Chemical Reactions
Chemical reactions are the basis of so many physical objects in the universe, including us as humans. Humans use decomposition reactions to digest food and rely on other chemical reactions to keep their blood pH stable. The way plants produce food for themselves is through a chemical reaction involving light and oxygen from the air. In many fireplaces and grills, when you light a fire, a chemical reaction is occurring between oxygen in the air and substances like propane. If you've ever seen a rusty car that's a bit on the older side, that rust is due to chemical reactions between oxygen in the air, water vapor, and iron on the car's surface.
Chemical reactions drive many of the processes in our own lives and are arguably the most important part of chemistry as a science, as the large majority of concepts in the science deal with the nature of chemical reactions. To learn more about them, click on the links below:
The image above represents a chemical reaction. If this image is one of the most confusing things in the world to you, then allow for an explanation. On the left side of the reaction are reactants, the initial species partaking in the reaction. Once the reactants interact, they form products on the right side. This sounds simple but there is a very important rule here that may slip your mind at first if you're a beginner.
Count the number of Oxygen(red) atoms on the left and count the number on the right. Do the same for Hydrogen(white) and Carbon(black) atoms. You'll see that the amount of each atom doesn't change as the reaction proceeds. This is simply due to the conservation of mass for the reaction. The reactants only interact with each other so they won't go anywhere. It is also worth noting that charge is also conserved in a chemical reaction so if there are ions involved, you have to make sure the total charge for the reactants equals the total charge for the products.
Also notice that there are two diatomic oxygen molecules on the left and that is represented in the chemical equation by the 2 as a coefficient. The same goes for water on the right side. What this tells you is that for every 1 methane(CH₄) molecule and every 2 oxygen(O2) molecules reacted, 2 water(H2O) molecules and 1 carbon dioxide(CO2) molecule are produced.
Note that chemical reactions can almost never have fractional coefficients(except in very, very special cases). If you find yourself using fractional coefficients, you have to multiply them to make the ratios equal but with integers. Also note that generally, you should reduce the coefficients if you can. For example, if every coefficient in a chemical reaction you write is an even integer value, then you can reduce them by factors of 2.
H2O → H2 + O2
However, that reaction was already balanced. Sometimes, you may find unbalanced chemical equations, like the one above with water producing hydrogen and oxygen gas.
How do you balance this equation? There are a handful of ways to do so but one of the simplest to use would be the table method.
Draw up a table like the one above where you track the elements involved, their proportions on the reactant side and product side, and check to see if they're balanced.
The reaction isn't balanced as we see on the table above. Let's double the reactants and see if that can balance the equation.
2H2O → H2 + O2
As you can see, the reaction isn't balanced. Let's double the coefficient of the water on the left. Well, now Oxygen is balanced but Hydrogen is unbalanced, so we have the reaction above. We have to make another change.
2H2O → 2H2 + O2
By doubling H2 on the right side, we balanced the reaction successfully now. By the way, you don't need to make separate tables on each trial. You can use one table and just keep editing that if you do a problem concerning this.
It is quite clear that the tabular method is simple and works. However, it can require tons and tons of trial and error, especially if the reaction is far more complicated than this. That gives two issues. The first is that trial and error can become extremely time-consuming and balancing reactions shouldn't be all too time-consuming. Second is that when you leave it up to excessive trial and error, you can easily make a mistake.
However, a far more foolproof method that is very efficient, especially for math people, is the "system of equations" method. This method breaks up the atoms in the reaction into variables and then just solve for the coefficients. Does this sound weird? It should because it's a somewhat nontraditional approach. However, if you're willing to read into it, look at the portion below.
Let's take that same reaction with water decomposing into hydrogen and oxygen gas, but now use the "system of equations" method.
H2O → H2 + O2
What you do is you assign a variable to each atom involved in the reaction. If your reactants involve polyatomic ions, you can assign a variable to just the polyatomic ion.
Here's how we assign the variables. First, put the variables as coefficients for each reactant and product. For this reaction, this means:
a⋅H2O → b⋅H2 + c⋅O2
So, we assigned one variable as each coefficient in the reaction. We'll be solving for these 3 variables with the method.
H: 2a = 2b
O: a = 2c
Ok, let's take a step back. So, what did we just do? We took each atom(or polyatomic ion) in the reaction. We then took its subscript multiplied by the variable that is the coefficient of the molecule it was found in. This is done on the left for reactants and on the right for products.
On the left-side reactants, there are 2 hydrogen atoms in a molecules of water. Thus, the value we assign is 2a for hydrogen. In similar fashion, we assign 2b to hydrogen on the product side, meaning 2a = 2b. The same is done for oxygen.
Now, we have 3 unknowns and 2 equations, which may seem alarming. However, with a bit of substitution, you can easily solve for this system. See below:
2a = 2b --> a = b
a=2c
It's good practice to set everything equal to one variable so you don't mix and match. With our simplification, we can easily now solve this system. You may be asking how, but remember that chemical equations are best balanced in their most reduced form.
If we set a =1, then b=1 and c = 1/2. If setting a to 1 makes it so that other variables aren't reduced integers, then just multiply every variable by the lowest factor you can to make them all reduced integers. In this case, multiply every variable by 2 so c = 1.
a = 2, b =2, c=1
And there are your coefficients.
2H2O → 2H2 + O2
It may seem like a long process, but if you practice it a bit, it becomes far more reliable than using the table method. It's effectively easier and roughly quicker, but only if you have decent experience with linear systems of equations and the equation is complex. If the reaction is simple, like this one, you can use the table method.
Citations/Attributions
Chemical reaction. Provided by: Wikipedia. Located at: https://en.wikipedia.org/wiki/Chemical_reaction . License: CC BY-SA: Attribution-ShareAlike
Chemistry 2e. Provided by: Openstax. Located at: https://openstax.org/books/chemistry-2e/pages/1-introduction. License: CC BY 4.0