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All chemical reactions occurring within the test tubes, industrial reactors, or nature can be described by reaction equations.
For example reaction of water synthesis can be written as
H2 + O2 -> H2O
This reaction contains the correct reactants - hydrogen and oxygen in its diatomic forms, and correct product - water molecule. We call such a reaction equation (unbalanced, but correctly listing all reactants and products) skeletal.
Knowing the skeletal reaction equation we know what the reactants are and what are the products, but for quantitative predictions we need to balance the reaction equation.
Reaction balancing is based on mass preservation. We know that atoms don't appear nor disappear. If the atom is present in a reactant (compound entering the reaction), it must be present in one of reaction products. The same happens with a charge - charge is also preserved, just like mass is.
We also know that compounds always have the same composition. A water molecule will always consist of one atom of oxygen and two atoms of hydrogen. It can be made when hydrogen reacts with oxygen - but these gases are usually present in the diatomic form of H2 and O2 molecules. To describe the reaction as it occurs we have to combine both mass preservation and molecules composition.
Let's take a look again at the above reaction of water synthesis - there are two oxygen atoms on the left side (in the form of O2) but only one atom of oxygen on the right side - one atom in one water molecule. Reaction equation is not balanced.
Now let's take a look at the same reaction with added coefficients:
2H2 + O2 -> 2H2O
We read it: two molecules of diatomic hydrogen react with one molecule of diatomic oxygen, producing two molecules of water. (Or, alternatively - two moles of diatomic hydrogen react with one mole of diatomic oxygen to produce two moles of water). Is this equation balanced? The ultimate test which allows you to check whether the reaction is correctly balanced or not, is to count all types of atoms on both sides of the equation - they must be identical.
Let's check oxygen - there is one molecule on the left side, containing two atoms - so there are two atoms of oxygen on the left side. On the right side there are two molecules, each containing one atom of oxygen - so there are two oxygen atoms on the rights side as well. As number of atoms of oxygen is identical on both sides reaction is balanced with respect to oxygen. Please check for yourself, that the same reaction is also balanced with respect to hydrogen, with four hydrogen atoms on both sides.
So when is the reaction balanced? Firstly, it must have the same number of atoms on both sides. Secondly, all coefficients must be integer. Finally, they have to be the smallest possible denominator.
H2 + 1/2O2 -> H2O
4H2 + 2O2 -> 4H2O
are incorrect - even if they are balanced in terms of number of atoms. Note, that such incorrect equations can appear during balancing - and they are perfectly valid as intermediate forms, they just have to be cleaned up before becoming the final version. In this case multiplying the first reaction equation by 2 and dividing the second one by 2 leads us to correctly balanced equations.
Please remember, that when balancing equations you should never touch subscripts, since that will change the composition and therefore the substance itself. All you can modify are the coefficients telling us how many molecules of the reagent entered the reaction, or have left it.
If there are any charged species, you should also check if the charge is balanced, just as atoms are. But there is one, important difference - charge may be negative and positive and sum of these charges can be zero. Atoms don't come in two - positive and negative - types.
Finally note that not all reaction equations can be balanced - for example
H2O2 -> H2O
will be never balanced, no matter what coefficients you will use. More on that in the when balancing fails section.
Once you will know how to balance equations on paper, you may check our equation balancing and stoichiometry calculator EBAS - it makes most calculations immediately.
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