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Understanding how ionic strength/primary salt effect affects hydrolysis/transiti
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K.L.
Understanding how ionic strength/primary salt effect affects hydrolysis/transiti
To start with, I don't think that website is very clear, and it looks like there are some ambiguities. Google kinetic salt effect and look at some of the other sites you find. Try this, for example: http://www.chem.boun.edu.tr/wp-content/uploads/2014/04/EXP1-Primary-Kinetic-Salt-Effect.pdf Quote
First things first is ionic strength and primary salt effect different things?
Yes. Ionic strength is a property of an ionic solution, defined by I = (1/2)Σmizi2 where mi is the concentration of ions of charge zi. A solution has an ionic strength irrespective of whether any reactions are going on in it. Kinetic salt effect is a kinetic phenomenon in which the rate of a reaction changes with changes in the ionic strength of the solution. I'm not sure about their explanation in terms of entropy. Enthalpy is involved as well; solvation results in a decrease in enthalpy and a decrease in entropy. If solvation occurs, that means ΔGsolv is negative; the enthalpic effect outweighs the entropic. Let's consider the two following reactions: A+ + B [A...B]+ products A+ + B- [A...B] products where the species in square brackets is the transition state. In the first case, A and TS both being charged, will both be stabilised by an increase in ionic strength, due to the presence of more counterions. To a first approximation, the free energy difference between reactants and TS will not be much changed, and nor will the rate constant. In case 2, both A and B are stabilised, while TS, being uncharged, is not. Thus ΔG of activation increases with ionic strength, and rate constant decreases. The next example is different, involving an intermediate in equilibrium with reactants. (You do know the difference between an intermediate and a transition state?) If this is so, that implies that the second step (E products) is rate-determining. Under these circumstances, what matters for the equilibrium is the free energy difference between the reactants and the intermediate - not the transition state between them. All that guff about "the rate at which the reactants come together" is irrelevant - what matters is the relative rates of the forward and reverse reactions, not their absolute values (as long as they are faster than step 2 - if step 2 is fast, there is no real equilibrium in step 1). As I say, you have been unfortunate in your choice of website. Try searching further.
To start with, I don't think that website is very clear, and it looks like there are some ambiguities. Google kinetic salt effect and look at some of the other sites you find. Try this, for example: http://www.chem.boun.edu.tr/wp-content/uploads/2014/04/EXP1-Primary-Kinetic-Salt-Effect.pdf Quote
First things first is ionic strength and primary salt effect different things?
Yes. Ionic strength is a property of an ionic solution, defined by I = (1/2)Σmizi2 where mi is the concentration of ions of charge zi. A solution has an ionic strength irrespective of whether any reactions are going on in it. Kinetic salt effect is a kinetic phenomenon in which the rate of a reaction changes with changes in the ionic strength of the solution. I'm not sure about their explanation in terms of entropy. Enthalpy is involved as well; solvation results in a decrease in enthalpy and a decrease in entropy. If solvation occurs, that means ΔGsolv is negative; the enthalpic effect outweighs the entropic. Let's consider the two following reactions: A+ + B [A...B]+ products A+ + B- [A...B] products where the species in square brackets is the transition state. In the first case, A and TS both being charged, will both be stabilised by an increase in ionic strength, due to the presence of more counterions. To a first approximation, the free energy difference between reactants and TS will not be much changed, and nor will the rate constant. In case 2, both A and B are stabilised, while TS, being uncharged, is not. Thus ΔG of activation increases with ionic strength, and rate constant decreases. The next example is different, involving an intermediate in equilibrium with reactants. (You do know the difference between an intermediate and a transition state?) If this is so, that implies that the second step (E products) is rate-determining. Under these circumstances, what matters for the equilibrium is the free energy difference between the reactants and the intermediate - not the transition state between them. All that guff about "the rate at which the reactants come together" is irrelevant - what matters is the relative rates of the forward and reverse reactions, not their absolute values (as long as they are faster than step 2 - if step 2 is fast, there is no real equilibrium in step 1). As I say, you have been unfortunate in your choice of website. Try searching further.
It might be helpful to ask what happens when two chemical species of the same charge collide as a function of ionic strength, in order to picture what is going on. In the second edition of Frost and Pearson's "Kinetics and Mechanism" there is a discussion on pages 150-51 that might be helpful. They show an equation for the rate constant of two ionic species that is valid for dilute solutions. Bronsted and Bjerrum were the first to derive it, and it appears to be quite similar to the equation that you found. It is only valid for dilute solutions, roughly 0.01 M. However, a modified form of this equation (which looks more like a hyperbola of some kind to my non expert eyes) can be used to 0.1 M: "Kinetic Salt Effect in Pharmaceutical Investigations" JT Carstensen, J. Pharmaceutical Sciences 1970 Aug; 59, pp. 1140-3.
It might be helpful to ask what happens when two chemical species of the same charge collide as a function of ionic strength, in order to picture what is going on. In the second edition of Frost and Pearson's "Kinetics and Mechanism" there is a discussion on pages 150-51 that might be helpful. They show an equation for the rate constant of two ionic species that is valid for dilute solutions. Bronsted and Bjerrum were the first to derive it, and it appears to be quite similar to the equation that you found. It is only valid for dilute solutions, roughly 0.01 M. However, a modified form of this equation (which looks more like a hyperbola of some kind to my non expert eyes) can be used to 0.1 M: "Kinetic Salt Effect in Pharmaceutical Investigations" JT Carstensen, J. Pharmaceutical Sciences 1970 Aug; 59, pp. 1140-3.
QuoteYes. Ionic strength is a property of an ionic solution, defined by I = (1/2)Σmizi2
where mi is the concentration of ions of charge zi.
A solution has an ionic strength irrespective of whether any reactions are going on in it.
Kinetic salt effect is a kinetic phenomenon in which the rate of a reaction changes with changes in the ionic strength of the solution.
I'm not sure about their explanation in terms of entropy. Enthalpy is involved as well; solvation results in a decrease in enthalpy and a decrease in entropy. If solvation occurs, that means ΔGsolv is negative; the enthalpic effect outweighs the entropic.
Let's consider the two following reactions:
A+ + B
A+ + B-
where the species in square brackets is the transition state. In the first case, A and TS both being charged, will both be stabilised by an increase in ionic strength, due to the presence of more counterions. To a first approximation, the free energy difference between reactants and TS will not be much changed, and nor will the rate constant.
In case 2, both A and B are stabilised, while TS, being uncharged, is not. Thus ΔG of activation increases with ionic strength, and rate constant decreases.
The next example is different, involving an intermediate in equilibrium with reactants. (You do know the difference between an intermediate and a transition state?) If this is so, that implies that the second step (E
As I say, you have been unfortunate in your choice of website. Try searching further.
QuoteYes. Ionic strength is a property of an ionic solution, defined by I = (1/2)Σmizi2
where mi is the concentration of ions of charge zi.
A solution has an ionic strength irrespective of whether any reactions are going on in it.
Kinetic salt effect is a kinetic phenomenon in which the rate of a reaction changes with changes in the ionic strength of the solution.
I'm not sure about their explanation in terms of entropy. Enthalpy is involved as well; solvation results in a decrease in enthalpy and a decrease in entropy. If solvation occurs, that means ΔGsolv is negative; the enthalpic effect outweighs the entropic.
Let's consider the two following reactions:
A+ + B
A+ + B-
where the species in square brackets is the transition state. In the first case, A and TS both being charged, will both be stabilised by an increase in ionic strength, due to the presence of more counterions. To a first approximation, the free energy difference between reactants and TS will not be much changed, and nor will the rate constant.
In case 2, both A and B are stabilised, while TS, being uncharged, is not. Thus ΔG of activation increases with ionic strength, and rate constant decreases.
The next example is different, involving an intermediate in equilibrium with reactants. (You do know the difference between an intermediate and a transition state?) If this is so, that implies that the second step (E
As I say, you have been unfortunate in your choice of website. Try searching further.
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