Oxidation, reduction and redox reactions
7.1 Oxidation and Reduction
7.1 Oxidation and reduction
- Oxidation and reduction have 4 definitions:
7.2 oxidation states
Oxidation Reduction
Gain of oxygen Loss of oxygen 7.3 redox equations
Loss of electrons Gain of electrons
Loss of hydrogen Gain of hydrogen
Increase in oxidation states Decrease in oxidation states
- Redox reaction ~ a
reaction with both Cu + ½O2 àCuO
reduction and oxidation The compound CuO is ionic so can be split into its ions.
recurring. Cu + ½O2 à (Cu2+ + O2-)
- Oxidising agent ~ a We can see that the Cu has lost 2 electrons, so its half equation is:
substance that oxidises Cu à Cu2+ + 2e-
something, itself being The oxygen has gained 2 electrons, so its half equation is:
reduced. They accept ½O2 + 2e- à O2-
electrons
- Reducing agent ~ a substance that reduces something, itself being oxidised. They donate electrons.
- Any redox reaction involves the transfer of electrons, to best understand this, we split the
equation into two parts, one half equation shows oxidation, the other reduction.
7.2 Oxidation States
- The oxidation states are a measure of how many electrons the element has involved in bonding.
- Rules for determining oxidation states:
- The oxidation state of uncombined elements is always zero.
o E.g., Mg(s)
- In monoatomic ions, the oxidation state is the same as the charge on the ion.
o E.g., Mg2+ is +2
- The sum of all the oxidation numbers in a neutral compound is 0.
o E.g., H2O (each H is +1, each O is -2) overall (2x+1)-2=0.
- The sum of all the oxidation numbers in a polyatomic ion is the charge on the ion.
o E.g., CO32- (each C is +4, each O is -2) overall +4+(3x-2) = -2
- The oxidation state of hydrogen in a compound is +1 except when it is bonded to a metal hydride
when it is -1.
o HCl hydrogen is +1
o NaH hydrogen is -1
- Group 1 elements in a compound are always +1.
- Group 2 elements in a compound are always +2.
- Aluminium in a compound is always +3.
- Fluorine is always -1 in a compound.
- Oxygen in a compound is -2 except when it is in a peroxide when it is -1 or bonded to fluorine.
o MgO, Mg is +2 therefore O is -2.
o H2O2, H is +1 and the compound is neutral therefore O is -1.
o F2O, F is always -1, the compound is neutral therefore O is +2.
- Chlorine is -1 unless it is in a compound with a more electronegative elements such as O.
o NaCl, Na is +1 therefore Cl is -1.
o NaClO, Na is +1, O is -2 therefore Cl is -1.
7.3 Redox Equations
- The oxidation states can be used to see which element is reduced and which is oxidised.
