Quantitative Aspects of

Chemical Change

6

An equation for a chemical reaction can provide us with a lot of useful information. It tells uswhat

the reactants and the products are in the reaction, and it also tells us the ratio in which the reactants

combine to form products. Look at the equationbelow:

Fe + S→ FeS

In this reaction, every atom of iron (Fe) will reactwith a single atom of sulphur (S) to form one molecule

of iron sulphide (FeS). However, what the equation doesn’t tell us, is the quantities or the amount of

each substance that is involved. You may for example be given a small sample of iron for the reaction.

How will you know howmany atoms of iron arein this sample? And howmany atoms of sulphur will

you need for the reaction to use up all the iron you have? Is there a wayof knowing what mass of iron

sulphide will be produced at the end of the reaction? These are all very important questions, especially

when the reaction is anindustrial one, where itis important to know the quantities of reactantsthat

are needed, and the quantity of product that will be formed. This chapter will look at how to quantify

the changes that take place in chemical reactions.

See introductory video:VPiwd at http://www.everythingscience.co.za

6.1 The Mole ESBBG

Sometimes it is important to know exactly howmany particles (e.g. atoms or molecules) are in asam-

ple of a substance, or what quantity of a substance is needed for a chemical reaction to take place.

You will remember fromGrade 10 that the atomic mass of an element, describes the mass of an atom

of that element relative to the mass of an atom of carbon-12. So the mass of an atom of carbon (atomic

mass is 12 u) for example, is twelve times greater than the mass of an atom of hydrogen, which has an

atomic mass of 1 u. Howcan this information beused to help us to knowwhat mass of each element

will be needed if we want to end up with the same number of atoms of carbon and hydrogen?

Let’s say for example, that we have a sample of12 g carbon. What mass of hydrogen will contain the

same number of atoms as 12 g carbon? We knowthat each atom of carbon weighs twelve times more

than an atom of hydrogen. Surely then, we will only need 1g of hydrogen for the number ofatoms

in the two samples to be the same? You will notice that the number ofparticles (in this case, atoms)

in the two substances isthe same when the ratioof their sample masses(12 g carbon: 1g hydrogen

= 12 : 1) is the same as the ratioof their atomic masses (12 u: 1 u = 12:1).

To take this a step further, if you were to weigh out samples of a numberof elements so that the mass

of the sample was the same as the atomic massof that element, you would find that the number of

particles in each sampleis 6. 022 × 1023. These results are shown in Table 6.1 below for a number

of different elements. So, 24. 31 g of magnesium (atomic mass = 24. 31 u) for example, has thesame

number of atoms as 40.08 g of calcium (atomicmass = 40. 08 u).