Lecture 22

Fuels. Reaction Rate. Electrolysis.

• Liquid, Solid, and Gaseous Fuels• Reaction Rates• Oxidation and Reduction

Chapter 11.6 11.15

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 4449 kJ/g of heat.Each gram of gasoline requires 15 g of air for complete combustion.

Fuel Combustion

Complete combustion reaction:2C8H18 + 25O2 16 CO2 + 18H2O

Incomplete combustion reaction:2 C8H18 + 20 O2 11 CO2 + 15 H2O + 3 CO + C2H6

Octane

Ethane

CO combines with the hemoglobin, replacing O2, and is lethal even in small concentrations (~1%).

Pollution ProblemIncomplete 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).

Gaseous Fuels

Gaseous and solid fuel produce mostly the same substances as a result of combustion (CO2 and H2O).

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.

Solid FuelsThe 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 23 (pure water pH=7).

Poisonous products can be converted into less harmful.2SO2 + O2 + 2CaCO3 2CaSO4 + 2CO2

Reaction RatesSome 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 reactionConcentration: the same as aboveSurface area: the greater it is, the faster the reactionCatalysts: can increase or decrease the reaction rate

Catalysts are substances that either speed up or slow down a reaction being unaffected.

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.

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.

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.

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.