6.4 Limited Reactant

Concept of Limiting Reactant:

• Limiting Reactant: In a chemical reaction with multiple reactants, the limiting reactant is the one that gets completely consumed during the reaction. Once it is used up, the reaction stops, and other reactants may remain in excess.

• The limiting reactant determines:

• How much of the products will be formed.

• How much residue of other reactants will be left.

Example 1:

• Problem: 4 oxygen molecules are mixed with 4 atoms of magnesium metal. Which is the limiting reactant?

• Reaction: 2Mg + O2 → 2MgO

• Solution: The equation tells us that 1 O2 molecule reacts with 2 Mg atoms. Therefore:

• 2 O2 molecules are needed to react with the 4 Mg atoms.

• The given 4 Mg atoms will consume 2 O2 molecules.

• This leaves 2 O2 molecules unused.

• Conclusion: Magnesium (Mg) is the limiting reactant because it is completely consumed in the reaction.

Example 2:

• Problem: 30 O2 molecules are mixed with 70 Mg atoms. Which is the limiting reactant?

• Reaction: 2Mg + O2 → 2MgO

• Solution:

• 1 O2 molecule reacts with 2 Mg atoms.

• To react with 70 Mg atoms, 35 O2 molecules are needed (70 ÷ 2 = 35).

• But only 30 O2 molecules are available.

• Conclusion: Oxygen (O2) is the limiting reactant because there is insufficient oxygen to react with all the magnesium atoms.

Example 3:

• Problem: 75 grams of chlorine gas (Cl2) are mixed with 5 grams of hydrogen gas (H2). Which is the limiting reactant?

• Reaction: H2 + Cl2 → 2HCl

• Solution:

• 2 grams of hydrogen (H2) need 71 grams of chlorine (Cl2) to react.

• 5 grams of hydrogen will require (71/2) * 5 = 177.5 grams of chlorine.

• Only 75 grams of chlorine are available.

• Conclusion: Chlorine (Cl2) is the limiting reactant because it is insufficient to react with all the hydrogen gas.

Do It Yourself:

• Task: Find out how much hydrogen gas (H2) is left over in the reaction between 75 grams of chlorine and 5 grams of hydrogen.