• Energy changes drive chemical reactions
• Common types of chemical reactions
• Oxidation and reduction
• Precipitation and solution
• Acid – base
• Polymerization and depolymerization
• Hydrocarbons
• Isomers

• Last lecture was an examination of the sorts of changes that pressure and temperature can bring to a group of atoms with a fixed composition. These changes are called phase changes. In this lecture we examine the sorts of changes that can occur when chemistry is varied. These changes are called chemical reactions.

• Chemical reactions occur when compounds form or breakup.
• Compounds breakup by breaking bonds and compounds form by the formation of new bonds: Eg. 2H₂ + O₂ = 2H₂O
• Each atom has a chemical energy associated with it. This energy is the sum of the energies of all of its electrons.
• Each electron has three types of energy associated with it:
• Orbital kinetic energy from the electron whizzing around.
• Attractive potential energy to the positively charged nucleus
• Repulsive potential energy with the other electrons
• The energy of a compound is the sum of the energy of all of its atoms.
• When two atoms form a chemical bond either:
• The final energy is greater than the two atoms alone or
• The final energy is less than the two atoms alone
• If the energy is less, then the reaction gives off energy and is called exothermic. E.g. the chemical reaction of a burning match.
• If the final energy is greater, energy must be supplied to drive the reaction and it is called endothermic. E.g. the chemical reaction of turning brown sugar into toffee.
• Energy must be conserved in the reaction.

• Oxidation and reduction:
• An atom that gives up electrons, often to oxygen, is oxidized

eg. Rust (Fe metal combines with atmosphere to become FeO), Combustion
• An atom that receives electrons from other atoms is reduced

eg. Iron smelting (iron oxides are processed to become pure iron + oxygen)
• OIL RIG (oxidation is loss, reduction is gain)

• Precipitation and solution: Eg dissolve salt in water.
• Acid-base
• Acids give up H⁺ ions (a single proton) when they are dissolved in water
• Examples include: Lemon juice, orange juice, vinegar, sulfuric acid – car batteries
• Bases give up hydroxide ions, (OH)^- when dissolved in water
• Examples include: Antacids, ammonia, drain cleaner
• When acids and bases are brought together in solution they neutralize each other to form water and a salt. For example, hydrocloric acid and lye react to become water and table salt:
HCl + NaOH = H₂O + NaCl
• In a solution, some the hydrogen and hydroxide ions remain unattached (they do not form water).  In pure water, there are about 10^-7 moles per liter of hydrogen ions (i.e. 0.0000001 moles/liter).  PH = -log(H⁺ concentration) so for pure water, the pH is 7.  The lower the value of pH the more acidic the solution. E.g. a solution with a pH = 4 is more acidic than one with a pH = 10.
• Polymerization and depolymerization
• A polymerization reaction is any reaction that forms very large molecules from small molecules
• Depolymerization is the, as yet, irreversible breakdown of polymers
• Hydrocarbons: made from carbon and hydrogen
• Examples include: methane CH₄, ethane C₂H₆, propane C₃H₈, n-butane C₄H₁₀, pentane C₅H₁₂, hexane C₆H₁₄, heptane C₇H₁₆, octane C₈H₁₈, …

• Isomer – same atoms, different structure
• Longer chains = higher melting points, kinks = lower melting points
• Refinement of petroleum by distillation: boil off and collect different compounds successively

• Octanes – ability to withstand compression without exploding

eg 100 octanes = good as pure isooctane
• There is a strong structural similarity between these hydrocarbon fuels and the sugar and fats that our bodies consume. The sugars that give us instant energy are simpler than the complex fatty molecules that are difficult to break down.