Chemical Kinetics

Catalysts

The rate of a chemical reaction increases by using a catalyst to lower the activation energy.

A catalyst is a substance that causes the rate of a chemical reaction to increase. A catalyst is not consumed during the overall reaction. A catalyst is commonly one of the reactants of an early step of a multistep reaction and is then produced at a later step.

Catalysts are commercially important. Production of many chemicals relies on suitable catalysts. The Haber-Bosch process, for example, allows production of ammonia from nitrogen gas. This process forms the basis of most ammonia chemicals, including nitrogen-based fertilizers, which are partly responsible for increased farm productivity since 1900. The Haber-Bosch process uses a catalyst. The reaction will not occur at a reasonable rate without one.

A homogeneous catalyst is a catalyst that is in the same phase as that of the reactants and is present in the reaction medium. A homogeneous catalyst for a gas-phase reaction is a gas. A homogeneous catalyst for a liquid-phase or aqueous reaction is a liquid. Consider the decomposition of hydrogen peroxide (H2O2).
2H2O2(aq)2H2O(l)+O2(g)2{\rm H}_2{\rm O}_2(\mathit{aq})\rightarrow2{\rm H}_2\rm O(\mathit l)+{\rm O}_2(\mathit g)
On its own, this reaction occurs slowly. Bromide ions may be used to catalyze this reaction. The bromide ion provides an alternate pathway, turning this reaction into a multistep one.
2Br(aq)+H2O2(aq)+2H+(aq)Br2(aq)+2H2O(l)Br2(aq)+H2O2(aq)2Br(aq)+2H+(aq)+O2(g)\begin{array}{c}2\rm{Br}^-(\mathit{aq})+{\rm H}_2{\rm O}_2(\mathit{aq})+2\rm H^+(\mathit{aq})\rightarrow{\rm{Br}}_2(\mathit{aq})+2{\rm H}_2\rm O(\mathit l)\\\\{\rm{Br}}_2(\mathit{aq})+{\rm H}_2{\rm O}_2(\mathit{aq})\rightarrow2\rm{Br}^-(\mathit{aq})+2\rm H^+(\mathit{aq})+{\rm O}_2(\mathit g)\end{array}
Note that aqueous bromine (Br2) is produced during the reaction and then consumed. Bromine (Br2) is an intermediate. The bromine ion (Br) is initially consumed but is then reproduced. The bromine ion (Br) is also in the same phase as the reactants, making it a homogeneous catalyst. The presence of the bromine ion makes an elementary reaction into a multistep one. This may look like it complicates the reaction. However, the activation energy of the uncatalyzed reaction is high. It proceeds at a very slow rate. The activation energy of both reactions in the catalyzed reaction is lower, making the catalyzed pathway faster.

Potential Energy-Reaction Progress Graph for a Catalyzed Reaction

Catalysts often provide alternate pathways for chemical reactions. In the reaction represented by this graph, the energy required to break the peroxide (H2O2) bonds and form water (H2O) and oxygen gas (O2) is decreased by adding the catalysts bromide ions (Br-) and hydrogen ions (H+) because the activated complexes are different.
A heterogeneous catalyst is a catalyst that is in a different phase than that of the reactants. A heterogeneous catalyst is often a solid surface of a specific material. The solid surface catalyzes liquid-phase or gas-phase reactions by interacting with the reactant molecules. Consider the hydrogenation of ethene (C2H4).
C2H4(g)+H2(g)C2H6(g){\rm C}_2{\rm H}_4(\mathit g)+{\rm H}_2(\mathit g)\rightarrow{\rm C}_2{\rm H}_6(\mathit g)
In the gas phase, this reaction occurs slowly. A platinum or nickel surface, however, catalyzes this reaction. The platinum surface can adsorb hydrogen (H2) molecules, breaking them into hydrogen atoms as it does so. The surface also adsorbs ethene (C2H4) molecules and breaks the double bond in ethene. The hydrogen atoms and adsorbed ethene molecules can bond over the platinum surface, forming ethane (C2H6). Ethane can no longer be adsorbed by the surface and is released.

An enzyme is a biological molecule that catalyzes a biological reaction. An enzyme is typically a very large protein molecule with a very specific shape. The shape allows the enzyme to bond to specific molecules, called substrates. Enzymes serve many functions. Some break down their substrates. Some align their substrates with a specific orientation so that a reaction is more likely to occur. Some enzymes bond to other enzymes, activating or deactivating them.