Experiment-9: Thermal Decomposition of Baking Soda
Thermal decomposition of Baking soda, sodium hydrogen carbonate (NaHCO3) yields
sodium carbonate (Na2CO3), water, and carbon dioxide. The balanced equation for this
chemical reaction is
2 NaHCO3 (s)  Na2CO3 (s) + H2O (g) + CO2 (g)…………(equation 1)
After the thermal decomposition is complete, only sodium carbonate is left as a solid
product whereas water and carbon dioxide being gases will escape into the air.
In part–A, you will analyze ‘pure sodium hydrogen carbonate (NaHCO3)’. You will
calculate the % yield to check the validity of the experimental procedure being used for
thermal decomposition.
Example, thermal decomposition of 1.355 g NaHCO3 yields 0.824 g of solid Na2CO3.
Calculate theoretical yield and % yield of Na2CO3.
As per the balanced equation, 2 mol of NaHCO3 yields 1 mol of Na2CO3
Step-1: [g NaHCO3 / molar mass of NaHCO3 ] = mol NaHCO3
Step-2: [mol NaHCO3 x ½ ] = mol of Na2CO3 expected
Step-3: [mol of Na2CO3 expected x molar mass of Na2CO3] = g Na2CO3 expected =
theoretical yield of Na2CO3 = 0.855 g
{Calculate molar mass of sodium hydrogen carbonate (NaHCO3) and sodium carbonate
(Na2CO3) using a periodic chart. Plug-in all the numbers to find the answer for Step-3}
Step-4: [Actual yield/Theoretical yield] x 100 = % yield
[0.824 g / 0.855 g ] x 100 = 96.4 %
In part-B, you will find the percentage of sodium hydrogen carbonate (NaHCO3) in a
given unknown mixture, using the experimental procedure of thermal decomposition.
The loss of mass corresponds to water plus carbon dioxide.
As per the balanced
equation, 2 mol of NaHCO3 lose 1 mol of water and 1 mol of carbon dioxide. For the
sake of simplicity one may combine and write, 1 mol of H2O (18.02 g/mol) and 1 mol of
CO2 (44.01 g/mol) = 1 mol H2CO3 (62.03 g/mol). In other words, 2 mol of NaHCO3 will
lose 1 mol of H2CO3 , which is equal to 62.03 g.
Example, Thermal decomposition of 1.545 g unknown mixture containing NaHCO3 lost
0.255 g. Calculate the percentage of NaHCO3 in this unknown mixture.
Step-1: [g lost / molar mass H2CO3] = mol of H2CO3
Step-2: [mol of H2CO3 x 2] = mol of NaHCO3
Step-3: [mol of NaHCO3 x molar mass of NaHCO3] = g of NaHCO3 = 0.691 g
{Calculate molar mass of NaHCO3 and H2CO3 using a periodic chart. Plug-in all the
numbers to find the answer for Step-3}
Step-4: [g of NaHCO3 / g of sample] x 100 = % NaHCO3
[0.691 g / 1.545 g ] x 100 = 44.7 %
Your instructor will demonstrate the proper process of heating using a Bunsen
burner. Observe carefully and follow the instructions!
Procedure: Part A
1. Obtain a clean and DRY 16x150 mm test tube. Place this test tube in a clean and
DRY 100-ml or 150-mL beaker. Find the mass of ‘test tube and beaker’. Record
all the digits.
2. Using a dry spatula, transfer approximately 1 gram of ‘Baking soda, sodium
hydrogen carbonate (NaHCO3)’ into the test tube. Using the same scale, find the
exact mass of ‘test tube, beaker and Baking soda’. Record all the digits. Mass of
the ‘Baking soda’ should be no more than 1.500 g.
3. Clamp the test tube at the neck. Attach the clamp to an iron stand as shown in the
Fig.1. The mixture inside the test tube should be evenly spread so that the heating
will be uniform and more effective. Begin heating gently using a low blue flame
from a Bunsen burner. After about 5 minutes, gradually increase the flame to
high. Observe the reaction inside the test tube. Heat the test tube very carefully
for 3 minutes using high flame. After the reaction is complete, shut off the Bunsen
burner. Let the test tube cool for about 10 minutes. Start part-B while this test
tube cools down to room temperature.
4. Place the test tube in the same beaker. Using the same scale, find the exact mass
of ‘test tube, beaker and the residue’. Record all the digits.
Part B
5. Record the Unknown sample number you are planning to analyze.
6. Repeat step 1-4, substituting your Unknown sample for ‘Baking Soda’.
Disposal: Regular waste sink/ container.
Laboratory Report#9: Thermal Decomposition of Baking Soda
Last Name____________________, first name________________
Date of Experiment___________ Instructor’s Initials__________
Part A
Data
1. Mass of ‘test tube and beaker’
=_____________________g
2. Mass of ‘test tube, beaker and NaHCO3
=_____________________g
3. Mass of ‘NaHCO3’ used [#2-#1]
=_____________________g
After heating and cooling,
4. Mass of ‘test tube, beaker and sodium carbonate’ =_____________________g
Calculations
5. Mass of sodium carbonate produced (actual yield) [#2-#4] =____________________g
6. mol of NaHCO3 used = [#3 /84.01 g/mol] =______________
7. mol of Na2CO3 expected = mol NaHCO3 used x ½ = #6 x ½ = ____________
8. Mass of sodium carbonate theoretically expected [#7 x 105.99 g/mol] =___________g
9. Percent yield of sodium carbonate=
= [(actual yield/theoretical yield) x 100] = [(#5/#8) x100] =__________%
Part-B (your Unknown #_______)
Data
10. Mass of ‘test tube and beaker’
=_______________g
11. Mass of ‘test tube, beaker and unknown mixture’
=_______________g
12. Mass of ‘unknown mixture’ used [#11-#10]
=_______________g
After heating and cooling,
13. Mass of ‘test tube, beaker and residue’
=_______________g
Calculations
14. Mass of H2CO3 produced [#12-#14]
=_______________g
15. mol of H2CO3 (convert #14 into moles) [#14/62.03 g/mol] =__________mol
16. mol of NaHCO3 present in the unknown mixture = mol of H2CO3 x 2
=
[#15x 2] =____________mol
17. g of NaHCO3 present in the unknown mixture (convert #16 into grams)=
= [#16x 84.01 g/mol] =______________g
18. % of NaHCO3 in the unknown mixture [(#17/#13)x100]=___________%
Show all your work and use proper units and significant figures!
1.
Calculate the gram molar mass of the following chemicals using a periodic chart:
a. NaHCO3 ---------------------------------------------------------------------------------------------------------- grams/mol
b. Na2CO3 ------------------------------------------------------------------------------------------------------------grams/mol
c. H2CO3
2.
-------------------------------------------------------------------------------------------------------------- grams/mol
Thermal decomposition of 2.765 g NaHCO3 yields 1.234 g of solid Na2CO3.
Calculate theoretical yield and % yield of Na2CO3.
3. Thermal decomposition of 2.968 g of a mixture containing NaHCO3 lost 0.453 g.
Calculate the percentage of NaHCO3 in this unknown mixture.