chemistry topic 4

ACIDS AND BASES

  • the pH scale is a measure of how acidic or alkali a solution is
  • pH 0-6 is an acid 0 being the strongest
  • pH 7 is neutral
  • pH 8-14 is an alkali, 14 being the strongest 
  • an indicator changes colour depending on the pH of the substance, a pH probe attached to a pH meter can be used to measure pH electronically meaning the result in more accurate given its a numerical value and not just a colour
  • an acid is a substance that forms aqueous solutions with a pH of less than 7, it forms H+ ions in water
  • a base is a substance with a pH greater than 7, an alkali is a base that dissolves in water to form a solution with a pH greater than 7, alkalis form OH- ions in water
  • neutralisation= acid + base -> salt + water
  • neutralisation= H+ + OH- -> H20
  • when an acid neutralises a base the products are neutral ie they have a pH of 7
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TITRATIONS

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STRONG ACIDS AND WEAK ALKALIS

  • strong acids ionise completely in water, they release H+ ions 
  • weak acids do not fully ionise in solution only a few H+ ions are released
  • the ionisation of a weak acid is a reversible reaction which lies to the left of equalibrium 
  • for every decrease of one on the pH scale, the concentration of H+ ions increase by a factor of 10
  • factor H+ ion concentration changes by = 10^-x
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REACTIONS OF ACIDS

  • metal oxides and metal hydroxides are both bases
  • acid + metal oxide -> salt + water
  • eg hydrochloric acid + copper oxide -> copper chloride + water
  • acid + metal hydroxide -> salt + water
  • eg hydrochloric acid + sodium hydroxide -> sodium chloride + water
  • acids and metal carbonates produce carbon dioxide
  • acid + metal carbonate -> salt + water + carbon dioxide
  • eg hydrochloric acid + sodium carbonate -> sodium chloride + water + carbon dioxide
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THE REACTIVITY SERIES

  • the reactivity series shows how well a metal reacts
  • in order from most to least reactive it goes: potassium, sodium, lithium, calcium, magnesium, carbon, zinc, iron, hydrogen, copper
  • the higher up in the reactivity series shows how easily positive ions are formed
  • the more reactive a metal, the faster the reaction
  • acid + metal -> salt + hydrogen
  • metal + water -> metal hydroxide + hydrogen
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SEPARATING METALS FROM METAL OXIDES

  • oxidation is the gain of oxygen, loss of electrons
  • reduction is the loss of oxygen, gain of electrons
  • a reaction that seperates a metal from its oxide is a reduction reaction
  • metals higher than carbon in the reactivity series have to be extracted using electrolysis
  • metals lower than carbon in the reactivity series can be extacted by reduction using carbon
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REDOX REACTIONS

  • if electrons are transferred,its a redox reaction
  • Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons) OIL RIG
  • a more reactive metal will displace a less reactive metal from its compound
  • displacement reactions are redox reactions
  • in displacement reactions its always the metal ion that gains electrons and is reduced 
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ELECTROLYSIS

  • electrolysis means splitting up with electricity
  • during electrolysis, an electric current is passed through an electrolyte (a molten or dissolved ionic compound)
  • positive ions in the electrolyte will move towards the cathode (the negative electrode) and will gain electrons (they are reduced)
  • negative ions in the electrolyte will move towards the anode (the positive electrode) and will lose electrons (they are oxidised)
  • an ionic solid cant be electrolysed as the ions are in fixed positions and cannot move
  • molten compounds can be electrolysed as the ions can move freely and carry a charge, molten ionic liquids are always broke up into their elements
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ELECTROLYSIS OF AQUEOUS SOLUTIONS

  • in aqueous solutions,as well as the ions form the ionic compound, there will be hydrogen ions (H+) and hydroxide ions (OH-) from the water 
  • at the cathode if h+ ions and metal ions are present hydrogen gas will be produced if the metal ions form an elemental metal that is more reactive than hydrogen, if the metal ions form an elemental metal that is less reactive than hydrogen eg copper, a solid layer of the pure metal will be produced instead
  • at the anode if the OH- and hailide ions (Cl-,Br-,I-) are present molecules of chlorine, bromine will be formed. if no hailide ions are presents then OH- ions are discharged and oxygen will be formed along with water
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