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Key Idea 7
Enthalpy
Concept Question: What is enthalpy and why do chemists use it?

Although many chemical reactions release energy, few reactions release as much energy as the combustion of a fossil fuel. Evidently, the usefulness of fossil fuel combustion is not only that energy is released, but that a large amount of energy is released. However, in this lesson, we have not yet learned how chemists can determine or measure the amount of energy released or absorbed by a chemical reaction. How does a chemist do this?

 

How do we know?

Previously we discussed how calorimetry can be used to determine whether a reaction releases or absorbs energy, but calorimetry can also be used to measure the amount of energy that is released or absorbed. In a calorimeter, a chemical reaction is conducted in the presence of water, so that the energy absorbed or released by the reaction produces a temperature change in the water. If the water’s temperature goes up, the reaction released energy; if the water’s temperature goes down, the reaction absorbed energy.

However, we also are able to calculate the heat transfer that is associated with the temperature change of a substance, using the equation q = mcΔT. Therefore, if we perform an exothermic reaction in a known mass of water (m) and measure the temperature change (ΔT), we can calculate the energy absorbed by the water, using water’s specific heat capacity (c). Since the energy absorbed by the water must have come from the particles involved in the reaction, we also know the amount of energy released by the reaction. However, there is a subtle problem associated with this approach.

 

Question for Thought

Why might the temperature change of the water in a coffee-cup calorimeter not be an accurate measurement of the total energy released by a reaction? Hint: Consider the two ways in which energy can be transferred and what type of energy transfer typically causes an increase in temperature.