So far we have learnt about ions as single atoms that have lost or gained an electron, causing imbalance of charge. We also know that on a larger scale, when lots of atoms get together and have more electrons than protons, we also get charge imbalance for example the electrodes in our experiment and the charged plate in the test.
And in all the cases, on the small scale and the large scale, opposite charges attract and like charges repel.
But we have not yet considered the case in between. What about a case of compounds being charged?
When ionic compounds get together, they do so to balance their charges, but there are some groups of elements that get together regularly and are still not balanced in charge. So even though they are combined into a new molecule they are still ions.
Lets take as an example one compound that we have come across already, Sodium Hydroxide.
Sodium hydroxide has the chemical formula NaOH.
What charge does the ion of sodium have? It's in group I so the ion of sodium is created by losing an electron and is hence positive and has a valency of +1. Now we have this other part of the equation, the OH bit. What you have to imagine is that their is a molecule of OH that goes around as a group on it's own. The OH behaves as if it was one ion on it's own. So it's compound and an ion, or a so called compound ion.
The OH molecule is called a HYDROXIDE.
When this combines with Na, it creates NaOH. What charge then does this compound ion have? It has to be the exact opposite of Na to balance out the charge on the Na. So OH as a compound ion has the charge -1.
The compound ion can be written in brackets and treated as if it were just one ion
e.g. Hydroxide = (OH)-1
The charge on a compound ion you just need to remember, unlike with the other ions where you can work it out. Now that we know this piece of info about hydroxide ions, we can do question 9 very easily. Aluminium Hydroxide would be the combination of Al with hydroxide
Al is in Group 3 so its ion has a valency of +3
(OH) has a valency of -1
So you write these (metal first remember) as
Al3+(OH)-1
Flip the numbers down
Al(OH)3
et Voila! That was easy huh? Just ignore the numbers in the bracket and treat the whole thing as if it’s just one ion, paying attention only to the charge outside the bracket.
Now here's some more info on some compound ions that you may want to use to help you answer questions 8-12
| Compound ion | Charge |
| Hydroxide | (OH)- |
| Carbonate | (CO3)2- |
| Sulphate | (SO4)2- |
| Nitrate | (NO3)- |
| Ammonium | (NH4)+ |
Remembering these compound ions and being able to spot them in an equation will help a lot later on in chemistry IGCSE with balancing equations and with identifying and naming compounds, so it’s worth remembering them early.
No comments:
Post a Comment