Tiêu đề 4.CHEMICAL KINETICS.pdf ✅

ChemContent | Chemical Kinetics ChemContent 1 CHEMICAL KINETICS Rate of Chemical Reaction Rate of Chemical Reaction • Alternatively, the rate of reaction can also be expressed as • Consider a hypothetical reaction, assuming that the volume of the system remains constant. R → P One mole of the reactant R produces one mole of the product P. • If [R]1 and [P]1 are the concentrations of R and P at time t1 and [R]2 and [P]2 are their concentrations at time t2, then The rate of increase in concentration of any one of the products. The rate of reaction is the change in concentration of a reactant or productin unit time. The rate of decrease in concentration of any one of the reactants.
ChemContent | Chemical Kinetics ChemContent 2 The square brackets in the above expressions are used to express molar concentration. • Δ[R] is a negative quantity because the concentration of reactants is decreasing. • Equations 1 and 2 represent the average rate of a reaction, rav. This average rate depends on the change in concentration of reactants or products and the timetaken for that change to occur. Units of Rate of a Reaction • From Equations 1 and 2, it is clear that the units of rate are concentration time−1 . • For example, if concentration is in mol L −1 and time is in seconds, then the units are mol L −1s −1 . • In gaseous reactions, the concentration of gases is expressed in terms of their partial pressures; hence, the units of the rate equation will be atm s −1 . Instantaneous Rate of Reaction • Consider the hydrolysis of butyl chloride (C4H9Cl). • We have provided the concentrations over different intervals of time below.
ChemContent | Chemical Kinetics ChemContent 3 • We can determine the difference in concentration over different intervals of time, and thus, we determine the average rate by dividing Δ[R] by Δt. • It can be seen from experimental data that the average rate falls from 1.90 × 10−4 mol L −1s −1 to 0.4 × 10−4 mol L−1s −1 . • However, the average rate cannot be used to predict the rate of reaction at a particular instant as itwould be constant for the time interval for which it is calculated. • Hence, to express the rate at a particular moment of time, we determine the instantaneous rate. • It is obtained when we consider the average rate at the smallest time interval, say dt, when Δt approaches zero. Therefore, for an infinitesimally small dt, the instantaneous rate is given by • By drawing the tangent at time t on either of the curves for the concentration of R versus time t orconcentration of P versus time t and calculating the slope of the curve, we can determine the instantaneous rate of reaction. • Hence, in this example, rinst at 600 s is calculated by plotting the graph of the concentration of butylchloride as against time t. • A tangent is drawn on the curve at a point t = 600 s.
ChemContent | Chemical Kinetics ChemContent 4 • Now consider a reaction, Here, the stoichiometric coefficients of the reactants and products are the same; hence, the rate of reaction is given as Therefore, we can say that from the above equation that the rate of disappearance of any of the reactants is the same as the rate of appearance of the products. • Consider another reaction, In this reaction, two moles of HI decompose to produce one mole each of H2 and I2, i.e. the stoichiometric coefficients of reactants or products are not equal to one; hence, we need to divide the rate of disappearance of any of the reactants or the rate of appearance of products by their respective stoichiometric coefficients. Because the rate of consumption of HI is twice the rate of formation of H2 or I2, to make them equal, the term Δ[HI] is divided by 2. The rate of this reaction is given by • For a gaseous reaction at constant temperature, concentration is directly proportional to the partial pressure of a species, and hence, the rate can be expressed as the rate of change in partial pressure of the reactant or the product. Instantaneous Rate of Reaction