Limiting Reactant Lab
Chemistry teacher Jamie Castle created this dataset and online version of the classic limiting reactants lab for high school chemistry. This is the first of many chemistry datasets yet to come in our free library. We hope it is useful to you or your chemistry colleagues!
Background
When we represent chemical reactions using balanced equations, the quantities of the reactants will always be fully consumed in order to produce the quantities of the products in the equation. When we run chemical reactions in the lab, however, it is much more typical that we use up one reactant entirely while an excess of any other reactants remains. In fact, scientists often create this type of scenario on purpose!
The limiting reactant of a reaction is the reactant that is used in full and thus limits the amount of product that can be produced. Once the limiting reactant is completely consumed, the reaction will cease to progress. An excess reactant is one that is not fully consumed over the course of a reaction. The theoretical yield of a reaction is the amount of product that is ideally produced when the limiting reactant runs out, though it’s not likely you’ll achieve this yield. Instead, your actual yield - the amount of product you actually produce in experimental conditions - will be lower than the theoretical yield. You can determine the percent yield of an experimental reaction by comparing the actual yield to the theoretical yield using the equation (AY/TY) x 100%.
In this activity, you’ll consider a hypothetical scenario in which aqueous acetic acid (CH3COOH) is combined with solid sodium bicarbonate (NaHCO3, aka baking soda) in a beaker to yield gaseous carbon dioxide, liquid water, and dissolved sodium acetate (NaCH3COO). One way to determine the amount of carbon dioxide gas produced in this reaction is by using water displacement, as shown below.
The Dataset
In your analysis, you’ll examine the collected data of six different groups (A-F) that ran six trials (1-6) each of the same reaction performed with a constant amount of sodium bicarbonate and a varying amount of acetic acid. When setting up your graph, think carefully about which of the variables is independent and which is dependent.
The Activity
1. Write a balanced equation for the reaction of acetic acid with sodium bicarbonate, including phase notations.
2. Open the dataset, click on the Table tab at the top of the screen and examine the data. Note that there are six groups of data, each with six trials. Determine the mass of sodium bicarbonate and the mass of acetic acid used in each of the six trials.
3. Now, click on the Graph tab at the top of the screen to switch to the graph view. Be sure that the Scatter/Box/Bar or Categorical Bubble icon is selected; this will ensure you make a scatter plot. Click the Show buttons beneath the variable names to show the independent variable on the X-axis and the dependent variable on the Y-axis of the graph. Be sure each variable is showing on the correct axis. If it’s not, you can correct that on the panel to the right side of your graph. Next, click on the Show button under the Group column and select the Z-axis (on the right-side panel). This will show each group’s data with its own color. Finally, check the Connect Dots box.
There are two independent variables in this dataset, but one of them will produce a much more informative graph than the other. Use the more informative graph to answer the next question below.
4. Use your knowledge of stoichiometry and the graph produced above (in #3) to determine the limiting and excess reactant in each of the six trials run by each group.
5. Calculate the theoretical yield for each of the six trials.
6. Return to the Table view. Add a seventh group of data to the dataset with the theoretical yield for each of the six trials.
To do this, click on the three-line “hamburger” menu on the upper left. Select “Resize table” to add 6 rows at the end of the dataset (creating rows 37-42) Fill in each of the cells for Lab Group with “theoretical”, fill in the cells for Trial from 1 to 6, and fill in the Mass of Acetic Acid and Mass of Sodium Bicarbonate following the same patterns as the rest of the dataset. For Volume of CO2, you will need to use the appropriate theoretical yield that you calculated above in question #5.
7. Examine the graph with the seventh group of (theoretical) data added. By visual approximation, which Lab Group appeared to have the highest percent yield across the six trials? Which Lab Groupappeared to have the lowest percent yield across the six trials?
8. Calculate the percent yield of trials 3-6 for one Lab Group of your choice.
9. What happens to the amount of carbon dioxide produced as you increase the amount of acetic acid used in the reaction? Explain this trend using the data in the graph and your knowledge of stoichiometry.
10. Propose two reasons why each group’s results weren’t the same for trials 3-6. (Hint: Think about the experimental procedure involved in running this experiment.)