Chemguide: Core Chemistry 14 - 16

Answers to Set 2 of the equations problems

You need to balance:

Al + H₂SO₄     Al₂(SO₄)₃ + H₂

Balanced version:

2Al + 3H₂SO₄     Al₂(SO₄)₃ + 3H₂

You need to balance:

Fe₂(SO₄)₃ + KOH     Fe(OH)₃ + K₂SO₄

Balanced version:

Fe₂(SO₄)₃ + 6KOH     2Fe(OH)₃ + 3K₂SO₄

Well done if you got this right. This is a good example of where it is much easier to count groups rather than mess around with individual atoms. If you treat the unchanged hydroxide and sulfate groups as single things, it comes out much more easily.

You need to balance:

C₄H₁₀ + O₂     CO₂ + H₂O

You would need to do this in stages. First balance the carbons and hydrogens:

C₄H₁₀ + O₂     4CO₂ + 5H₂O

The count the oxygens. There are 13 on the right-hand side. That needs 6½ oxygen molecules.

C₄H₁₀ + 6½O₂     4CO₂ + 5H₂O

Now double everything to get rid of the half.

Balanced version:

2C₄H₁₀ + 13O₂     8CO₂ + 10H₂O

Well done if you got it right. The next two examples are reinforcing the same sort of thinking.

You need to balance:

K + H₂O     KOH + H₂

The problem here is that there is one too many hydrogens on the right-hand side. You can again use the trick of having half an H₂ molecule to get around it.

K + H₂O     KOH + ½H₂

Then double everything to get rid of the half.

Balanced version:

2K + 2H₂O     2KOH + H₂

You need to balance:

HNO₃    H₂O + NO₂ + O₂

If you put the hydrogens and nitrogens right, you will have . . .

2HNO₃    H₂O + 2NO₂ + O₂

You now have 6 oxygens on the left, but 7 on the right. By this time, you know what to do about it!

2HNO₃    H₂O + 2NO₂ + ½O₂

Balanced version:

4HNO₃    2H₂O + 4NO₂ + O₂

I have hammered away at this in the last three examples, not because these are important cases, but so that you know how to deal with the odd occasion when it comes up during a chemistry course at this level.

Return to the problems . . .