Chemical Reactions

Chemical Reaction Definition

During a chemical reaction, the atoms of the reactants rearrange themselves, forming new bonds and breaking existing ones to create one or more different substances, known as products.

When this happens, we can see changes in colour, gas formation, temperature changes, or even the formation of a precipitate. Or, in some cases, the result looks the same as before.

Chemists use chemical equations to show how what happens to the reactants and products involved in a chemical reaction.

• Reactants are substances that react together to form products.
• Products are the new substances formed after the chemical reaction takes place.

Chemical Reaction Balance

A very important skill to have when it comes to chemical reactions is the ability to balance chemical equations.

To balance chemical equations, there are some steps we need to follow. As an example, take a look at the chemical equation below: $$ \text{K + H}_{2}\text{O} \to \text{KOH + H}_{2} $$

Step 1: Count the number of atoms present on each side of the chemical equation.

First, we need to count the number of atoms for each type of atom present in the reactant and on the product side. To find out the number of atoms for each type, we can multiply the coefficient (which, in this case, is 1) by the number on the subscript.

To make it easier to remember, we can make a simple table below the chemical equation.

Figure 4. Counting the number of atoms present on each side of the equation

If the number of atoms on each side is the same, then it means that your equation is already balanced. Now, if the number of atoms on each side is different, then you need to go to step two.

Step 2: Determine how many atoms you must add in order to balance the chemical equation.

In order to balance this chemical equation, we have to look for multiples of the coefficient. In this example, the coefficients are 1.

The only way to get the same amount of H atoms on both sides you be to add a coefficient of 2 to the H₂O molecules, and a coefficient of 2 to the KOH molecules. This way, we would get 4 H atoms on each side.

However, this would also change the number of K atoms on the product side, and the number of oxygen atoms on both sides. To overcome this, we can add a coefficient of 2 to the K on the reactant side. Now, the reaction should be balanced!

Figure 5. Adding coefficients to balance a chemical equation

Types of Chemical Reactions

There are five basic types of chemical reactions:

• Combination or synthesis reactions
• Decomposition reactions
• Combustion reactions
• Single-replacement reactions
• Double-replacement reactions

Synthesis Reactions

Synthesis reactions are reactions in which two or more substances combine to form a new substance. The general formula for synthesis reactions is: \( A + B \to C \).

For example, the formation of hydrogen bromide (HBr) is the result of a synthesis reaction between hydrogen and bromine.

$$ H_{2}\text{ (g)} \text{ }+ \text{ }Br_{2} \text{ (l)}\to 2\text{ }HBr \text{ (g)} $$

Oxidation-Reduction (redox) Reactions

In redox reactions, electrons are transferred from one species (oxidized) to another species (reduced). A change in oxidation state indicates that a redox reaction has taken place.

• Oxidation is the loss of electrons
• Reduction is the gain of electrons

For example, the reaction between zinc (Zn) and manganese oxide (MnO₂) is a type of oxidation-reduction reaction.

$$ Zn\text{ }( s)\text{ + }2\text{ }MnO_{2}\text{ }(s)\text{ }\to \text{ }ZnO \text{ }(s)\text{ + }Mn_{2}O_{3}\text{ }(s) $$

Decomposition Reactions

Next, we have decomposition reactions. Decomposition reactions are chemical reactions in which one substance gets converted into 2 or more substances, and the general formula for this is: \( AB \to A + B \).

The decomposition of calcium carbonate (CaCO₃) into calcium oxide (CaO) and carbon dioxide (CO₂) is an example of a decomposition reaction.

$$ CaCO_{3}\text{ } (s)\to \text{ } CaO \text{ (s)} \text{ + } CO_{2} \text{ (g)} $$

Combustion Reactions

Combustion reactions are reactions involving a hydrocarbon reacting with oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O). These reactions give out energy as heat and light.

A common combustion reaction is the reaction between the hydrocarbon methane (CH₄) and oxygen (O₂).

$$ CH_{4}\text{ }(g) + O_{2}\text{ } (g) \text{ }\to\text{ } CO_{2} \text{ }(g)\text{ }+\text{ }H_{2}O \text{ }(g) $$

Single Replacement Reactions

Single replacement reactions occur when one element in a compound is replaced by another element. The general formula for single displacement reactions is: \( \color{Teal} A \color{orchid} B +\color{Blue} C \color{black} \to \color{Teal} A \color{black} + \color{orchid} B \color{Blue}C \)

In single replacement reactions, the element that tends to form cations will replace the cation in a compound. Similarly, the element that tend to form anions will replace the anion in a compound.

For example, in a reaction between zinc chloride (ZnCl₂) and copper (Cu), copper replaces the Zn to form cooper chloride (CuCl₂).

$$ ZnCl_{2}\text{ } (s)\text{ + } Cu \text{ } (s) \text{ } \to \text{ }CuCl_{2}\text{ } (s)\text{ } + \text{ } Zn\text{ } (s) $$

Double Replacement Reactions

Double replacement reactions occur when two compounds reacts in aqueous solutions, causing the cations and anions of the two reactants to "trade partners or switch places" to form two new compounds. The general formula is this case is: \( \color{Teal} A\color{Orchid} B\color{black}\text{ } + \text{ }\color{Blue} C\color{Orange} D\color{black} \text{ }\to \text{ }\color{Teal} A\color{Orange} D\color{black}\text{ } + \text{ }\color{Orchid} B\color{Blue} C \).

For example, the reaction between mercury (II) nitrate and diammonium sulfide in aqueous solutions to yield mercuric sulfide and ammonium nitrate is a type of double replacement reaction.

$$ Hg(NO_{3})_{2} \text{ }(aq)\text{ + }(NH_{4})_{2}S\text{ }(aq) \text{ }\to HgS \text{ }(s) \text{ + } 2NH_{4}NO_{3} \text{ } (aq) $$

Neutralization Reactions

Neutralization reactions are a special type of double replacement reactions in which one reactant is an acid and the other is a base, giving salt and water as products. The equation below is an example of a neutralization reaction.

$$ HCl \text{ (acid) + }LiOH\text{ (base) } \to LiCl \text{ (salt)}+ H_{2}O \text{ (water)} $$

Chemical Reaction of Photosynthesis

Now that you are aware of the different types of chemical reactions that can occur around you, let's take a look at the chemical reaction happens in photosynthesis.

The process and chemical reaction of photosynthesis is shown in the figure below.

Chemical Reaction Examples

Lastly, let's look at some examples involving chemical reactions. Let's start with the following reaction: \( \text{Zn + AgNO}_{3}\to \text{Ag + Zn(NO}_{3}\text{)}_{2} \).

Can you guess which type of chemical reaction it is? Well, if you guessed that it is a decomposition reaction, you are right!

Now, let's amplify the difficulty a bit and solve a problem similar to what you might see in your exam.

Now, I hope that you feel more confident in your understanding of chemical reactions!