Concept of Chemical Equation

A chemical equation is a concise way to describe a chemical reaction. It shows the conversion of reactants (starting substances) into products (new substances).

Key terms

1. Reactants: The substances you start with, written on the left-hand side of the equation.
2. Products: The substances formed in the reaction, written on the right-hand side of the equation.
3. Arrow (→): Separates the reactants from the products and indicates the direction of the reaction.
4. State Symbols: Indicate the physical state of each substance in the equation:
   • (s): Solid
   • (l): Liquid
   • (g): Gas
   • (aq): Aqueous (dissolved in water)

Chemical equations must

1. Tell the truth or represent the facts
2. Show symbols of elements and formula of products and reactants
3. Be balanced
4. Bear state symbols (s), (l), (g) and (aq) for substances dissolved in water (aqueous – water).

Balancing chemical equations

Chemical equations must follow the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction. Therefore, the number of atoms of each element on the reactants' side must equal the number on the products' side.

Steps for balancing a chemical equation

1. Write the Unbalanced Equation: Write the chemical formulas of reactants and products.
2. List the Atoms of Each Element: Count the number of atoms for each element on both sides of the equation.
3. Balance One Element at a Time: Start with the element that appears in only one reactant and one product. Use coefficients (numbers in front of formulas) to balance the atoms.
4. Balance Oxygen and Hydrogen Last: If oxygen or hydrogen appear in multiple compounds, balance them after other elements.
5. Check Your Work: Ensure all elements are balanced and that coefficients are in the simplest ratio.

Example 1: Reaction of hydrogen and oxygen to form water

Step 1: Write the Unbalanced Equation

$H_2 + O_2 \rightarrow H_2O$

Step 2: Count Atoms

• Left side: H=2, O=2
• Right side: H=2, O=1

Step 3: Balance Oxygen

Place a 2 in front of $H_2O$

$H_2 + O_2 \rightarrow 2H_2O$

Now, oxygen is balanced: O=2

Step 4: Balance Hydrogen

Place a 2 in front of $H_2$

$2H_2 + O_2 \rightarrow 2H_2O$

Now, hydrogen is balanced: H=4

Final Balanced Equation:

$2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$

Example 2: Combustion of methane

Unbalanced Equation:

$CH_4 + O_2 \rightarrow CO_2 + H_2O$

Step-by-Step Balancing:

1. Balance carbon: C=1 on both sides. $CH_4 + O_2 \rightarrow CO_2 + H_2O$
2. Balance hydrogen: Place a 2 in front of $H_2O$ $CH_4 + O_2 \rightarrow CO_2 + 2H_2O$
3. Balance oxygen: O=4 on the right, so place a 2 in front of $O_2$ $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

Final Balanced Equation:

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

Reversible and irreversible reactions

1. Irreversible Reactions: Proceed in one direction only, from reactants to products. Symbol: → Example: Combustion of magnesium: $2Mg(s) + O_2(g) \rightarrow 2MgO(s)$
2. Reversible Reactions: Can proceed in both directions. Products can revert to reactants. Symbol: ⇌ Example: Formation of ammonia: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$