C4

• pH scale


  Measure of how acidic or alkaline a solution is
• pH scale


  • Goes from 0 to 14
  • Lower pH = more acidic
  • Higher pH = more alkaline
  • Neutral substance (e.g. pure water) has pH 7
• Neutral substances

  • Normal rain
• Indicator


  Dye that changes colour depending on whether it's above or below a certain pH
• Wide range indicators


  • Contain a mixture of dyes that gradually change colour over a broad range of pH
  • Useful for estimating the pH of a solution
• pH probe


  Attached to a pH meter to measure pH electronically
• Acid


  • Substance that forms an aqueous solution with a pH of less than 7
  • Acids form H+ ions in water
• Base


  Substance that will react with an acid to form a salt
• Alkali


  • 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
• Skin is slightly acidic (pH 5.5)
• When an acid neutralises a base (or vice versa), the products are neutral (pH 7)
• When you mix an acid with an alkali, hydrogen ions from the acid react with hydroxide ions from the alkali to make water
• The leftover bits of the acid and alkali make a salt
• Titrations


  A method of analysing the concentrations of solutions
• Titrations


  • Used to find out concentrations
  • Allow you to find out exactly what volume of acid is needed to neutralise a measured volume of alkali-or vice versa
  • You can then use this data to work out the concentration of the acid or alkali
• How to do a titration

  1. Add a set volume of the alkali to a conical flask and add indicator

  2. Fill a burette with some acid of known concentration

  3. Use the burette to add the acid to the alkali a bit at a time, going slowly when the end-point (colour change) is about to be reached

  4. Record the final volume of acid in the burette and use it, along with the initial reading, to calculate the volume of acid used to neutralise the alkali
• You should repeat titrations to increase accuracy and find a mean volume
• Anomalous results

  Results that don't fit in with the rest
• Single indicators


  Used for titrations to see a sudden colour change at the end-point, unlike universal indicator which shows a gradual change
• Single indicators


  • Phenolphthalein
  • Methyl orange
  • Litmus
• Acids produce protons (H+ ions) in water
• Strong acids

  Ionise completely in water, all acid particles dissociate to release H+ ions
• Weak acids


  Do not fully ionise in solution, only a small proportion of acid particles dissociate to release H+ ions
• Ionisation of a weak acid

  Reversible reaction, sets up an equilibrium between undissociated and dissociated acid
• pH


  Measure of the concentration of H+ ions in the solution
• Decrease of 1 on the pH scale

  Concentration of H+ ions increases by a factor of 10
• The pH of a strong acid is always lower than the pH of a weaker acid if they have the same concentration
• Strong acids ionise completely, but weak acids don't
• The concentration of a solution and the strength of an acid are different things
• Metal oxides react with water to form bases (alkalis)
• Actions it of chemistry ever
• Ions, H.
• Until they meet water
• Gas isn't acidic
• More on equilibria
• Tum to p:127
• Factor of 10.
• That has a pH of 5.
• Tor of 100.