True or false? (explain) (a) Because Arrhenius equation contains the gas constan
ID: 532090 • Letter: T
Question
True or false? (explain) (a) Because Arrhenius equation contains the gas constant R, the Arrhenius equation applies only to gas - phase reactions. (b) The Arrhenius equation holds exactly. (c) The pre - exponential factor A has the same units for all reactions. (d) In homogeneous catalysis, the catalyst does not appear in the rate law. (e) In homogeneous catalysis, doubling the catalyst concentration will not change the rate. (f) A catalyst does not appear in the overall reaction. (g) In homogeneous catalysis, the catalyst does not appear in any of the steps of the mechanism.Explanation / Answer
Too many questions, answering as many possible.
a. False. Although R is called gas constant it has applicability on liquid or solid also. R was first discovered during study of gases thats why its called gas constant. Arhenius equation is applicable for many liquid phase reactions.
b. False. Arhenius equation is an empirical(determined experimentally) equation to show the change of reaction rate with change of temperature.
Rate, k = Ae-E/RT ....1)
A is pre-exponential factor, T is temperature, R is gas constant, E is Arhenius activation energy. Though most of the reaction follow the equation, few examples are there which totally disobey it. e.g explosion reaction, enzyme catalysed reactions etc. As the equation is experimental, even if a reaction follows it, the value may change little from the expected values of reaction rate.
c. False. From the equation 1) we can see that unit of A is same as the unit of reaction rate as the exponential part does not have any unit. Now different order reaction has different unit. So A will not have same unit for all kind of reactions.
d. False. In homogenous catalysis the catalyst remain in the same phase as that of reactant. In case of a simple substrate the mechanism is as follows:
S + C = A + B (equilibrium reaction)
A + D = P + E
The substrate (S) forms complex (A) with the catalyst (C) very fastly requiring low energy of activation. The intermediate complex A either decomposes into product directly with reataining the catalyst, or by reacting with another D molecule. Either way the substrate concentration is needed to determine the rate of reaction.
e. False. As the catalyst concentration is present in the rate law, its change will obviously change the rate.
f. True
g. False
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