Chemistry

11

The Decomposition Reaction of Baking Soda, NaHCO3

Oscar ShiPeriod 2

Laboratory AssignmentDue Date: 2011/4/14

Introduction

The objective of this experiment is to find the actual chemical equation that governs the manner in which baking soda (sodium hydrogen carbonate NaHCO3) decomposes.

When NaHCO3 is heated, it decomposes to produce a gas and leaves behind a solid. Two possible equations for the decomposition are:

NaHCO3 (s) + heat NaOH (s) + CO2 (g)

NaHCO3 (s) + heat ½ Na2OH3 (s) + ½ H2O (g) + ½ CO2 (g)

Materials and Methods

The Procedure of the Laboratory

1. Put on safety goggles and a lab apron.2. Obtain a clean and dry crucible and lid. Crucibles very slowly adsorb small amounts of water vapour from the atmosphere and must be heated for a minute or two to get rid of adsorbed water molecules before you can get an accurate “initial mass” for a crucible. Crucibles are usually heated in clay triangles, which in turn are set on top of rings attached to ring stands.3. Hot crucibles can blister your bench top, so hot crucibles should be placed on the steel base of your ring stand using your crucible tongs at all times in order to cool down. 4. After the crucible has cooled down, accurately determine the mass of your crucible. Record this mass on your date sheet. If a very hot crucible is put on an electronic balance, the balance can be ruined. 5. Put about 5.00 g of sodium hydrogen carbonate in the cool crucible. Once again using the electronic balance, record the mass of the crucible plus baking soda accurately on your data sheet. Using the method of mass by difference, figure out the mass of the sample and enter this into your data sheet. 6. In order to decompose, sodium hydrogen carbonate must be heated until the crucible starts to glow an “orange-red”. Heat the crucible for at least 5 minutes making sure the lid is on with a slight gap to let the gas escape. If there is no lid then some of the baking soda “can” pop out thereby altering your calculations. 7. Cool down the crucible as above.8. After the crucible has cooled down, weight the crucible and lid and sample. Record this in your data sheet.9. Re-heat the crucible again for at least 5 minutes and let cool down. After the crucible has cooled down, take another weighing. Record this in your data sheet. Again, using the method of mass by difference, calculate the mass of the heated sample. 10. Dispose of the sample down the sink with water and wash the crucible and lid with water and put back all equipment and supplies.

crucible crucible lid clay triangles

ring ring stand crucible tongs

electronic balance sodium hydrogen carbonate

Pre-Laboratory Calculations

NaHCO3 (s) + heat NaOH (s) + CO2 (g)

5.00 g NaHC O3×1mol NaHC O3

84 gNaHC O3× 1mol NaOH

1 mol NaHC O3× 40 g NaOH

1mol NaOH=2.38 g NaOH

NaHCO3 (s) + heat ½ Na2OH3 (s) + ½ H2O (g) + ½ CO2 (g)

5.00 g NaHC O3×1mol NaHC O3

84 g NaHC O3×

12

mol N a2 C O3

1 mol NaHC O3×

106 g N a2C O3

1 mol N a2C O3=3.15 g N a2 C O3

Result/ Data

Measurements Mass (in grams)Mass of Empty Crucible (after initial heating) 19.208 gMass of Crucible + sample 24.208 gMass of sample 5.00 gMass of Crucible + sample (after 1st heating) 22.343 gMass of Crucible + sample (after 2nd heating) 22.355 gMass of sample after heating 3.147 g

Questions

1. Using the mass of the sample that you started with in your lab, calculate the theoretical yield of each of the solids in the two possible equations given in the introduction of the lab.

NaHCO3 (s) + heat NaOH (s) + CO2 (g)

5.00 g NaHC O3×1mol NaHC O3

84 gNaHC O3× 1mol NaOH

1 mol NaHC O3× 40 g NaOH

1mol NaOH=2.38 g NaOH

NaHCO3 (s) + heat ½ Na2OH3 (s) + ½ H2O (g) + ½ CO2 (g)

5.00 g NaHC O3×1mol NaHC O3

84 g NaHC O3×

12

mol N a2 C O3

1 mol NaHC O3×

106 g N a2C O3

1 mol N a2C O3=3.15 g N a2 C O3

2. From your results in number 1, which of the 2 equations is the correct decomposition reaction of baking soda? Why?

From my results in question number 1, NaHCO3 (s) + heat ½ Na2OH3 (s) + ½ H2O (g) + ½ CO2 (g) is the correct decomposition reaction of baking soda. Because the answer calculated from the equation is accurately close to the weighted mass of the sample after heating.

3. What is the actual yield of your final sample in this experiment?

According to the data collected, the actual yield of the final sample in this experiment is 3.147 grams.

4. What is your percent yield for this experiment?

Percent Yield= ActualYieldTheoritical Yield

=3.147 grams3.15 grams

×100 %=99.9 %Yield

Conclusion

Every step of this experiment was taken carefully and therefore the result must hold a high accuracy. Possible source of error cannot be excluded, however. The small particles of dirt or any other remained substance in crucible from previous laboratory that could not be examined by naked eye might be a cause to slight errors upon measurement readings. Temperature of the crucible was assumed to be acceptable to be placed on the electronic scale for mass measurement. It is unknown if the air currents whether or not influenced the reading of the scale during the experiment. The electronic scale itself or when under the influence of the environment, was another source of error. The reading on scale increased and decreased even when the mass placed still on the balance. The sources of errors above were assumptions and even if they did influence the accuracy, it would not have influenced that much of it. Therefore it concludes the readings and calculations on this laboratory are highly accurate.