Conjugate Acids and Conjugate Bases

 Hello! I am happy to be with you again today as we continue to consider the lesson that we started over the weekend about bases. Yesterday, I showed you that water could act either as an acid or a base depending on the circumstance. 


Let us now bring back the two reactions that we looked at in the lesson yesterday and see how they help us to go deeper in our study of bases and acids.

HA(aq) + H2O(l) --> H3O^+(aq) + A^-(aq)

NH3(aq) + H2O(l) --> NH4^+(aq) + OH^-(aq)


What is the relationship between HA and A^- as well as H2O and H3O^+? What is the relationship between NH3 and NH4^+ as well as H2O and OH^-? You will need to look very closely at each pair of species in order to make out the answer.


Do you notice that A^- is formed when HA looses a proton? Do you notice that H3O^+ is formed when H2O gains a proton? Do you notice that NH4^+ is formed when NH3 gains a proton? Do you notice that OH^- is formed when H2O looses a proton.


Now take your mind back to the Brønsted – Lowry definition of acids and bases;

- An acid donates protons

- A base accepts protons


Thus we can conclude that;

A^- is the conjugate base of HA since it only needs to gain a proton to form HA

H3O^+ is the conjugate acid of H2O since it only needs to loose a proton to form H2O.



From our discussion above, can you guess the relationship between;

- NH4^+ and NH3? 

- H2O and OH^-?

I would specially recognize anyone who gave the first correct answer in tomorrow’s lesson.


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