Chemical Reactions and Equations

Chemical reactions are processes in which substances, called reactants, are transformed into different substances, called products. These processes can involve the breaking of bonds in the reactants and the formation of new bonds in the products. Chemical reactions are fundamental to chemistry and are used in industries, environmental processes, and even in our everyday lives, such as cooking, breathing, and growing plants.

What is a chemical reaction?

A chemical reaction involves the rearrangement of atoms and a change in the chemical composition of substances. A classic example of a chemical reaction is the burning of hydrogen in oxygen to form water:

2H 2 + O 2 → 2H 2 O

In this reaction, hydrogen gas (_H2) combines with oxygen gas (_O2) to form water (_H2O).

Chemical equation

A chemical equation is a way of representing chemical reactions in a concise form. A chemical equation has reactants on the left, products on the right, and an arrow indicating the direction of the reaction.

For example, let's make magnesium oxide by reacting magnesium and oxygen:

2Mg + O 2 → 2MgO

This equation tells us that two molecules of magnesium react with one molecule of oxygen to form two molecules of magnesium oxide.

Parts of a chemical equation

A chemical equation consists of the following:

• Reactants: The starting substances in a reaction.
• Product: The substance formed by the reaction.
• Coefficients: Numbers placed before formulas to balance the equation.
• Subscripts: Numbers in a formula that indicate the number of atoms.

Example of a chemical equation

Consider the reaction of nitrogen gas and hydrogen gas to form ammonia:

N 2 + 3H 2 → 2NH 3

In this equation:

• Nitrogen gas (_N2) and hydrogen gas (_H2) are the reactants.
• Ammonia (_NH3) is the product.
• The equation is balanced because there are equal numbers of nitrogen and hydrogen atoms on both sides.

Balancing chemical equations

Balancing chemical equations is necessary because the law of conservation of mass states that matter cannot be created or destroyed in normal chemical reactions. To balance a chemical equation, make sure the number of atoms of each type on the reactant side equals the number on the product side.

Steps to balancing chemical equations

1. Write the unbalanced equation.
2. Count the number of each type of atom in the reactants and products.
3. Add coefficients to equalize the number of atoms on both sides.

For example, let's balance the combustion of ethane (C ₂ H ₆):

C 2 H 6 + O 2 → CO 2 + H 2 O

When we count the atoms, we see:

• Carbon: 2 in C ₂ H ₆, 1 in CO ₂.
• Hydrogen: 6 in C ₂ H ₆, 2 in H ₂ O.
• Oxygen: 2 in _O2, 2 in _CO2 and 1 in _H2O.

By adding the coefficients:

2C 2 H 6 + 7O 2 → 4CO 2 + 6H 2 O

The equation is now balanced.

Types of chemical reactions

Chemical reactions can be classified into several broad categories:

1. Combination reactions

In a combination reaction, two or more substances combine to form a single product. For example:

2H 2 + O 2 → 2H 2 O

Here hydrogen and oxygen gases combine to form water.

2. Decomposition reactions

In decomposition reactions, a single compound breaks down into two or more simpler substances. For example:

2HgO → 2Hg + O 2

Mercury(II) oxide decomposes into mercury and oxygen gas.

3. Displacement or substitution reactions

In these reactions, one element in the compound is displaced by another element. An example of this is the reaction between iron and copper (II) sulfate:

Fe + CuSO 4 → FeSO 4 + Cu

Iron displaces copper in copper sulphate.

4. Double displacement reactions

These reactions involve the exchange of ions between two compounds. A classic example of this is the reaction between barium chloride and sodium sulfate:

BaCl 2 + Na 2 SO 4 → BaSO 4 + 2NaCl

Barium sulfate and sodium chloride are formed.

5. Combustion reactions

In a combustion reaction, a substance reacts with oxygen to release energy in the form of heat and light. A typical reaction is that of ethane:

2C 2 H 6 + 7O 2 → 4CO 2 + 6H 2 O

Exothermic and endothermic reactions

Chemical reactions can either release or absorb energy.

Exothermic reactions

Exothermic reactions release energy in the form of heat. Combustion reactions are a common example. In the reaction of burning hydrogen in oxygen, the energy released is:

2H 2 + O 2 → 2H 2 O

Endothermic reactions

Endothermic reactions absorb energy from their surroundings. An example of this is photosynthesis:

6CO 2 + 6H 2 O + Energy (sunlight) → C 6 H 12 O 6 + 6O 2

Plants absorb sunlight and convert carbon dioxide and water into glucose and oxygen.

Law of conservation of mass

This law states that mass is neither created nor destroyed in a chemical reaction. This is important for balancing chemical equations, because the total mass of the reactants equals the total mass of the products.

Catalyst

Catalysts are substances that speed up chemical reactions without being consumed in the process. They provide an alternative reaction pathway with lower activation energy. Enzymes are biological catalysts in living organisms.

Balanced chemical equations and stoichiometry

Stoichiometry involves using a balanced equation to determine the proportion of reactants and products in a chemical reaction. For calculations, the coefficients in the balanced equation are used to determine moles, mass, and volume.

Conclusion

Chemical reactions and equations are important to understanding chemistry. They show how different substances interact and change, allowing us to predict product formation and calculate the quantities involved in reactions. Balancing equations ensures conservation of mass, which is essential for accurate chemical calculations. Recognizing different types of reactions can help understand the behavior of compounds and elements in different contexts.

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