Thermal Decomposition of Baking Soda



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.

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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 --------------------------------------------------------------------------------------------------------------grams/mol

2. 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.

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