In this lesson, we will learn:
• How to analyze a balanced chemical equation.
• What a “mole” is and how it is important to understanding chemical reactions.
• How to predict amounts of product made in a reaction with a given amount of reactant.
• How to apply the moles concept with conversion factors to find the mass of reactants and products involved in reactions.
• The coefficients
(the number before
the chemical symbol / formula) tell you how many molecules
of that chemical are used, as a ratio, in the reaction. If no number is there, it’s one molecule.
• Recall from lesson on Introduction to chemical reactions
that a chemical equation tells you the fixed ratio
of reactants the reaction uses, and products that the reaction produces.
In the made up example below, we are being told that to produce 2 molecules of C and 1 molecule of D, the reaction requires 1 molecule of A and 2 molecules of B IN THAT RATIO
A 2B → 2C D
• The actual amounts of reactants used or products made in a particular reaction can be changed, but the ratio between them cannot – changing plans to make twice as much of a product will need twice as much reactants. There is no way around this fact!
• A mole is just a number of atoms –
(six hundred and two billion trillion!) but that is it - it is just a number
in exactly the same way that a dozen or a pair is a number. It is used in chemistry because the mass of atoms and molecules are measured in grams per mole (written gmol)
Take carbon dioxide, CO
as an example. CO
has a molecular mass of 44 grams per mole. That means that 1 mole (six hundred billion trillion molecules) of carbon dioxide has a mass of 44 grams. Similarly, a sample of 22 grams of CO
would be half a mole of CO
(as 22g is half of 44g).
The relative mass, measured in grams per mole is vitally important for chemists because atoms and molecules are too small to be counted individually but we can measure the mass of substances very easily!
With the mass and grams per mole known, we can work out the number of moles – how many molecules are in the sample of the substance we are using.
• Use the chemical equation to find the ratio of reactant moles to product moles (the reaction stoichiometry) and then find their relative masses. Now, you can predict the mass of any reactants required or products expected for any reaction scale.