4.2.A.1 Representation of Physical and Chemical Processes by Balanced Equations:

1. Introduction to Chemical Equations:

i. Definition:
A chemical equation is the symbolic representation of a chemical reaction where reactants are written on the left and products on the right-hand side separated by an arrow. An equation will give a way on how substances change their chemical structure to produce a new substance through the chemical process. This further illustrates that it follows the rule of mass conservation. Generally speaking, atoms of any given element on the reactant’s side are equal to atoms of that given element on the product’s side.

ii. Purpose:
a. Chemical Representations: The chemical equation represents a shorthand for the representations of chemical reactions.
b. Understanding the Mechanism of the Reaction: They show how reactants turn into products, which reactants are utilised, as well as what conditions these will require.
c. Prediction of Results: They help chemists predict what results from a reaction based on the reactants involved and the conditions used.
d. Balancing Reactions: They balance chemical reactions to make sure the law of conservation of mass is followed.

iii. Symbols Used in Chemical Equations:
a. Reactants and Products: Formulas of substances involved in the reaction are written. Reactants are placed on the left, and products on the right side.
Example:

$\text{2H}_2 + \text{O}_2 \rightarrow \text{2H}_2\text{O}$

2H2​+O2​→2H2​O
b. Arrow (→): This sign marks the distinction between reactants and products. The direction of the reaction is shown.
    Single Arrow, →         The reaction proceeds in only one direction.
   Double Arrow, ⇌        The reaction is reversible: reactants and products interconvert throughout the course of the reaction.

c. State Symbols:
 (s) Solid

 (l) Liquid
 (g) Gas
 (aq): Aqueous solution (dissolved in water)
d. Coefficients: Numbers in front of chemical formulas to balance the equation. They represent the number of molecules or moles.
     Example: In

$\text{2H}_2 + \text{O}_2 \rightarrow \text{2H}_2\text{O}$

2H2​+O2​→2H2​O, the coefficient 2 in front of

$\text{H}_2$

H2​ and

$\text{H}_2\text{O}$

H2​O balances the equation.

e. Plus Sign (+): Is used between multiples of reactants or products.

f. Heat or Catalyst: Sometimes a symbol is used to indicate the requirement for either heat or catalyst.
Δ (triangle): This is a triangle and means heat is added to the reaction.
Pt or other symbols: This indicates that a catalyst was involved in the reaction.

2. Types of Reactions:

Chemical reactions can be classified into a variety of types based on the change of reactants to the products. Some of the common ones are:

i. Synthesis (Combination) Reaction:

Definition: A synthesis reaction is the combination of two or more simple substances to make a more complex product.
General Form: A + B → AB
Illustration: Water from hydrogen and oxygen

$$2H_2(g)+O_2(g)\to 2H_{2}O(l)$$

Characterizing Feature: One reactant results in two or more products.

ii. Decomposition Reaction:
Definition: A single compound breaks down into two or more simpler substances in a decomposition reaction.
General Form:

$$AB \rightarrow A + B$$

Characteristic: A single reactant breaks down to produce several products.

iii. Combustion Reaction:
Definition: Combustion reaction is the burning of a substance, typically a hydrocarbon, in which it reacts with oxygen to form carbon dioxide, water, and energy in the form of heat and light.
General Form:
Cx​Hy​+O2​→CO2​+H2​O

Example: The combustion of methane (natural gas):

$$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$$

Characteristic Feature: It is the process wherein an element displaces another element that is found in a chemical compound.

iv. Single Displacement (Replacement) Reaction:
A single displacement or replacement reaction involves the one element from a compound displacement another element.

General Form:  A+BC→AC+B

 Example: Zinc displaces hydrogen in the reaction with hydrochloric acid:       Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Key Feature: One element displaces another that is in a compound.

 v. Double Displacement (Metathesis) Reaction:
Definition: In a double displacement reaction, the ions or molecules of two compounds replace each other to form two new compounds.
General Form:         AB+CD→AD+CB

Example: When silver nitrate reacts with sodium chloride to form silver chloride and sodium nitrate:

$${\text{AgNO}}_3(aq)+\text{NaCl}(aq)\to \text{AgCl}(s)+{\text{NaNO}}_3(aq)$$

Key Feature: Two compounds trade parts to make new compounds, frequently resulting in a precipitate.

vi. Acid-Base (Neutralization) Reaction:
Definition: An acid-base reaction is a chemical reaction where an acid reacts with a base to give water and a salt.
General Form:

$$\text{Acid}+\text{Base}\to \text{Salt}+H_{2}O$$

Example: Neutralization of hydrochloric acid with sodium hydroxide:

$$HCl(aq)+NaOH(aq)\to NaCl(aq)+H_{2}O(l)$$

Characterizing Feature: An acid reacts with a base to form a neutral solution, usually water and salt.

vii. Redox (Oxidation-Reduction) Reaction:
Definition: Redox is the transfer of electrons from one substance to another. One substance loses its electrons, called oxidation, while another gains the electrons, known as reduction.
Example: Combustion reaction between hydrogen and oxygen to form water:
The reaction is

$$2H_2(g)+O_2(g)\to 2H_{2}O(l)$$

3. Balancing Chemical Equations:

Balancing chemical equations is the process of ensuring that the law of conservation of mass is met. This law stipulates that during a chemical reaction, matter cannot be created or destroyed. Instead, the total mass of the reactants should be equal to that of the products. While balancing a chemical equation, we want to make sure that the number of atoms for each element on both sides of the equation is the same.

i. The Law of Conservation of Mass:
a. Principle: The atoms of each element in the reactants must have the same quantity as the number of atoms of that element in the products.
b. Implication: There must be equality in the masses of the reactants and that of the products since atoms cannot be formed and destroyed in any chemical reaction.

a. How to Balance Chemical Equations:
1. Write the Unbalanced Equation: Write the unbalanced chemical equation. On the left side of the equation, put the reactants, and on the right side, the products.
Example: Hydrogen and oxygen react to produce water:

$$H_2 + O_2 \rightarrow H_2O$$

Identify all the species involved in the reaction on both sides of the equation.
In the experiment, we have hydrogen (H) and oxygen (O).

2. Count the Number of Atoms of Each Element: Count how many atoms of each element are on both sides of the equation.

• On the left:
  $H_2$

  H2​ has 2 hydrogens, and

  ${\mathrm{<span\; style="font-family:\; Georgia,\; \text{'}Times\; New\; Roman\text{'},\; \text{'}Bitstream\; Charter\text{'},\; Times,\; serif;\; font-size:\; 16px;">O2\; <span\; style="font-size:\; 16px;">has\; 2\; oxygens.</span></span>}}_{}$
• On the right:
  ${}_{}\mathrm{<span\; style="font-family:\; Georgia,\; \text{'}Times\; New\; Roman\text{'},\; \text{'}Bitstream\; Charter\text{'},\; Times,\; serif;\; font-size:\; 16px;">H2O\; <span\; style="font-size:\; 16px;">has\; 2\; hydrogens\; and\; 1\; oxygen.</span></span>}$

3. Balance One Element at a Time:
Balance elements that appear in only one reactant and one product.
Balance Hydrogen: There are 2 hydrogens on the left (in H2 ), and 2 hydrogens on the right (in H2O ), so hydrogen is already balanced.
Balance Oxygen: There are 2 oxygens on the left in ( O2 ) but only 1 oxygen on the right in ( H2O ). To balance oxygen, place a coefficient of 2 in front of ( H2O ) on the right:   H2​+O2​→2H2​O

 4. Check and Balance Hydrogen Again:
We have balanced the oxygens, but not the hydrogens. We now have on the right 2 times 2 = 4 hydrogens, but only 2 on the left
In order to balance hydrogen, write a coefficient of 2 in front of H_2 on the left:      2H2​+O2​→2H2​O

5. Check That All Elements Are Balanced:
Now check that all the elements are balanced:
Left: ( 2H2 ) gives 4 hydrogens and ( O2 ) gives 2 oxygens.
Right:( 2H2O ) gives 4 hydrogens and 2 oxygens.
             Everything’s balanced!

6. Write the Final Balanced Equation:
The final, balanced equation is:    2H2​+O2​→2H2​O

This equation now obeys the law of conservation of mass because there are the same number of each type of atom on both sides of the equation.

ii. Balancing Tips:

i. Balance Complex Molecules Last: Start with elements that appear in only one reactant and one product, and leave complex molecules (such as hydrocarbons) for last.
ii. Coefficients Must Be Whole Numbers: Coefficients of the equation should be whole numbers. If you get fractional coefficients, multiply the whole equation by a common factor to make the coefficients whole numbers.
iii. Check Your Work: Once balanced, always count the atoms again to be sure that both sides have an equal number of each type of atom.

4. Stoichiometry:

Stoichiometry is the branch of chemistry where it deals with calculating reactants and products during a chemical reaction. It bases on mole ratios found from a balanced chemical equation. Stoichiometric calculations aid in chemists being able to figure out how much of reactant produced during a chemical reaction.

i. Key Concepts in Stoichiometry:

a. Mole Ratio: The ratio of the amounts (in moles) of any two reactants in a chemical reaction is known as a mole ratio. It directly follows from the coefficients in the balanced chemical equation.

b. Moles: The amount of substance in terms of moles, which is a standard unit in chemistry. One mole of any substance contains

$6.022 \times 10^{23}$

 entities, known as Avogadro’s number.
c. Molar Mass: Molar mass is the mass (in grams) of one mole of a substance. For example, the molar mass of water is water is

$18.015 \, \text{g/mol}$

d. Limiting Reactant: The reactant used up first in a reaction; this determines the amount of product formed.

e. Excess Reactant: The reactant which is in excess at the end of a reaction after all the reagents have reacted.

5. Steps for Solving Stoichiometric Calculations:
Stoichiometry problems are typically solved by the following steps:
i. Write a Balanced Chemical Equation
Make sure that the chemical equation is balanced with proper mole ratios for each reactant and product.
ii. Convert Given Information into Moles
If the quantities are in grams or liters, convert them to moles using the molar mass for solids or liquids, or the molar volume for gases (22.4 L at STP).

iii. Relate Reactants and Products through Mole Ratios
Use the mole ratio of the balanced equation to change moles of one substance to moles of another.
iv. Convert Moles of Products or Reactants to Desired Units
Convert the number of moles of a product or reactant into grams, liters, or molecules, depending on what is required.

6. Symbolism for Physical Processes:

In chemistry, physical changes involve changes in the state of matter, without altering the chemical composition. These changes are represented using specific symbols:

i.States of Matter:

• Solid:
  $(s)$

  — e.g.,

  $\text{H}_2\text{O} (s)$

  (ice)

• Liquid:
  $(l)$

  — e.g.,

  $\text{H}_2\text{O} (l)$

  (water)

• Gas:
  $(g)$

  — e.g.,

  $\text{CO}_2 (g)$

  (carbon dioxide)

• Aqueous (dissolved in water):
  $(aq)$

  — e.g.,

  $\text{NaCl} (aq)$

  (salt in water)

ii. Phase Changes:

• Melting:
  $(s) \rightarrow (l)$

  — e.g.,

  $\text{H}_2\text{O} (s) \rightarrow \text{H}_2\text{O} (l)$
• Freezing:
  $(l) \rightarrow (s)$

  — e.g.,

  $\text{H}_2\text{O} (l) \rightarrow \text{H}_2\text{O} (s)$
• Vaporization:
  $(l) \rightarrow (g)$

  — e.g.,

  $\text{H}_2\text{O} (l) \rightarrow \text{H}_2\text{O} (g)$
• Condensation:
  $(g) \rightarrow (l)$

  — e.g.,

  $\text{H}_2\text{O} (g) \rightarrow \text{H}_2\text{O} (l)$
• Sublimation:
  $(s) \rightarrow (g)$

  — e.g.,

  $\text{CO}_2 (s) \rightarrow \text{CO}_2 (g)$
• Deposition:
  $(g) \rightarrow (s)$

  — e.g.,

  $\text{H}_2\text{O} (g) \rightarrow \text{H}_2\text{O} (s)$