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Enthalpy is a concept used in science and engineering when heat and work need to be calculated.

When a substance changes at constant pressure, enthalpy tells how much heat and work were added or removed to the substance.

Enthalpy is similar to energy, but not the same. When a substance grows or shrinks, energy is used up or released. Enthalpy accounts for this energy. Because of this, scientists often calculate the change in enthalpy, rather than the change in energy.

The name comes from the Greek word "enthalpos" (ενθαλπος), meaning "to put heat into". The idea and the word was made up by the Dutch scientist Heike Kamerlingh Onnes in 1909.

Enthalpy and chemical reactions[change | change source]

When a chemical reaction happens, a substance can become warmer or colder. As a result, heat will flow to things around it, or from things around it, until its temperature is the same again. If the pressure stays the same, this amount of heat tells how much the enthalpy changed.

For example, if gasoline is burned in the open air, heat comes off the burned gasoline until it has cooled off again. If 100 kilojoules of heat come off it, then the enthalpy of the gasoline became less by 100 kilojoules. The enthalpy of the reaction was therefore ∆H = –100 kJ.

If a chemical reaction gives off heat, and warms up things around it, the enthalpy becomes less. The value of ∆H is negative. This kind of reaction is called exothermic.

If a chemical reaction uses up heat, and cools down things around it, the enthalpy becomes more. The value of ∆H is positive. This kind of reaction is called endothermic.

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