4- Chemical Changes

• Metal + oxygen → metal oxide 

  When metals react with oxygen we form a metal oxide and an oxidation reaction takes place. The metal atom has been oxidised 

  • Oxidation: Metal has gained oxygen 
  • Reduction Reaction: losing oxygen 
• Reactivity series: Potassium, Sodium, Lithium, Calcium, Magnesium, Aluminium, Carbon, Zinc, Iron, Lead, Hydrogen, Copper, Silver, Gold, Platinum
• Reactivity with Acid

  
  

  Potassium, Sodium, Lithium = Dangerously fast reaction with acids

  Calcium = Extremely vigorous reaction

  Magnesium = rapid reaction

  Zinc = quite rapid reaction

  Iron  = quite slow reaction

  Copper = no reaction
• Reactivity with Water

  Potassium, Sodium, Lithium = react rapidly with water at room temp

  Calcium = reacts quite rapidly

  Magnesium, Zinc , Iron, Copper = no reaction with water
• • A more reactive element with displace (push out) a less reactive element from its compound (this is according to the reactivity series, eg- iron would get displaced by all the metal above it on the list) 
• Oxidation: is the loss of electron

  Reduction:  is the gain of electrons 

  • S + 2e- → S2- oxygen has been reduced as it has gained electrons  
  • OIL RIG - oxidation is loss of, reduction is gain 
• oxidation is loss, reduction is gain
• In acids (aqueous solutions) acids produce hydrogen ions (H+)

  • HCl →  H+ + Cl-
• Bases are chemicals that can neutralise acids producing a salt and water, (usually metal oxides or metal hydroxides.)

  • bases are insoluble (cannot dissolve in water)

  
  

  Bases that are soluble in water are also alkalis  

  • In aqueous solution, alkalis produce hydroxide ions (OH-)
• Ph levels: 0-6 acids, 7 neutral, 8-14 alkali 
• equation for neutralisation

  • Acid + Base → Salt + Water

  H+(aq) + OH-(aq) → Salt + H20

  Acid= H+

  Base = OH-
• salts produced:

  
  

  Hydroloric acid = chloride

  sulfuric acid = sulfate
• Displacement example

  Hydrochloric acid + magnesium →  magnesium chloride + hydrogen

  • magnesium displaced hydrogen as it is high on the reactivity series
• Rate of reaction with dilute acids:

  magnesium - very rapid reaction = Easily forms Mg2+ 

  zinc - quite rapid = Quite easily forms Zn2+ 

  Iron- slow reaction = Less easily forms Fe2+ 
• When we react an acid with either a base or an alkali, we make a salt and water

  Acid + (base or alkali) → salt + water

  
• Salts: contain a positive ion which comes from base or alkali

  • they can also contain a negative ion which comes from the acid
• negative ions produced by acids:

  hydrochloric acid → ....chlorides

  sulfuric acid → ....sulfates

  nitric acid → ....nitrates
• Metal Carbonate- each contains a metal ion

  When acids react with a metal carbonate they make a salt, water and carbon dioxide

  (Acid + Metal → Salt + Hydrogen (water) + carbon )

  
• Required Practical 1 Making Soluble Salts

  All salts contain a positive ion and a negative ion 

  
  

  react hydrochoril acid with copper oxide in container

  add copper oxide untill in excess

  filter excess copper oxide

  use bunsen burner to gently heat solution (will evaporate)

  leave to crystalise
• In aqueous solutions, acid molecules ionised (split) and release the hydrogen ion H+ 

  • Strong acids fully ionise in aqueous solutions

  
• Weak acids partially ionised in aqueous solutions. Only some particles dissociate to release H+ ions, meaning this reaction is a reversible reaction.

  • Examples: carbonic acid, ethanoic acid and citric acid
• The pH scale gives as idea of the concentration of hydrogen ions (H+)

  • As the pH scale decreases by one unit, the concentration of hydrogen ions increases by ten times

  ! acids are less than 7 on the pH scale

  
• The concentration of an acid tells us the amount of acids molecules in a given volume of solution

  • A dilute acid will have fewer acid molecules in a given volume than a concentrated acids even if the strength of the acid is the same 
  • This is because  dilute acid contains a small amount of acid in a given volume.
• Introducing Electrolysis

  • Solid ionic compounds cannot conduct electricity, this is because the ions are locked in place and not free to move- they have strong electrostatic forces of attraction. 
  • When an ionic compound is melted or dissolved in water, the forces of attraction are broken and the ions are free to move- these liquids and solutions can now conduct electricity - these are called electrolytes 
• PANIC: Positive (is) Anode Negative Is Cathode.

  Negative electrode- (cathode) are covered with electrons, positive electrode- (anode) lack electrons

  • Positive ions are attracted to negative cathode, then they gain electrons  (reduction)
  • Negative ions lose electrons (oxidation)
• • Electrode: a rod of metal or graphite through which an electric current flows into or out of an electrolyte
  • Electrolyte: ionic compound in molten or dissolved solution that conducts the electricity
• • Anode: the positive electrode of an electrolysis cell
  • Anion: negatively charged ion which is attracted to the anode
  • Cathode: the negative electrode of an electrolysis cell
  • Cation: positively charged ion which is attracted to the cathode

  
• Electrolysis involves using electricity to break down electrolytes to form elements.

  Why do we use electrolysis:

   to separate metal ions from a solution or molten substance and deposit them as pure metal at the electrodes.
• Electrolysis of Aluminium Oxide

  anything more than carbon on the reactivity series are extracted using electrolysis

  Aluminium is extracted from the compound aluminium oxide by electrolysis, contains aluminium ion (AI3+) and oxide ion (O2-)

  • anode must be replaced regularly, the oxygen molecules produced at the anode react with the graphite(form of carbon) electrode forming carbon diox

  
• electrolysis is expensive:

  melting compounds require a lot of energy and a lot of energy is required to produce electric current 
•  Electrolysis of Aqueous Solutions (copper sulphate)

  • Aqueous solutions are dissolved in water- water molecules ionise to form hydrogen ions and hydroxide ions

  At the cathode: copper ions are discharged at the cathode as copper is less reactive than hydrogen and form copper metals (reduction) They gain 2 electrons

  • equation: Cu2 + 2e- → Cu

  At the anode: hydroxide ions are discharged at the anode and form oxygen gas (oxidation)

  
• Electrolysis of Aqueous Solutions (sodium chloride)

  Aqueous solutions are dissolved in water- water molecules ionise to form hydrogen ions and hydroxide ions

  To work out which ion will react you have to go to the reactivity series

  • if the metal (eg: sodium) is more reactive = hydrogen is produced at the cathode
  • If the solution contains halide ions then the halogen will be 

  produced at the anode (group 7)

  1. Hydrogen is produced at the cathode
  2. At the anode a chloride gas is produced 

  
• Required Practical 3: Electrolysis

  Summary:

  If you electrolyse copper (II) chloride solution, then you get chlorine gas discharge at the anode and copper discharge at the cathode 

  
• Repeat Electrolysis with sodium chloride solution 

  1. Pour approx 50cm3 of sodium chloride solution into a beaker 
  2. Insert carbon graphite rod- (electrodes) carbon graphite are inert (won't react) must not touch each other or it will produce a short circuit
  3. Positive anode: bubble of gas being produced- this bleaches blue litmus paper, this tells us the gas is chlorine, chlorine is a halide so it is discharged at the anode
  4. Gas bubbles at cathode- this is hydrogen gas, because sodium is more reactive than hydrogen, hydrogen gas is produced.