Chemical systems are subject to opposing influences: the tendency to minimize their enthalpy is opposed by the tendency to maximize their entropy. The factors which determine the equilibrium constant of a reaction and its variation with changes in conditions follow from the principles of thermodynamics. Ethylene, for example, has a positive standard free energy of formation, whereas that for ethane is negative; that is, ethylene is unstable with respect to carbon and hydrogen and ethane is stable. Certain compounds are unstable to heat not because they contain any intrinsically weak bonds but because decomposition can lead to the formation of a strongly bonded molecule. Tautomerism in which the tautomers differ in the position of a hydrogen atom is referred to as prototropy, for the mechanisms of interconversion involve the removal of a proton from one position and the gain of a proton at the other.