Single Replacement - Chemistry

1. Fundamental Concepts

Definition: A reaction where an element reacts with a compound, displacing another element from it to form a new compound and a new element.

General Equation: $$A + BC \rightarrow AC + B$$

Types:

Metal Replacement: A metal replaces another metal (e.g., $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$).

Hydrogen Replacement: An active metal replaces hydrogen in an acid (e.g., $Mg + 2HCl \rightarrow MgCl_2 + H_2$).

Halogen Replacement: A halogen (Group 17) replaces another halogen (e.g., $Cl_2 + 2NaBr \rightarrow 2NaCl + Br_2$).

Activity Series:

The primary tool used to predict if a reaction will occur.

Rule: A more reactive element (higher on the list) can displace a less reactive element (lower on the list).

Hydrogen Exception: Metals before Hydrogen (H) can react with acids to produce $H_2$ gas.

No Reaction (NR):

If the single element is less reactive than the element in the compound, no reaction takes place.

Example: $Cu + NaCl \rightarrow NR$ (Copper is less reactive than Sodium).

Charge Conservation:

When forming the new compound, the ionic charges must be balanced correctly using the criss-cross method.

Easy: Basic Prediction & Balancing

Question: Predict the products and balance the equation: $Mg + AgNO_3 \rightarrow$?

Answer:

1. Mg is more reactive than Ag, so a reaction occurs.

2. Products are $Mg(NO_3)_2$ and $Ag$.

3. Balanced Equation: $Mg + 2AgNO_3 \rightarrow Mg(NO_3)_2 + 2Ag$

Medium: Application of Activity Series

Question: Determine if a reaction occurs. If yes, write the balanced chemical equation: $Al + Fe_2O_3 \rightarrow$?

Answer:

1. Check Series: Al is before Fe, so the reaction occurs.

2. Products are $Al_2O_3$ and $Fe$.

3. Balanced Equation: $2Al + Fe_2O_3 \rightarrow Al_2O_3 + 2Fe$ (Known as the Thermite Reaction).

Hard: Complex Ions & Halogens

Question: Write the balanced chemical equation for Chlorine gas reacting with a solution of Potassium Iodide.

Answer:

1. Identify: This is a non-metal (halogen) replacement.

2. Check Series: Cl is above I, so the reaction occurs.

3. Swap: Cl replaces I.

4. Balanced Equation: $Cl_2(g) + 2KI(aq) \rightarrow 2KCl(aq) + I_2(aq)$

Check the Series First: Always consult the Activity Series chart. If the single element is not more reactive, immediately write "NR".

Determine Charges: Identify the ionic charge of the element (e.g., Group 1 = +1, Group 2 = +2, Al = +3, Halogens = -1).

Criss-Cross Rule: Apply the criss-cross method to write the correct chemical formula for the new compound.

Balance Last: Ensure the product formulas are correct before attempting to balance the overall equation.