We've looked at how atoms bond and the types of molecular structures they form. Now it's time to write down the reactions to show how many atoms are reacting and what products result from those reactions.
If you'd like a quick overview of the key types of reactions, check out the ExplainersEnigma video. Otherwise, continue onward!
ExplainersEnigma | "Every Type of Chemical Reaction Explained in 9 Minutes"
Writing and Balancing Chemical Equations
A chemical equation involves the reactants, which are your starting materials. The products are the materials that result from the reaction. One simple example is the burning of natural gas — this is the combustion reaction between methane and oxygen. When you burn methane with oxygen, this produces water and carbon dioxide:
CH4 + 2O2 → CO2 + 2H2O
In this equation, notice the following components:
plus signs (+) separate the individual reactants and products
Coefficients are the numbers of each molecule (two oxygen molecules and two water molecules in this example)
The Arrow shows the direction the reaction goes, from reactant to product.
Wayne Breslyn | "How to Balance Chemical Equations"
Types of Chemical Reactions
Overview
Not all reactions happen the same way. In this section, we're looking at three major types of chemical reactions. With all reaction types, bear in mind that electrons get transferred as atoms reconfigure themselves into new combinations.
Precipitation: When dissolved liquids react to form solid substances. (Think of it like solid rain falling in the ocean.)
Acid-Base: When you mix and acid and a base, the hydrogen ion H+ gets transferred from one chemical to another.
Oxidation-Reduction (Redox): when electrons get transferred between atoms in such a way that the oxidation number changes. Basically, if the effective electrical charge of the atom changes, then it's a redox reaction.
Precipitation Reactions
Remember that section about solutions? When we bring together different compounds in a liquid, sometimes you get a reaction rather than a simple mixture. In a precipitation reaction, the liquids react and cause a solid to form.
CrashCourse | "Precipitation Reactions"
Acid-Base Reactions
When you add an acid to water, it releases hydrogen ions (H+) into the solution. Adding a base leads to increased hydroxide ions (OH-). In an acid-base reaction, the H+ ions are exchanged between atoms and molecules to create different compounds.
CrashCourse | "Acid-Base Reactions in Solution"
Oxidation-Reduction (Redox) Reactions
The oxidation number indicates the effective charge of each atom within a molecule — essentially, how many extra electrons the atom has gained or lost from bonding with other atoms. In oxidation-reduction reactions, the effective number of electrons changes, sometimes with dramatic results, such as in combustion reactions. The term "oxidation" comes from oxygen — and oxygen is well known for fire and corrosion.
Melissa Maribel | "Oxidation and Reduction (Redox) Reactions Step-by-Step Example"
Stoichiometry and Reaction Yields
Stoichiometry: Calculating Reaction Quantities
Let's say you have a some quantity of charcoal for a barbecue, and you want to know how much air is needed to burn all that charcoal. If you have the reaction formula, you can use that starting mass of carbon to determine how much mass of oxygen it will react with.
This is where moles really help. Using the molar masses from your reaction, you can calculate just how much reactant is needed for a certain amount of product, or you can see how much product you'll get from the amount of reactants you've put together.
In the Ketzbook videos, we first look at the basic concept of stoichiometry, and then how to use masses in your calculations.
Limiting Reagents and Percent Yields
When you bring two reactants together, it's unlikely that everything will completely react. Instead, you'll have some constraints:
A Limiting Reactant is the reactant you don't have enough of. So the reaction will use up all of limiting reactant, but some of the other reactants will still be left over.
Percent Yield indicates the amount of product you actually get when compared to how much you could theoretically produce.
Professor Dave Explains | "Limiting Reagents and Percent Yield"
Wrapping Up the Course
Congratulations!
If you've gone through all the units and reached this point, then you've completed the course! Let's briefly review some of the skills you've picked up:
You've learned some of the history and applications of chemistry.
You know the differences between atoms, molecules, and ions.
You've explored the role of electrons in atomic properties, chemical bonds, and molecular structures.
You've learned to write and balance chemical equations, and then apply these equations to real-world reaction quantitites.
Whether you've used this course to earn science, writing, or history credit, I hope you've gained a greater appreciation for the science of chemistry and how it shapes our world!