Reaction Kinetics

Reaction Kinetics

Learning Outcomes: Candidates should be able to explain and use the terms rate of reaction, rate equation, order of reaction, rate constant, half-life of a reaction, rate-determining step, activation energy, catalysis Construct and use rate equations of the form: rate = k[A]^m[B]^n (limited to simple cases of single step reactions and of multi-step processes with a rate-determining step, for which m an n are 0, 1, 2) including 1. deducing the order of a reaction by the initial rates method 2. Justifying for zero- and first- order reactions, the order to reaction from concentration-time graphs 3. Verifying that a suggested reaction mechanism is consistent with the observed kinetics 4. predicting the order that would result from a given reaction mechanism 5. calculating an initial rate using concentration data 1. Show understanding that the half-life of a first-order reaction is dependent of concentration 2. use the half-life of a first-order reaction in calculations Calculate a rate constant using the initial rates method devise a suitable experimental technique for studying the rate of a reaction, from given information explain qualitatively, in terms of collision, the effect of concentration changes on the rate of a reaction Show understanding, including reference to the Boltzmann distribution,of what is meant by the term activation energy explain qualitatively, in terms both of the Boltzmann distribution and of collision frequency, the effect of temperature change on a rate constant (and hence, on the rate) of reaction 1. Explain that, in the presence of a catalyst, reaction has a different mechanism, i.e., one of lower Ea, giving a larger rate constant 2. Interpret this catalytic effect on a rate constant in terms of the Botlzmann distribution outline the different modes of action of homogeneous and heterogeneous catalysis including: 1. The Haber Process 2. The catalytic removal of oxides of nitrogen in the exhaust gases from car engines 3. The catalytic role of atmospheric oxides of nitrogen in the oxidation of atmospheric sulfur dioxide 4. The catalytic role of Fe3+ in the I-/S2O82- reaction Describe enzymes as biological catalysts which may have specific activity Explain the relationship between substrate concentration and the rate of an enzyme-catalysed reaction in biochemical systems

Introduction: Reaction Kinetics is the study of the rates of chemical reactions, which include the factors that affect them and the mechanisms by which the reactions occur. Rate of reaction can be defined as the change in concentration of a particular reactant or product per unit time. It is a positive quantity that expresses how the concentration of a reactant or a product changes with time. Normally it is convenient to express reaction rates in mol1dm-3s-1. Instantaneous rate is the rate at a particular time i.e., the rate of a particular instant during the reaction. (The steeper the gradient, the faster is the reaction.) Average rate is the change in concentration of a reactant or a product over a time interval. Initial rate is the instantaneous rate at time t=0. It is the instantaneous rate at the start of the reaction, when an infinitesimally small amount of reactant has been used up. It is obtained by measuring the gradient of the tangent drawn to the curve at time t=0. It can be approximated by the average rate provided: 1. the time interval is small enough and 2. the time interval starts from t=0. have to draw out ***

Rate Equation and its components Relating rate and reactant concentration (have to draw out) The rate equation for a reaction is a mathematical expression that shows the exact dependence of the reaction rate on the concentrations of all the reactants; it relates the rate of reaction to the concentration of reactants raised to the appropriate power Rate constant is the constant of proportionality in the rate equation. Order of reaction with respect to a given reactant is the power to which the concentration of that reactant is raised in the rate equation. The overall order of a reaction is the sum of the powers of the concentration terms in the rate equation. Half-life (or t1/2) of a reaction is the time taken for the concentration of a reactant to fall to half its initial value.

Zero-order reactions A zero-order reaction is one in which the rate is independent of [reactant], i.e., the rate is unaffected by changes in the concentration of the reactant A --> Products => rate=k, where k is the rate constant. In this case, the reaction proceeds at a constant rate k at a constant temperature First-order reactions A first-order reaction is one in which the reaction rate is directly proportional to the concentration of a single reactant (say A) raised to the power of one i.e, reaction rate inversely proportionate to [A] A --> Product => rate = k[A]. where k is the rate constant. t1/2 is constant throughout reaction. t1/2= (In2)/k = (0.693)/k Second-order reactions A second-order reaction is one in which the reaction rate is proportional to the product of the concentrations of two reactants or to the concentration of a single reactant raised to the power of 2 2A --> products where overall order of reaction is 2 => rate = k[A]^2 2nd t1/2 = 2 x 1st t1/2 Pseudo-order reactions Presence of a large excess of a solvent Solvent is a reactant Presence of a catalyst

Reaction Mechanism of a reaction is a collection of elementary steps in the proper sequence showing how reactant particles are converted into products. The reaction mechanism is the explanation of how a reaction takes place. A proposed reaction mechanism must be consistent with the observed kinetics. it must satisfy both the stoichiometric equation and the rate equation (basically theoretical fulfillment) Elementary steps means the distinct steps in which all chemical reaction takes place in. It cannot be broken down into yet simpler processes. Each elementary step describes a single molecular event that involves breaking and/or making covalent bonds, such as one reactant particle decomposing or two reactant particles colliding and combining. Molecularity of an elementary step in a reaction mechanism is the number of reactant particles taking part in that step unimolecular step involves one reactant species in that step, e.g., H2O2 --> 2OH bimolecular step involves 2 reactant species in that step termolecular step involves 2 reactant species in that step (some termolecular ES occur, but they are extremely rare because the probability of 3 particles colliding simultaneous