Have you ever tried to make soap at home? The art of soap-making is a type of chemical reaction! First, look at the structure of a soap molecule.

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Soap is a type of salt made up of fatty acids. The structure of soap consists of a nonpolar hydrocarbon chain containing a carboxylic acid group. The carboxylic acid group forms an ionic bond with a sodium or potassium ion. So, soap has a hydrophobic end that does not dissolve in water (hydrocarbon chain) and a hydrophilic end (ionic salt). When soap comes into contact with grease and water, the hydrophobic end of the soap interacts with the nonpolar molecules of grease and suspends the grease/oil, allowing it to be washed away by water!

Soap forms by the reaction between the triglycerides in fat/oil and an alkali such as aqueous sodium hydroxide or potassium hydroxide.

The chemical reaction for the making of soap is:

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First, we will talk about chemical reactions and how to balance them.Then, we will look at the different types of chemical reactions and give some examples.We will also learn about oxidation numbers and how to calculate them!

Definition of a Chemical Reaction

Chemical reactions are fascinating processes that occur when different substances combine to form a new substance. These reactions can result in a variety of changes, such as color changes, gas formation, temperature changes, or even the formation of a precipitate. Chemical equations are used to show what happens during a chemical reaction. Reactants are the chemicals on the left side of the equation, while products are the chemicals on the right side of the equation. For example, in photosynthesis, six moles of carbon dioxide and six moles of water react to make one mole of glucose and six moles of oxygen as products. Balancing chemical equations is important, as it ensures that the law of conservation of mass and the law of conservation of charge are followed. This means that the mass and charge of the products are equal to the mass and charge of the reactants. With practice, anyone can learn to balance equations and tackle different problems involving chemical reactions.

Steps to Balance Chemical Equations

Now that we have found the coefficients that make the number of moles of each element equal on both sides, we can write the balanced chemical equation:

2N2 + 3H2 → 2NH3

By adding a coefficient of 2 to NH3, and a coefficient of 3 to H2, the equation is now balanced. Remember, balancing chemical equations is important to ensure that the law of conservation of mass and the law of conservation of charge are followed. With practice, anyone can learn to balance equations and tackle different problems involving chemical reactions.

Types of Chemical Reactions and Examples

Now that you know the definition of a chemical reaction and how to balance chemical equations, let's dive into the different types of chemical reactions that you might encounter in your chemistry journey!

The essential types of chemical reactions include:

Synthesis reactions (also called combination reactions)Decomposition reactions Single and double replacement reactions Combustion reactions Redox reactions Acid-base reactions (also called neutralization reactions)Precipitation reactions

We will talk about the general difference between these types of reactions. However, for your AP exam, you should focus more on the last three types of reaction: redox, acid-base, and precipitation. Synthesis Reactions Synthesis reactions are chemical reactions that occur when two or more reactants produce a single product.   The general formula for synthesis reactions is: A + B → AB.

Decomposition Reactions

Decomposition reactions are the opposite of synthesis reactions.

So, instead of two substances reacting to form one product, decomposition reactions are chemical reactions where one reactant is broken down to form many products. In this type of reaction, the general formula is: AB → A + B.

Replacement Reactions

Single replacement reactions are chemical reactions where the element by itself replaces one of the elements in the compound.

The general formula for single replacement reactions is: A + BC → AC + B.

Double replacement reactions are chemical reactions that occur in an aqueous solution.

In this type of reaction, the cation (positive ions) and anion (negative ions) of one compound switch places with the cation and anion of the other compound to yield two different products. The general formula for double replacement reactions is:

AB + CD → AC + BD

Combustion Reactions

In a combustion reaction, oxygen (O2) will react with any chemical to form carbon dioxide (CO2) and water (H2O). The general formula is A + O2 → CO2 + H2O in combustion reactions.

Up until now, the types of chemical reactions that we described were simple. Now, let's look at the types of chemical reactions that are a little more complex and commonly appear in the AP exam!

Oxidation-Reduction (Redox) Reactions

In redox reactions, there is a transfer of electrons that happens between the reactants involved in the reaction. Combustion reactions are actually a type of oxidation-reduction reaction!

There is a pretty cool mnemonic that can help you remember what occurs in redox reactions!

OIL RIG ⇨ Oxidation Is Loss, Reduction Is Gain

This means that the element that loses electrons will be oxidized, while the element that gains electrons will be reduced. Elements that undergo oxidation will increase their oxidation number and become more positively charged. Elements that are reduced will decrease their oxidation number and become more negatively charged.

Oxidation numbers can tell us which chemicals have lost or gained electrons in a chemical reaction. The oxidation number of an atom shows you what would be the charge of an atom in a special case. We give both electrons of an electron pair (covalent bond) to the more electron negative atom.

For example, when we look at the structure of CO2, we notice that both oxygen atoms have double bonds with carbon. So, this means that both oxygen atoms will have two extra electrons than what you would expect them to have. On the other hand, carbon has four electrons less, giving it a +4 oxidation state.

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Calculating Oxidation Numbers

To find the oxidation number, we can follow some general rules:

First, notice whether the ion or compound has a charge. Then, look for atoms with set oxidation numbers. If any, write these oxidation numbers above the atoms. Finally, calculate the oxidation state for the remaining atom. The sum of all oxidation numbers should be equal to the charge of the ion or compound. If a compound is neutral, then it should be equal to zero.

When assigning fixed oxidation numbers, there are some common rules we need to follow.

Metals in group I have an oxidation number of +1Metals in group II have an oxidation number of +2Al has an oxidation number of +3F has an oxidation number of +1With the except of metal hydrides, H has an oxidation number of +1Cl usually has a -1 oxidation number, except when present in compounds containing O or F.O has an oxidation number of -2, except in peroxides and compounds containing F.

Let's solve an example!

Calculate the oxidation number of chromium (Cr) in the dichromate ion (Cr2O72-).Does the ion have a charge? Yes, it has a charge of -2. So, keep in mind that the sum of all oxidation numbers will be equal to -2.Do any of these atoms have a fixed oxidation number? Yes. In this case, oxygen has an oxidation number of -2.Now, solve for the oxidation number of Chromium:

Electronegativity

Electronegativity increases from left to right in the periodic table and decreases down a group. Notice that noble gases don't even appear in the image below. They do not bond, so they do not have electronegativity values.  Electronegativity also increases with the increase in effective nuclear charge.

Electronegativity is the ability of an atom to attract a shared pair of electrons.

Acid-base Reactions

In an acid-base reaction (also known as a neutralization reaction), protons transfer between the chemical species in the chemical reaction. When an acid and a base react together, they form salt and water as products: acid + base → water + salt.  Acid-base reactions are considered a type of double displacement reaction!

When weak acids and bases react with each other, equilibrium is reached. At this point, the reactants and products are reacting at the same rate, and the reaction becomes reversible.

When a strong acid and a strong base reaction occurs, the reaction is not considered reversible because the acids dissociate completely during the chemical reaction.

If you want to learn more about acid-base reactions, check out the article "Acid-Base Reactions."

What happens when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH)? HCl is a strong acid, and NaOH is a weak acid. When these two reactants are allowed to react, they dissociate entirely and form sodium chloride (NaCl) and water (H2O).

Precipitation Reactions

Last but not least, let's talk about precipitation reactions. As the name suggests, precipitation reactions form a precipitate (solid) as the product. A precipitate is defined as an insoluble substance that separates itself from the solution.

What happens when you add silver nitrate (AgNO3) to aqueous sodium chloride (NaCl)?When AgNO3 reacts with aqueous NaCl, the products formed are AgCl (a white precipitate) and aqueous NaNO3

Chemical reactions happen all around us and now that you know about them, next time you wash your hands or stare at some wood burning in a fireplace, think about the cool types of chemical reactions happening there!

Great key takeaways! Here's a slightly revised version:

• Chemical reactions occur when different chemicals are combined to create a new substance.
• Chemical equations have a reactant side and a product side.
• Balancing chemical equations requires ensuring that the number of moles of each element on the reactant side is the same as the number on the product side.
• There are three main types of chemical reactions: redox reactions, acid-base reactions, and precipitation reactions. It's important to be familiar with each type.

ReferencesArbuckle, D., & Albert.io., The Ultimate Study Guide to AP® Chemistry, 1 March 2022Moore, J. T., & Langley, R. (2021). McGraw Hill: AP Chemistry, 2022Malone, L. J., Dolter, T. O., & Gentemann, S., Basic concepts of Chemistry (8th ed.), 2013Swanson, J. W., Everything you need to Ace Chemistry in one big fat notebook, 2020

Frequently Asked Questions about Chemical Reactions

What happens during a chemical reaction?

During a chemical reaction, different substances combine to form a new substance. When this happens, we see changes in color, gas formation, temperature changes, or even the formation of a precipitate. 

Which will increase the rate of a chemical reaction?

Increasing temperature, concentration, and surface area will increase the rate of a chemical reaction. 

What is a chemical reaction?

A chemical reaction is when different chemicals are combined to produce a new substance.

How do enzymes speed up chemical reactions?

Enzymes are catalysts that work by increasing the rate of a reaction. They achieve this by lowering the activation energy needed to start a reaction. 

What are the reactants in a chemical reaction?

In a chemical reaction, reactants are the chemicals on the left side of the equation. Reactants will react with each other to form the product.

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