What Makes a Reaction Fast? The Things That Change How Quickly It Runs

Thermodynamics tells you whether a reaction releases energy overall and therefore whether it tends to proceed, and it says nothing whatever about how long that will take. A mixture of hydrogen and oxygen at room temperature is strongly favoured to form water and will sit indefinitely without doing so, because the molecules must first be broken apart before they can recombine and nothing at that temperature supplies enough energy for the first step. Diamond is thermodynamically less stable than graphite and converts at a rate too slow to observe. The gap between what is favoured and what happens is the domain of kinetics, and confusing the two produces persistent misunderstandings, including the assumption that an energetically favourable process must be quick.

A handful of factors account for most of what can be controlled:

• •Temperature, which increases the fraction of collisions carrying enough energy to react and has the largest effect
• •Concentration, since more particles in a given volume means more collisions
• •Pressure for gases, which is concentration by another name
• •Surface area for solids, since only the exposed surface can participate
• •Catalysts, which offer an alternative route with a lower energy requirement without being consumed
• •Light, for reactions where a photon supplies the energy directly

The central idea is that reacting molecules must pass through an arrangement of higher energy than either the starting materials or the products, and the height of that barrier determines how many collisions succeed. At any temperature the molecules have a spread of energies, with most near the average and a tail extending upwards, and only those in the tail above the barrier can react. Raising the temperature does not merely shift the average, it disproportionately enlarges that tail, which is why a modest temperature rise produces a large rate increase, with a rough working rule that many reactions roughly double in rate for every ten degrees. The relationship between temperature and rate was quantified in the nineteenth century and the resulting expression remains the standard tool, with the barrier height extracted from measurements at several temperatures.

A catalyst is frequently described as speeding a reaction without being consumed, which is true and leaves out the interesting part. It participates fully, forming bonds with the reacting species and being released at the end, so it is consumed and regenerated in each cycle rather than standing aside. What it provides is a different route with a lower barrier, which means more collisions have enough energy and the rate rises, and the crucial point is that it lowers the barrier equally in both directions, so it speeds the reverse reaction by the same factor and cannot shift where the reaction settles. It changes how fast equilibrium is reached and not what equilibrium is. Enzymes are catalysts of extraordinary specificity and effectiveness, accelerating reactions by factors that would otherwise take geological time, and industrial catalysts underpin fertiliser production, fuel refining and the treatment of vehicle exhaust.

Determining a rate means measuring how a concentration changes with time, and the methods depend on the timescale. Slow reactions are followed by sampling and analysing. Faster ones are followed continuously by measuring something that changes as the reaction proceeds, including colour, pressure, conductivity or the rotation of polarised light. Very fast reactions require mixing the reagents in milliseconds and observing immediately, or initiating the reaction with a pulse of light and watching with another, which reaches into timescales shorter than a millionth of a millionth of a second. The results matter well beyond the laboratory, since shelf life, the setting of concrete, the curing of adhesives, corrosion, the effectiveness of a drug and the design of every industrial process are all rate questions, and a process that is favourable but slow is frequently the whole engineering problem.

Whether a reaction is favoured and how fast it runs are separate questions, which is why hydrogen and oxygen sit together unchanged. Temperature, concentration, surface area and catalysts are the controllable factors, with temperature having the largest effect. Molecules must pass an energy barrier, and warming disproportionately enlarges the tail of molecules carrying enough energy to cross it.