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What are the main sources of error that might cause a discrepancy between the predictions and experimental results in this lab, and how could you improve them?
Stoichiometry of Vinegar and Baking Soda
• In chemistry, reactions are frequently written as an equation, using chemical symbols. The reactants are on the left side of the equation, and the products are on the right.
Reactants → Products
• The law of Conservation of Mass tells us that matter is neither created nor destroyed in a chemical reaction. Because of this, a proper chemical equation must be balanced; the number of atoms of an element on one side of the equation (Reactants side) has to match the number of atoms of that element on the other side (Products side).
• Stoichiometry describes the quantitative relationship between reactants and/or products
in a chemical reaction. The purpose of this lab is to determine the correct chemical reaction for sodium hydrogen carbonate and acetic acid. In this lab, you will use the concept of “stoichiometry” for your experimental results to determine the exact ratios of the products of a chemical reaction between acetic acid and sodium bicarbonate, choosing between three possibilities. All parts of this experiment revolve around the reaction that occurs when aqueous acetic acid (vinegar) CH3COOH(aq), reacts with sodium hydrogen carbonate (also known as “sodium bicarbonate”), NaHCO3. You have probably seen this reaction in elementary school — add a few drops of red food coloring, and you have the classic “volcano reaction.” Or you can perform it easily in your kitchen by mixing vinegar (dilute acetic acid) and baking soda (sodium bicarbonate). The most noticeable sign of the reaction is vigorous bubbling, a result of very rapid carbon dioxide generation. Gaseous carbon dioxide is one of the products, as you can see with your own eyes. (You can prove the gas to be carbon dioxide by collecting it in a flask and inserting a burning match into the flask. The flame will be immediately extinguished.) Aside from carbon dioxide, what else is produced by the reaction? CH3COOH (aq) + NaHCO3(s) → CO2(g) + ??? A chemist approaching this problem would most likely form some hypotheses about the other products, and then design experiments to evaluate which hypothesis is best supported by experimental evidence. For this experiment, we will supply three possible reactions, shown below. Notice that they are all balanced.
A. CH3COOH (aq) + NaHCO3(s) → 2 CO2(g) + CH2O(aq) + Na+(aq) + 3H+(aq)
B. CH3COOH (aq) + NaHCO3(s) → CO2(g) + H2O(l) + NaCH3COO(aq)
C. CH3COOH (aq) + 2 NaHCO3(s) → CO2(g) + Na2CO3(aq) + H2O(l) + 2 CH2O(aq)
Your job is to determine which of these three possibilities is correct, using some simple laboratory measurements. One way to approach this problem is to focus on the chemical differences between the three Proposals:
• Reactions A and C produce formaldehyde (CH2O), but Reaction B does not. • The products of Reaction A are acidic; those of B and C are basic.
These things could be tested, but an even simpler method would be taking advantage of the different amounts of carbon dioxide produced, relative to sodium bicarbonate:
In Reaction A, 1 mole NaHCO3 produces 2 moles CO2
In Reaction B, 1 mole NaHCO3 produces 1 mole CO2
In Reaction C, 2 mole NaHCO3 produces 1 mole CO2
In this experiment, you will measure the ratio of moles NaHCO3 used to moles of CO2 produced. If it is approximately 1:2, you may conclude that Reaction A is correct; if the ratio is around 1:1, you can bet that Reaction B is correct, and if it’s about 2:1, you should choose Reaction C. Keep in mind that your results may not give you exact whole number mole-to-mole ratios because the precision of your equipment is limited, and you are doing this experiment for the first time. In fact, your results may be off by as much as 20% for this experiment, but you will still be able to choose between the three reactions (A, B, or C) with a fair amount of confidence if you work carefully and collect good data. How will you determine moles of NaHCO3 and CO2? The number of moles of sodium bicarbonate is easily calculated from its molar mass and the measured mass you scoop out of the container to use. (The other reactant, acetic acid will be used in excess, so its exact amount will have no relationship to the amount of carbon dioxide generated.) To determine the mass of CO2 in each reaction, please notice that, for each proposed reaction above, carbon dioxide is the only gas, and all other reactants and product are liquids, solids, or aqueous. As the reaction occurs, carbon dioxide will bubble out of the reaction solution and escape into the laboratory. This means the mass of your reaction mixture after the reaction will be smaller due to the loss of carbon dioxide, and a simple subtraction tells you how much carbon dioxide was produced: mass of CO2 = mass of rxn mixture before − (mass of rxn mixture after)
One small complication is that some of the CO2 produced will remain dissolved in the reaction mixture because carbon dioxide is somewhat soluble in water. This means that the mass you calculate by subtraction in the above equation is somewhat too low—i.e., you have not accounted for the carbon dioxide that goes into the water. You will account for this with a correction factor in your calculations. (This is listed on the Data Report.)
• React a known amount of sodium hydrogen carbonate (baking soda, NaHCO3) with excess vinegar.
• Use the loss in mass to determine the amount of carbon dioxide gas released. • Determine the correct chemical reaction for sodium hydrogen carbonate and acetic acid
Included in the lab kit • Weigh boats • Beakers • Balances • Baking soda (NaHCO3) • Vinegar (CH3COOH)
Safety goggles must be worn during this investigation.
Read all instructions for this laboratory activity before beginning. Follow the instructions closely and observe established laboratory safety practices, including use of appropriate personal protective equipment described in the Safety and Procedure sections.
Acetic acid is corrosive. In the event of contact with skin or eyes, the affected area should be immediately rinsed with water for 15 minutes.
Do not eat, drink, or chew gum while performing this activity. Wash your hands with soap and water before and after performing the activity. Clean up the work area and all glassware with soap and water after completing the investigation.
1. On a balance, tare a weigh boat. Add about 2 g of sodium bicarbonate (NaHCO3) and record the exact mass in your note book.
2. Pour 50 mL of acetic acid into the beaker and stack the weigh boat (with the sodium bicarbonate still in it.)
3. Carefully measure the mass of the whole stack together: beaker with acetic acid, plus
weigh boat and sodium bicarbonate on top. Record the mass in your notebook.
4. Without spilling any of the sodium bicarbonate or reaction mixture and initiate the reaction by slowly transferring all the sodium bicarbonate into the acetic acid. (Note that the solution may overflow if you dump the sodium bicarbonate too quickly.) To cut
down on fizzing, which may eject solution from the bottle, put the weigh boat back on top of the beaker and swirl the reaction mixture gently.
5. After a few minutes, when the reaction is complete and no more bubbling is observed,
weigh and record the total mass of the beaker, reaction mixture, and empty weigh boat.
6. Pour the reaction mixture down the drain. Rinse the glassware thoroughly with water (no soap required) and dry completely. Never put a wet beaker onto a balance!
7. Repeat for two more trials.
Determine the correct chemical reaction for sodium hydrogen carbonate and acetic acid
* Calculate by adding 0.0040 moles to the “Moles of CO2 gas released”. This correction accounts for the amount of CO2 that dissolves in 50 mL of aqueous solution at room temperature.
1. Show the calculation for the number of moles of CO2 for Trial #1.
2. Show the calculation for the number of moles of NaHCO3 for Trial #1.
3. Show the calculations for other steps and repeat the calculations for trials #2 and #3.
Trial 1 Trial 2 Trial 3
Mass of NaHCO3 (g)
Mass of beaker + acetic acid + NaHCO3 + weigh boat before rxn (g)
Mass of beaker + rxn mixture + weigh boat after rxn (g)
Mass of CO2 gas released (g)
Moles of CO2 gas released (g)
Moles of CO2, corrected for amount dissolved*
Moles of NaHCO3 used
Present, discuss and explain your results. Be sure to compare the calculated number of moles of CO2 and NaHCO3 in the table above. Which of the three possible reactions, A, B or C, is best supported by these results? Include the balanced chemical equations. (Note that results may not be exactly whole numbers.)
POST LAB QUESTIONS
1. “Matter cannot be neither created nor destroyed during a reaction”. Have you used this concept in this lab? (Yes/ No). Explain your answer.
2. What are the main sources of error that might cause a discrepancy between the
predictions and experimental results in this lab, and how could you improve them?
3. How many grams of NaOH are needed to completely react with 50.0 g of H2SO4 to form Na2SO4 and H2O?
Follow the below steps. I. Write down the balanced equation for the reaction. II. Calculate the number of moles of H2SO4. III. Calculate the moles of NaOH required. IV. Calculate the mass of NaOH required.