Reactions of acids

Acids undergo several important reactions with metals and bases:

1. Reactions with reactive metals - Produce a salt and hydrogen gas.

Metal + acid ➔ salt + hydrogen

Example:

• Mg_(s) + 2HCl_(aq) ➔ MgCl_2(aq) + H_2(g)

1. Reactions with metal oxides and hydroxides - Produce a salt and water in a neutralisation reaction.

Metal oxide/hydroxide + acid ➔ salt + water

Examples:

• MgO_(s) + 2HCl_(aq) ➔ MgCl_2(aq) + H₂O_(l)
• Mg(OH)_2(s) + 2HCl_(aq) ➔ MgCl_2(aq) + 2H₂O_(l)

1. Reactions with metal carbonates and hydrogencarbonates - Produce a salt, water, and carbon dioxide gas in a neutralisation reaction.

Metal carbonate/hydrogencarbonate + acid ➔ salt + water + carbon dioxide

Examples:

• Na₂CO_3(aq) + 2HCl_(aq) ➔ 2NaCl_(aq) + CO_2(g) + H₂O_(l)
• NaHCO_3(aq) + HCl_(aq) ➔ NaCl_(aq) + CO_2(g) + H₂O_(l)

Writing ionic equations for acid reactions

Consider the reaction between magnesium and hydrochloric acid:

Mg_(s)_ + 2HCl_(aq)_ ➔ MgCl_2(aq)_ + H_2(g)_

The actual reacting species in strong acids such as HCl are hydrogen ions (H⁺). This can be shown using ionic equations.

To write the ionic equation for a reaction involving a strong acid, follow these steps:

1. Write the total ionic equation which includes all dissolved ions. For example:

Mg_(s) + 2H⁺_(aq) + 2Cl^-_(aq) ➔ Mg²⁺_(aq) + 2Cl^-_(aq) + H_2(g)

1. Identify and cancel out spectator ions (those not participating in the reaction) to give the net ionic equation. For example:

Mg_(s) + 2H⁺_(aq) ➔ Mg²⁺_(aq) + H_2(g)

For weak acids, the molecular form must be used in ionic equations as they don't fully dissociate. For example:

Mg_(s) + 2CH₃COOH_(aq) ➔ Mg²⁺_(aq) + 2CH₃COO^-_(aq) + H_2(g)

Balancing equations for acid-base reactions

Follow these steps to balance acid-base equations:

1. Balance all non-metals except H and O.
2. Balance all metals (go back to step 1 if other coefficients change).
3. Balance H (go back to step 1 if other coefficients change).
4. Check O atoms balance (go back to step 1 if not).

This method requires the fewest trials for most acid-base reactions.

Worked example 1 - Balancing an acid-base equation

H₃PO₄ + MgCO₃ ➔ Mg₃(PO₄)₂ + CO₂ + H₂O

Balance the equation shown above.

Step 1: Balance P and Mg:

2H₃PO₄ + 3MgCO₃ ➔ Mg₃(PO₄)₂ + 3CO₂ + H₂O

Step 2: Balance H:

2H₃PO₄ + 3MgCO₃ ➔ Mg₃(PO₄)₂ + 3CO₂ + 3H₂O

Step 3: Check O is balanced

The equation is balanced because there are 8 x O atoms on each side.

Worked example 2 - Writing full and ionic equations for an acid-base reaction

Write the full and ionic equations for the reaction of aqueous lithium hydroxide (LiOH) with sulfuric acid (H_2_SO4).

Step 1: Write the full unbalanced equation

LiOH_(aq)_ + H_2_SO_4(aq)_ ➔ Li_2_SO_4(aq)_ + H_2_O_(l)_

Step 2: Balance Li

2LiOH_(aq)_ + H_2_SO_4(aq)_ ➔ Li_2_SO_4(aq)_ + H_2_O_(l)_

Step 3: Balance H

2LiOH_(aq)_ + H_2_SO_4(aq)_ ➔ Li_2_SO_4(aq)_ + 2H_2_O_(l)_

Step 4: Check O is balanced

The equation is balanced because there are 4 x O atoms on each side.

Step 5: Write total ionic equation

2Li^+^_(aq)_ + 2OH^-^_(aq)_ + 2H^+^_(aq)_ + SO_4_^2-^_(aq)_ ➔ 2Li^+^_(aq)_ + SO_4_^2-^_(aq)_ + 2H_2_O_(l)_

Step 6: Cancel out spectator ions and simplify

2OH^-^_(aq)_ + 2H^+^_(aq)_ ➔ 2H_2_O_(l)_

OH^-^_(aq)_ + H^+^_(aq)_ ➔ H_2_O_(l)_

Identifying parent acids and bases

Salts are often produced by neutralisation. The parent acid and base can be determined from the salt's formula or name.

Method 1 - Using formula of salt:

1. Split the salt into its component ions. For example: Li₂SO₄ ➔ 2Li⁺ + SO₄^2-
2. Determine the parent acid and base by pairing OH^- with cations and H⁺ with anions based on their charges. For example:

Li⁺ + OH^- ➔ LiOH (parent base).

2H⁺ + SO₄^2- ➔ H₂SO₄ (parent acid).

Method 2 - Using name of salt:

• The cation name (e.g., "lithium") indicates the parent base (LiOH).
• The anion name (e.g., "sulfate") specifies the parent acid (H₂SO₄).

For ammonium salts, the parent base can be either NH₃ or NH₄OH.