6.6.2 Exploring Ionic Equations

A chemical equation in which substances dissolved in water are written as individual ions is called an ionic equation.

Example:

HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)

Write the substances that are soluble in water in their dissociated form:

H⁺_(aq) + Cl^-_(aq) + Na⁺_(aq) + OH^-_(aq) → Na⁺_(aq) + Cl^-_(aq) + H₂O_(l)

Remove common ions from both sides. These ions do not actually take part in the chemical reaction and are called spectator ions.

Write the net ionic equation:

H⁺_(aq) + OH^-_(aq) → H₂O_(l)

Simple explanation: In ionic equations, you show ions separately. Ions that appear on both sides unchanged (spectator ions) are cancelled out, leaving the net ionic equation.

Intermediate Level

Why Equations Must Be Balanced

The law of conservation of mass states that matter cannot be created or destroyed. In a chemical reaction, the number of atoms of each element must be the same on both sides.

Simple explanation: Atoms are not created or destroyed — they just rearrange. So both sides must have the same number of each type of atom.

Steps to Balance a Chemical Equation

• Write the unbalanced equation.
• Count atoms of each element on both sides.
• Balance one element at a time.
• Use coefficients (not subscripts).
• Start with the lowest coefficient.
• Check all atoms at the end.

Worked Example: CH₄ + O₂ → CO₂ + H₂O

Step 1: Unbalanced equation:

CH₄ + O₂ → CO₂ + H₂O

Step 2: Count atoms:

Step 3: Balance H — put 2 before H₂O:

CH₄ + O₂ → CO₂ + 2H₂O

Step 4: Balance O — put 2 before O₂:

CH₄ + 2O₂ → CO₂ + 2H₂O

Step 5: Check:

Balanced!

Ionic Equations

Molecular equation:

HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)

Ionic equation:

H⁺ + Cl^- + Na⁺ + OH^- → Na⁺ + Cl^- + H₂O

Net ionic equation:

H⁺_(aq) + OH^-_(aq) → H₂O_(l)

Spectator ions: Na⁺ and Cl⁻ — they appear on both sides unchanged and are cancelled out.

Simple explanation: Spectator ions "watch" the reaction but don't take part in it.

Important Points

• - Chemical equation: symbolic representation of a chemical reaction.
• - Reactants: substances that initiate the reaction (left side).
• - Products: substances formed during the reaction (right side).
• - Arrow (→): shows the direction of the chemical change.
• - State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous.
• - Balanced chemical equation: same number of atoms of each type on both sides.
• - Coefficients: numbers in front of formulas; used for balancing.
• - Never change subscripts when balancing.
• - Law of conservation of mass: matter cannot be created or destroyed.
• - Ionic equation: substances dissolved in water written as individual ions.
• - Spectator ions: ions that appear on both sides unchanged; cancelled in net ionic equation.
• - Example: CH₄ + 2O₂ → CO₂ + 2H₂O
• - Net ionic equation for HCl + NaOH: H⁺ + OH⁻ → H₂O

Remember

• - Chemical equation = symbolic representation of a reaction.
• - Reactants (left) → Products (right).
• - State symbols: s, l, g, aq.
• - Balance one element at a time.
• - Use coefficients, not subscripts.
• - Never change subscripts.
• - Balanced equation: same number of each atom on both sides.
• - Spectator ions: appear on both sides unchanged.
• - Net ionic equation: shows only the reacting ions.

Common Mistakes

• 1. Changing subscripts to balance. Subscripts define the substance — you must never change them. Use coefficients instead.
• 2. Forgetting state symbols. Always include (s), (l), (g), or (aq).
• 3. Balancing all elements at once. Balance one element at a time.
• 4. Forgetting to check at the end. Always recount atoms after balancing.
• 5. Confusing coefficients with subscripts. Coefficients are in front (can change); subscripts are inside formulas (cannot change).
• 6. Forgetting spectator ions in ionic equations. They are cancelled out to give the net ionic equation.

Quick Revision

• Chemical equation: symbolic representation of a chemical reaction.
• Reactants: substances that initiate the reaction (left side).
• Products: substances formed during the reaction (right side).
• Arrow: shows direction of chemical change.
• State symbols: (s), (l), (g), (aq).
• Balancing: use coefficients; balance one element at a time; never change subscripts.
• Example: CH₄ + 2O₂ → CO₂ + 2H₂O
• Ionic equation: substances dissolved in water written as individual ions.
• Spectator ions: ions that appear on both sides unchanged.
• Net ionic equation: H⁺ + OH⁻ → H₂O

60-Second Revision

• Chemical equation = symbolic representation.
• Reactants → Products.
• State symbols: s, l, g, aq.
• Balance one element at a time.
• Coefficients, not subscripts.
• Spectator ions appear on both sides unchanged.
• Net ionic: H⁺ + OH⁻ → H₂O.
• Warning: never change subscripts to balance an equation.