Writing Reaction Equation for Polyprotic Acids/Bases

[Manali Ghadiali 1E]

In the course reader, page 158, in order to determine the overall reaction, there are two steps shown. In the first equation, one proton is donated and then another in the second equation. By adding these equations together, the final (overall reaction) equation is found.

Does this need to be done to determine the overall reaction for all polyprotic substances or can the reaction equation be found in one step?

[Sangita_Sub_3H]

Hi! So, in order to figure out the overall reaction for polyprotic substances, it would probably be easier to use two steps in order to balance the overall reaction equation and to figure out the K overall. However, two steps are not necessary; if you know how many H+ ions the substance accepts or donates, then as long as you balance the resulting equation appropriately, you can do it in one step. Hope this helps!

[Janice Kim 3I]

Writing out the two steps for polyprotic acids is usually very helpful in visualizing what cancels out and what is added together. However, if you are just finding the K value for the overall equation, you can just multiply the individual Ka or Kb values.