Reaction rate
Chemical kinetics is the part of physical chemistry that concerns how rates of chemical reactions are measured and predicted, and how reaction-rate data can be used to deduce probable reaction mechanisms.
The rate of reaction differs from the rate of increase of concentration of a product P by a constant factor (the reciprocal of its stoichiometric number) and for a reactant A by minus the reciprocal of the stoichiometric number.
The stoichiometric numbers are included so that the defined rate is independent of which reactant or product species is chosen for measurement.
The above definition is only valid for a single reaction, in a closed system of constant volume.
where When applied to the closed system at constant volume considered previously, this equation reduces to:
For a single reaction in a closed system of varying volume the so-called rate of conversion can be used, in order to avoid handling concentrations.
When side products or reaction intermediates are formed, the IUPAC[8] recommends the use of the terms the rate of increase of concentration and rate of the decrease of concentration for products and reactants, properly.
If the basis is a specific catalyst site that may be rigorously counted by a specified method, the rate is given in units of s−1 and is called a turnover frequency.
The number of reacting species, their physical state (the particles that form solids move much more slowly than those of gases or those in solution), the complexity of the reaction and other factors can greatly influence the rate of a reaction.
However, the main reason that temperature increases the rate of reaction is that more of the colliding particles will have the necessary activation energy resulting in more successful collisions (when bonds are formed between reactants).
For example, coal burns in a fireplace in the presence of oxygen, but it does not when it is stored at room temperature.
The increase in temperature, as created by a match, allows the reaction to start and then it heats itself because it is exothermic.
As such, it may speed up the rate or even make a reaction spontaneous as it provides the particles of the reactants with more energy.
[citation needed] This energy is in one way or another stored in the reacting particles (it may break bonds, and promote molecules to electronically or vibrationally excited states...) creating intermediate species that react easily.
For example, platinum catalyzes the combustion of hydrogen with oxygen at room temperature.
For gas phase reaction the rate equation is often alternatively expressed in terms of partial pressures.
Of all the parameters influencing reaction rates, temperature is normally the most important one and is accounted for by the Arrhenius equation.
For an elementary (single-step) reaction, the order with respect to each reactant is equal to its stoichiometric coefficient.
For a bimolecular reaction or step, the number of collisions is proportional to the product of the two reactant concentrations, or second order.
A termolecular step is predicted to be third order, but also very slow as simultaneous collisions of three molecules are rare.
By using the mass balance for the system in which the reaction occurs, an expression for the rate of change in concentration can be derived.
[10] In chemical kinetics, the overall reaction rate is often explained using a mechanism consisting of a number of elementary steps.
There are also more complex equations possible, which describe the temperature dependence of other rate constants that do not follow this pattern.
With the reactants moving faster this allows more collisions to take place at a greater speed, so the chance of reactants forming into products increases, which in turn results in the rate of reaction increasing.
A rise of ten degrees Celsius results in approximately twice the reaction rate.
The transition state or activated complex shown on the diagram is the energy barrier that must be overcome when changing reactants into products.
The pressure dependence of the rate constant for condensed-phase reactions (that is, when reactants and products are solids or liquid) is usually sufficiently weak in the range of pressures normally encountered in industry that it is neglected in practice.
where V̄ denotes the partial molar volume of a species and ‡ (a double dagger) indicates the activation-state complex.
For the above reaction, one can expect the change of the reaction rate constant (based either on mole fraction or on molar concentration) with pressure at constant temperature to be:[9]: 390 In practice, the matter can be complicated because the partial molar volumes and the activation volume can themselves be a function of pressure.
Reactions can increase or decrease their rates with pressure, depending on the value of ΔV ‡.
