R: see gas constant

Reaction mechanism: the series of fundamental steps, usually involving no more than two particles, through which a reaction proceeds from reactants to products.  The balanced chemical equation indicates only the ratio between the reactant molecules, and the product molecules that are formed.  It rarely, if ever, indicates how the reaction actually proceeded.   In other words, the balanced equation does not tell what molecules are colliding.

Reaction rate: change in an observable property per unit of time, for example, change in concentration per second (mol/L/sec).  For a reaction represented by  R P the average rate of disappearance of R could be represented by:

where:

• - D[R] (or D[P] ) is the change in some observable property (generally a concentration).  The negative sign indicates that the concentration of reactants is decreasing.
• D t is the change in time.

Because the concentration of both reactants and products will change during the reaction, most average rates will change with time. The instantaneous rate in the notation of calculus is:

The reaction rate depends on both the concentration and a reaction rate constant.

Reduction: reduction is one half of the oxidation-reduction process.  In an oxidation, a substance has an decrease (a reduction of) its oxidation number.  In a reduction a substance gains electrons.  Thus the following is a reduction reaction:

where the hydrogen reduces from an oxidation number of +1 to 0.  It is not possible to carry out a reduction without some other substance being oxidized.

Reverse reaction: the reaction in the "non-traditional" direction, that is from right to left for the products turning back into reactants. The opposite of the forward reaction.

Reactants  Products

Reversible: chemical reactions can go in either direction. Although we call the substances on the left the reactants, and those on the right the products, this is only for convenience. Actually the materials on either side can react.