Nature of the reactants The reaction rate varies depending upon what substances are reacting. Acid/base reactions, the formation of
salts, and
ion exchange are usually fast reactions. When covalent bond formation takes place between the molecules and when large molecules are formed, the reactions tend to be slower. The nature and strength of bonds in reactant molecules greatly influence the rate of their transformation into products.
Physical state The
physical state (
solid,
liquid, or
gas) of a reactant is also an important factor of the rate of change. When reactants are in the same
phase, as in
aqueous solution, thermal motion brings them into contact. However, when they are in separate phases, the reaction is limited to the interface between the reactants. Reaction can occur only at their area of contact; in the case of a liquid and a gas, at the surface of the liquid. Vigorous shaking and stirring may be needed to bring the reaction to completion. This means that the more finely divided a solid or liquid reactant the greater its
surface area per unit
volume and the more contact it with the other reactant, thus the faster the reaction. To make an analogy, for example, when one starts a fire, one uses wood chips and small branches — one does not start with large logs right away. In organic chemistry,
on water reactions are the exception to the rule that homogeneous reactions take place faster than heterogeneous reactions (those in which solute and solvent are not mixed properly).
Surface area of solid state In a solid, only those particles that are at the surface can be involved in a reaction. Crushing a solid into smaller parts means that more particles are present at the surface, and the frequency of collisions between these and reactant particles increases, and so reaction occurs more rapidly. For example,
Sherbet (powder) is a mixture of very fine powder of
malic acid (a weak organic acid) and
sodium hydrogen carbonate. On contact with the
saliva in the mouth, these chemicals quickly dissolve and react, releasing
carbon dioxide and providing for the fizzy sensation. Also,
fireworks manufacturers modify the surface area of solid reactants to control the rate at which the fuels in fireworks are oxidised, using this to create diverse effects. For example, finely divided
aluminium confined in a shell explodes violently. If larger pieces of aluminium are used, the reaction is slower and sparks are seen as pieces of burning metal are ejected.
Concentration The reactions are due to collisions of reactant species. The frequency with which the molecules or ions collide depends upon their
concentrations. The more crowded the molecules are, the more likely they are to collide and react with one another. Thus, an increase in the concentrations of the reactants will usually result in the corresponding increase in the reaction rate, while a decrease in the concentrations will usually have a reverse effect. For example,
combustion will occur more rapidly in pure oxygen than in air (21% oxygen). The
rate equation shows the detailed dependence of the reaction rate on the concentrations of reactants and other species present. The mathematical forms depend on the
reaction mechanism. The actual rate equation for a given reaction is determined experimentally and provides information about the reaction mechanism. The mathematical expression of the rate equation is often given by :v = \frac{\mathrm{d}c}{\mathrm{d}t} = k \prod_i c_i^{m_i} Here k is the
reaction rate constant, c_i is the molar concentration of reactant
i and m_i is the partial order of reaction for this reactant. The
partial order for a reactant can only be determined experimentally and is often not indicated by its
stoichiometric coefficient. In highly diluted solutions, such as at concentrations below the micromolar level, molecular collisions are primarily governed by
diffusion. Under these conditions, the apparent reaction order deviates from the stoichiometric expectation because reactant molecules require additional time to traverse longer distances before encountering one another. This behavior can be described by
Fick's laws of diffusion and is consistent with fractal reaction kinetics, which yield fractional reaction orders.
Temperature Temperature usually has a major effect on the rate of a chemical reaction. Molecules at a higher temperature have more
thermal energy. Although collision frequency is greater at higher temperatures, this alone contributes only a very small proportion to the increase in rate of reaction. Much more important is the fact that the proportion of reactant molecules with sufficient energy to react (energy greater than
activation energy:
E >
Ea) is significantly higher and is explained in detail by the
Maxwell–Boltzmann distribution of molecular energies. The effect of temperature on the reaction rate constant usually obeys the
Arrhenius equation k = A e^{-E_{\rm a}/(RT)}, where A is the
pre-exponential factor or A-factor, Ea is the activation energy, R is the
molar gas constant and T is the
absolute temperature. At a given temperature, the chemical rate of a reaction depends on the value of the A-factor, the magnitude of the activation energy, and the concentrations of the reactants. Usually, rapid reactions require relatively small activation energies. The 'rule of thumb' that the rate of chemical reactions doubles for every 10 °C temperature rise is a common misconception. This may have been generalized from the special case of biological systems, where the
α (temperature coefficient) is often between 1.5 and 2.5. The kinetics of rapid reactions can be studied with the
temperature jump method. This involves using a sharp rise in temperature and observing the
relaxation time of the return to equilibrium. A particularly useful form of temperature jump apparatus is a
shock tube, which can rapidly increase a gas's temperature by more than 1000 degrees.
Catalysts A
catalyst is a substance that alters the rate of a chemical reaction but it remains
chemically unchanged afterwards. The catalyst increases the rate of the reaction by providing a new
reaction mechanism to occur with in a lower
activation energy. In
autocatalysis a reaction product is itself a catalyst for that reaction leading to
positive feedback. Proteins that act as catalysts in biochemical reactions are called
enzymes.
Michaelis–Menten kinetics describe the
rate of enzyme mediated reactions. A catalyst does not affect the position of the equilibrium, as the catalyst speeds up the backward and forward reactions equally. In certain organic molecules, specific substituents can have an influence on reaction rate in
neighbouring group participation.
Pressure Increasing the pressure in a gaseous reaction will increase the number of collisions between reactants, increasing the rate of reaction. This is because the
activity of a gas is directly proportional to the partial pressure of the gas. This is similar to the effect of increasing the concentration of a solution. In addition to this straightforward mass-action effect, the rate coefficients themselves can change due to pressure. The rate coefficients and products of many high-temperature gas-phase reactions change if an inert gas is added to the mixture; variations on this effect are called
fall-off and
chemical activation. These phenomena are due to exothermic or endothermic reactions occurring faster than heat transfer, causing the reacting molecules to have non-thermal energy distributions (
non-Boltzmann distribution). Increasing the pressure increases the heat transfer rate between the reacting molecules and the rest of the system, reducing this effect. Condensed-phase rate coefficients can also be affected by pressure, although rather high pressures are required for a measurable effect because ions and molecules are not very compressible. This effect is often studied using
diamond anvils. A reaction's kinetics can also be studied with a
pressure jump approach. This involves making fast changes in pressure and observing the
relaxation time of the return to equilibrium.
Absorption of light The activation energy for a chemical reaction can be provided when one reactant molecule absorbs light of suitable
wavelength and is promoted to an
excited state. The study of reactions initiated by light is
photochemistry, one prominent example being
photosynthesis. ==Experimental methods==