Energy
Definition and meaning of Energy in chemistry.
Energy is the capacity of a system to do work or transfer heat, and in chemistry it exists in interconvertible forms including kinetic, potential, thermal, and chemical energy stored in bonds.
In more detail
Every chemical reaction and phase change involves an energy change, absorbed or released as heat, light, or work, which chemists track using state functions such as internal energy and enthalpy. The first law of thermodynamics states that energy is conserved: it cannot be created or destroyed, only transferred between a system and its surroundings or converted from one form to another. Chemical energy, stored in the arrangement of electrons and bonds, is released when stronger bonds form in products than existed in reactants, as in combustion.
Key facts
| Field | Physical Chemistry |
|---|---|
| SI Unit | joule (J) |
| Common forms | kinetic, potential, thermal, chemical |
| Governing law | First law of thermodynamics (conservation of energy) |
When methane burns, CH4 + 2O2 -> CO2 + 2H2O, chemical potential energy stored in its bonds converts to thermal energy and light, releasing about 890 kJ per mole of methane consumed.
Frequently asked questions
What is the difference between energy and heat?
Heat is energy transferred between a system and its surroundings because of a temperature difference; energy is the broader capacity to do work and can exist as kinetic, potential, thermal, or chemical energy, of which heat transfer is just one manifestation.
What units are used for energy in chemistry?
The SI unit is the joule (J), with kilojoules per mole (kJ/mol) common for reaction energies; the calorie (1 cal = 4.184 J) is also still widely used, especially in nutrition and older literature.