topic 4- chemical changes

• pH


  Using an indicator to measure how acidic or alkaline a solution is
• pH scale


  • 0
  • 1
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
  • 9
  • 10
  • 11
  • 12
  • 13
  • 14
• Acidic


  pH less than 7
• Alkaline


  pH greater than 7
• Neutral


  pH 7
• Indicators


  • Change colour depending on pH
  • Some are wide range and gradually change colour over a broad pH range
• pH meter


  Measures pH electronically and gives a numerical value
• Acid


  Substance that forms aqueous solutions with a pH less than 7
• Alkali


  Substance with a pH greater than 7 that dissolves in water
• Neutralisation


  Acid + base → salt + water
• When an acid neutralises a base (or vice versa), the products are neutral, i.e. they have a pH of 7</b>
• Titration


  Method of analysing the concentrations of solutions
• Titration procedure


  1. Add alkali to conical flask

  2. Fill burette with known concentration acid

  3. Add acid to alkali slowly

  4. Note volume of acid used

  5. Repeat for consistent results

  6. Calculate mean
• Single indicator


  Used in titrations to show a sudden colour change at the end-point
• Strong acid


  Ionises completely in water
• Weak acid


  Does not fully ionise in solution, only a small proportion of particles dissociate
• Acid strength


  Determines proportion of molecules that ionise in water
• Acid concentration


  Determines total number of dissolved molecules and number of ionised molecules
• Metal oxides and metal hydroxides


  Bases that react with acids in neutralisation reactions
• Reaction of acid and metal oxide/hydroxide


  Acid + metal oxide/hydroxide → salt + water
• Reaction of acid and metal carbonate


  Acid + metal carbonate → salt + water + carbon dioxide
• Making soluble salts from insoluble bases


  1. Heat dilute acid

  2. Add insoluble base until no more reacts

  3. Filter out excess solid

  4. Evaporate solution to get salt crystals
• Reactivity Series


  • Lithium
  • Calcium
  • Magnesium
  • Carbon
  • Hydrogen
  • Zinc
  • Iron
  • Copper
• Reactivity Series


  Lists metals in order of their activity towards other substances
• Reactivity of metals


  • Determined by how easily they lose electrons - forming positive ions
  • The higher the reactivity series a metal is, the more easily they form positive ions
• When metals react with water or acid, they lose electrons and form positive ions
• The higher a metal is in the reactivity series


  The more easily it reacts with water or acid
• Comparing the relative reactivity of different metals


  1. Reacting with either an acid or water

  2. Putting them in order from most reactive to least reactive
• Carbon and hydrogen are non-metals but are often included in the reactivity series
• Reactivity of metals


  • Very reactive
  • Fairly reactive
  • Not very reactive
• Metals reacting with acids

  1. Produces a salt and hydrogen gas

  2. The speed of reaction is indicated by the rate of hydrogen bubbles given off

  3. The more reactive the metal, the faster the reaction
• Reactions of metals with acids


  • Potassium, sodium, lithium and calcium react explosively
  • Magnesium, zinc and iron react less violently
  • Copper won't react with cold, dilute acids
• Magnesium reacts significantly with cold dilute acids such as HCl and H2SO4, producing hydrogen gas
• Zinc and iron react slowly with dilute acids, but more strongly if heated
• Investigating reactivity of metals

  1. Measuring temperature change of reaction with acid or water over time

  2. More reactive metal = greater temperature change
• Metals reacting with water

  1. Produces metal hydroxide and hydrogen

  2. Reactive metals like potassium, sodium, lithium and calcium will react with water

  3. Less reactive metals like zinc, iron and copper won't react with water
• Most metals are not found in the earth as pure lumps, they have to be extracted from compounds
• Oxidation


  Gain of oxygen
• Reduction


  Loss of oxygen
• Extracting metals from metal oxides


  Reduction reaction separates metal from oxide