(40) GCSE Chemistry Revision "Limiting reactant"

Limiting Reactants

Definition

• Limiting Reactant: The reactant that is completely used up in a chemical reaction, which limits the amount of product formed.

• Excess Reactant: The reactant that is not completely used up and remains after the reaction.

Understanding Chemical Equations

• Example Equation: ( \text{Mg} + \text{Cl}_2 \rightarrow \text{MgCl}_2 )

  • 1 mole of magnesium reacts with 1 mole of chlorine to produce 1 mole of magnesium chloride.

  • Reactants: Magnesium (Mg) and Chlorine (Cl₂)

  • Product: Magnesium Chloride (MgCl₂)

Practical Application in Reactions

• In the real world, reactants are seldom measured precisely.

• It’s common to use more of one reactant than the other.

Example Scenario:

• Using 0.5 moles of magnesium and 1 mole of chlorine:

  • Reactants Used: 0.5 moles of magnesium (limiting), 1 mole of chlorine (excess)

  • Product Formed: 0.5 moles of magnesium chloride

Predicting Outcomes of Reactions

Example Problem 1: Zinc and Iodine Reaction

• Equation: ( \text{Zn} + \text{I}_2 \rightarrow \text{ZnI}_2 )

• Given: 0.5 moles of zinc, 1 mole of iodine

• Limiting Reactant: Zinc (0.5 moles leads to 0.5 moles of zinc iodide)

• Calculate Mass:

  • Relative formula mass of ( \text{ZnI}_2 ) = 319

  • Mass = ( 0.5 \text{ moles} \times 319 = 159.5 \text{ grams} )

Example Problem 2: Hydrochloric Acid and Sodium Chloride Reaction

• Equation: ( \text{HCl} + \text{Na} \rightarrow \text{NaCl} )

• Given: 0.25 moles of hydrochloric acid

• Limiting Reactant: Hydrochloric acid (0.25 moles leads to 0.25 moles of sodium chloride)

• Calculate Mass:

  • Relative formula mass of NaCl = 58.5

  • Mass = ( 0.25 \text{ moles} \times 58.5 = 14.6 \text{ grams} ) (to one decimal place)

Example Problem 3: Copper Production from Copper Sulfate and Magnesium

• Equation: ( \text{CuSO}_4 + \text{Mg} \rightarrow \text{Cu} + \text{MgSO}_4 )

• Given: 0.5 moles of copper sulfate, 1 mole of magnesium

• Limiting Reactant: Copper sulfate (0.5 moles leads to 0.5 moles of copper)

• Calculate Mass:

  • Relative atomic mass of copper = 63.5

  • Mass = ( 0.5 \text{ moles} \times 63.5 = 31.75 \text{ grams} )

Summary

• By understanding limiting and excess reactants, one can predict the outcome and yield of a chemical reaction.

• Essential calculations involve converting moles to mass using the formula: ( \text{mass} = \text{number of moles} \times \text{relative formula mass} ) and recognizing the role of each reactant in the reaction.