Salts

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  • Created by: Holly45
  • Created on: 25-03-15 14:21

Acids and alkalis

  • the pH scale goes from 0-14
  • pH is a measure of how acidic or alkaline a solution is
  • the strongest acid has a pH 0
  • strongest alkali has pH 14
  • the die in the indicator changes colour depending on wether it's above or below a certain pH
  • a universal indicator is a combination is dyes which give the colours
  • acids for H+ ions in water
  • a base is a substance with a pH greater than 7
  • an alkali is a base that disolves in water to for OH- ions in water
  • H+ + OH- -----------> H2O
  • acid + base ---------> salt + water
  • neutralisation products are neutral with a pH 7
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Acids reacting with metals

  • acid + metal --------> salt + hydrogen
  • the more reactive the metal, the faster the reaction will go
  • copper does not react with dilute acids at all because it's less reactive than hydrogen
  • the speed of a reactiom is indicated by the rate at with bubbles are given off
  • hydrochloric acid can react with magnesium, aluminium and zinc to form chlorides
  • sulphuric acid can react with magnesium, zinc and aluminium to form sulphates
  • nitric acids product nitrate salts when neutralised but can also produce nitrogen oxides
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Oxides hydroxides and ammonia

  • some metal oxides and metal hydroxides dissolve in water
  • even bases that won't dissolve in water will still react with acids
  • all metal oxides and metal hydroxides react with acids to form salt and water
  • acid + metal oxides ------> salt and water
  • acid + metal hydroxide --------> salt + water
  • ammonia dissolves is water to make an alkaline solution
  • ammonia + nitric acid -------> ammonia nitrate
  • ammonia nitrate is a good fertiliser because it has nitrogen from 2 sources
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Making salts

ACIDS:

  • you need to pick the right acid and metal
  • you add the metal, metal oxide or hydroxide to the acid. The solid will dissolve in the acid. You will know the acid has neutralised because the excess solid will just sink to the bottom
  • filter out the metal to get the salt solution. To get the pure, solid crystals off the salt, evaporate some of the water and then leave

ALKALIS:

  • you have to add exactly the right amount of alkali to just neutralise the acid
  • you use an indicator to show the reaction has finished
  • evaporate the water to crystallise the salt

INSULUBLE SALTS:

  • precipitation reaction
  • you need to pick 2 solutions containing ions that you need
  • filter, wash and dry out
  • precipitation reactions can be used to remove poison ions from drinking water
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Electrolysis

  • if you pass an electric current though an ionic substance that's mooten or in a solution, it breaks down into the elements it's made from
  • requires liquid to conduct electricity which is called an electrode
  • electrolytes contain free ions which are usually the molten or dissolved ionic substance
  • the free electrons conduct electricity
  • electrons are taken away from ions at the positive electrode and given to the other ions at the negative electrons
  • as ions loose or gain electrons they become atoms or molecules

OXIDATION IS LOSS
REDUCTION IS GAIN

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Electrolysis of sodium chloride solution

  • at the negative electode if metal ions and H+ ions are present, the metal ions will stay in solution if the metal is more reactive than hydrogen
  • the more reactive an element, the keener it is to stay as ions
  • at the positive electrode, if OH- and halide ions are present then molecules of chlorine, bromine or iodine will be formed
  • if no halide is present, then oxygen will be formed
  • when sodium chloride is dissolved and electrolysed it will produce: hydrogen, chlorine and sodium hydroxide
  • half equations show the reactions at the elecrode
  • sodium hydroxide is a very strong alkali and is used widely for soap
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Extraction of aluminium and electroplating

  • aluminium is always found naturally in compounds
  • it's main ore is bauxite and after mining and purifying, a white powder is left
  • Al2O3
  • aluminium has to be extracted using electrolysis
  • aluminium oxide is dissolved in molten cryolite
  • doing this brings the temperature down and makes it cheaper and easier
  • the electrodes are made out of carbon which is a good conductor of electricity
  • aluminium forms at the negative electrode and oxygen forms at the positive
  • oxygen reacts with the carbon to produce carbon dioxide

ELECTROPLATING:

  • uses electrolysis to coat the surface of one metal with another metal
  • the negative electrode is the metal object you want to plate and the positive is the pure metal you want to be plated with
  • electrode must contain ions of the plating metal
  • it's much cheaper to plate a brass cup with silver then to make a solid silver cup
  • metals like copper conduct electricity
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