A chemical reaction is a process that converts the molecular or ionic structure of one substance to another substance. Chemical reactions usually yield one or more products, with a structure and properties different from the reactants. Reactions may go to completion or proceed in the forward or reverse direction until they reach equilibrium. An important reactive intermediate in many organic reactions is a positively charged carbon atom, a carbocation. A carbanion is a high energy intermediate, unstable, and very reactive. Since a carbanion has an excess of electrons, it will readily react with an electron-deficient carbon atom, so it is classified as a nucleophile. Energy can be transferred by collision with another atom or molecule. The energy released by such collisions is utilized in bond breaking and bond-making.