Stepwise ionization is the process by which a polyprotic acid loses its protons one at a time in sequential steps. Each step has its own distinct ionization constant.
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Polyprotic acids undergo multiple ionizations, each with a different equilibrium constant ($K_a$).
The first ionization constant ($K_{a1}$) is always stronger than subsequent ones ($K_{a2}$, $K_{a3}$, etc.).
In solutions of polyprotic acids, the concentration of hydrogen ions from each step must be considered to find the total $[H^+]$.
Common examples of polyprotic acids include sulfuric acid ($H_2SO_4$), phosphoric acid ($H_3PO_4$), and carbonic acid ($H_2CO_3$).
The pH of a solution containing a polyprotic acid can often be approximated by considering only the first ionization step.
Review Questions
What is meant by stepwise ionization in the context of polyprotic acids?
Why is $K_{a1}$ always larger than $K_{a2}$ for a given polyprotic acid?
How do you determine the total hydrogen ion concentration in a solution of a polyprotic acid?
Related terms
Polyprotic Acid: An acid that can donate more than one proton per molecule.
$K_a$ (Acid Dissociation Constant): A quantitative measure of the strength of an acid in solution, specifically how completely it dissociates into ions.
$pH$: A scale used to specify the acidity or basicity of an aqueous solution, defined as $-\log[H^+]$.