Equations

• • Reaction conditions or the name of a catalyst can be written above the arrow in a reaction
• Writing symbol equations

  • When writing symbol equations, you should:
  • Ensure reactants are on the left of the equation and products are on the right
  • Write the following non-metals as molecules: H2(hydrogen ), N2 (nitrogen)O2 (oxygen)F2 (fluorine), Cl2 (chlorine), Br2 (bromine) and I2 (iodine)
  • Include state symbols
  • Solid = (s)
  • Liquid = (l)
  • Gas = (g)
  • Aqueous = (aq)
• A symbol equation must be balanced to give the correct ratio of reactants and products
• When balancing equations do not add or remove the small numbers because it changes what the substance is. E.g you wouldn't write H2O2 to balance it
• • Ions which are present but do not take part in reactions are called spectator ions
• • Half equations are used to show what happens to the electrons in reactions where or ions are gaining or losing Examples of half equations are:

  
  

  Pb2+ + 2e- → Pb

  
  

  2Br- → Br2 + 2e-

  • They are called half equations, because they represent only half of what is happening in a reaction involving electron transfer
• Ionic equations only show the ions that take part in reactions

  They help to simplify complicated processes where many substances are present, but only certain ions are actually reacting with each other

  e.g

  HCl + NaOH  → NaCl + H2O

  Written out as an ionic equation would be

  
  

  H+ + OH- → H2O
• Oxidation and reduction reactions take place at the same time. This is because if one substance gains electrons another has to lose electrons
• A redox reaction is when oxidation occurs at the same time as reduction
• Redox reactions also take place in displacement reactions
• HCl + NaOH  → NaCl + H2O

  is written without sodium and chlorine in its half equation ( H+ + OH- → H2O)

  • This is because sodium and chloride ions were present at the beginning and also at the end of the reaction, so they are unchanged
  • This means that the sodium and chloride ions are spectator ions as they are present but do not take part in the reaction