A student determines the composition of a copper chloride compound. Tell whether each of the situations described below would cause the student’s experimental value for the percent of copper in the compound to be greater than less than, or unchanged from the theoretical value. Explain.
a. The student fails to heat to constant mass thereby leaving some water in the copper product
b. the student heats the copper product too long turning the product green and indicating the formation of some copper(II) carbonate.
a. The student fails to heat to constant mass thereby leaving some water in the copper product:
This situation would cause the student's experimental value for the percent of copper in the compound to be greater than the theoretical value. This is because the presence of water in the copper product would increase the overall mass of the compound, leading to a higher calculated percentage of copper. To explain why this happens, we need to understand the method the student is using to determine the percentage of copper in the compound.
The student is most likely using a process called "gravimetric analysis" to determine the percentage of copper. In this process, the student heats the compound to constant mass to drive off any water present. By measuring the mass before and after heating, and comparing it to the mass of the copper chloride compound, the student can calculate the percentage of copper in the compound.
If the student fails to heat the compound to constant mass, some water will remain in the copper product. When the student measures the mass, it will be higher than the actual mass of the copper chloride compound. As a result, the calculated percentage of copper will be higher than the true theoretical percentage.
b. The student heats the copper product too long, turning the product green and indicating the formation of some copper(II) carbonate:
This situation would cause the student's experimental value for the percent of copper in the compound to be less than the theoretical value. The appearance of a green color and the formation of copper(II) carbonate indicates that some of the copper in the compound has reacted with carbon dioxide in the air, leading to the formation of copper(II) carbonate.
During the heating process, copper chloride is supposed to decompose to produce copper metal. However, if the heating is prolonged, the copper metal can react further with carbon dioxide in the air, resulting in the formation of copper(II) carbonate. This additional compound adds extra mass to the product, thereby increasing the overall mass of the compound. Consequently, the calculated percentage of copper will be lower than the theoretical value since the actual amount of copper present in the compound will be less due to the formation of copper(II) carbonate.