Chemical equations: Iron extraction from iron oxide

The Earth's crust contains a lot of iron, about five per cent of its mass. But the iron found in nature is never in pure form. The iron is always bonded together with other elements, in chemical compounds. One common iron compound consists of iron and oxygen - iron oxide. You can extract the pure iron, by heating the iron oxide together with coal.

The iron oxide becomes pure iron. Where does the oxygen go? That reacts with the carbon from the coal and forms carbon dioxide. That is: iron oxide plus carbon... becomes...

iron plus carbon dioxide. There are several kinds of iron oxide. The most common has two 'irons' for each three 'oxygens'. Iron is written as Fe, so the chemical formula for this type of iron oxide is Fe2O3. Iron oxide does not consist of molecules containing only two iron and three oxygen.

But Fe2O3 is the smallest unit we can use to express the substance's chemical formula. One Fe2O3 is called a formula unit of iron oxide. Using chemical formulae for all four substances, the chemical equation reads: Fe2O3 + C becomes Fe + CO2 If you have a certain amount of iron oxide, you can figure out the amount of carbon needed to extract the metallic iron. And how much pure iron you will get. In order to calculate these, you'll need to balance the equation.

When we balance a chemical equation, we count each type of atom, and make sure that the numbers of them are the same before, and after the reaction. One thing before we start: the iron and oxygen are actually in ionic form in iron oxide. But we can call them 'atoms' just for the purpose of balancing the equation. For each formula unit of iron oxide that we start with, we'll get two iron atoms. We write the coefficient two in front of the iron atoms on the right side of the equation.

What about the oxygen atoms? Three of them to start with... One molecule of carbon dioxide is not enough to take on three oxygen atoms. Let's try two carbon dioxide molecules instead. No, then there will be too many oxygen atoms on the right.

We need to fit three oxygen atoms from the iron oxide, into a substance that has two oxygen atoms in each molecule. How do we solve that? Pause the video and see if you can work that out yourself! We can start out with two formula units of Fe2O3. Then we'll have a total of six oxygen atoms before the reaction.

Six oxygen atoms fit nicely into three carbon dioxide molecules. But remember! Now that we've changed the amount of iron oxide that we start with... ... we also have to change the number of iron atoms in the products, to four. Finally, we need to balance the carbon atoms.

We'll need three carbon atoms to form three carbon dioxide molecules. Let's check the numbers. Four iron atoms on each side of the reaction arrow. Six oxygen atoms before the reaction, and six after. Three carbon atoms on each side.

The equation is balanced! Two iron oxide plus three carbon form four iron plus three carbon dioxide. Did you follow what we did with the oxygen atoms? The problem was getting three oxygen atoms from the reactants... ... to fit into a molecule with two oxygen atoms.

We solved the problem by doubling the amount of iron oxide. Two formula units of iron oxide are needed to form three carbon dioxide molecules. Actually, there is another way to solve the same problem. But it's a bit more complicated, so we'll save that for later.