Lesson 7.3/4 - Limiting and Excess Reactants

Limiting & Excess Reactants

Introduction to Limiting Reactant

  • In real life, chemical reactions rarely have reactants in stoichiometric amounts.

  • Example: BBQ reaction: C<em>3H</em>8(g)+5O<em>2(g)→3CO</em>2(g)+4H2O(g)C<em>3H</em>8(g) + 5 O<em>2(g) \rightarrow 3 CO</em>2(g) + 4 H_2O(g)

  • Question: What happens when you run out of propane?

Limiting Reagent (LR)

  • The reactant in a chemical reaction that determines the amount of product formed.

  • This reactant is completely used up (consumed) in the chemical reaction.

Excess Reagent (ER)

  • The reactant present in more than the required amount for a complete reaction to occur.

  • This reactant remains after the reaction is over.

  • The excess reactant cannot be used in stoichiometry problems.

Examples of Limiting and Excess Reactants

  1. 2 car bodies, 7 tires

  2. No gasoline in your vehicle but oxygen is still present for combustion

  3. 4 hamburger patties, 2 hamburger buns

  4. 6 crackers, 28 chocolate squares, 2 marshmallows to make Perfect S’mores (2 Graham Crackers, 4 Chocolate Squares, 1 Marshmallow)

Answers to Examples

The limiting and excess reactants in the previous scenarios:

  1. Limiting: car bodies, Excess: tires

  2. Limiting: gasoline, Excess: oxygen

  3. Limiting: hamburger buns, Excess: hamburger patties

  4. Limiting: marshmallows, Excess: crackers and chocolate squares

Complete & Incomplete Combustion

  • Figure 1: When you light a Bunsen burner, you start with an orange flame then adjust the air intake to get a hot, blue flame. This demonstrates the effect of changing the reactant ratio on the reaction.

Identifying Limiting and Excess Reactants in a Chemical Reaction

  • Consider the reaction: 2H<em>2(g)+O</em>2(g)→2H2O(g)2 H<em>2(g) + O</em>2(g) \rightarrow 2 H_2O(g)

  • Figure 4: The synthesis of water from its elements. The limiting reagent, oxygen, is completely used up in the reaction. Some hydrogen, the excess reagent, remains after the reaction is complete.

Significance of Limiting Reactant

  • The reactant that runs out first (the limiting reactant) dictates how much product will be produced.

  • Example: C<em>3H</em>8(g)+5O<em>2(g)→3CO</em>2(g)+4H2O(g)C<em>3H</em>8(g) + 5 O<em>2(g) \rightarrow 3 CO</em>2(g) + 4 H_2O(g)

    • We need 1 mol of C<em>3H</em>8C<em>3H</em>8 for every 5 mol of O2O_2.

    • If we only had 4 mol of O<em>2O<em>2, then O</em>2isthelimitingreactant</p></li></ul></li></ul><h4id="69748be4−82d7−4d22−8ccc−4b2feade60fb"data−toc−id="69748be4−82d7−4d22−8ccc−4b2feade60fb"collapsed="false"seolevelmigrated="true">SampleProblem1</h4><ul><li><p>Apieceofmagnesiummetalreactswithhydrochloricacidandcompletelydisappears:<br>O</em>2 is the limiting reactant</p></li></ul></li></ul><h4 id="69748be4-82d7-4d22-8ccc-4b2feade60fb" data-toc-id="69748be4-82d7-4d22-8ccc-4b2feade60fb" collapsed="false" seolevelmigrated="true">Sample Problem 1</h4><ul><li><p>A piece of magnesium metal reacts with hydrochloric acid and completely disappears:<br>Mg(s) + 2 HCl(aq) \rightarrow H2(g) + MgCl2(aq)
      a) Identify the limiting reactant.
      b) Identify the excess reactant.
      c) What mass of hydrogen gas will be produced from the reaction of 25.0 g of magnesium? (2.08 g)

    7.4 - Calculations Involving Limiting Reagents

    To determine which reactant is the limiting reagent:

    1. Find the moles of each reactant

    2. Divide each by their coefficient from the balanced equation

    3. The lower of the two is the limiting reagent, the other is in excess

      • The excess reagent cannot be used in stoichiometry problems

    Sample Problem 1 - Problems Involving Amounts

    Determine the amount of titanium metal produced when 2.8 mol of titanium (IV) chloride reacts with 5.4 mol of magnesium.

    • Step 1 - Write a balanced equation listing given value(s) and required value(s)

    • Step 2 - To determine the limiting reagent, first use the amount of one reactant to find the stoichiometric amount of the other

    • Step 3 - Use the amount of limiting reagent to find the amount of required substance

    Sample Problem 2 & 3

    • Chloroform, CHCl_3,reactswithchlorinetoformcarbontetrachlorideandhydrogenchloride.Inanexperiment25gofchloroformand25gofchlorineweremixed.<br>a)Whichisthelimitingreactant?<br>b)Whatisthemaximumyieldofcarbontetrachloride?<br>c)Determinethemassofexcessremaining.</p></li></ul><h4id="138955b8−d8b0−41e9−9ded−10faa0e2ad9a"data−toc−id="138955b8−d8b0−41e9−9ded−10faa0e2ad9a"collapsed="false"seolevelmigrated="true">SampleProblem4</h4><ul><li><p>6.73gofmagnesiumreactswith8.15gofoxygeninacombustionreaction.<br>a)Determinethelimitingreagent.<br>b)Calculatethemaximummassoftheproductformed.<br>c)Calculatetheamountofexcessreagentremaining.</p></li></ul><h4id="a3dcc3f5−8c06−475a−abc3−24c5e1922c4d"data−toc−id="a3dcc3f5−8c06−475a−abc3−24c5e1922c4d"collapsed="false"seolevelmigrated="true">SampleProblem5−AmountsInvolvedintheSynthesisofWater</h4><ul><li><p>, reacts with chlorine to form carbon tetrachloride and hydrogen chloride. In an experiment 25 g of chloroform and 25 g of chlorine were mixed.<br>a) Which is the limiting reactant?<br>b) What is the maximum yield of carbon tetrachloride?<br>c) Determine the mass of excess remaining.</p></li></ul><h4 id="138955b8-d8b0-41e9-9ded-10faa0e2ad9a" data-toc-id="138955b8-d8b0-41e9-9ded-10faa0e2ad9a" collapsed="false" seolevelmigrated="true">Sample Problem 4</h4><ul><li><p>6.73 g of magnesium reacts with 8.15 g of oxygen in a combustion reaction.<br>a) Determine the limiting reagent.<br>b) Calculate the maximum mass of the product formed.<br>c) Calculate the amount of excess reagent remaining.</p></li></ul><h4 id="a3dcc3f5-8c06-475a-abc3-24c5e1922c4d" data-toc-id="a3dcc3f5-8c06-475a-abc3-24c5e1922c4d" collapsed="false" seolevelmigrated="true">Sample Problem 5 - Amounts Involved in the Synthesis of Water</h4><ul><li><p>2 H2(g) + O2(g) \rightarrow 2 H2O(g) with 8.15 g of oxygen in a combustion reaction.
      2Mg(s)+O2(g)→2MgO(s)2Mg(s) + O_2(g) \rightarrow 2MgO(s)


      a) Determine the limiting reagent.

      b) Calculate the maximum mass of the product formed.

      c) Calculate the amount of excess reagent remaining.