How to Balance Chemical Equations
A balanced chemical equation tells how many units of each atom, molecule or formula unit and how many are needed to form a specific product.
So, in a chemical equation, first you’re usually going to have coefficients. Now, you don’t always have coefficient but many times you do, and you have two of them in this equation right here, a two here and a two here. So those are coefficients. Now this little number right here is a subscript. And if you think about the prefix sub like in submarine, it means below. So if you think about the word subscript, it also has the prefix sub, which means it’s below. You can see how it’s smaller, and below here. Now a coefficient is as big as the other letters, so that’s a coefficient right here, it comes before the molecule or before the atom. And it tells you how many of something that you have. So it’s different than a subscript, in that right here this subscript means you have two chlorine atoms together in a molecule, so you have a chloride molecule. Now over here you don’t have two sodium atoms together in a molecule, instead you just have two sodium atoms doing their own thing floating around. So that’s what the two means: you have two units of something. So right here you have a sodium chloride molecule, it means you have two sodium chloride molecules. Now for the purpose of counting atoms to make sure you have a balanced chemical equation, make sure you apply the coefficient to every atom in that molecule. So this two right here means you have two sodium atoms, and it means you have two chlorine atoms.
So the next thing you’re always going to have in an equation is an arrow. I’m going to put an “s” in parentheses here because sometimes you have more than one. And an arrow just shows the direction of a reaction, and it’s going to move from the reactive side of the equation to the product side. So here’s the reactant side because these are the things that are reacting together, and here’s the product, or what they form. Now sometimes you may have two arrows pointing in opposite directions or a double-headed arrow where it’s one line, but there’s arrowheads pointing in both directions, and that shows that it’s a reversible reaction. Next we’re going to get to the subscripts. So we only have one subscript here, and it’s right here. And I kind of already told you what that is. Now sometimes you don’t have a subscript. Like right here you don’t have a subscript. When there’s no subscript, you can just assume that you could actually put a one there. So we could actually write a one right here and that would mean there’s one sodium atom there. It’s kind of an understood one. You don’t really need it there. And the same thing with coefficients. Right here you don’t have a coefficient, but it’s an understood one. So you could just put a one right there, and it would be the same thing as not having a one at all. So now finally we need to make sure our equation is balanced.
And remember, a balanced chemical equation tells how many units of each atom, molecule, or formula unit are present, and how many are needed to form a specific product. And so this right here tells us how many are needed to form a specific product. Because to get a sodium chloride molecule, you need one sodium and one chlorine atom to join together. And see since we have the coefficient of two, it means we have two sodium chloride molecules, but it shows us that really you can have see what that coefficient is telling us is that we have two sodium chloride molecules looking like that, so you only need one sodium and one chlorine molecule, or one sodium and one chlorine atom to join together to form sodium chloride. So that’s what it means when it says how many are needed to form a specific product. And a balanced chemical equation does tell how many units of each atom, molecule, or formula unit are present. And we can look right here, and see that we have two sodium atoms over here, and two sodium atoms over here. We have two chlorine atoms over here, and we have two chlorine atoms over here. And so this is a balanced chemical equation because we have equal numbers of each atom on each side of the equation. So this equation is balanced. And if there are equal numbers of each type of atom on the left and right sides of the reaction arrow, then it is a balanced equation. Now, a balanced equation represents the Law of Conservation of Mass which states that in any chemical or physical change, matter cannot be created or destroyed. So if you were wondering why we have to have the same numbers of each atom on the left and right sides of the reaction arrow, it’s because matter cannot be created or destroyed.
So whatever’s on the reactive side of the equation must somehow be factored in to the product side of the equation.