Double Replacement - Chemistry

1. Fundamental Concepts

Definition: A reaction where the positive ions (cations) and negative ions (anions) of two different compounds switch places.

General Form: $$AB + CD \rightarrow AD + CB$$

Prerequisite: Reactants are usually aqueous ionic compounds (dissolved in water).

2. Key Concepts

Driving Forces: For a double replacement reaction to occur visibly, one of the following must happen:

1. Precipitate : An insoluble solid is formed (use Solubility Rules).

2. Gas : A gas is produced (e.g., $CO_2$, $H_2S$).

3. Water : A molecular compound (usually water in neutralization) is formed.

Neutralization: A specific type of double replacement where an Acid reacts with a Base to produce a Salt and Water ($H^+$ and $OH^-$ combine).

3. Examples

Easy
Reaction: Silver Nitrate + Sodium Chloride
Equation: $$AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)$$
Note: $AgCl$ is a white precipitate (insoluble).

Medium
Reaction: Hydrochloric Acid + Potassium Hydroxide
Equation: $$HCl(aq) + KOH(aq) \rightarrow KCl(aq) + H_2O(l)$$
Note: $H^+$ pairs with $OH^-$ to form water.

Hard
Reaction: Calcium Carbonate + Acetic Acid (Vinegar)
Equation: $$CaCO_3(s) + 2HC_2H_3O_2(aq) \rightarrow Ca(C_2H_3O_2)_2(aq) + H_2O(l) + CO_2(g)$$
Note: This involves complex ions ($C_2H_3O_2^-$) and gas evolution ($CO_2$).

4. Problem-Solving Techniques

Criss-Cross Charges: When writing products, always balance the charges of the new cation-anion pairs (e.g., $Al^{3+} + SO_4^{2-} \rightarrow Al_2(SO_4)_3$).

Check Solubility Rules: Use the solubility chart to determine if a product is solid ($s$) or aqueous ($aq$).

Balance the Equation: Ensure the number of atoms is equal on both sides (do this after writing the correct chemical formulas for products).

Net Ionic Equation: Cancel out "Spectator Ions" (ions that appear unchanged on both sides) to write the simplified reaction.