Hello! Bea here, and welcome back to integrated
physics and chemistry. I want you to think back to our lesson on physical and chemical changes.
Do you remember what a chemical change is? A chemical change is when one substance changes
and becomes a new, different substance. In this unit, we are going to learn about the
reactions that cause these chemical changes. But before we get started, let's look at
our goals for this lesson. By the end, you'll be able to: define a chemical
reaction; define reactants and products; apply the law of conservation of mass
when balancing chemical equations. A chemical reaction is when two or more substances
react to form new substances. For example, magnesium and oxygen react to form magnesium
oxide. However, sometimes a chemical reaction is more involved and includes a lot of different
substances. To make chemical reactions more user-friendly, chemists use chemical equations. A
chemical equation is a symbolic representation of chemical reactions. In a chemical equation, the
chemical formula for each reacting substance, or reactant, is written, followed by the substance
that is produced, or product, in the reaction. The standard format for a chemical equation
is to write the reactants on the left, connected by addition signs, then an
arrow pointing from left to right, followed by the products connected by addition
signs. When reading a chemical equation, you use the word "yield" for the arrow. For example,
magnesium plus oxygen yields magnesium oxide. All chemical reactions must follow the law
of conservation of mass. This law states that mass is neither created or destroyed
in a chemical reaction. What this means is that every atom present in the reactants must
also be present in the products. For example, in the chemical equation 2
H2 plus O2 yields two H2O, you have four hydrogen atoms and two
oxygen atoms in the reactants and products. Let's look at the reaction between magnesium and
oxygen one more time. How many atoms of magnesium are in the reactants? There is one magnesium atom.
How about oxygen? There are two atoms of oxygen. Now, let's take a look at the products. There is
one atom of magnesium and one atom of oxygen in the products. Does this chemical equation
follow the law of conservation of mass? No, somehow we lost an atom of oxygen along the
way. Do you have any idea how we can fix this? Well, what happens if the equation looks like this
instead? By adding a 2 in front of the magnesium, we are saying that we would be adding
two magnesium atoms to the reaction. The 2 in front of the Mg O means we have two
magnesium oxide compounds. So, each atom in the formula is multiplied by two, which means we have
two magnesium atoms and two oxygen atoms. This means that we have two magnesium atoms and two
oxygen atoms in our reactants and two magnesium oxide compounds, which gives us two magnesium
atoms and two oxygen atoms in our products. Now, our reaction meets the law of conservation
of mass! This is called a balanced equation. This process of making sure that a chemical
equation meets the law of conservation of mass is called balancing chemical equations. To balance
a chemical equation, you can follow these steps. First, make a list of all the atoms that are
present in the reactants and the products. Next, look at the atoms of each element to
see if you can add a coefficient, or a large number in front of the chemical formulas, to
balance the number of atoms of each element. When you add a coefficient to each compound,
update the list of elements in that compound. One important rule when balancing equations is
that you can only add coefficients in front of compounds. You cannot change the formula of
a compound by changing the subscript. This is because if you change the subscript,
you change the compound. For example, 2O2 means two oxygen molecules, while O4 is not
a molecule. Once your reactants list matches your products list, your equation is balanced! Let's
work through these steps to balance the equation N2 plus H2 yields NH3. Let's make a list
of the number of atoms of each element in the reactants and products. In our reactants, we
have two nitrogen atoms and two hydrogen atoms. In our products, we have one nitrogen
atom and three hydrogen atoms. Let's try to balance our nitrogen atoms first.
We have two in our reactants and just one in our products. We can place a 2 in front of
NH3 in the products. Let's update our list. By placing a 2 in front of the molecule, we now
have two of those molecules. This means we have two nitrogen atoms, one in each molecule, and
six hydrogen atoms, three in each molecule. Our nitrogen atoms match, but
what about our hydrogen atoms? Oh, no! Now they don't match!
Any ideas of what we can do? We can place a 3 in front of the H2 in the
reactants. By placing a 3 there, we have three of those molecules, giving us a total of six
hydrogen atoms in our reactants to match our six hydrogen atoms in the products. Let's update our
list. Wow, look at that! Our equation is balanced. OK, I think you're ready to try one on your own! Try to balance the chemical equation
Li plus AlCl3 yields LiCl plus Al. Remember, make a list and update it every time
you add a coefficient. When your reactant list matches your product list, the equation is
balanced. Pause the video and give it a try. Welcome back! Check your answer
against mine. How did you do? If balancing equations is still a little
confusing, don't worry! You will have some more practice in the lesson PDF. And our next lesson
is a PhET simulation on balancing equations. Both of these are great opportunities
to continue practicing this concept. As we went through the lesson today, we learned
how the law of conservation of mass applies to chemical reactions and then practiced balancing
chemical equations. In our next lesson, you will continue practicing this concept while working
through a PhET simulation. Until next time, remember, the universe is vast and full
of surprises, so never stop exploring! See you next time!
