To estimate the absolute rate coefficient k_11, the pseudo first order constant

Question

To estimate the absolute rate coefficient k_11, the pseudo first order constant is a) multiplied times the concentration of the reactant in excess. b) multiplied times the concentration of the reactant in defect. c) divided times the concentration of the reactant in defect. d) divided times the concentration of the reactant in excess. Q(24) Which temporal profile is expected to be linear when observing a reaction under pseudo first order conditions? a) [A]^-1 vs t b) [A] vs t c) ln[A] vs t d) ln[A] vs t^-1 Q(25) Why is necessary to have the concentration of one of the reactants in great exce comparison with the other one to establish pseudo first order conditions? a) to assume that the concentration of the reactant in defect is constant over time b) to assume that k' is constant over time c) to assume that K" is constant over time d) to assume that the concentration of the reactant in great excess is constant over time Q(26) What are possible shortfalls of working under pseudo first order conditions? a) introducing a systematic error b) if the reactant in excess contains impurities that are reactive toward the reactant in de these impurities can play a significant role in determining a faster constant c) the disappearance of one of the two reactants is not observed d) all of the above are shortfalls of working under pseudo first order conditions Q(27) What is a possible indicator that the pseudo first order conditions are not met? a) an absolute rate coefficient close to the literature value b) a ln[A] vs t plot non linear c) the disappearance of at least 90 % of the reactant in defect d) all of the above are shortfalls of working under pseudo first order conditions

Explanation / Answer

23) option C

Let us consider a pseudo first order reaction A + B -----> products

whose rate expression should be rate = kII [A][B]

However one of the componentsB concentration is in large excess, such that practically there is no change in its concentration during reaction , that is [B] is assumed to be constatnt.

Thus to get absolute value of kII we have to dividekI with [B] as kI = kII[B]

24) option C

since the reaction is under peudo first order conditions the integrated rate equation is

k = 2.303/t [ ln a/a-x] which on expansion gives ln (a-x) = ln(a) - kt which is like y=mx +c graph.

Thus a plot between ln[A]t vs. t will be a straight line with negative slope and the intercept being ln[A]o

25) option D

One component is taken in large excess, so that its change during the reaction is assumedd constant.

26) option c

27) option b

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