Chemical kinetics

• Chemical Kinetics is the branch of Chemistry that deals with the rate of chemical reaction, factors affecting the rate and the mechanism of a reaction.
• The rate of a chemical reaction is the change in concentration of any one of the reactants or product in unit time.
• The rate of a chemical reaction can also be expressed as the rate of decrease in concentration of any one of the reactants or the rate of increase in concentration of any one of the product.
• The proper orientation of reactant molecules leads to bond formation.
• To account for this, a probability or steric factor (P) is introduced, resulting in the equation Rate (r) = PZ AB e − Ea/RT.
• A is related to collision frequency in the Arrhenius equation.
• Improper orientation of reactant molecules causes them to bounce back and no products are formed.
• The rate of a bimolecular elementary reaction, A + B → Products, can be expressed as Rate (r) = Z AB e − Ea/RT, where Z AB represents the collision frequency of reactants, A and B and e − Ea/RT represents the fraction of molecules with energies equal to or greater than Ea.
• In collision theory, activation energy and proper orientation of the molecules together determine the criteria for an effective collision and hence the rate of a chemical reaction.
• For a chemical reaction, if ∆x is the change in concentration of reactant or product in time interval ∆t, the rate of reaction (r) = ∆x/∆t.
• For a hypothetical reaction R → P, the rate of reaction can be expressed by decrease in concentration of reactant or by increase in concentration of product.
• The rate of reaction (r) = Rate of disappearance of R r = Decrease in concentration of R Time taken Or, r = –∆ [R] ∆ t [Since, concentration of reactant decreases with time, ∆ [R] is negative.
• The rate of reaction cannot be negative, hence it is multiplied with –1.
• Different ordered reactions have different integrated rate law equations.
• The units of rate constant for different ordered reactions are: Zero order reaction: mol/L-1 s-1, First order reaction: 1 s-1, Second order reaction: 2 mol-1 L s-1.
• An example of a first order reaction is the thermal decomposition of HI on gold surface.
• For a first order reaction, the rate of the reaction is proportional to the first power of the concentration of the reactant.
• The constant of integration in the equation for a first order reaction is C, which can be evaluated when the initial conditions are known.
• The constant of integration in the equation for a zero order reaction is C, which can be evaluated when the initial conditions are known.
• The rate expression for a zero order reaction is r= - d[R] and the rate law is r= k[R] 0.
• Zero order reactions are reactions in which the rate of reaction is independent of concentration of the reactants.
• Integrated Rate Equations are equations relating the rate of a reaction and concentration of reactants.
• An example of a zero order reaction is the decomposition of gaseous ammonia on a hot platinum surface at high pressure.
• The rate expression for a first order reaction is r= - d[R] and the rate law is r= k[R].
• The differential rate equation for a zero order reaction is d[R] = - k.dt and on integrating it, [R] = - kt + C.
• For an nth order reaction, the rate is given by the equation: k[A]^n.
• The differential rate equation for a first order reaction is - d[R] = k[R] dt and on integrating it, - ln[R] = kt + C.
• The unit of rate constant is mol/L/sec, which can be rewritten as mol/L^1-n s-1.
• The rate of reaction = Rate of appearance (formation) of P = Increase in concentration of P Time taken i.e., r = ∆[P] ∆ t.
• The rate expressions are also called Average rate of reaction.
• For a gaseous reaction, at constant temperature, concentration is directly proportional to the partial pressure.
• The rate of reaction can also be expressed as the rate of change in partial pressure of the reactant or product.
• If concentration is expressed in mol/L and time is in second, the unit of rate of reaction is mol/L/s [mol L - 1 s - 1 ].
• In general the dimension of rate of reaction is concentration/ time.
• For gaseous reaction, the concentration is replaced by partial pressure and so the unit of rate of reaction is atm/s.
• The rate of a reaction at a particular instant of time is called Instantaneous rate of a reaction.
• If we plot a graph between ln k against 1/T, we get a straight line graph with the slope of the graph being -Ea and the y-intercept being ln A.
• Collision Theory was developed by Max Trautz and William Lewis and is based on kinetic theory of gases.
• A catalyst is a substance that increases the rate of a reaction without itself undergoing any permanent chemical change.
• In the Arrhenius equation, the factor e-Ea/RT corresponds to the fraction of molecules that have kinetic energy greater than Ea.