2.2 c: Limiting Reactant

Limiting Reactant
2.2 c: explore the concept of the mole and relate this to Avogadro’s constant to describe, calculate and manipulate masses, chemical amounts and numbers of particles in: (ACSCH007, ACSCH039)

– moles of elements and compounds 𝑛 = 𝑚 /𝑀𝑀 (n = chemical amount in moles, m = mass in grams, MM = molar mass in gmol-1 ) Stoichiometry

– percentage composition calculations and empirical formulae

– limiting reagent reactions

​What we will cover

• definition

  • limiting reactant

  • theoretical yield

  • actual yield ​

  • percent yield

• calculations

• example questions

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​An analogy about limiting reactants

To make 5 pancakes you need:

• 1 cup of flour

• 2 eggs

• 1/2 tsp of baking powder ​

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Say you have 3 cups of flour, 10 eggs, and 4 tsp of flour. How many pancakes can you make? ​

Only 15 pancakes.

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The 3 cups of flour = the limiting reactant

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And 15 pancakes is the theoretical yield ​

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Imagine we then accidentally burn 3 of the pancakes, we would only have 12. This is the percentage yield ​

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So our percentage yield = (12/15)​ x 100 = 80%

​Example: LR from initial mole values

Ti + 2Cl₂ --> TiCl₄

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Given 1.8 mol of titanium and 3.2 mol of chlorine​, what is the limiting reactant of the reaction, and the theoretical yield of TiCl₄ in moles?

​Cl₂ must be the limiting reactant, because it produces the least amount of product.

1.6 moles is the theoretical yield. ​

Open Ended

​Consider the reaction:

2 Al + 3 Cl₂ --> 2 AlCl₃ ​

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If you begin with 0.552 mol of aluminium, and 0.887 mol of chlorine, what is the limiting reactant and theoretical yield of AlCl₃ in moles?

​Solution

​Example: LR from initial masses

​Consider the reaction and reactant masses given below

Open Ended

Ammonia is synthesised by the reaction:

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2 NO + 5 H₂ --> 2 NH₃ + 2H₂O

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What maximum amount of ammonia in grams can be synthesised from 45.8g of NO and 12.4g of H₂? ​

​Solution:

Open Ended

Consider the reaction:

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​Cu₂O + C --> 2 Cu + CO

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When 11.5g of C are reacted with 114.5g of Cu₂O​, 87.4g of Cu are obtained. Find the limiting reactant, the theoretical yield, and the percentage yield.

​Solution

​What we will cover next

Concentration of solutions and molarity ​