Expt 043 -- Decomposition of Sodium Bicarbonate

Mass relationships are at the heart of chemistry. A known mass of sodium bicarbonate is heated to produce sodium carbonate. The mass of Na₂CO₃ found is compared to the amount predicted.

Safety

Hot glass and hot objects can cause burns. Hot glass and cold glass look exactly alike. Hold a finger near a previously-heated object for several seconds before touching it to detect heat. Do not grasp hot objects. Do not place hot objects on balance pans. Wear goggles and apron. Know the location of the fire equipment; review use of that equipment before beginning work. Wash hands after the experiment.

Procedure

1. Obtain a glass Pasteur pipet with a short tip. set up a ring stand with a ring and Nichrome gauze for heating. The gauze should be about 3 cm above the end of the barrel of a bunsen burner. Using a hot bunsen burner flame, heat the empty pipet to drive off water. Heat the tip for brief periods only. The tip will bend and seal if it is heated too long. Sweep the flame from the large end to the tapered end. Repeat several times. Cool to room temperature.

   !!!Click here to See Movie.

2. Determine the mass of the pipet to the nearest 0.001 g.

   !!!Click here to See Movie.

3. Preweigh a sample of 0.45-0.55 g of sodium bicarbonate on a balance paper. Using a small spatula, transfer the sodium bicarbonate into the pipet through its large open end. Sodium bicarbonate is filled in the same manner as CuSO₄•5H₂O.

   !!!Click here to See Movie.

4. Determine the mass of the pipet plus sodium bicarbonate to the nearest 0.001 g.

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5. Using a non-luminous flame, heat the sodium bicarbonate. Begin at the open end, and heat slowly and carefully moving the burner toward the tapered end. There are no special indicators of the completeness of this reaction. Water condenses, and this condensate must be heated away. When the entire sample has been heated for 5 minutes, sweep out the remaining water by moving the flame from the large end of the tube to the tapered end. Repeat until no water is observed.

   !!!Click here to See Movie.

6. Turn off the burner and allow the pipet to cool to room temperature.
7. Determine the mass of the pipet plus sodium carbonate to the nearest 0.001 g.
8. Repeat the heating and cooling steps until a constant weight is obtained.

Data Analysis

1. Predict the mass of sodium carbonate expected to result from complete reaction of the sodium bicarbonate.
2. Compare the mass obtained from that predicted.
3. Discuss the reason for any discrepancy.

Questions

1. Write a balanced equation for the sodium bicarbonate decomposition reaction.
2. Predict the mass of sodium carbonate expected from heating 0.500 g sodium bicarbonate.
3. Suppose the mass actually obtained after one heating was 0.325 g. Suggest a possible reason for the problem.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 043 Decomposition of Sodium Bicarbonate

Watch the movies.

Use this sample data for the calculations and to answer the questions.

mass heated pipet- 2.890 g

mass pipet + NaHCO₃- 3.382 g

mass pipet + Na₂CO₃ 1st- 3.204 g

mass pipet + Na₂CO₃ 2nd- 3.199 g

Answer the questions.

Curriculum-

This activity fits in as soon as mass relations are studied. Excellent quantitative results are expected. Related experiments include the decomposition of malachite (Experiment 042) and the dehydration of copper sulfate pentahydrate (Experiment 041).

Safety-

• Hot glass and hot objects can cause burns. Hot glass and cold glass look exactly alike. Hold a finger near a previously-heated object for several seconds before touching it to detect heat. Do not grasp hot objects. Do not place hot objects on balance pans. Wear goggles and apron. Know the location of the fire equipment; review use of that equipment before beginning work. Wash hands after the experiment.
• Burns are the biggest problem, possibly followed by cuts from broken tubes. When the period begins, remind students to test previously heated objects for heat before grasping them.

Time-

Teacher Preparation: 10 minutes

Class Time: 45 minutes (Cooling periods may be used to discuss data analysis or other topics.)

Materials-

• sodium bicarbonate, NaHCO₃, 0.6 g/reaction.
• Pasteur pipet with short stem
• burner, ring, ringstand, Nichrome gauze, ignitor or matches
• top-loading balance sensitive to 0.001 g.
• weighing paper
• small spatula

Disposal-

Dissolve the solid in water and discarded at the sink with large volumes of water.

Lab Hints-

• Warn students to heat the tips cautiously and slowly. The tips will bend and seal shut easily.

  !!!Click here to See Picture.

• Although balances sensitive to the nearest milligram are desired, this experiment gives adequate results when only centigram balances (nearest 0.01 g) are available.

Data Table-

mass heated pipet- 2.890 g

mass pipet + NaHCO₃- 3.382 g

mass pipet + Na₂CO₃ 1st- 3.204 g

mass pipet + Na₂CO₃ 2nd- 3.199 g

Data Analysis-

mass pipet + NaHCO₃ 3.382 g

mass heated pipet 2.890 g

mass NaHCO₃ 0.492 g

mass pipet + Na₂CO₃ 2nd 3.199 g

mass heated pipet 2.890 g

mass Na₂CO₃ 0.309 g

2 NaHCO₃ --> Na₂CO₃ + CO₂ + H₂O

0.492 g NaHCO₃ x (1 mole Na₂CO₃/2 moles NaHCO₃)

= 0.492 g NaHCO₃ x (105.99 g Na₂CO₃ /168.02 g NaHCO₃)

= 0.310 g Na₂CO₃

The mass of Na₂CO₃ found, 0.310, is within balance error of the amount predicted.

Answers-

Q1. Write a balanced equation for the sodium bicarbonate decomposition reaction.

A1. 2 NaHCO₃ --> Na₂CO₃ + CO₂ + H₂O

Q2. Predict the mass of sodium carbonate expected from heating 0.500 g sodium bicarbonate.

A2. 0.500 g NaHCO₃ x (Na₂CO₃/2 NaHCO₃)

= 0.500 g NaHCO₃ x (105.99 g Na₂CO₃ /168.02 g NaHCO₃)

= 0.315 g Na₂CO₃

Q3. Suppose the mass actually obtained after one heating was 0.325 g. Suggest a possible reason for the problem.

A3. A mass that is too large indicates that the sample may require reheating. Heat until a constant weight is reached.

CoopLearn-

The data from many groups should be gathered and analyzed to get a good impression of the precision of the experiment.

Assign some groups the related experiments 41 and 42.

Key Words 1-

stoichiometry, mass relations, decomposition reaction

Elements-

Na C