Lesson 1: Introduction, Equations, Reactions

Video Lesson

Lesson Overview

This lesson is divided into three parts. The first part provides a very general view of the entire unit and further presents the meaning of chemical reaction, reactant, product, chemical equation, and reaction’s stoichiometry.

The second section is about chemical equations. It introduces chemical equations and gives conventions used to write the chemical equations. It explains the reason for balancing chemical equations. It presents the different methods used to balance chemical equations as inspection. method, the least common multiple (LCM) method, and the algebraic method.

The third section of the unit treats the types of chemical reactions. It provides the classification of chemical reactions as direct combination, decomposition, displacement, and double decomposition reactions. It gives the definitions and conditions for the reactions including the nature of reactants for each type of reaction.

Competencies

At the end of this lesson you will be able to:

• Define chemical reaction;
• Explain physical and chemical changes using examples
• Write chemical equations for a variety of chemical reactions
• List the four types of chemical reactions;
• Define combination, decomposition, single displacement, and double decomposition reactions and give examples for each.

Brainstorming Question

• What are the differences between the raw and ripened fruit? Is the change reversible?
• What is the difference between a chemical equation and a chemical reaction?
• Which law of chemical reaction is applied when a chemical equation is balanced?

Key Terms

1.1. Chemical Reaction

Change is a fundamental aspect of nature, observed in various forms such as plant growth, wood burning, iron rusting, food rotting, and liquid evaporation. Scientists classify changes as either physical or chemical. Physical changes, like evaporation and melting, do not create new substances. In contrast, chemical changes, such as milk turning into curd and photosynthesis, result in new substances with different chemical compositions. Chemical changes, driven by reactions between substances, are characterized by new substance formation, heat or light production, color changes, and temperature variations.

Chemical reaction is the process in which reacting substances, called reactants, are converted into new substances, called products. The characteristics of the products are completely different from those of the reactants. A chemical reaction is represented by a shorthand notation called chemical equation:

Reactants → Product

A chemical equation uses chemical symbols to show what happens during a chemical reaction. A balanced chemical equation can be used to describe the relationships between the amounts of reactants and products. The quantitative study of reactants and products in a chemical reaction is called reaction stoichiometry.

According to the Law of Conservation of Mass, atoms are neither created nor destroyed during a chemical reaction, so the number of atoms of each element remains the same before and after the reaction.

1.1.2 Balancing Chemical Equations

A. Balancing Chemical Equations by Inspection Method

Balancing a chemical equation involves equalizing the number of atoms on both sides by adding appropriate coefficients. The inspection method, also known as the trial and error or hit and trial method, is a common way to balance equations. It involves four steps: writing the word equation, writing the correct symbols or formulas for reactants and products (unbalanced equation), counting and tabulating the number of each type of atom on both sides and adjusting the coefficients of reactants and products NOT subscripts to balance the equation. This method works best for simple equations.

Example:  Write a balanced chemical equation for the reaction between iron and water to form iron (IV) oxide and hydrogen gas using the inspection method.

Step 1: Iron   +   water      →    Iron(IV) oxide   +    Hydrogen

Step 2: Fe   +   H₂O     →    FeO₂    +    H₂

Step 3: Fe   +   2H₂O     →    FeO₂    +    2H₂

B. Balancing chemical equations by the LCM method

By taking the LCM of the total valence of the reactants and products and dividing it by the total valence of the reactants and products, the coefficients for the balanced chemical equation are derived. This is known as The LCM method. Follow the following steps to balance a chemical equation using the LCM Method.

Step 1: Represent the reaction by a word equation.

Step 2: Write the correct symbols or formulas for the reactants and products.

Step 3: Place the total valence of each atom above it.

Step 4: Find the LCM of each total valence and place it above the arrow.

Step 5: Divide the LCM by each total valence number to obtain the coefficients for each of the reactants and products. Place the obtained coefficients in front of the respective chemical formulas.

C. Balancing Chemical Equations Using Algebraic Method

This method of balancing chemical equations involves assigning algebraic variables as stoichiometric coefficients to each species in the unbalanced chemical equation. These variables are used in mathematical equations and are solved to obtain the values of each stoichiometric coefficient. Consider the formation of ammonia from hydrogen and nitrogen as an example.

Step 1: Write the unbalanced equation with the correct symbols of the reactants & products:

N₂     +     H₂    →     NH₃ 

Step 2: Assign algebraic variables to each species as coefficients (a, b, c) in the unbalanced equation:

a N₂     +     b H₂    →     c NH₃

Then, equations for each element should be set so that they are equal on both the right and left-hand sides.

N: 2a = c (subscript of N is 2 on the left side and 1 on the right side) H: 2b = 3c (subscript of H is 2 on the left side and 3 on the right side)

Step 3: Choose the smallest variable and assign an arbitrary number in order to determine the remaining variables.

In the above case, a is the smallest coefficient. Assuming a = 1, the values of b and c can be obtained as follows. c = 2 x 1= 2, 2b = 3c = 3 x 2 = 6; b = 6/2 = 3

Since a, b, and c have no common multiples, they can be substituted into the equation as follows.

N₂    +    3H₂    →   2NH₃

Exercise: Write a balanced chemical equation for the reaction between aluminum and oxygen using all the 3 methods discussed above.

1.1.3. Types of Chemical Reactions

The four basic types of chemical reactions are combinations, decompositions, single displacement, and double displacement reactions, which are described as follows.

1. Direct Combination Reactions

Combination reactions are those reactions in which two types of pure substances react directly and form a single substance

A + B → AB

Examples

2Na      +          Cl₂       →     2NaCl

 CaO    +          CO₂     →     CaCO₃

2. Decomposition Reactions

A decomposition reaction (or analysis) is a reaction that involves the breaking down of a single compound into two or more elements or simpler compounds. A decomposition reaction can be carried out using heat (thermal decomposition), light (photodecomposition), electricity (electrical decomposition), or a catalyst. But most decomposition reactions are carried out when heat is supplied and this heat energy is indicated by a ‘delta’ (Δ) symbol above the arrow. The general form of the equation for a decomposition reaction is:

Examples:

2HgO    →    2 Hg   +    O₂

CaCO₃    →     CaO   +    CO₂

3. Single Displacement Reactions

A reaction in which one element displaces another element from its compound is known as a single displacement or replacement reaction. Such a reaction is represented by the following two general forms.

I. If A is a metal, it will displace B to form AC, provided A is a more active metal than B.

A + BC → AC + B

II. If A is a non-metal, it will displace C to form BA, provided A is a more active non-metal than C.

A + BC → BA + C

In general, a more reactive element displaces a less reactive element from a compound.

Examples:

Zn + 2HCl → ZnCl₂ + H₂

2Na + 2H₂O → 2NaOH + H₂

F₂ + CaCl₂  → CaF₂ + F₂

4. Double Displacement Reactions

A double displacement reaction is a reaction in which two compounds react together to form two new compounds by exchange of the positive and negative ions of each reactant. Such a reaction is also known as a double replacement reaction or metathesis. This type of reaction can be written in the following general form of equation:

AB + CD → AD + CB

Examples:

AgNO₃    +     NaCl       →      AgCl      +      NaNO₃

Na₂CO₃  +     2HCl       →      2NaCl    +    H₂O    +     CO₂