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What do we know?

The problem with measuring the energy transferred by a reaction in a coffee-cup calorimeter comes from the fact that energy can be transferred in two forms: heat and work. The equation, q = mcΔT, only accounts for the heat transferred to or from the water in the calorimeter. Consider an exothermic reaction: the reaction releases a certain amount of energy, but the temperature change of the water only accounts for the heat released by the reaction. But what if the reaction also performs work? Then, the total energy released by the reaction is the heat released to the surroundings (measured as a temperature change of the water) plus whatever work was performed on the surroundings.

 Thermite reaction, which released energy and both heat and work

Many of the chemical reactions conducted in a laboratory produce gases. When the reaction mixture is open to the atmosphere, the gases produced by the reaction displace the gases of the atmosphere and the system performs the work of expansion on the surroundings. When this occurs, the energy released by an exothermic reaction is greater than the energy that is transferred to the surroundings as heat and can be measured as a temperature change, because the energy released is also used to perform work.

Reactions that are performed open to the atmosphere are said to be under constant pressure, because the atmospheric pressure exerted on the system does not change. Under these constant pressure conditions, work can be performed by expanding the system. To determine the total energy transfer accompanying a reaction at constant pressure, a chemist would need to account for both the heat that is transferred to or from the system and the work performed by or on the system, which causes a change in the system’s volume. While a chemist could determine the energy transferred as heat by measuring the surroundings’ change in temperature, it is difficult to measure the work performed by a reaction. Consequently, it is also inconvenient to determine the total energy released or absorbed by a reaction at constant pressure.