hChemical kinetics deals with the rates of chemical processes. Any chemical process is considered to consist of number of one or more single-step step which are known as elementary reactions elementary processes or elementary steps. Elementary reactions may involve dissociation or isomerisation of a single reactant molecule, which is referred as unimolecular step. Also there may be a single collision between two molecules, which is referred as a bimolecular step.
It should be noted that majority of the reactions that are written as a single reaction equation actually consists of a series of elementary steps or process.
The main objectives for the study of chemical kinetics are:
(a) To analyze the sequence of elementary steps giving rise to the overall reaction. i.e. to determine the reaction mechanism and (b) To determine the absolute rate of the reaction and also for the individual elementary steps. …show more content…
In general chemical reactivity is controlled by two broad factors:
(a) Thermodynamics: This factor allows us to answer which state is more stable whether the reactants or products and
(b) Kinetics: Thermodynamics gives the idea about the feasibility of the reaction. Also it gives the idea about the factors which controls the rate of a reaction. Practically in order for a reaction to occur, the reaction must be thermodynamically and kinetically favored. The Thermodynamic factors which control reactions are enthalpy, entropy and the temperature. The kinetic factors which controls the rate of reaction are, the concentration of reactants, the reaction mechanism, the energy barrier required to be overcome i.e. its activation energy, and temperature
Rate of a reaction
The rate of a chemical reaction, is change in concentration in unit time. Hence the rate of a reaction has units of concentration per unit time i.e. mol dm-3 s-1. For gaseous reactions, alternative units of concentration are often used, likes units of pressure – Torr, mbar or Pa). In order to determine the rate of reaction we can monitor the change in concentration of one of the reactants or products as a function of time. Stoichiometry of a reaction The stoichiometry is the number of moles of each reactant and product appearing in the balanced reaction equation. Taking the stoichiometry in to consideration the reaction rate is modified and can be defined as the rate of change of the concentration of a reactant or product divided by its stoichiometric coefficient. In the above reaction, the rate (ν) is expressed as ν = -d [N2] /dt = - 1/3 d[H2]/dt = 1/2 d[NH3] /dt The negative sign indicates that the decrease in concentration of one of the reactants. The reaction rate needs to be a positive quantity. Rate laws The rate law is an expression which relates the rate of a reaction to the concentrations of the chemical species present, which includes reactants, products, and catalysts i.e. ν = k [A]a [B]b [C]c ... Hence the rate is proportional to the concentrations of the reactants. The powers to which each concentration term is being raised is the corresponding order. It may be a zero, a whole number or a fraction. It should be observed that the orders will not reflect the stoichiometry of the reaction equation. Incase of a multi-step process, the overall reaction equation is the net result of all of the elementary reactions in the mechanism. By knowing the sequence of elementary steps that constitutes the mechanism of a reaction, we can deduce the rate law. Conversely, if the mechanism
It should be noted that majority of the reactions that are written as a single reaction equation actually consists of a series of elementary steps or process.
The main objectives for the study of chemical kinetics are:
(a) To analyze the sequence of elementary steps giving rise to the overall reaction. i.e. to determine the reaction mechanism and (b) To determine the absolute rate of the reaction and also for the individual elementary steps. …show more content…
In general chemical reactivity is controlled by two broad factors:
(a) Thermodynamics: This factor allows us to answer which state is more stable whether the reactants or products and
(b) Kinetics: Thermodynamics gives the idea about the feasibility of the reaction. Also it gives the idea about the factors which controls the rate of a reaction. Practically in order for a reaction to occur, the reaction must be thermodynamically and kinetically favored. The Thermodynamic factors which control reactions are enthalpy, entropy and the temperature. The kinetic factors which controls the rate of reaction are, the concentration of reactants, the reaction mechanism, the energy barrier required to be overcome i.e. its activation energy, and temperature
Rate of a reaction
The rate of a chemical reaction, is change in concentration in unit time. Hence the rate of a reaction has units of concentration per unit time i.e. mol dm-3 s-1. For gaseous reactions, alternative units of concentration are often used, likes units of pressure – Torr, mbar or Pa). In order to determine the rate of reaction we can monitor the change in concentration of one of the reactants or products as a function of time. Stoichiometry of a reaction The stoichiometry is the number of moles of each reactant and product appearing in the balanced reaction equation. Taking the stoichiometry in to consideration the reaction rate is modified and can be defined as the rate of change of the concentration of a reactant or product divided by its stoichiometric coefficient. In the above reaction, the rate (ν) is expressed as ν = -d [N2] /dt = - 1/3 d[H2]/dt = 1/2 d[NH3] /dt The negative sign indicates that the decrease in concentration of one of the reactants. The reaction rate needs to be a positive quantity. Rate laws The rate law is an expression which relates the rate of a reaction to the concentrations of the chemical species present, which includes reactants, products, and catalysts i.e. ν = k [A]a [B]b [C]c ... Hence the rate is proportional to the concentrations of the reactants. The powers to which each concentration term is being raised is the corresponding order. It may be a zero, a whole number or a fraction. It should be observed that the orders will not reflect the stoichiometry of the reaction equation. Incase of a multi-step process, the overall reaction equation is the net result of all of the elementary reactions in the mechanism. By knowing the sequence of elementary steps that constitutes the mechanism of a reaction, we can deduce the rate law. Conversely, if the mechanism