Redox Reactions

Chemical Reactions There are 3 major classes of chemical reaction: There are 3 major classes of chemical reaction: 1. Precipitation 2. Acid-base 3. Redox

Oxidation Refers to the gain of oxygen in a chemical reaction Refers to the gain of oxygen in a chemical reaction EXAMPLE: EXAMPLE: Iron + oxygen Iron + oxygen Iron(II) oxide Write the chemical equation (unbalanced) Fe + O 2 FeO The iron has gained oxygen and has been oxidised

Reduction Is the opposite of oxidation Is the opposite of oxidation This means that it involves the lose of oxygen This means that it involves the lose of oxygen Write a chemical equation for the reduction of calcium oxide. Balance the equation CaO Ca + O 2 22

Redox reactions Write a similar equation for the reaction of zinc and iron oxide. Label the oxidation and reaction steps Whenever a chemical is oxidised, another chemical must be reduced, and vise vera EXAMPLE: Mg + CuO MgO + Cu Mg + CuO MgO + Cu

Introducing electrons Mg + CuO MgO + Cu The oxidation and reduction steps can be written out separately: Mg MgO Mg MgO CuO Cu CuO Cu Re-write the above two steps, but add charges to the ions involved OXIDATION REDUCTION Mg 2+ O 2- Cu 2+ O 2-

In the oxidation step: Mg + O 2 Mg 2+ O 2- Mg + O 2 Mg 2+ O 2- The magnesium atoms have turned into magnesium ions Have they gained or lost electrons? OXIDATION IS ALWAYS LOSS of electrons O.I.L. Where have the electrons gone to?

They have been taken by the copper: Cu 2+ O 2- Cu Cu 2+ O 2- Cu The copper has turned from ions into atoms 2+ NEUTRAL + 2- Cu e Cu REDUCTION IS ALWAYS GAIN of electrons R.I.G.

Practice Write similar ion-electron equations for: Write similar ion-electron equations for: Calcium ions turning into calcium atoms Sodium ions turning into sodium atoms Aluminium ions turning into aluminium atoms Potassium atoms turning into potassium ions Nickel atoms turning into nickel(II) ions All of these REDUCTIONS can be found in the data booklet

Writing ion-electron equations All reductions are found in data booklet All reductions are found in data booklet To write the oxidation, reverse the equation To write the oxidation, reverse the equation Cu e Cu Cu Cu e Both sides of the equation should be neutral when the charges are added together Both sides of the equation should be neutral when the charges are added together REMEMBER: OIL RIG REMEMBER: OIL RIG

Problems 1. Look at the following equation: Zn + Ag 2 O ZnO + Ag Zn + Ag 2 O ZnO + Ag (a) Which chemical is being oxidised? (b) Write an ion-electron equation for the oxidation step. (c) Write an ion-electron equation for the reduction step. 2. Assessment tests 10.3 and 10.4