Don't understand what is exchanging with what in this acid/base equili

[alex_alex]

NH4+ + H2O (kr) = (kf) NH3 + H3O+;

the same reaction can be written as:
NH4+ + HO- (kr) = (kf) NH3 + H2O;

where (kf) - rate of forward reaction and (kr) - rate of reverse reaction.

1 Question. NH4+ or NH3 exchanging with H2O?

If NH4+ is exchanging with H2O:
NH4+ (k-1) =(k1) H2O,

where (k1) - rate of exchange from NH4+ to H2O and (k-1) - rate of exchange from H2O to NH4+.


If NH3is exchanging with H2O:
NH3(k-1) =(k1) H2O,

where (k1) - rate of exchange from NH3 to H2O and (k-1) - rate of exchange from H2O to NH3.

2 Question. What is the relationship between k1, k-1 and kf, kr?

 

[Borek]

NH4+ + H2O (kr) = (kf) NH3 + H3O+;

the same reaction can be written as:
NH4+ + HO- (kr) = (kf) NH3 + H2O;

I don't see why you assume it is the same reaction with the same rate constants. They are definitely related to the same equilibrium, but that's all.

 

[alex_alex]

I don't see why you assume it is the same reaction with the same rate constants. They are definitely related to the same equilibrium, but that's all.

At pH=7, I assume that 1st one is the most likely case. I am not sure I understand NH⁴₊ or NH³ is exchanging H⁺ with H₂O?

 

[Borek]

To be honest, I have no idea what the exact mechanism is - at the same time I don't think it matters. Equilibrium doesn't depend on these details, and it is equilibrium that we are typically interested in.

Please note there is no such thing as H⁺ nor OH^- in the solution. H₃O⁺ is only an approximation, as proton is most likely solvated by more than one water molecule. OH^- is solvated as well, so it is present as - at least - H₃O₂^-. We ignore all these and it doesn't interfere with our understanding of acid/base reactions, why should it matter for ammonia?

 

[LudusRex]

The relationship between k1 and k-1 is that they are the forward and reverse rates of the same reaction, respectively. This means that k1 is the rate at which the reactants (NH4+ or NH3) are converted into products (H2O or H3O+) and k-1 is the rate at which the products are converted back into reactants. Similarly, kf and kr are also the forward and reverse rates of the same reaction, respectively. The difference is that kf and kr are more general terms used to represent the rates of any chemical reaction, while k1 and k-1 are specific to this particular acid/base equilibrium reaction.