The answer is free energy. In fact, in thermodynamics, the free energy of a system is defined as the energy that can be converted to do work. Helmholtz definition of free energy, for instance, is:
where U is the internal energy, T the temperature and S is the entropy
<u>the chemical element of atomic number 81, a soft silvery-white metal that occurs naturally in small amounts in pyrite and other ores. Its compounds are very poisonous.</u>