Dear Carbonated and Confused,

Thank you for your question! For the reactions you mention, some knowledge of physical properties can also help to address your question, so let’s begin with those. The decomposition reactions are shown in the equations below. In your experiment, the goal is for students to heat the mixture, decomposing only the bicarbonate salt, and then use stoichiometry with their results to determine the original composition of the carbonate/bicarbonate mixture.

Desired decomposition: 2 KHCO₃ → K₂CO₃ + H₂O + CO₂

Undesired decomposition: K₂CO₃ → K₂O + CO₂

Although potassium salts and sodium salts are often interchangeable for the reactions carried out in high school labs, yours is a case where the sodium salts might be a better choice. Table 1 compares physical properties of the carbonate and bicarbonate compounds of potassium and sodium. These properties show that some complications you may encounter using the potassium salts for this lab can be avoided by using the corresponding sodium salts.

Table 1. Relevant physical properties of potassium salts and sodium salts.

In both cases, the appropriate temperature range to selectively and quickly decompose the bicarbonate portion of the mixture is between about 300^oC and 800^oC. The main differences, relevant to your question, are in the properties of the carbonate salts. Sodium carbonate decomposition proceeds very slowly unless a catalyst is added. This means that accidental Na₂CO₃ decomposition is less likely than accidental K₂CO₃ decomposition. Additionally, Na₂CO₃ is relatively stable in air, while K₂CO₃ is deliquescent, readily drawing in moisture from the air. Deliquescence can affect both the initial mass of the mixture and the final mass after bicarbonate decomposition.

Using a mixture of NaHCO₃ and Na₂CO₃ will lead to more reliable results, particularly in a high school setting where it is difficult to control the flame temperature and where samples may need to be left overnight. It is still important, though, to consider the flame temperature.

Although there are several types of lab burners, those that use a central classroom gas line generally have similar outputs. The type of source gas can change the flame temperatures, but most will provide a flame with a maximum temperature between about 1300^oC and 1500^oC at the most efficient setting. Remember that the temperature of the flame does not exactly correlate to the temperature of the object being heated, due to differing thermal conductivities of containers or glassware, but it can be a reasonable guideline to use.

Figure 1. Graphic showing the hottest part of a lab burner flame is at the tip of the inner cone.

One simple way to ensure that your sample does not reach the temperatures that cause unwanted decomposition is to distance the sample from the flame. The region above the flame is called the thermal plume, and temperatures drop off quickly in this column. If you instruct students to place the sample at a height approximately 10-15 cm above the inner cone of the flame (Figure 1) the sample temperatures should fall in the proper region for your experiment.

If you’d like to be more scientific about it, you could measure or estimate temperatures in 1-inch increments above the flame to determine the best height to place your sample.

• Direct measurements of flame or thermal plume temperature can be made using a thermocouple, like those from Vernier or Pasco.
• Estimates of flame temperature can be made by holding different metal samples at designated heights to see whether they melt. Commonly available options are: copper wire (1085^oC), aluminum wire (660^oC), zinc foil or pieces (420^oC), and tin pellet or foil (232^oC).
• Direct measurements of sample surface temperatures can be made using a high temperature non-contact IR thermometer.

The advice given above also applies to the AACT resource, Baking Soda Stoichiometry. This lab uses the same process, but the sample contains only sodium bicarbonate. In this lab, first year chemistry students select which of three theoretical balanced equations best models the mass data they collect from decomposing sodium bicarbonate.

Thanks for writing in with your lab question!

Yours in Chemistry,

Labby