We have seen reduction and oxidation as the removal and addition of oxygen. This is a perfectly good definition but we can use the words in a slightly more subtle way. We can talk about reduction as the gain of electrons and oxidation as the loss of electrons. Oil RiG – oxidation is loss, reduction is gain.
This needs a little explanation perhaps. Atoms consist of a positively charged nucleus surrounded by layers of negatively charged electrons. In an atom, the charges balance so the overall charge of the atom is neutral. When elements react, it often involves electrons moving from one atom to the other. Metals usually give up electrons (and are oxidised) and non-metals gain electrons (and are reduced). When the atoms lose or gain electrons they either become positively or negatively charged. We now call them ions. Groups of atoms (like sulphate SO4 or carbonate CO3) can also have an overall charge and be called ions.
When sodium and chlorine react, the sodium atom gives up the single electron in its outer shell so that it has a more stable arrangement of 8 electrons in its new outer shell. This forms a positive sodium ion – Na+. The chlorine atom has 7 electrons in its outer shell so just needs one to form the stable 8 electron arrangement. It then becomes a negatively charged chloride ion Cl–. The oppositely charged ions cling to one another in their solid form but when dissolved in water they completely dissociate to form an ionic solution.
When sodium and chlorine react, the sodium is oxidised (because it loses an electron) and the chlorine is reduced (because it gains an electron).
Consider a reaction like zinc with sulfuric acid. Sulfuric acid (H2SO4) is an ionic compound so each molecule splits up in water into two positive hydrogen ions (H+) and one negative sulfate ion (SO42-). When the zinc is added it is able to displace the hydrogen ions by donating them each an electron. This produces two atoms of hydrogen, which bubbles away as a gas (H2) and the zinc atom gains a double positive charge (Zn2+). This leaves zinc sulfate solution (ZnSO4) which will have split into separate zinc and sulfate ions. We can say that the hydrogen ions have gained electrons and been reduced to hydrogen. The zinc has lost two electrons to be oxidised into zinc ions.
Questions…
- How many atoms are there in one molecule of sulphuric acid?
- From the diagrams, what are the atomic numbers of sodium and chlorine?
- The relative atomic mass (RAM) of an element is the number of protons and neutrons added together. What is the RAM of carbon?
- When zinc and sulfur are mixed and heated, which element is oxidised and which is reduced?
- When copper chloride solution and solid aluminium react, which element(s) are oxidised and which reduced?
- Why is there no overall reduction or oxidation in the reaction between copper oxide and hot sulphuric acid?
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