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The higher the proton affinity, the stronger the base and the weaker the conjugate acid in the gas phase.The (reportedly) strongest known base is the ortho-diethynylbenzene dianion (E pa = 1843 kJ/mol), [3] followed by the methanide anion (E pa = 1743 kJ/mol) and the hydride ion (E pa = 1675 kJ/mol), [4] making methane the weakest proton acid [5] in the gas phase, followed by dihydrogen.
As an example, consider the gas-phase reaction NO 2 + CO → NO + CO 2.If this reaction occurred in a single step, its reaction rate (r) would be proportional to the rate of collisions between NO 2 and CO molecules: r = k[NO 2][CO], where k is the reaction rate constant, and square brackets indicate a molar concentration.
An example of a simple chain reaction is the thermal decomposition of acetaldehyde (CH 3 CHO) to methane (CH 4) and carbon monoxide (CO). The experimental reaction order is 3/2, [4] which can be explained by a Rice-Herzfeld mechanism. [5] This reaction mechanism for acetaldehyde has 4 steps with rate equations for each step :
The order of reactivity, as shown by the vigour of the reaction with water or the speed at which the metal surface tarnishes in air, appears to be Cs > K > Na > Li > alkaline earth metals, i.e., alkali metals > alkaline earth metals, the same as the reverse order of the (gas-phase) ionization energies.
The order of reaction is a number which quantifies the degree to which the rate of a chemical reaction depends on concentrations of the reactants. [2] In other words, the order of reaction is the exponent to which the concentration of a particular reactant is raised. [2]
n th-order reaction (r = kC A n), where k is the reaction rate constant, C A is the concentration of species A, and n is the order of the reaction; isothermal conditions, or constant temperature (k is constant) single, irreversible reaction (ν A = −1) All reactant A is converted to products via chemical reaction; N A = C A V
Usually, this mechanism is used in gas phase decomposition and also in isomerization reactions. An example of isomerization by a Lindemann mechanism is the isomerization of cyclopropane. [11] cyclo−C 3 H 6 → CH 3 −CH=CH 2. Although it seems like a simple reaction, it is actually a multistep reaction: cyclo−C 3 H 6 → CH 2 −CH 2 −CH ...
A similar set can be constructed for reactions with higher order stoichiometry in which case the excess varies predictably over the course of the reaction. While e may be any value (positive, negative, or zero) generally positive or negative values smaller in magnitude than one equivalent of substrate are used in reaction progress kinetic analysis.