Chemical reaction and equilibrium constant expression

If the reactants and products are all substances capable of an independent existence, then in principle, the answer is always "yes". This answer must be qualified, however, by the following considerations:

Chemical reaction and equilibrium constant expression

The orders for the individual reactants, unity for E and unity for A, are known as partial orders and the sum of all the partial orders of a reaction is the overall order.

Second-order rate constants, such as k in Eqn 6have the dimensions of reciprocal concentration multiplied by reciprocal time, whereas first-order rate constants, such as k in Eqn 5have the dimensions of reciprocal time. This difference in dimensions is not normally evident from the symbols used to represent rate constants, and care must therefore be taken to avoid making improper comparisons between rate constants of different orders.

It is sometimes convenient to multiply a second-order rate constant by an appropriate concentration to produce a quantity with the dimensions of a first-order rate constant, e.

The units of rate constants vary with the order of reaction in the same way that their dimensions vary. For a first-order rate constant, such as k in Eqn 5the units are s For a second-order rate constant, such as k in Eqn 6the units are dm3 mol-1 s-1 or L mol-1 s-1 or M-1 s The concept of order of reaction but not molecularity can also be applied to certain reactions that occur by composite mechanisms, provided that the rate is proportional to reactant concentrations raised to powers which need not be integral.

However, this is rarely the case with enzyme-catalysed reactions and the concept of order cannot therefore be applied strictly to such reactions overall. Nonetheless the individual steps of composite reactions have orders when considered in one direction.

For processes that do not have a true order it is sometimes convenient to define an apparent order with respect to a reactant A as or aswhich is equivalent. For many enzyme-catalysed reactions the true order with respect to any substrate approximates to unity at very low concentrations and to zero at very high concentrations, but is not defined at intermediate concentrations.

The apparent order, on the other hand, exists at any concentration. It is sometimes useful to consider the rates of unidirectional elementary steps of a composite reaction in isolation. When the use of the term rate of reaction for such rates would cause ambiguity the term chemical flux or chemiflux may be used instead.

In enzyme kinetics the need for an unambiguous terminology occurs mainly in discussions of the use of rates of transfer of isotopic labels as probes of the chemical fluxes in different parts of the composite reaction. The reaction numbers should be used as subscripts to k, for rate constants, or v, for the individual rates chemical fluxes.

The preferred scheme for ordinary use is: For some kinds of computer application and for theoretical discussions of enzyme mechanisms it is sometimes convenient to number the different forms of the enzyme rather than the elementary steps and then to denote the step from e.

E3 to E4 as 34, etc. With this scheme the numbering of enzyme forms must be given explicity and the rate constants and rates listed above might become 2 k12, k21, k23, k32.

If there are more than nine enzyme forms in the mechanism the subscripts should be separated by a comma, e. The following scheme, in which odd subscripts refer to forward steps and even subscripts to reverse steps: It is unrealistic to expect any universal system of numbering rate constants to be equally satisfactory in all circumstances.

For example, in a mechanism where enzyme forms in different states of protonation can undergo analogous reactions it may be clearer to assign the same numbers to the analogous steps and distinguish between them by the use of primes, etc.


Regardless of what system is used, rate constants should never be referred to except in explicit relation to a mechanism or to a kinetic equation. Steady-State Approximation If an intermediate is always present in amounts much less than those of the reactants other than the enzyme the rate of change of its concentration is much smaller than that of the reactants.

This condition is ensured whenever, as is usually the case in enzyme-catalysed reactions, the concentration of substrate is much higher than that of the enzyme; it is not necessary for the amount of intermediate to be small compared with the amount of enzyme.

The use of this approximation to obtain an overall rate expression is known as the steady-state treatment or the steady-state approximation. At the very beginning of the reaction the concentration of EA in the above scheme is rising from zero to its steady-state value.

The steady-state approximation is not valid during these early times and the kinetics are known as pre-steady-state kinetics or transient-phase kinetics. The transient phase of an enzyme-catalysed reaction usually occupies a very brief period of time usually a small fraction of a secondand special techniques must be used for investigating this phase of the reaction Section 9.

The rate of reaction of an enzyme-catalysed reaction is not defined during the transient phase, because there is not a one-to-one stoichiometry between the reactants see section 2. In the steady state a one-to-one stoichiometry is established and the rate of reaction can be defined.

This rate, extrapolated back to zero time, is called the initial rate and given the symbol v0. The subscript 0 is normally omitted when no other kinds of rate are at issue, i.

The substrate dissociation constant should not be confused with the Michaelis constant KmA see section 4. References for this section 1.In a chemical reaction, chemical equilibrium is the state in which both reactants and products are present in concentrations which have no further tendency to change with time, The equilibrium constant expression is therefore usually written as.

Reversible reactions, equilibrium, and the equilibrium constant K.

Chemical reaction and equilibrium constant expression

How to calculate K, and how to use K to determine if a reaction strongly favors products or reactants at equilibrium. Factors that affect chemical equilibrium. For reactions that are not at equilibrium, we can write a similar expression called the reaction quotient Q Q Q.

For a system at equilibrium: both forward and reverse reactions are occurring simultaneously rate of forward reaction must equal rate of reverse reaction OR Rate forward = Rate reverse concentrations of reactants and products remain constant with time Changes in Rate and Concentration as a System.

Note: Any reaction that does not involve the transfer of electrons (= change in oxidation numbers) qualifies as a non-redox reaction.

Chemical reaction and equilibrium constant expression

Combination reactions qualify as non-redox reactions when all reactants and products are compounds and the oxidation numbers do not change. Gaseq is a Windows program for combustion equilibrium calculations. I regret that Gaseq is no longer being maintained.

It was originally written in Visual Basic 3 on Windows and there have been about 6 versions of Windows since. Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical kaja-net.comal kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition states, as well as the construction of mathematical models that can describe the .

Chemical kinetics - Wikipedia