. Electrolysis An electrode is a conductor through which electric current enters or leaves a solution. Negative electrode is called cathode, positive  anode. 2NaCl  2Na + Cl2 Cl Cl + e Na + e  Na Electroplating is depositing of a metal on an object made of another metal.

10. Electrochemical Cells Oxidation-reduction reactions can produce electric currents. Currents can be used by adding conducting wires. All kinds of batteries based on oxidation-reduction reactions are electrochemical cells. Fuel cell use reactants continuously. Hydrogen-oxygen cell combines 1 kg of H with 8 kg of O and produces 200 MJ of electric energy.

11. Summary The most efficient currently used fuels are pollutant and need to be replaced by safer ones. Reaction rates depend on the temperature, concentration, surface area, and the presence of a catalyst. Fuel cells are the most promising energy suppliers of the future.