Chemical Changes

• pH Scale

  • Goes from 0-14
  • Measures how acidic or alkaline a solution is
  • Lower the pH = More acidic
  • Higher the pH = More alkaline
  • Neutral substance = pH of 7
• How to Measure pH - Indicator

  • A dye that changes colour depending on whether its above or below a certain pH.
  • Some contain a mixture of dyes that means they gradually change colour over a broad range of pH.
  • These are called wide range indicators and they are useful for estimating the pH of a solution.
• How to Measure pH - pH Probe

  • A pH probe attached to a pH meter can be used 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.
  • This makes it more accurate than an indicator.
• Acids and Bases

  • An acid is a substance that forms aqueous solutions with a pH of less than 7.
  • Acids form H+ ions in water.
  • A base is a substance with a pH greater than 7.
  • An alkali is a base that dissolves in water to form a solution with a pH greater than 7.
  • Alkalis form OH- ions in water.
  • The reaction between acids and bases is called neutralisation: acid + base -> salt + water
  • Neutralisation between acids and alkalis can be seen as H+ and OH- ions: H+(aq) + OH-(aq) -> H2O(l)
  • When an acid netralises a base (or vice versa), the products are neutral.
• What do Acids produce in Water?

  Protons. Acids ionise in aqueous solutions - they produce hydrogen ions ,H+, which is just a proton.
• Examples of Strong Acids

  • Sulfuric Acid
  • Hyrochloric Acid
  • Nitric Acid
• Examples of Weak Acids

  • Ethanoic Acid
  • Citric Acid
  • Carbonic Acid
• Strong & Weak Acids

  • Strong acids ionise completely in water. All acid particles dissociate to release H+ ions.
  • Weak acids do not fully ionise in solutions. Only a small proportion of acid particles dissociate to release H+ ions.
  • The ionisation of a weak acid is a reversible action, 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.
  • Strong acids are more reactive of weak acids of the same concentration.
• The pH of an acid or alkali is a measure of the concentration of H+ ions in the solution. For every decrease of 1 on the pH scale, the concentration of H+ ions increases by a factor of 10. So, an acid that has a pH of 4 has 10 times the concentration of H+ ions of an acid that has a pH of 5. The general rule for this is: Factor H+ ion concentration changes by = 10-x

  X is the difference in pH. So if a pH falls from 7 to 4, the difference is -3. So the pH of a strong acid is always less than the pH of a weaker acid if they have the same concentration.
• What does Acid Strength tell you?

  It tells you what proportion of the acid molecules ionise in water.
• What does Concentration Measure (with acid)?

  It measures how much acid there is in a certain volume of water. It is basically how watered down the acid is.
• What do you get when a Metal Oxide/Hydroxide reacts with Acid?

  Salt and Water
• Examples of Bases

  • Metal Oxides
  • Metal Hydroxides
  • Metal Carbonates
• What do Acids and Metal Carbonates produce?

  Salt, Water and Carbon Dioxide.
• Making Soluble Salts (Practical)

  1. You need to use the right acid and insoluble base, such as an insoluble metal oxide, hydroxide or carbonate.
  2. Gently warm the dilute acid using a bunsen burner, then turn off the bunsen burner.
  3. Add the insoluble base to the acid a bit at a time, until no more reacts. You'll know when all the acid has been neutralised b/c even after stirring, the excess solid will just sink to the bottom of the flask.
  4. Then filter out the excess solid to get the salt solution.
  5. Use crystallization to get pure, solid salt crystals.
• Reactivity Series (Most to Least)

  • Potassium (K)
  • Sodium (Na)
  • Lithium (Li)
  • Calcium (Ca)
  • Magnesium (Mg)
  • Carbon (C)
  • Zinc (Zn)
  • Iron (Fe)
  • Hydrogen (H)
  • Copper (Cu)
• Reactivity Series

  • Lists metals in order of their reactivity towards other substances.
  • The higher up the reactivity series a metal is, the more easily they form postive ions.
  • When metals react with water or acid, they lose electrons and form positive ions. So the higher a metal is in the reactivity series, the more easily it reacts with water or acid.
  • If you compare the relative reactivity of different metals with either an acid or water and put them in order from most reactive to the least reactive, the order you get is the reactivity series.
• How Metals React with Acids

  • Some metals react to produce a salt or hydrogen gas.
  • The speed of reaction is indicated by the rate at which the bubbles of hydrogen are given off.
  • The more reactive the metal, the faster the reaction will go.
  • You can investigate the reactivity of metalsby measuring the temp change of the reaction with an acid or waterover a set time period.
  • If you use the same mass and surface area of metal each time, then the more reactive the metal, the greater the temp change should be.
• What gets produced when Metals react with Water?

  Metal Hydroxides and Hydrogen Gas.
• What Metals (from the Reactivity Series) react with Water?

  • Potassium
  • Sodium
  • Lithium
  • Calcium
• What is Oxidation?


  The gain of oxygen and the loss of electrons.
• What is Reduction?


  The loss of oxygen and the gain of electrons.
• Reduction with Carbon


  Some metals can be extracted from their ores chemically by reduction using carbon. The ore is reduced as oxygen is removed from it, and carbon gains oxygen so it is oxidised.
• What Metals can be extracte using Electrolysis?

  • Potassium
  • Sodium
  • Lithium
  • Calcium
  • Magnesium
• What Metals can be extracted using Reduction by Carbon?

  • Zinc
  • Iron
  • Copper
• Why are some Metals mined in their Elemental Form?

  Because some metals are so unreactive that they are in the earth as the metal itself.
• What is a Redox Reaction?

  A reaction with both reduction and oxidation.
• What type of Reactions are Displacement Reactions?

  They are redox reactions.
• What is the rule of Displacement Reactions?

  A more reactive metal will displace a less reactive metal from its compound.
• Example of Displacement Reaction

  If you put iron in a solution of copper sulfate (CuSO4) the more reactive iron will displace the less reactive copper. You end up with iron sulfate solution (FeSO4) and copper metal. In this reaction the iron loses 2 electrons to become a 2+ ion - it is oxidised. The copper ion gains these 2 electrons to become a copper atom - it is reduced
• In displacement reactions, it's always the metal ion that gains electrons and is reduced. The metal atom always loses electrons and is oxidised.