Balancing Chemical Equation

What is a balanced equation?

A chemical equation is balanced when the number of atoms of each type on each side of the equation is the same. Which means if you have 12 hydrogens on the left hand side of the equation, you must have 12 hydrogens on the right hand side, if there are 4 oxygens on the left, there must 4 oxygens on the right, and so on. This is because of the law of conservation of mass - you can't make or destroy atoms during a chemical reaction. But you can't just add atoms at random to each side, you have to work with the molecules of the reactants. Also, you will find it very tricky to try to balance a word equation, it is very much easier to use a chemical equation with chemical symbols, as then you will be able to see how many atoms of each type are in each chemical.

Example 1
Unbalanced Equation:- C₃H₈ + O₂ ---> H₂O + CO₂

There are three carbons on the left, but only one on the right.
There are eight hydrogens on the left but only two on the right.
There are two oxygens on the left but three on the right.
Balanced Equation:- C₃H₈ + 5O₂ ---> 4H₂O + 3CO₂

How do we balance the equation?

Balancing chemical equations isn't difficult, once you know the way to do it. Start by finding out how many atoms of each type are on each side of the equation. Some teachers recommend making a little table listing the numbers of each atom for the left hand side and for the right hand side.


Next, look for an element which is in only one chemical on the left and in only one on the right of the equation. (But it is usually a good idea to leave hydrogen and oxygen until you've done the others first.)
To balance that element, multiply the chemical species on the side which doesn't have enough atoms of that type by the number required to bring it up to the same as the other side. The number is called the coefficient.

BUT
If you have to multiply by, say, 2 1/2, do so, THEN multiply EVERYTHING on each side of the equation by two to get rid of the half.

We don't like having halves in equations, as you can't get half a molecule.
Now look for the next element or species that is not balanced and do the same thing.
Repeat until you are forced to balance the hydrogen and oxygens.
If there is a complex ion, sometimes called a polyatomic ion, on each side of the equation that has remained intact, then that can often be balanced first, as it is acts as a single species. The ions NO₃^- and CO₃^2- are examples of a complex ion.


A VERY useful rule is to leave balancing oxygen and hydrogen to the last steps as these elements are often in more than one chemical on each side , and it is not always easy to know where to start. Some people also say you should leave any atom or species with a valancy of one one until the end, and also generally leave anything present as an element to the end.


In Example 1 above, you would balance the carbons first, by putting a 3 in front of the CO₂, then balance the hydrogens by putting a 4 in front of H₂O and finally the oxygens (which are in more than one compound on the right, so we leave them until last) by putting a 5 in front of the O₂.

Example 2
Unbalanced equation:- H₂SO₄ + Fe ---> Fe₂(SO₄)₃ + H₂

Balance the SO₄ first (as it is a complex ion and it is in one chemcial species on each side)
3H₂SO₄ + Fe ---> Fe₂(SO₄)₃ + H₂
Now balance the Fe (which is also in one chemical on each side)
3H₂SO₄ + 2Fe ---> Fe₂(SO₄)₃ + H₂
Finally, balance the hydrogen (although it is in one chemical species on each side, it is usually a good idea to leave it until last)
Balanced Equation:- 3H₂SO₄ + 2Fe ---> Fe₂(SO₄)₃ + 3H₂


We alter the coeficients in the equation.
Do NOT touch the subscripts for the atoms in a chemical species, or you will change it into an different chemical. That would be a bit like saying I want six chicken legs for a meal, so I'll go get a six-legged chicken. As chickens have two legs, you will need three normal, two-legged, chickens, not a six-legged mutant monster, probably from outer space.
If you start by trying to balance something which is in more than one species on one side, you can't easily tell which species you should have more of, and so can end up going round in circles, continually altering things. If this happens, just start again, but balancing atoms or complex ions that are in one species on each side. (This is important or it will not work.)


EVALUATION:

Question Excerpt From Balancing Chemical Equations

1. 1.     ______NH3             -->           ______N2             +            ______H2
2. 1.     ______ H2S             +            ______ O2          -->             ______ H2O          +         ______ S
3. What would happen if we did not follow the law of conservation of mass when taking apart and then putting together a car for repair?
4. ___ CH4 + ___O2 --> ___CO2 + ___H2O
5. 1.     ______ C            +             ______ O2             -->            ______ CO2 write the correct prefixes below
6. What law says we need to have balanced chemical equations?
7. ______ CH­2O            +             ______ HCl             -->            ______ CH4O             +            ______ Cl2