27. Limiting Reactants & Balanced Equations

1. What is a Limiting Reactant?

  • Definition: A limiting reactant is the substance that is completely used up first in a chemical reaction. It "limits" the amount of product that can be formed.

  • Excess Reactant: This is the reactant that is left over after the reaction has stopped. There is more than enough of it to react with the limiting reactant.

  • Identifying the Stop Point: A reaction stops as soon as the limiting reactant is exhausted. For example, if you drop calcium carbonate into acid, the fizzing stops once all the solid has disappeared, even if there is still acid left.

2. Calculating Product Mass from a Limiting Reactant

To calculate the mass of a product produced, you must follow these steps using the limiting reactant:

Step 1: Write and Balance the Equation

  • Ensure the chemical equation is balanced to find the correct molar ratios between reactants and products.

Step 2: Find the Moles of the Limiting Reactant

  • Use the formula: Moles = Mass / Mr

  • Example: If you start with 115g of sodium (Ar = 23), you have 5 moles of sodium (115 / 23 = 5).

Step 3: Use Molar Ratios to Find Moles of Product

  • Look at the coefficients in the balanced equation to find the ratio.

  • Example: If the ratio of Sodium to Sodium Oxide (Na2O) is 4:2 (or 2:1), then 5 moles of sodium will produce 2.5 moles of sodium oxide.

Step 4: Convert Product Moles back to Mass

  • Use the rearranged formula: Mass = Moles x Mr

  • Example: To find the mass of 2.5 moles of Na2O (Mr = 62):

    • Mass = 2.5 x 62 = 155g.

3. Contextual Clues in Questions

  • "Reacted in Air": If a question says a metal is burned in air, you can assume oxygen is in excess (because there is plenty in the atmosphere) and the metal is the limiting reactant.

  • Direct Statements: Often, a question will explicitly state that one reactant is "in excess" to simplify the calculation.