Week 33 – Limiting Reactants

You went through the Mole Tunnel and came out the other side. Along the way, you got hungry. You go to the refrigerator and find a 10-pack of hot dogs. You check the cupboard and find an unopened 12-pack of hot dog buns. A hot dog requires 1 hot dog and 1 bun. How many complete hot dogs can you make? Do you have exactly the right amount of hot dogs and buns to use everything up?  Will you be left with too many hot dogs or too many buns? Hmmm….10 hot dogs, 12 buns…looks like you’re going to have 2 leftover hot dog buns. Turns out, when it comes to hot dogs and hot dog buns, hot dogs are the limiting reactant.  You run out of hot dogs first. The number of hot dogs determines how many complete hot dogs you can make.

The same goes for chemical reactions. When it comes to reactions, if you combine reactants, you can either combine them in the exactly right amount to use both up, or you are left over with something. The chemical that runs out first is the limiting reactant.

Let’s return to the single replacement reaction we examined as we navigated the Mole Tunnel:

Cu(s) + 2AgNO3(aq) → Cu(NO3)2(s) + 2Ag(s)

Imagine you have a beaker containing 4 mol of AgNO3.  To the beaker you add 75 g of Cu.  Will the addition of this amount of Cu completely react with the AgNO3?  Will either reactant by left over?  Let’s find out.

  • Step 1: Calculate the number of moles of Cu are in 75 g.  The molar mass of Cu is 63.55 g/mol, so 75 g x (1 mol / 63.55 g) = 1.18 mol Cu
  • Step 2: Evaluate the chemical equation to determine the mole ratios.  How many moles of Cu does it take to fully react with AgNO3?  1 mol Cu : 2 mol AgNO3
  • Step 3: Compare the mole ratio with the amount of AgNO3?  We are told we have a beaker containing 4 mol of AgNO3.  According to the mole ratio, we need 2 mol Cu to fully react with 4 mol of AgNO3.
  • Step 4: Compare the mole ratio with the amount of Cu added.  You added 1.18 mol Cu to 4 mole of AgNO3.  You need 2 mol of Cu to fully react with the AgNO3, and the 1.18 mole we added is less than 2 mol.  Therefore, Cu is the limiting reactant in this experiment.

What does that mean?  All of the copper will fully react to form Cu(NO3)2.  How many moles of Cu(NO3)2?  You added 1.18 moles of Cu as the reactant, and the mole ratio of Cu:Cu(NO3)2 is 1:1, so you will produce 1.18 mol of Cu(NO3)2.

More examples:

Chemistry Challenge!  Try this one!

Your turn!  Complete the Limiting Reactant Google Form before returning to Week 33 – Stoichiometry to continue working.

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