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How do we know?

The energy change that occurs during a chemical reaction can be directly measured using a technique called calorimetry.The simplest calorimeter which can be used to determine the energy change of a reaction is a “coffee-cup” calorimeter, consisting of two nested Styrofoam coffee cups, a loose fitting lid and a thermometer. Using this technique, two solutions, each containing a reactant compound at the same temperature, are poured into the calorimeter. Since the system is assumed to only consist of the particles which are involved in the reaction, the water in the calorimeter is considered a part of the surroundings and we can determine whether the system released or absorbed energy by measuring the temperature of the water. If the thermometer records an increase in temperature, the surroundings (the water) absorbed energy which was released by the system. Since the system released energy, the reaction between the two compounds must have been exothermic. If, however, the thermometer records a decrease in temperature, the system must have absorbed energy from the surroundings, indicating that the reaction was endothermic.


Why should we care?

The fact that energy is either absorbed or released during chemical reactions has profound practical importance. Because many chemical reactions absorb energy, we often need to provide energy to make certain useful products via chemical reactions. However, many chemical reactions such as the combustion of fossil fuels release huge amounts of energy. As was mentioned in the introduction to this lesson, fossil fuels have a high energy density. This is the reason we use fossil fuels to power our vehicles and generate electricity. Unfortunately, the huge amounts of energy obtained from fossil fuel combustion make it difficult to transition to alternative energy sources, which do not produce greenhouse gases and contribute to climate change like fossil fuels do. Currently, most alternative energy sources, such as wind, solar or geothermal power, are not able to harness as much energy and produce as much electricity as fossil fuels can. Nuclear reactions, on the other hand, can release much more energy than the combustion of fossil fuels, but there are other dangers to consider that are associated with nuclear power.