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

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In summary, the conversation discusses the exchange of NH4+ and NH3 with H2O, and the relationship between the rate constants (k1, k-1) and (kf, kr) in the equilibrium reaction. The participants also mention the potential presence of H+ and OH- in the solution and their solvation by water molecules, but ultimately conclude that these details do not affect the understanding of acid/base reactions.
  • #1
alex_alex
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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?
 
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  • #2
alex_alex said:
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.
 
  • #3
Borek said:
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?
 
  • #4
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?
 
  • #5


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.
 

1. What is an acid/base equilibrium?

An acid/base equilibrium refers to the balance between the concentration of acid and base molecules in a solution. In this equilibrium, both acid and base molecules are constantly exchanging protons (H+) with each other, resulting in a stable pH level.

2. How does the exchange of protons occur in an acid/base equilibrium?

In an acid/base equilibrium, the exchange of protons occurs through a process called dissociation. This is when an acid molecule donates a proton to a base molecule, forming a conjugate base and a conjugate acid. This exchange continues until a balance is reached.

3. What factors can affect the acid/base equilibrium?

The concentration of acid and base molecules, temperature, and the presence of other substances can all affect the acid/base equilibrium. For example, adding more acid will shift the equilibrium towards the acid side, while adding more base will shift it towards the base side.

4. How is the equilibrium constant (Keq) related to the acid/base equilibrium?

The equilibrium constant is a measure of the ratio of products to reactants at equilibrium. In the case of an acid/base equilibrium, it represents the balance between the concentration of acid and base molecules. A higher Keq value indicates a stronger acid or base, while a lower Keq value indicates a weaker acid or base.

5. What is the significance of an acid/base equilibrium in real-life applications?

An acid/base equilibrium is essential in many biological and chemical processes, such as digestion, blood pH regulation, and water treatment. Understanding the principles of acid/base equilibria can also help in predicting and controlling the behavior of different substances in various reactions.

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