9.4.9 Chemical Equations and Chemical Reactions
This is the ninth lecture from Chapter 4: “Stoichiometry” in the new Class 9 Chemistry book (Punjab Board – PCTB). It discusses the basics of chemical equations as well as chemical reactions. The lecture also includes a multiple-choice quiz, short question and long question notes.
MCQs Based Quiz
Short Questions
Why do chemists use chemical equations?
Chemists use chemical equations to:
- Study the chemical change
- Study the different factors that control this change
What is a chemical equation?
The method of representing a chemical change by using the symbols of elements and chemical formulas of compounds is called a chemical equation.
Example:
$\text{2C} + \text{O}_2 \longrightarrow \text{2CO}$
What is the purpose of a chemical equation?
A chemical equation tells us about how a chemical change is converting elements or compounds (reactants) into some other elements or compounds (products)
Example:
$$
\underset{\text{Reactants}}{4\mathrm{Al}\ + \ 3\mathrm{O}_2}
\ \longrightarrow \
\underset{\text{Products}}{2\mathrm{Al}_2\mathrm{O}_3}
$$
What are reactants?
The chemical substances that react in a chemical reaction are called reactants. They are written on the left side of the chemical equation.
Example:
$$
\underset{\text{Reactants}}{4\mathrm{Al}\ + \ 3\mathrm{O}_2}
\ \longrightarrow \
\underset{\text{Products}}{2\mathrm{Al}_2\mathrm{O}_3}
$$
What are products?
The chemical substances that are produced in a chemical reaction are called products. They are written on the right side of the chemical equation.
Example:
$$
\underset{\text{Reactants}}{4\mathrm{Al}\ + \ 3\mathrm{O}_2}
\ \longrightarrow \
\underset{\text{Products}}{2\mathrm{Al}_2\mathrm{O}_3}
$$
What are the different components of a chemical equation?
A chemical equation contains following components:
- Reactants (on the left side)
- Products (on the right side)
- An arrow (from reactants to the products)
How would you show the physical state of the chemical species in a chemical reaction?
Physical states of chemical species are written as subscript. Solid, liquid and gas are written as s, l and g respectively. Aqueous (aq) indicates the dissolved ions.
$\text{Zn}_{\text{(s)}} + H_2SO_{4 (aq)} \longrightarrow \text{ZnSO}_{4 (s)} + \text{H}_{2 (g)} $
What is a reversible reaction?
A chemical reaction that can move both ways i.e., the reactants react to give products and products react to give reactants back is called reversible reactions. They are represented by a double arrow ().
$$
\mathrm{N_2}(g) + 3\mathrm{H_2}(g) \rightleftharpoons 2\mathrm{NH_3}(g)
$$
Give an example of chemical equation involving reaction involving ions.
AgNO3 and NaCl react in water to produce products.
$$\mathrm{AgNO_3}(aq) + \mathrm{NaCl}(aq) \rightarrow \mathrm{AgCl}(s) + \mathrm{NaNO_3}(aq)$$
AgCl is insoluble in water and forms a precipitate.
Descriptive Question
Q1. Write a comprehensive note on chemical equations. Also explain the basics of chemical reactions.
A chemical reaction is a chemical change in which substances (reactants) react to produce some other substances (products).
The method of representing a chemical change by using the symbols of elements and chemical formulas of compounds is called a chemical equation.
Chemists use chemical equations to:
- Study the chemical change
- Study the different factors that control this change
So, in other words, a chemical equation tells us about how a chemical change converts elements or compounds into some other elements or compounds.
Components of Chemical Equation:
A complete chemical equation has following components:
Reactants: Substances that react are called reactants. They are written on the left side of the equation.
Products: Substances that are produced because of that reaction are called products. They are written on right side of the equation.
Arrow: An arrow represents the direction of reaction. It is drawn from reactants (left side) to the products (right side).
$$\underset{\text{Reactants}}{4\mathrm{Al}\ + \ 3\mathrm{O}_2}
\ \longrightarrow \
\underset{\text{Products}}{2\mathrm{Al}_2\mathrm{O}_3}$$
Writing a Complete Chemical Equation:
Following points must be remembered while writing a chemical equation:
- The chemical equation must follow the law of conservation of mass. Which means no atom should be destroyed or created during a chemical reaction. The total number of atoms on left side (reactants) must be equal to total number of atoms on right side (products). In simpler words, the equation must be balanced.
- The formulas of elements and compounds must be written correctly.
- There should be correct mole ratio among, reactants, products and between the reactants and products.
- The chemical equation must also indicate the direction of the reaction.
- Physical states of chemical species are written as subscript. Solid, liquid and gas are written as s, l and g respectively. Aqueous (aq) indicates the dissolved ions.
For example the following equation is written by implementing all of these points:
$\text{Zn}_{\text{(s)}} + H_2SO_{4 (aq)} \longrightarrow \text{ZnSO}_{4 (s)} + \text{H}_{2 (g)} $
This equation tells us that one mole of zinc reacts with one mole of sulphuric acid to produce one mole of zinc sulphate and one mole of hydrogen gas.
Reversible Reactions:
Chemical reactions in which reactants react to produce products and products again react to form reactants are called reversible reactions. Such type of reactions move both ways and indicated by a double arrow $\rightleftharpoons$.
$\mathrm{N_2}(g) + 3\mathrm{H_2}(g) \rightleftharpoons 2\mathrm{NH_3}(g)$
Reactions Involving Ions:
Following is chemical equation for a reaction involving ions:
$\mathrm{AgNO_3}_{(aq)} + \mathrm{NaCl}_{(aq)} \rightarrow \mathrm{AgCl}(s) + \mathrm{Na^+}_{(aq)} + \mathrm{NO_3^-}_{(aq)}$
AgNO3 and NaCl are ionic compound and soluble in water. But when they react with each other in water, they produce AgCl which is insoluble in water and comes out as precipitate.