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The change in enthalpy$∆H^0$ is the heat ofthereaction,which may be defined as the amount of heat evolved or consumed in the course of a reaction. $∆H^0$is usually given in or . Forming a bond is always exothermic (negative value of$∆H^0$ ) and releases energy,whereas breaking a bond is always endothermic (positive value of $∆H^0$) and absorbs energy.

Entropy may be defined as a measure of randomness, disorder, or freedom of motion. Two smaller molecules (or fragments, such as radicals) have more freedom of motion (greater entropy) than one larger molecule.

Two molecules (ethene and ) have more freedom of motion than a single ethane molecule. Therefore, the change in entropy$∆S^0$ is negative, and the entropy term is positive$-T∆S^0$.

Again, in this reaction, one $\mathrm{Ï€}C=C$bond and one$\mathrm{Ï€\pm á}=\mathrm{H}$ bond break, while two strong bonds are formed$\mathrm{σ}C-H$. This implies that$∆H^0$ has a negative value.