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Section 7: Calorimetry
Figure 7-10. Coffee Cup Calorimeter
When a chemical reaction occurs in the coffee cup, the heat generated by the reaction will be absorbed by the water in the cup. Because Styrofoam is a good insulator, nearly all the thermal energy from the reaction will stay in the cup and not be lost to the surroundings during the experiment.
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The amount of heat released or absorbed by any chemical process can be measured using an insulated container called a "calorimeter." If the reaction in question occurs in an aqueous solution, an effective calorimeter can be as simple as a Styrofoam coffee cup and a thermometer (Figure 7-10). In this case, because the Styrofoam cup is an open container, the pressure is constant, and measuring the heat is the same as measuring the change in enthalpy of a chemical reaction (ΔH).
When the reaction occurs, the change in temperature will be proportional to the enthalpy released by the reaction. For example, let's say that one mole of a substance can react inside 100 g of water inside the calorimeter so that the water absorbs all the heat, and the temperature of the water increases by 5 degrees. Using the mass of the water (m), the specific heat of the water (c), and the change in temperature (ΔT), we can calculate the heat released by the reaction:
q = mcΔT = 100g • 4.18 J/g °C • 5°C = 2090 J = 2.09 kJ
Because one mole of the substance dissolved, the value of ΔH is -2.09 kJ/mol. Note that this value is negative because the reaction gave off the enthalpy that the water absorbed.
Figure 7-11. Bomb Calorimeter
A bomb calorimeter makes it possible to measure changes in a system’s internal energy due to a reaction.
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However, many chemical reactions, such as combustion reactions, do not take place in a solution. To more efficiently measure the heat released by combustion reactions, chemists use a bomb calorimeter, which is a sealed vessel that contains a smaller container called a "bomb." The bomb (not related to a military weapon) is a container designed to withstand high pressure, and is equipped with valves for adding gases and electrical contacts for initiating combustion reactions. (Figure 7-11)
The basic principle is the same: A chemical reaction heats a quantity of water in an insulated container. In this case, however, the reaction takes place inside a sealed container, or bomb. The bomb contains the chemical to be analyzed and enough oxygen to make sure the sample burns completely. The bomb sits submerged in a container of water, and ignition wires start the combustion. Because the reaction takes place in a rigid, sealed container, no pressure-volume work is done by the reaction; all the energy will be released as heat, and none as work. In other words, a bomb calorimeter always measures the heat that is released by a reaction, but in this case the heat represents not the change in enthalpy (ΔH), but the change in internal energy (ΔU). Conveniently, there are simple calculations that can be done to convert the internal energy change into the enthalpy change chemists need.
Chemists have measured the change in enthalpy for thousands of different reactions and have collected them into tables that can be found in almost any chemistry book. So, there is no need for a chemist to perform calorimetry on a particular reaction, as that value can be found in a table of enthalpies of reactions. (Section 8 will explain how an enthalpy change for a reaction that is not in the table can be determined without doing a new calorimetry experiment.)