C4- Chemical Changes

• What is the pH scale?

  A measure of how acidic or alkaline a solution is:

  • The lower the pH, the more acidic it is
  • The higher the pH, the more alkaline it is
  • A neutral substance has pH 7
• Describe the structure of the pH scale

  Acids:

  • pH 0-2: car battery acid, stomach acid
  • pH 3: Vinegar, lemon juice
  • pH 4: Acid rain
  • pH 5-6: normal rain

  pH 7 Neutral: pure water

  Alkaline:

  • pH 8-9: washing-up liquid
  • pH 10: Pancreatic juice
  • pH 11: Soap powder
  • pH 12: bleach
  • pH 13-14: Caustic soda
• How can an indicator be used to measure the pH of a solution?

  A indicator is a dye that changes colour depending on whether it's above or below a certain pH:

  • Some indicators contain a mixture of dyes that means they gradually change colour over a broad range of pH
  • These are called wide range indicators
  • The most common one for pH is the universal indicator
• How can a pH probe and meter be used to measure the pH of a solution?

  A pH probe is attached to a pH meter to measure pH electronically:

  • The probe is placed in the solution you are measuring and the pH is given on a digital display as a numerical value, meaning it's more accurate than an indicator
• What is an acid?

  A substance that forms aqueous solutions with a pH less than 7

  Acids form H+ ions in water
• What is a base?

  A substance with a pH greater than 7
• What is an alkali?

  A base that dissolves in water to form a solution with a pH greater than 7.

  Alkalis form OH- ions in water
• What is the reaction between acids and bases called?

  Neutralisation:

  acid + base --> salt + water
• What do acids produce in aqueous solutions?


  They produce hydrogen ions (protons) in water

  They ionise in aqueous sokut
• What do strong acids do in water?

  They ionise completely in water. All acid particles dissociate to release H+ ions
• What do weak acids do in water?

  Weak acids do not fully ionise in solution. Only a small proportion of acid particles dissociate to release H+ ions

  The ionisation of a weak acid is a reversible reaction, which sets up an equilibrium between the undissociated and dissociated acid

  Since only a few of the acid particles release H+ ions, the position of equilibrium lies well to the left
• Describe why strong acids are more reactive than weak acids

  Reactions of acids involve the H+ ions reacting with other substances. If the concentration of H+ ions is higher, the rate of reaction will be faster, so strong acids will be more reactive than weak acids of the same concentration
• What is pH?


  The measure of the concentration of H+ ions in the acidic/alkaline solution
• What happens when the pH scale decreases?


  . For every decrease of 1 on the pH scale, the concentration of H+ ions increase by a factor of 10

  . For a decrease on 2 on the pH scale, the concentration of H+ ions increases by a factor of 100

  
  

  Factor H+ ion in concentration changes by= 10^-x

  x= difference in pH
• What does acid strength tell you?

  What proportion of the acid molecules ionise in water
• What are concentrated acids?

  How much acid there is in a certain volume of water
• What are metal oxides and hydroxides?


  Soluble compounds that are alkalis, that dissolve in water
• What is the formula including metal oxides?

  Acid + Metal Oxide --> Salt + Water
• What is the formula including metal hydroxides?

  Acid + Metal Hydroxide --> Salt + Water
• What do acid and metal carbonates form?

  Acid + Metal Carbonate ---> Salt + Water + Carbon Dioxide
• What are metal oxides, hydroxides and carbonates examples of?

  Bases
• Describe the practical 'Making soluble salts from an insoluble base'

  . Pick a suitable acid and insoluble base (e.g. insoluble metal oxide/hydroxide/carbonate)

  . Gently warm the dilute acid using a bunsen burner, then turn the burner off

  . Add the insoluble base to the acid a bit at a time, until no more reacts (the base is in excess). When the acid has been neutralised, excess solid will sink to the bottom of the flask