Weeks 34-35 – Limiting Reactants Revisited

Last week, you were introduced to the concept of limiting reactants.  In a chemical reaction, the limiting reactant is the chemical that is completely used up in the process of creating products.  When the limiting reactant is gone, the reaction ends.  Let’s imagine the chemical reaction of sodium bicarbonate (baking soda) and acetic acid (vinegar):

NaHCO3(s) + C2H4O2(aq) → NaC2H3O2(aq) + H2O(l) + CO2(g)

Let’s first notice that this equation is already balanced.  It does not require any coefficients to have equal numbers of atoms of each element on both sides of the equation.  One mole of solid sodium bicarbonate NaHCO3(s) reacts with one mole of aqueous acetic acid C2H4O2(aq) to produce one mole of aqueous sodium acetate NaC2H3O2(aq), one mole of liquid water H2O(l), and one mole of carbon dioxide gas CO2(g).

To determine how much of each substance we need to actually conduct this experiment, let’s calculate the molar masses of sodium bicarbonate and acetic acid:

  • NaHCO3 = 22.99 + 1.008 + 12.01 + (16.00 x 3) = 84.01 g/mol
  • C2H4O2 = (12.01 x 2) + (1.008 x 4) + (16.00 x 2) = 60.06 g/mol

Therefore, if we wanted to react one mole of sodium bicarbonate with one mole of acetic acid, we would need to combine 84.01 g of sodium bicarbonate with 60.06 g of acetic acid.  Easy!  We also know that we will produce one mole of carbon dioxide gas in this reaction – there will be bubbles!  Thinking back to our work with gas laws in Unit 3, we know that one mole of any gas at standard temperature and pressure will occupy a volume of 22.4 L.  So there will be a lot of gas bubbles!  Here is the reaction:

What did you observe?  Hopefully you noticed the geyser of bubbles – that was hard to miss!  Did you also notice the white solid material at the bottom of the flask?  That shouldn’t be there if the reactants fully reacted as expected.  The science tells us that one mole of sodium bicarbonate will fully react with one mole acetic acid.  Why was there so much unreacted sodium bicarbonate remaining?  Why was acetic acid the limiting reactant in this chemical reaction?

To answer this question, we need to look closely at our reactant labels:

The baking soda is pure sodium bicarbonate.  The distilled white vinegar has some fine print that says “distilled with water to 5% acid strength” – in other words, the distilled white vinegar actually contains 5% acetic acid and 95% water!  In our balanced equation, we assumed we were using 100% acetic acid, not 5% acetic acid.  Because 5% is 1/20 of 100%, we need to combine 20 times more of the 5% acetic acid (60.06 g x 20 = 1201.2 g) with 84.01 g of sodium bicarbonate to fully react both chemicals.  Or, we could keep the amount of acetic acid the same (60.06 g) and use 20 times less of the sodium bicarbonate (84.01 g / 20 = 4.2 g).  Let’s do that instead, to keep the amounts of each reagent reasonable:

What did you observe?  Can you explain it?  As we wind down the school year, this is an opportunity for you to earn credit in the lab report section of your chemistry grade.  This is an optional assignment and will be worth 50 bonus points. If your grade is not where you want it, and your lab report scores have been less than amazing, this is your chance to make up a lot of ground.  Please take advantage of it.

Lab Report Requirements:

    • Purpose: What are we trying to accomplish with this lab?
    • Procedure: Write out the steps, in order and in detail, that were followed in this experiment.
    • Results: What exactly did you observe?  Be clear, be descriptive, and include images if possible.
    • Conclusion: Clearly explain the results.  Include the concepts of chemical reaction, reactants, products, molar mass, and limiting reactant.

Note: If you have access to baking soda and vinegar and a safe space to work, you are welcome to substitute your own experimental data for what you were provided above.

Need some help setting up your lab report?  The links below will give you either a highly structured template specific to this lab, or a more generic template applicable to any lab report.  If you decide to use either one, click the link and then select File > Make a Copy and get to it!

Finally, if you have read all the way down to this point, enjoy!

When finished, return to Weeks 34-35 – How Much Is Too Much? and continue working.

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