acids and redox

acids

  • All acids contain hydrogen
  • When dissolved in water an acid releases H+ ions into the solution
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strong and weak acids

  • strong acids completely disassociate (release all their hydrogens) into an aqueous solution
  • Weak acids only partially dissociate (release a few hydrogens) in aqueous solution
  • most organic acids are weak acids like ethanolic acid 
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bases and alkalis

  • metal oxides, metal hydroxides, metal carbonates and ammonia are bases
  • a base neutralises an acid to form a salt
  • an alkali is a base that dissolves in water releasing hydroxide ions into the solution
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neutralisation of acids with metal oxides and hydr

  • acid is neutralised by a metal oxide or metal hydroxide to form a salt and water only
  • acid + alkali -> salt + water
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neutralisation of acids with carbonates

  • carbonates neutralise acids to form a salt, water and carbon dioxide gas
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titration

  • titration is used to accurately measure the volume of one solution that reacts exactly with another solution
  • titrations can find the concentration of a solution
  • to identify unknown chemicals 
  • find the purity of a substance
  • checking purity is an important aspect of quality control especially for compounds manufactures for human use like medicines, food, and cosmetics
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preparing a standard solution

  • a volumetric flask is used to make a solution very accurately 
  • volumetric flasks are manufactured to certain tolerances
  • The 100cm^3 flask has a tolerance of + or - 0.2^3
  • the 250cm^3 flask has a tolerance of + or - 0.3cm^3
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the mean titre

  • only use the most accurate titres
  • repeat titrations until two agree within 0.1cm^3
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titration calculations

  • work out the amount in moles of the solute in the solution for which you know both the concentration and the volume
  • use stoichiometry to work out the amount in moles of the solute in other solutions
  • work out the unknown information about the solute in the other solution
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oxidation number

  • can be thought of as the number of electrons involved in bonding to a different element
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rules from elements

  • the oxidation number is always zero for elements
  • in a pure element, any bonding is to atoms of the same elements
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rules for compounds and ions

  • each atom in a compound has an oxidation number
  • a sign is placed before the number

O = -2

H = +1

F = -1

Na+ K+ = +1

Mg2+ Ca2+ = +2

Cl- Br- = -1

H in metal hydroxides = -1

O in peroxides = -1

O bonded to F = +2

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working out oxidation numbers

  • the sum of oxidation numbers = total charge
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redox reactions - reduction and oxidation

  • oxidation is the addition of oxygens, the loss of electrons and an increase in oxidation number
  • the reduction is the removal of oxygen, the gain of electrons and decrease in oxidation number
  • redox reactions involve reduction and oxidation
  • if one process happens, so must the other
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