Balancing Chemical Equations

Two very important Laws of Chemistry Law of Conservation of Matter Law of Constant Proportions When balancing a chemical equation we need to keep in mind two very important Laws of Chemistry… Law of Conservation of Matter Law of Constant Proportions

Law of Conservation of Matter Matter cannot be created or destroyed The Law of Conservation of Matter states that matter cannot be created or destroyed.

Chemical Equations This is the way it works in chemical equations too. We have substances (called reactants) that react with each other to form new chemical substances (called products) We can write this reaction down in a chemical equation with the reactants on the left and the products on the right.

Chemical Reaction During a reaction chemical bonds are broken or undone and then re-formed in a different arrangement In other words, a reaction just reorganises atoms… but through it the total mass of all the reactants must EQUAL the total mass of all the products.

Take a look… Take a look at this chemical equation… H2 + O2 → H2O How many hydrogen atoms are reacting? How many oxygen atoms are reacting? And now count how many hydrogen atoms are on the product-side of the equation? In other words, how many hydrogen atoms are being produced in this reaction? How many oxygen atoms are being produced in this reaction?  

Is it the same? Are the amounts the same? … NO! Although the Hydrogen is the same on both sides, we have 2 Oxygen atoms reacting in this reaction and only 1 Oxygen atom in the product being formed. Does this make sense? What happened to the other oxygen? It cannot just disappear into thin air!

What now? Now you may think you can solve this problem by simply adding a 2 after the oxygen in the product like this… H2O2… Or what about changing O to O2 like this… H2O2? This is where the second important Law comes in: The Law of Constant Proportions!

Law of Constant Proportions In any particular chemical compound, all samples of that compound will be made up of the same elements in the same proportion or ratio. The Law of Constant Proportions states that in any particular chemical compound, all samples of that compound will be made up of the same elements in the same proportion or ratio. This means in simpler terms that in any molecules of a particular substance that in the forming of a molecule of a particular substance, the elements will always bond in the same relationship or ratio.

The Law of Constant Proportions For example, a water molecule will always be a chemical combination of the elements hydrogen and oxygen in the ratio 2:1. So water will always be H2O and never H2O2 or H3O. Therefore in order to balance this equation we put a 2 in front of the hydrogen and a 2 in front of the water.

So keeping these two important laws in mind, let’s balance an equation… Hydrogen gas reacts with chlorine gas to form hydrogen chloride gas in the following unbalanced equation: H2 + Cl2 → HCl Let’s do an inventory… What do we have on the reactant-side and how much is there of it compared to the product-side?  

Clearly you can see that what is reacting in this reaction is NOT equal to what is forming in the product so some adjustments will need to be made …

According to the Law of Constant Proportions we cannot touch the composition of the molecules so let’s put a box around each molecule to protect it from being changed… Now we can only balance this equation by putting a number in front of the box to increase the number of molecules produced. That means… there are now two molecules of hydrogen chloride formed for every one molecule of hydrogen and one molecule of chlorine Let’s do an inventory again…

Balanced Equation… Let’s do an inventory again… Now this equation is balanced!