Heat
Definition and meaning of Heat in chemistry.
Heat is the transfer of thermal energy between a system and its surroundings that occurs because of a temperature difference between them.
In more detail
Heat is not a substance stored in matter but energy in transit, and it always flows spontaneously from a region of higher temperature to one of lower temperature until thermal equilibrium is reached. It must be distinguished from temperature: temperature measures the average kinetic energy of particles, while heat is the actual quantity of energy exchanged. In chemistry, the heat absorbed or released by a reaction at constant pressure equals its enthalpy change, ΔH, which is why calorimetry (measuring heat flow) is used to determine reaction enthalpies.
Key facts
| Field | Physical Chemistry |
|---|---|
| SI Unit | joule (J) |
| Common Symbol | q (or Q) |
| Sign Convention | q > 0 for heat absorbed (endothermic); q < 0 for heat released (exothermic) |
When a hot iron bar is dropped into cool water, heat flows from the iron to the water until both reach the same final temperature, with the iron cooling and the water warming.
Frequently asked questions
Is heat the same thing as temperature?
No. Temperature measures the average kinetic energy of particles in a substance, while heat is the energy that flows between objects because of a temperature difference.
How is heat related to enthalpy?
At constant pressure, the heat absorbed or released by a chemical process is equal to its enthalpy change, ΔH, which is why calorimeters measure heat to determine reaction enthalpies.