Coffee Cup Calorimeter

Question

Some chemical reactions release thermal energy and some reactions gain thermal energy. Sam and Alex want to make a calorimeter, which is a device that can measure thermal energy (heat) changes during chemical reactions involving a sample. Sam and Alex design their device to measure these changes. A drawing of their calorimeter is shown below.

The illustration titled 'Coffee Cup Calorimeter' shows nested Styrofoam cups as insulation and a cork stopper that seals the top cup. Inside the cup is a sample cube and water that surrounds the sample. A stirrer and thermometer are inserted through the cork stopper into the cup. The bottoms of the stirrer and thermometer are in the water.

As a reaction occurs involving the sample, changes occur in the system based on whether the reaction gains or releases thermal energy.

Constructed Response Question
A. Identify the system as open or closed and explain why this type of system is helpful for measuring thermal energy transfer during a reaction.

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Constructed Response Question
B. Explain why the cork stopper is needed in this device’s design.

Answer 1

A. System Identification and Explanation

The coffee cup calorimeter can be classified as a closed system. In a closed system, energy can be exchanged with the surroundings in the form of heat, but the mass of the reactants and products cannot move in or out of the system. This characteristic is vital for measuring thermal energy transfer during a chemical reaction because it allows for accurate observation of temperature changes without the interference of mass exchange. Since the calorimeter is designed to contain the reactants (the sample and water) while preventing matter from entering or escaping, it ensures that all thermal energy changes due to the chemical reaction are captured within the system. Consequently, the measured temperature change directly correlates with the heat absorbed or released during the reaction, allowing Sam and Alex to precisely quantify thermal energy changes.

B. Explanation of the Cork Stopper's Purpose

The cork stopper is an essential component of the calorimeter's design for several reasons:

1. Seal the System: The cork stopper seals the top of the calorimeter, preventing the escape of vapor or gaseous products that may form during the reaction. This ensures that all products and reactants remain within the system, maintaining the integrity of the closed environment.

2. Minimize Heat Loss: By sealing the cup, the cork stopper helps to minimize heat exchange with the external environment. This insulation contributes to more accurate measurements of temperature changes by preventing external temperature fluctuations from affecting the water's temperature, thereby allowing for a clearer understanding of the heat transfer occurring within the system.

3. Access for Measurement: The cork stopper has openings for the thermometer and stirrer, enabling the monitoring of temperature changes and mixing of the reactants. This allows Sam and Alex to observe the thermal effects of the reaction without compromising the closed nature of the system.

In summary, the cork stopper enhances the calorimeter's functionality by sealing the system, reducing heat loss, and allowing for precise measurement and observation of thermal energy changes during chemical reactions.