Strategy for Balancing Chemical Reactions

Balancing chemical equations involves ensuring that the number of atoms of each element is the same on both sides of the equation. Here's a step-by-step procedure using the example:

Lab Lore

Antoine Lavoisier revolutionized chemistry by identifying oxygen and disproving phlogiston theory, establishing the law of conservation of mass. Condemned during the French Revolution, he was guillotined in 1794. Legend holds that he attempted one last experiment on the scaffold, trying to blink after decapitation to test how long consciousness persisted.

\[ \text{NH}_3 + \text{O}_2 \rightarrow \text{N}_2 + \text{H}_2\text{O} \]

Step 1: List the number of atoms of each element on both sides.

Left Side (Reactants):

  • Nitrogen (N): 1 (from NH₃)
  • Hydrogen (H): 3 (from NH₃)
  • Oxygen (O): 2 (from O₂)

Right Side (Products):

  • Nitrogen (N): 2 (from N₂)
  • Hydrogen (H): 2 (from H₂O)
  • Oxygen (O): 1 (from H₂O)

Step 2: Identify the elements that are unbalanced.

  • Nitrogen: 1 (left) vs. 2 (right)
  • Hydrogen: 3 (left) vs. 2 (right)
  • Oxygen: 2 (left) vs. 1 (right)

Step 3: Balance the elements one at a time.

a) Balance Nitrogen (N):

There are 2 nitrogen atoms on the right (N₂), so we need 2 nitrogen atoms on the left. Adjust NH₃ to 2NH₃:

\[ 2\text{NH}_3 + \text{O}_2 \rightarrow \text{N}_2 + \text{H}_2\text{O} \]

Now:

  • Nitrogen: 2 (left) vs. 2 (right) → Balanced.

b) Balance Hydrogen (H):

There are 6 hydrogen atoms on the left (2NH₃ → 2 × 3 = 6). Adjust H₂O to 3H₂O to balance hydrogen:

\[ 2\text{NH}_3 + \text{O}_2 \rightarrow \text{N}_2 + 3\text{H}_2\text{O} \]

Now:

  • Hydrogen: 6 (left) vs. 6 (right) → Balanced.

c) Balance Oxygen (O):

There are 2 oxygen atoms on the left (O₂). There are 3 oxygen atoms on the right (3H₂O → 3 × 1 = 3). Adjust O₂ to \( \frac{3}{2} \)O₂ to balance oxygen:

\[ 2\text{NH}_3 + \frac{3}{2}\text{O}_2 \rightarrow \text{N}_2 + 3\text{H}_2\text{O} \]

Now:

  • Oxygen: 3 (left) vs. 3 (right) → Balanced.

Step 4: Eliminate fractions by multiplying the entire equation by 2:

\[ 4\text{NH}_3 + 3\text{O}_2 \rightarrow 2\text{N}_2 + 6\text{H}_2\text{O} \]

Final Balanced Equation:

\[ 4\text{NH}_3 + 3\text{O}_2 \rightarrow 2\text{N}_2 + 6\text{H}_2\text{O} \]

Verification:

Left Side:

  • Nitrogen: 4 (from 4NH₃)
  • Hydrogen: 12 (from 4NH₃)
  • Oxygen: 6 (from 3O₂)

Right Side:

  • Nitrogen: 4 (from 2N₂)
  • Hydrogen: 12 (from 6H₂O)
  • Oxygen: 6 (from 6H₂O)

The equation is now balanced!