Writing Balanced Chemical Reactions

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  • 0:01 Chemical Reactions Are…
  • 1:09 Components of a…
  • 2:24 Balancing Chemical Equations
  • 5:14 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Chemical reactions are occurring around us all the time. We express these reactions in chemical equations, which must be balanced, meaning that while both sides may not have the same arrangement of atoms, they will have the same number and type.

Chemical Reactions Are Balanced

Have you ever watched an ice cube melt? It's pretty cool, right? As that solid form of water changes temperature, it melts into a puddle of liquid water right in front of you. This is what we call a physical change, which is when a substance changes its physical phase but not its chemical composition. But, sometimes there is a change in the chemical composition, meaning a chemical reaction has occurred. During this chemical change, the original bonds between atoms are broken, and new molecules are formed.

Chemical reactions are really cool. But the coolest part about them is that they can be written out very nicely as equations. These equations are read from left to right, with an arrow in the middle indicating which way the reaction is going. And, most importantly, they MUST be balanced. In other words, Even though you start with one thing and end with another, the CONTENTS on both sides are the same.

For example, when you bake a cake, you put in eggs, flour, milk, sugar, and butter. In the oven, it cooks and turns from batter to a delicious dessert. But, the cake that comes out has the same amount of all those ingredients that you mixed together initially, they're just sort of rearranged.

Components of a Chemical Equation

The 'ingredients' of a chemical reaction, or substances that react are called the reactants. Easy enough, right? These appear on the left side of the equation. Your finished cake side of the equation, the newly formed substances, are called the products. They're what you produce during the reaction.

Remember how I said that chemical equations have to be balanced? This is because just like everything else, chemical reactions follow the Law of Conservation of Mass, which states that matter cannot be created or destroyed. Basically, this means that what you give is what you get, even if it looks a little different in the end!

For example, if you have a water molecule that is made of two hydrogens and one oxygen, and you break that molecule apart, you still end up with two hydrogens and one oxygen, even if they're no longer bonded together. Every chemical reaction works this way. Each atom in the reaction must be present in the same quantity on both sides of the equation. It doesn't matter how you rearrange them, if you have 10 nitrogen, calcium, argon, or any other type of atom on the reactant side, you have to have 10 nitrogen, calcium, argon, or any other type of atom on the product side.

Balancing Chemical Equations

Now that you know what a chemical equation is, and that is has to be balanced, let's look at a real-world example of how we would go about balancing a chemical equation. You may recall that photosynthesis is the process of converting sunlight into chemical energy. This chemical reaction is a very important one because many living things get their energy from eating plants that photosynthesize.

During photosynthesis, plants take in carbon dioxide (CO2 ) and water (H2O) and from these produce sugar (C6H12O6) and oxygen (O2) - the very oxygen we breathe.

Therefore, our reactants of the equation (the 'ingredients' of the reaction) are CO2 and H2O, while the products (what is produced in the reaction) are C6H12O6 and O2. In the basic chemical equation for photosynthesis, we start with 6 CO2 and 6 H2O, so the left side of our equation looks like this:

6 CO2 + 6 H2O

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