-The empirical formula of a hydrated compound is written in a unique way:
-The empirical formula of the compound is separated from the water of crystallisation by a dot.
- The relative number of water molecules of crystallisation is shown after a dot.
Oxidation number:
in a chemical formula each atom has an oxidation number. An atom uses electrons to bond with atoms from other elements.
-The oxidation number is the number of electrons that an atom uses to bond with atoms of another element
-Oxidation numbers are worked out by the following rules:
Molecular ions
in a carbonate ion CO32- the overall charge is 2-
- The sum of the oxidation numbers must add to the overall charge
- There are 3 oxygen atoms, each with an oxidation number of -2. This gives a total contribution of -6
-The oxidation number of carbon must be +4 to giv the overall charge of 2-
Oxyanions
- Oxyanions are negative ions that contain one element along with oxygen
- The names of oxyanions usually end in –ate to indicate oxygen
- As with transition element ions, an element may form oxyanions in which the element has different oxidation numbers. Without the oxidation number the name would be ambiguious. For example:
NO2- Nitrate(III) – N: oxidation number = +3
NO3- Nitrate(V) – N: oxidation number = +5
Redox reactions
Oxidation and reduction
Oxidation and reduction are now used to describe any reactions in which electrons are transfers.
- oxidation is the loss of electrons
-reduction is the gain of electrons
-A reaction in which both oxidation and reduction take place is called a reduction reaction.
Electron transfers in redox reactions
Magnesium reacts with chlorine to form magnesium chloride Mg+Cl→MgCl2
This is a redox reaction, but the equation for the overall reaction conceals this fact
At an electronic level, electrons have been transferred from the magnesium atoms to the chlorine atoms. The half-euations below show this clearly:
Mg→Mg2+ +2e- Oxidation
Cl2 +2e- →2Cl- Reduction
Mg is a reducing agent- it has reduced the Cl2 to 2Cl- by donating its electrons
Cl2 is an oxidation agent- it has oxidised Mg to Mg2+ by removing its electrons.
Metals tend to be oxidised- losing electrons to form positive ions
Non-Metals tend to be reduced- gaining electrons to form negative electrons.
Oxidation numbers in a redox reactions
Oxidation and reduction can also be described in terms of exidation number:
- Reduction is a decrease in oxidation number
- Oxidation is an increase in oxidation number
You can identify the oxidation and reduction processes using oxidation numbers. You assign an oxidation number to each atom in a oxidation reaction. You can then follow any changes to these oxidation numbers.
The oxidation and reduction processes for the reaction of magnesium and chlorine are shown→
Redox reactions of metals with acids
Reactive metals react with many acids in redox reactions.
-The metal is oxidisedforming positive ions.
-The hydrogen in the acid is reduced forming the element hydrogen and a gas
Reaction of magnesium metal with dilute hydrochloric acid Reaction of magnesium with dilute sulphuric acid
Acids react with reactive metals by metal+acid→slat+H2