Why is the reaction going left?

[Counta]

In the book Handbook of Soil Science: Properties and Processes it says that the reaction NH₄⁺ <--> NH₃ + H⁺ is going to the left, consuming protons. It mentions something with the reactions Pk_a = 9.5 but doesn't explain further why it will go towards ammonium. Can someone explain this for me?

[Elite Engineer]

The Pk_a is a measure of the acid dissociation constant, meaning: The higher the value, the more tightly the protons are held, and the lower the value, the less tightly they're held.

 

In other words, High Pk_a = weak acid (Amien group); Low Pk_a = strong acid (HCl). 9.5 is kinda low, but not very strong. So Im guessing since it's not a very strong acid, protons would more likely bond to the NH₃ thna not.

 

 

Relative to chemical equilibrium, the reaction could be balancing out due to high concentration on the right side of the equation, I guess...

 

~EE

[hypervalent_iodine]

A small point: it's pKa, not Pka. The letters and their capitalisation (or lack thereof) have specific meanings. K is for equilibrium, k is used to represent reaction rates and p means -log10 of whatever it is to the left of.

 

A pKa of 9.5 is not low. Ammonia (NH3) is a weak base and its conjugate acid, ammonium (NH4+), a weak acid. Whether the equilibrium outlined goes towards the right or left depends on the pH of the solution. Understanding why it goes to the left can be explained in a few ways, but it is first necessary to understand what is meant by pKa in reference to pH. At the point where pH = pKa, exactly half of the acid has dissociated into its conjugate base. When the pH is lower than the pKa, it means that there is more H+ on the product side and so the reaction is pushed towards the left (according to Le Chatilier's principle). Similarly, when the pH of the solution is above the pKa, there is less H+ and so the reaction will go to the right. Since the OP referenced soil, we can probably assume the pH is lower than 9.5. Thus, the reaction would be pushed to the left and you have more of the ammonium ion.

[Counta]

Thanks!
Just to make sure; because the pH is lower than the reactions pKa (which it most likely is) the reaction will go left?

[John Cuthber]

Yes. (all other things being equal)

and you can calculate the extent of the reaction too.

http://www.chemteam.info/AcidBase/Henderson-hasselbalch-problems.html