9: LAB 9 - DECOMPOSITION OF BAKING SODA
- Page ID
- 506197
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The purpose of this experiment is to:
- Demonstrate decomposition of a compound.
- To experimentally determine the stoichiometry of the thermal decomposition of baking soda, \(\ce{(NaHCO3)}\).
- Estimate the percentage yield of sodium carbonate from a decomposition reaction.
INTRODUCTION
Stoichiometry is based on the law of conservation of mass, which states that the total mass of a reactant is equal to the total mass of products, where relationships among the quantities of reactants and products typically form a mole ratio, stoichiometric coefficients. Stoichiometric coefficients are also helpful in determining the mole ratio between the reactants and products. The mole ratio is important because it enables chemists to calculate the number of moles of product formed from a given number of reactants, or the number of moles required to produce a certain product.
When two reactants are mixed, one may be used up completely, while the other may remain in excess. The amount of product formed will be limited by the reactant that is used up first. A balanced chemical equation helps students determine the amount of product formed (actual yield), which reactant runs out first (limiting reagent), and which remains (excess reagent).
The thermal decomposition (decomposition by heating) of sodium bicarbonate \(\ce{(NaHCO3)}\) is a common chemical reaction. Sodium bicarbonate, more commonly known as baking soda, is widely listed as an ingredient in baked goods recipes. As the food item is being cooked or baked, the baking soda undergoes decomposition, releasing gas and causing the food item to “rise” and have a “light” texture.
In this experiment, we will use stoichiometry to determine the percent yield of sodium carbonate (Na2CO3) formed. When heated, baking soda, \(\ce{NaHCO3}\) (sodium bicarbonate or sodium hydrogen carbonate) decomposes, yielding Sodium carbonate \(\ce{(Na2CO3)}\), water (H2O), and carbon dioxide (CO2).
2 \(\ce{NaHCO3}\) (s) 
Na2CO3 (s) + H2O (g) + CO2 (g)
Decomposition by heating is called thermal decomposition. During the thermal decomposition of baking soda, \(\ce{NaHCO3}\), the products water (H2O) and Carbon dioxide (CO2) are released as gases, and the sodium carbonate is left as a solid. The amount (mass) of \(\ce{(Na2CO3)}\) obtained is the actual yield.
The theoretical yield, the maximum amount of \(\ce{(Na2CO3)}\) that could be produced under the given conditions, is calculated using the mass of baking soda used and mole ratios from the balanced chemical equation, as shown in Example 1.
Percent Yield of Sodium Carbonate, \(\ce{(Na2CO3)}\), obtained by decomposition of Baking Soda, \(\ce{NaHCO3}\)
Example 1:
A student heated 3.255 g of baking soda (Sodium bicarbonate),\(\ce{NaHCO3}\), in a lab to produce 1.875 g of sodium carbonate, Na2CO3. Calculate the percent yield.
Solution:
Percent Yield = 
x 100
To find the percent yield, we need to calculate the theoretical yield, as the actual yield is given as the mass of product obtained after the chemical reaction.
The theoretical yield in this case is the amount of Na2CO3 produced from 3.255 g of baking soda, using the molar masses of baking soda and sodium carbonate and the coefficients of the balanced chemical equation as a mole ratio.
2 \(\ce{NaHCO3}\) (s) Na2CO3 (s) + H2O (g) + CO2 (g)
g NaHCO3 
mol NaHCO3
mol Na2CO3 g Na2CO3
The balanced chemical equation indicates that two moles of \(\ce{NaHCO3}\) yield one mole of Na2CO3. Using the molar masses of \(\ce{NaHCO3}\) and Na2CO3, we can calculate the theoretical yield.
Theoretical yield = 3.255 g \(\ce{NaHCO3}\) x 
x 
x 
= 2.053 g
Percent Yield = x 100 = 
x 100 = 91.330 %
EQUIPMENT* AND CHEMICALS NEEDED
Safety goggles
Milligram Balance
Spatula
Bunsen burner and tubing
Ring stand and clamp
Large test tube
Test tube holder
Baking soda (Sodium bicarbonate, sodium hydrogen carbonate, \(\ce{NaHCO3}\) )
* Images of equipment needed in this lab are in the appendix (the equipment may differ slightly or be subject to changes; follow your instructors’ directions).
SAFETY PRECAUTIONS
1) Wear chemical splash goggles throughout the experiment.
2) Gloves are provided if you wish to wear them.
3) Please review Material Safety Data Sheets for additional questions.
4) Exercise caution when using the Bunsen burner and when handling heated objects. Use extreme caution while using a Bunsen burner and handling a hot ring stand and test tube.
5) Do not touch the crucible or any metal that may remain hot. Use heat-resistant gloves if necessary. Discard the waste in the proper waste containers.
6) Be sure all glassware is clean and all equipment is returned to its proper place.
7) Clean the lab benches and check the laminated sheets to ensure all equipment is in your lab drawer before leaving the lab.
8) Wash your hands once you leave the lab.
EXPERIMENTAL PROCEDURE
Percent Yield of Sodium Carbonate, Na2CO3, obtained by decomposition of Baking Soda, \(\ce{NaHCO3}\)
1) Put on safety goggles.
2) Set up the Bunsen burner and ring stand as shown in Figure 1.
Figure 1. Set up for thermal decomposition of sodium bicarbonate. A test tube with sodium bicarbonate clamped to a ring stand is heated using a Bunsen burner.
3) Weigh a large, clean, empty, dry test tube and record its exact mass.
4) Add 1.5-2.0 grams of baking soda into the test tube using a spatula.
5) Weigh the test tube with the baking soda and record the combined mass displayed on the milligram balance.
6) Attach the test tube with baking soda to the clamp on the ring stand (figure 1) and gently heat the test tube mixture with the Bunsen burner flame.
7) You will notice water droplets collecting at the end of the test tube. Heat until no further change is observed, indicating the completion of thermal decomposition.
8) Make sure all the moisture is gone before you stop heating. If there is any moisture in the test tube or sample, carefully use the flame to dry it. Any moisture will result in increased mass and lead to incorrect calculations.
9) Allow the test tube and its contents to cool to room temperature for 10 -15 minutes.
10) Weigh the test tube with the decomposed baking soda, which is the test tube now containing the sodium carbonate, Na2CO3, residue, and record the mass.
11) Gently heat this test tube for 10 minutes, cool it to room temperature, and record its mass as the second heating. The masses of sodium carbonate and Na2CO3 residue along with the test tube should be almost constant (within 0.05g). If not, a third heating will be needed. Follow the above steps for a third heating.
12) Calculate the theoretical and percent yields (refer to example 1) for both trials and calculate the average percent yield in the data table.
13) Discard the solid as indicated by your instructor.
PRE-LAB QUESTIONS
Name: ____________________________________
- Define the following:
a) Actual yield:
b) Theoretical yield:
c) Percent yield:
- What happens if moisture or water remains in the test tube?
- 2.5g of sodium bicarbonate is equivalent to how many moles of sodium bicarbonate? (Show your work)
- What theoretical yield and percent yield would be if 2.525 g of sodium bicarbonate is decomposed to 1.487 g sodium carbonate, Na2CO3? (Show your work*)
DATA AND OBSERVATIONS
Name: _________________________Lab Partner(s): ______________________________
Percent Yield of Sodium Carbonate, Na2CO3, obtained by decomposition of Baking Soda, \(\ce{NaHCO3}\)
1. Mass of empty test tube [m1] =
2. Mass of test tube + baking soda (before heating) [m2] =
3. Mass of baking Soda, \(\ce{NaHCO3}\) [m3 m2-m1]* =
4. Mass of test tube and sodium carbonate, Na2CO3 (After heating first heating) [m4] =
5. Mass of test tube and sodium carbonate, Na2CO3 (After heating second heating) [m5] =
Mass of test tube and sodium carbonate, Na2CO3 (After heating third heating if needed) [m6] =
6. Mass of sodium carbonate, Na2CO3 [m = m5 – m1] or [m = m6 – m1]* =
7. Theoretical yield of Na2CO3 (Refer to Example 1), show your work*.
8. Percent yield of Na2CO3 (Refer to Example 1), show your work*.
POST-LAB QUESTIONS
- A student stopped the reaction early without ensuring the decomposition of the sample of sodium bicarbonate, \(\ce{NaHCO3}\), in the test tube. How would this affect the results? Explain.
- 8.00 g of sodium bicarbonate, \(\ce{NaHCO3}\) when decomposed produced 4.257 g of Na2CO3. What is the percent yield of Na2CO3? (Show your work*)
- Potassium chlorate, KClO3, decomposed according to the following equation.
2 \(\ce{KClO3}\) (s) 
2KCl (s) + 3O2 (g)
What mass of potassium chloride would be theoretically produced if 25.253 g of \(\ce{KClO3}\) ? (Show your work*)
*Show all your calculations and work where possible for full credit.
Please click here to access the Pre-Lab, Data Tables, and Post-Lab in Word or PDF format. Complete them and upload according to your instructor's instructions.