Chemical Equations

The main thing to remember when writing chemical equations is that in a chemical reaction nothing is lost or gained even though changes occur. It's like taking a big blob of plasticine and moulding something using all of the plasticine. You still have the same amount of plasticine, but you've made it look like something different. In chemical reactions, you still have the same number of atoms after the reaction as you did before the reaction, but they've just combined in different ways.

Writing a chemical reaction is simple. Just write out the reactants and products using chemical formulae, as I explained in my post Formulae. Draw an arrow between the reactants and products, NOT an equals sign. Also, write the little state symbols (s for solid, g for gas, l for liquid and aq for aqueous solution).

But how do you know which state symbols to write? Well, water is a liquid. So are molten substances and some other substances like bromine water and chlorine water. There are exceptions here, like if there's a combustion reaction you won't get water as a product but you'll get steam. Most metals (Fe, Al etc.) are solids. I think the only exception here is Hg (mercury) which is a liquid at room temperature. Most of the gases you'll encounter are recognisable, like oxygen, nitrogen and ammonia. For unknown compounds, you could try checking your solubility table to see whether you should mark them as aqueous or solid. Obviously, if it's insoluble, mark it down as a solid, while if it's soluble, then mark it down as aqueous.

Here is the equation of the reaction between hydrochloric acid and calcium carbonate:

But wait! I said earlier that you need equal numbers of atoms in the reactants as in the products, as atoms aren't gained or lost during a reaction. But in my equation above, there's only 1 H in the reactants and 2 in the products! Same goes for Cl!

It's easily fixed. What we do now is add coefficients:

Now we have 2 H and 2 Cl atoms on both sides of the equation!

Remember, it's coefficients you need to add, not subscripts. Adding subscripts would change the chemical formula, and you don't want that.

The coefficient shows that you need twice as many molecules of HCl as compared to calcium carbonate for the reaction to take place. This will come in handy when we talk about limiting reagents later.

Okay, that's most of the basics covered for chemical reactions. Later I'll go over the standard reactions and whatnot. Also limiting reagents. Argh, limiting reagents...