Compounds

Compounds

1. Formed when atoms from different elements join together

   • Like CO2
   • Elements are made if...
     • ...The atoms are all the same and not joined up
       • An element made up of atoms
     • ...The atoms are joined and there is only one type
       • An element made up of molecules
         • When two or more atoms join together, a molecule is made.
           • The 'join' is known as a chemical bond
           • A compound made up of molecules
             • Different atoms joined together
       • If two identical elements combine its not a compound
         • Identical atoms of the same element are often found combined
           • It doesn't make them a compound and their name doesn't change
     • A mixture is not a compound because the elements aren't joined up

2. Formed from chemical reactions

   • Chemical reactions involve chemicals (called reactants) combining together or splitting apart.
     • To form one or more new substances (called products)
       • When a new compound is made elements combine
         • The new compounds produced by any chemical reaction are always different from the reactants.
           • Example
             • Iron reacting with sulphur
               • Iron is magnetic. It reacts with sulfur to make iron sulfide
                 • Not magnetic
                   • Equations
                     • Iron + Sulphur -> Iron Sulphide
                       • Fe + S -> FeS
         • Compounds can be split back up into their original elements but it won't just happen by itself
           • you have to supply a lot of energy to make the reaction go in reverse
   • When elements undergo a chemical reaction, the products will always have a chemical formula
     • Example
       • H2O for water
   • Chemical Reactions
     • Atoms are not created or destroyed
       • They're still there at the end
         • Bonds get broken and made in the reaction as atoms rearrange themselves in going from the reactants to the products
           • The atoms themselves aren't altered
             • Example
               • zinc + Copper sulphate -> zinc sulphate + copper
                 • The mass doesn't change in a chemical reaction
                   • This is because the total number of atoms is the same before and after the reaction
     • Involve a change in energy
       • Reactions always take or take in energy
         • This energy is usually transferred by heating which causes the temperature in a reaction to go up or down
     • Visible changes can occur in the reaction mixture
       • These show that a chemical reaction has taken place
         • Example
           • When magnesium reacts with blue copper sulphate solution, the solution goes colourless, copper coats the magnesium strip and the temperature rises.
             • The mass stays the same
     • Three common examples
       • Combustion
         • Combustion is burning
           • a fuel reacts with oxygen to release energy
         • Three things are needed
           • Fuel Heating Oxygen
             • Hydrocarbons are fuels containing only hydrogen and carbon
               • When it's hot enough there's enough oxygen, hydrocarbons combust to give water and carbon dioxide
                 • Equation
                   • hydrocarbon + oxygen -> carbon dioxide + water (+energy)
         • Useful because energy is transferred away by heating and light
           • Its the process behind candles, wood fires, car engines, coal power plants...
       • Oxidation
         • When a substance reacts and combines with oxygen, its called an oxidation reaction
           • Examples
             • Combustion is an oxidation reaction
             • Rusting
               • Iron reacts with oxygen in the air to form iron oxide
                 • Equation
                   • iron + oxygen -> iron oxide (rust)
       • Thermal decomposition
         • When a substance breaks down into at least two other substances when heated
           • The substance isn't actually reacting with anything, but it is a chemical change
             • Some metal carbonates break down on heating.
               • Carbonates are substances with CO3 in them, like copper (II) carbonate (CuCO3) and zinc carbonate (ZnCO3)
                 • They break down into a metal oxide and carbon dioxide
                   • This usually results in colour change
     • Exothermic reaction
       • An exothermic reaction is one which transfers energy to the surroundings
         • Energy is usually given out by heating
           • exothermic reactions involve a rise in temperatures
             • Best example is combustion
               • This gives out a lot of energy
       • Many neutralisation reactions and oxidation reaction are exothermic
       • Everyday uses
         • Hand warmers

3. Naming Compounds

   • Rule 1
     • When two different elements combine the ending is usually 'something-ide'
       • Example
         • Iron and Sulphur = Iron Sulphide
   • Rule 2
     • When three or more different elemts combine - and one of them is oxygen - the ending will usually be 'something-ate'
       • Example
         • 1 copper, 1 sulpher, 4 oxygen = copper sulfate