Chemical Equations

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Chemical Equations
PbBr2
CaSO4

• How to write chemical formulae

Ag2SO4
AlCl3
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What do we want to achieve?

• We want to be able to find the formulae of common
  ions using the data books.
• We then want to use these formulae to decide how
  many of each ion are needed in the finished
  formula.
• Now we should be able to write the correct symbol
  formula for the ionic compound.

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How do we find the formulae for the ions?

• Turn to the contents page of the Data Book and
  look for Formulae of Common Ions.
• Turn to the relevant page. Find the ion you are
  looking for on the table.
• The symbol for the ion is given as is its charge.

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Lets look at a familiar metal

• For example look up aluminium (used in airplane
  wings).
• You will find Al3 which tells you that
  aluminium has the symbol Al and the charge 3.

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So what will the finished formula look like?
KBr

• The compound will have two ions in it.
• One will have a plus charge and the other will
  have a negative charge.
• These charges must balance each other out in the
  final formula.
• The charges are not shown in the finished
  formula.
• We need to look more closely at balancing.

MgSO4
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So how do we balance the charges?

• Lets look at sodium chloride. We need to look up
  the formulae for sodium ions and chloride ions.
• Sodium is Na which means that the symbol for
  sodium is Na and the ion has a 1 charge.
• Chloride is Cl- which means that the symbol for
  chloride is Cl and it has a 1- charge.
• As we have one plus charge and one minus charge
  they balance each other out.

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These balance so now what?
You need one sodium ion Na ...
Put them together...
NaCl
and one chloride ion Cl-.
but dont write the charges in now that they are
balanced.
And so you get...
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Well this all seems a little too easy

• I mean, what happens when the charges arent the
  same as each other so they dont balance each
  other out?

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Balancing charges is just like balancing weights
If you have one 3kg mass and a pile of 1kg
masses, how could you make a balance sit level?
1kg
3kg
1kg
1kg
You would need to put the 3kg mass on one side
and three 1kg masses on the other side
So now its balanced!
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Just do the same thing with charges
Have a look at aluminium chloride
Cl-
Chloride is Cl-
Aluminium is Al3
Al3
Cl-
Cl-
So whilst we have three plus charges we only have
one minus charge.
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Clearly then we need three minus charges, and if
each chloride only carries one minus charge, how
many of them do we need?
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So how do we write this down?
Get rid of the charges now that we are ready to
write the final formula
AlCl3
We need one Al so thats easy, you just write Al
We need three Cls and you show this by writing
down the Cl with a subscript 3 after it as so Cl3
Put them together and what do you get?
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Lets try another!
Okay. Try sodium sulphide.
Sodium is Na
Sulphide is S2-
Are the charges balanced?
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Try it out
Na
S2-
Na
Two plus charges and two minus charges - its
balanced, and so you write
Well weve got two minus charges but only one
plus charge so we need one more sodium ion to
give us one more plus charge.
Na2S
No
Is it balanced?
So what do we need more of?
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Is it really that easy?
Well there is one more trick you need to know.
It concerns ions that have more than one capital
letter in their symbols.
Its easiest to show you with another example
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Have a look at calcium nitrate
Calcium is Ca2
Nitrate is NO3-. This means that nitrate is made
of one N atom and three O atoms. It carries a
charge of 1-.
So we have two plus charges and one minus charge.
Clearly we need one more nitrate ion so that we
have two minus charges.
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How do we write the formula then?
One calcium and two nitrates - it couldnt be
easier. You just write...
Wrong!!!
CaNO32
This looks like there is one Ca atom, one N atom
and 32 O atoms.
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So what can we do about that?
You need to use brackets
You mean 2 lots of NO3
So put a bracket around the NO3
And you get...
Ca(NO3)2
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Now you know what to do, try these

• Questions
• Potassium hydroxide
• Calcium sulphide
• Magnesium carbonate
• Lithium bromide
• Iron (III) phosphate
• Zinc oxide

• Answers
• KOH
• CaS
• MgCO3
• LiBr
• FePO4
• ZnO

Now you can do these, try some slightly more
tricky ones...
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youll need to do more to balance these.

• Questions
• Lead bromide
• Sodium oxide
• Aluminium fluoride
• Lithium sulphide
• Aluminium oxide

• Answers
• PbBr2
• Na2O
• AlF3
• Li2S
• Al2O3

Just one more set to try...
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dont forget brackets.

• Questions
• Ammonium Sulphate
• Barium hydroxide
• Iron (II) nitrate
• Aluminium carbonate

• Answers
• (NH4)2SO4
• Ba(OH)2
• Fe(NO3)2
• Al2(CO3)3

And thats all there is to it!
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Youve
cracked it!!!!