Chapter 4 - Acids and Redox

• Acids:

  • All acids contain hydrogen in their formulae
  • When acids dissolve in water, they release hydrogen ions as protons, H+
  • Acids are hydrogen ion donors
• Strong acids: Completely dissociates in solution to give H+ ions
• Weak acids: An acid that only partially dissociates in solution to give H+ ions
• Bases

  • Metal oxides, metal hydroxides, metal carbonates and ammonia are examples of bases
  • A base neutralises an acid to form a salt and water only
  • A base is a hydrogen ion acceptor
• Alkalis: A base that dissolves in water releasing hydroxide, OH- ions into solution
• Neutralisation: H+ from an acid reacts with a base to form a salt and water
• Acid + Metal Oxide --> Salt + Water
• Acid + Metal hydroxide --> Salt + Water
• Acid + Metal carbonate --> Salt + Water + Carbon Dioxide
• The unit of concentration is mol dm-3. The unit of volume therefore needs to be dm3
• Preparing a standard solution

  • Weigh mass of crystals to 2 d.p.
  • Dissolve in distilled water
  • Transfer to 250 cm3 volumetric flask
  • Make up to the mark with more distilled water so that the bottom of the meniscus is on the mark – dropping pipette for final stage
  • Stopper and invert several times to mix
• How to do a titration

  1. Transfer a known volume to a conical flask using a volumetric pipette
  2. Add the other solution to a burette and record the initial burette reading
  3. Add a few drops of indicator to a conical flask
  4. Titrate until you reach the end point with the final drop changing the colour
  5. Determine the titre
  6. Repeat until you have concordant results – results agreeing to 0.1 cm3
• Oxidation number

  • Oxidation number is based on a set of rules that apply to atoms
  • It can be thought of as the number of electrons involved in bonding to a different element.
  • Oxidation numbers help when writing formulae and can be used to check that all electrons are accounted for
• Atoms in elements have an oxidation number of 0
• Oxidation numbers of compounds:

  • Oxidation numbers of a compound is always zero overall
  • Each atom in a compound has an oxidation number
• In simple ions, the oxidation state is the same as the charge on the ion
• Special cases

  • H in metal hydrides has an oxidation state of -1
  • O in peroxides has an oxidation state of -1
  • O bonded to F has an oxidation state of +2
• Reduction is the gain of electrons, Oxidation is the loss of electrons
• Oxidation is the increase in oxidation number, Reduction is the decrease in oxidation number