Which Statement Describes How a Basic Coffee Cup Calorimeter Works?
A calorimeter is a tool that measures the amount of heat flow in a chemical reaction. The most common calorimeters include a coffee cup calorimeter and a bomb calorimeter.
Coffee Cup Calorimeter
A coffee cup calorimeter is basically polystyrene or a Styrofoam cup with a lid. It is simply one cup inside another with a cover to give insulation to the content mixed in it. For a basic coffee cup calorimeter, a Styrofoam insulator and a sensitive thermometer complete the device. A more complex machine will have a Dewar flask, a double-walled container that has reduced pressure in between the two walls.
Measure some amount of water, partially fill the cup, cover it, and insert a thermometer through the cup’s lid until the bulb is below the water surface. When a chemical reaction takes place inside the coffee cup calorimeter, the water absorbs the heat, which is produced as a result of the chemical reaction. The increase in the water temperature is used to calculate the amount of heat that has been used in the brewing process to make the coffee products. It also shows the amount of heat lost to the water when the temperature increases during the process.
Heat flow is calculated using the following equation:
q = (specific heat) x m x Δt
Where:
q is heat flow
m is mass in grams
Δt is the change in temperature.
The specific heat is the level of heat needed to raise the temperature of 1 gram of a substance 1 degree Celsius. The specific heat of water is 4.18 J/(g·°C).
For example, consider a chemical reaction that occurs in 100 grams of water with an initial temperature of 20.0 C. The reaction is allowed to proceed in the coffee cup calorimeter. As a result of the reaction, the temperature of the water changes to 26.0 C. The heat flow is calculated:
qwater = 4.18 J/(g·°C) x 100 g x (26.0 C – 20.0 C)
qwater = +2.5 x 103 J
The products of the reaction produced 2,500 J of heat, which the water absorbed. The enthalpy change, ΔH, for the reaction is equal in degree but opposite in sign to the water’s heat flow:
ΔHreaction = -(qwater)
Remember that for an exothermic reaction, ΔH < 0, qwater is positive. The water absorbs heat emitted from the reaction, and there is a visible rise in temperature. For an endothermic reaction, ΔH > 0, qwater is negative. The water emits heat for the reaction, and this results in a decrease in temperature.