Chemistry revision notes

  • Electrolysis
  • Percentage yield
  • Atom economy
  • Mass of moles
  • Percentage composition
  • Crystilisation
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  • Created by: vlada
  • Created on: 15-05-12 09:19


"Splitting up a compound using electricity"

e.g. sodium chloride >> sodium + chlorine 


Electrolysis only works with ionic compounds. Has to be heated above 800°C so uses up a lot of energyIt costs lots of money to seperate it this way because lots of electrical energy is used. 


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Haber Process - making ammonia (NH3)

Ammonia is used for fertilisers. The haber process is a reversible reaction. Has to condense into a liquid to seperate the elements. 


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  • Covalent bonding (two non metal atoms) - when atoms react by sharing electrons.
  • Ionic bonding (metal and non metal atoms) - when atoms react by losing or gaining electrons. 
  • Metallic bonding - is the force of attraction between free electrons and metal ions. Metallic bonds are strong and have high melting and boiling points.
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Ionic Compounds

  • High melting and boiling points. 
  • Electrostatic attractive forces are large, so lots of energy requires to break them.

3D structure arranged in a "lattice" 

           (                             (

Ionic compounds will dissolve in wate because the ions are free to move

Ionic compounds also conduct electricity in water                         

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Giant metallic structures

Atoms are arranged in layers so can slide over eachother one a force is on it. 

This makes metals malleable and ductile.

Alloys are made when metals are mixed with other elements, disrupting the layers. This makes the particles less able to move. 

Nano particles - very small particles. 

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Giant covalent substances

Diamond - every carbon atom is bonded to 4 other carbon atoms. There are no free electrons. Have strong covalent bonds, so diamond is hard. Diagram showing diamond structure (                                  (

Graphite - every carbon atom is bonded to 3 other carbon atoms. There is one free electron per atom. These delocalised electrons mean graphite conducts electricity. The forces between layers are weak, meaning they can slide over each other. 

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1 mole of any substance always has the same number of particles.  

H20 = H + H + O

       = 1 + 1 + 16 

       = 18 (relative molecular mass) so 1 mole of H20 is 18g.

Percentage composition 

What is percentage mass of magnesium oxide (MgO) is magnesium? 

MgO + 24 + 16 = 40 

% Mg is 24/40 x 100 = 60% 

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Percentage yield - how much product is made compared to how much is predicted. 

Yield is affected by:

  • reversible reaction 
  • unexpected products 
  • impure reactants 

Atom economy - the amount of strarting reactants that end as product. 

  • aim is to have few wasted reactants and lots of product 
  • good for company finances and environment 
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Endothermic and Exothermic

Exothermic reactions transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. e.g. neutralisation. 

Endothermic reactions take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. e.g. electrolysis. 

The decomposition of ammonium chloride is a reversible reaction:

ammonium chlorideEquilibrium symbol ( + hydrogen chloride

Ammonium chloride decomposes when it is heated, so the forward reaction is endothermic - energy must be transferred from the surroundings for it to happen. The backward reaction is exothermic - energy is transferred to the surroundings when it happens.

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The process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. 

Crystals are made if the solution is dissolved in water. 

If not dissolved then filtration method is used instead. 

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State symbols are used in symbol equations:

  • (s) means solid
  • (l) means liquid 
  • (g) means gas
  • (aq) means aqueous (dissolved in water)

acid + metal oxide → salt + water

hydrochloric acid + sold iron oxide → iron chloride + water

acid + metal hydroxide → salt + water

hydrochloric acid + sodium hydroxide → sodium chloride + water

acid + metal → salt + hydrogen 

hydrochloric acid + magnesium → magnesium chloride + hydrogen

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