2.2 Reaction Kinetics
Reaction kineticsA Branch of Chemistry and Thermodynamics that studies the rate of chemical reactions and their influencing factors. play a vital role in establishing the mechanism of a chemical reaction, and aiding the design of suitable equipment, such as reactors, as well as informing the operating conditions and control methods of such machinery. This lesson will introduce the concepts used to achieve this.
Reaction Order and Molecularity
The classification of a reaction is based on the reaction kinetics, where a reaction can be one of four mechanisms:
- Irreversible
- Reversible
- Simultaneous
- Consecutive
This classification is related to the reaction order, which can be related to the molecularityNumber of molecules coming together to react in an elementary reaction. of the reaction. The molecularity is the number of molecules coming together to react in an elementary reaction. The reaction order is sometimes identical to the molecularity, but in other terms may be a zero order reaction, where the rate of reaction does not vary with concentration. The Power Law Rate Equation shown in Equation 1 is used to determine the reaction order, where the sum of the exponents for each reactant concentration is the reaction order.
\(r = k C^p_a C^q_b\)
The half life (\(t_{1/2}\)) is the time taken for the quantity of a substance to decay to half of its original amount. For first order and second order reactions, the relationship between an initial concentration of a substance (e.g. \([A]_0\)) is relatively simple. The derivation for both first in Equation 2.
\(- \frac{d[A]}{dt} = k[A]\)
\(\int_{[A]_0}^{[A]} \frac{1}{[A]} d[A] = -\int_{0}^{t} k dt\)
\(ln(\frac{[A]}{[A]_0}) = -kt\)
\(\text{At} \: t_{1/2} \: [A] = \frac{[A]_0}{2}\)
\(\text{Thus} \: ln(2) = kt_{1/2}\)