Most chemical reactions have an activation barrier

A chemical reaction is a significant rearrangement of otherwise stable electron configurations in molecules or atoms. In order for any rearrangement of bonding electrons to take place, one or more of these things have to happen first:

close approach of atoms/molecules, which requires overcoming electron-electron repulsion (Coulomb) forces

bending or stretching of one or more chemical bonds

partial breakage or formation of one or more chemical bonds

Here's a quick example that we'll expand upon below. Consider the reaction in which a chloride ion (Cl^-) attacks a methyl-bromide molecule (CH₃Br) to displace the bromide and form methyl chloride (CH₃Cl)

In the diagram, Cl^- has to approach CH₃Br, and to approach closely, there has to be enough relative translational energy (they have to be approaching fast enough) to overcome electron-electron repulsion. As a Cl-C bond begins to form, VSEPR theory suggests that the C-H bonds will bend due to repulsion of electron clouds, and finally, one bond (Cl-C) is formed while another (C-Br) is broken. All of these contribute to what we refer to as an energetic "barrier," or an "activation barrier" to the reaction.

Activation energy ↔ rate constant

The activation energy is related to the rate of a chemical reaction. That just makes sense. In one extreme – no activation barrier at all – a reaction occurs very rapidly. And in the limit in which the activation energy is very high, the reaction should be very slow.

Arrhenius related the rate constant to the activation energy through the Arrhenius equation:

The transition state

Let's take another look (see diagram below) at a single-displacement reaction. This time a hydroxyl ion (OH^-) will attack bromomethane (CH₃CH₂Br) to displace the bromine (forming a Br^- ion) and ethanol (CH₃CH₂OH). It's very similar to the methyl bromide ↔ methyl chloride reaction in the previous section. The reaction is

OH^- + C₂H₅Br ⇌ C₂H₅OH + Br^-

Arrhenius noted that there must be some transient structure which the reacting compounds form that is unstable, and could, with equal probability, fall apart into either reactants or products.

Bromoethane and ethanol are stable products of either the forward (shown) or reverse reaction. The transition state is less stable and of high energy. Once it is formed, there is roughly equal probability of it falling apart either into reactants or products.

The role of a catalyst

A catalyst can come in many forms, and we'll get to some of those in a bit, but the role of a catalyst is simple: it alters the transition state to reduce its energy, thus making the barrier to the reaction lower.

Here's an example of how a catalyst might alter the activation energy of an exothermic reaction:

Heterogeneous catalysis

One of the simplest types of catalysis to understand is heterogeneous catalysis. Heterogeneous means "of different character," and here it refers to the fact that the reactants and products are of a different physical phase than the catalyst itself. A nice example is the catalytic converter in a gasoline-powered car.

The reason the catalytic converter is there is to convert toxic, smog-forming carbon monoxide (CO) to carbon dioxide (CO₂) by combination with oxygen in the reaction

2 CO _(g) + O_{2 (g)}   ⇌   2 CO_{2 (g)}

In the gas phase and at fairly low partial pressures, this reaction proceeds exceedingly slowly because the activation energy is high and there are pretty stringent requirements on the alignment of an already improbable three-body collision:

To speed this reaction along (substantially), we employ metals like platinum (Pt) and Rhodium (Rh). These metal surfaces consist of metal atoms packed in a tight crystal-like lattice, as in the cartoon below (only the surface atoms are shown).

Homogeneous catalysis

A homogeneous catalyst is one that is of the same phase (gas, liquid, solid) as the reaction it catalyzes.

The most common kind of homogeneous catalysis is acid catalysis. Here's an example, the hydrolysis of the ester methyl acetate (or methyl ethanoate).

This reaction relies on free protons from the autoionizaton of water. You can see the reaction mechanism in the wide figure below. the auto ionization of water is

H₂O   ⇌   H⁺ + OH^-

The equilibrium constant expression for it is

Enzymes

Our last class of catalyst is the most important for living things, enzymes. Enzymes are protein molecules (or sometimes protein / RNA complexes) that catalyze biological reactions.

While they're really in a class by themselves, enzymes can be considered to be heterogeneous catalysts. They are large molecules that present shaped surfaces or pockets on surfaces, into which reactants fit, and are pre-aligned to facilitate a reaction.

A good example is the ribosome, a very large protein + RNA complex that catalyzes the building of proteins from the code in DNA. The ribosome itself is built of more than half a million atoms. The picture gives a broad overview.

There is a groove into which the messenger RNA (mRNA) fits and can move along its axis. Other pockets contain transfer RNAs (tRNAs), which are attached, via the genetic code, to one of 20 amino acids, the building blocks of proteins.

The mRNA is "ratcheted" through its groove. At each stop, the appropriate tRNA aligns with the mRNA code,