CC8- acids

• acids are substances that produce H+ ions when they dissolve in solution.

  alkalis are substances that produce OH- ions, or hydroxide ions when they dissolve in solution.
• the higher the concentration of H+ ions, the lower the pH.

  as the concentration of H+ ions decreases by a factor of 10, the pH of the solution will increase by 1.
• the higher the concentration of OH- ions, the higher the pH.

  OH- ions react with H+ to form water and have a pH above 7, which is considered a neutral pH.
• the pH of a solution can be found using indicators or a pH meter.

  litmus: acid is red, alkali is blue

  methyl orange: acid is red, alkali is yellow

  universal: acid is red, alkali is purple

  phenolphthalein: acid is colourless, alkali is pink
• acids that dissociate (split up) fully are strong acids

  acids that do not dissociate fully are weak acids
• concentrated acids have a high number of acid molecules dissolved in solution

  dilute acids have a low number of acid molecules dissolved in solution
• example question: explain how a weak acid can have a lower pH than a strong acid.

  1. a high concentration of H+ ions is needed for a low ph
  2. weak acids do not fully dissociate so do not produce a lot of H+ ions
  3. strong acids do fully dissociate so do produce a lot of H+ ions
  4. if the weak acid was concentrated and the strong acid was dilute, the weak acid could have more H+ ions in solution hence a lower pH than the strong acid
• bases are substances that neutralise acids to form salt and water.
• in general, there are 3 types of base:

  metal hydroxides e.g lithium hydroxide

  metal oxides e.g calcium oxide

  metal carbonates e.g calcium carbonate
• when the base is a metal carbonate, carbon dioxide is produced.

  the test for carbon dioxide:

  bubble the gas through limewater

  if it turns cloudy the gas is CO2
• when bases are soluble in water, we call them alkalis.
• the reaction that takes place between an acid and a base to form salt and water is known as a neutralisation reaction.
• worked example: describe what happens in terms of ions when an acid and alkali react.

  • acids produce H+ ions
  • alkalis produce OH- ions
  • H+ reacts with OH- to produce H2O
• acids react with metals to form a salt and hydrogen gas, which means fizzing will be seen.

  testing for hydrogen gas:

  • insert a lit splint
  • a squeaky pop indicates gas is hydrogen
• if a salt does not dissolve you will see a solid forming over the course of the reaction- this is called a precipitation reaction.

  soluble in water:

  • all common sodium/ potassium/ammonium salts
  • all nitrates
  • most chlorides
  • most sulphates
  • sodium, potassium and ammonium carbonates
  • sodium, potassium and ammonium hydroxides

  insoluble in water:

  • silver, lead chlorides
  • lead, barium, calcium sulfates
  • most carbonates
  • most hydroxides
• to prepare an insoluble salt do the following:

  • mix two solutions in a beaker
  • filter mixture to remove solid precipitate
  • rinse the beaker with a little distilled water and pour through the funnel to ensure all of the precipitate has been collected
  • take the filter paper out and place in the oven to dry