Ionic equations and half equations

Fahad

Can someone please explain to me how we can work out these equations?

Thanks.

Reply 1

Untitled

deducing half equations? adding half equations? writing ionic equations? be a bit more specific

Reply 2

Jonatan

Fahad

Can someone please explain to me how we can work out these equations?

Thanks.

BAsicly a half equation is an equation showing how an atom (or molecule) is reduced or oxidised. As an example:

Na -> Na+ + e-

And

Cl2 + 2e- -> 2Cl-

In order to find these equations from an overall eqaution, you need to find the oxidation states of the atoms in the involved compounds.

As an example:

Na(s) + Cl2(g) -> NaCl(s)

On the left hand side it is fairly easy because each reactant is only composed on one type of atom (it gets much more difficult when you have molecules or eaven the worst nightmare organic compounds... )

Now, obviously, both reactants on the left hand side are neither in an oxidised or reduced state (since they are elements). However, on the right hand side Na is oxidised and has oxidation number +1 , Cl has oxidation number -1 (This you simply have to know. It can be determined by looking at electronegativity values, and you have some rules that work for most non-organic compounds (I will give some of them later)).

Thus we can deduce that the equation works by the two half equations stated earlier :

Na -> Na+ + e-

Cl2 + 2e- -> 2Cl-

These can be combined to give:

2Na + Cl2 -> 2Na+ + Cl2 + 2e- -> 2Na+ + 2Cl- -> 2NaCl

Or more simplified:

Na + 0.5Cl2 -> NaCl

Here are some rules that should help to determine oxidation numbers:

Overall oxidation numbers of ions equal the charge of the ion.

Flouride usualy has oxidation number -1

Oxygen usually has oxidation number -2 (exceptions include peroxides and flouride compounds)

Hydrogen typically has oxidation number +1, except when combined with alkali earth metals (As an example in LiH2 Hydrogen has oxidation number -1 if I remember correctly).

When two atoms are bounded together, assume all electrons in the bond are located at the most electronegative atom, and add the oxidation number corresponding to the net charge of the atom due to the extra charges (This one is a bit difficult to grasp at first, just think of water were you have H2O such that the Oxygen gets oxidation number -2 and the hydrogen atoms each have oxidation number +1 ).

Reply 3

Fahad

Thank you very much, Jonatan :smile:

Reply 4

Jonatan

Fahad

Thank you very much, Jonatan :smile:

No problem, had this crap last year....

BTw, I forgot to say that the ion rule also works for uncharged dissolved molecules.

As an example the overall oxidation number of Methanol is 0.

H3COH

The 4 hydrogen atoms have oxidation number +1, the oxygen atom has -2, and so the carbon must have oxidation number -2 is the molecule as a whole shall have overall zero oxidation number (please correct me if Im wrong here folks).

Ionic equations and half equations

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