Hello wonderful people! Today I'm going to explain polyprotic acids. It's quite an interesting topic so let's dive in!
Why you should understand polyprotic acids:
Polyprotic acids are a very complex type of acid. They can donate multiple hydrogen ions and also have the ability to create a buffer system. Understanding how a polyprotic acid behaves in solution and each of the dissociations gives incredible insight into the behavior of both acids and buffer systems.
Key Ideas
- What is a polyprotic acid
- Example of a polyprotic acid
- Ka's of the different dissociations
- Ice charts and polyprotic acids
Defining a Polyprotic Acid
A polyprotic acid is an acid that is capable of donating multiple hydrogen ions. This is not to be confused with acids that have multiple hydrogens. Acids can have multiple hydrogens and not be a polyprotic acid. For example, CH3COOH is a weak acid called acetic acid. It has 4 hydrogens in its chemical formula. However, only one of them is acidic.
If only one of the hydrogens in a formula is acidic, it is typically separated from the other hydrogens. For example, in the formula for acetic acid above three hydrogens appear at the front of the formula while one appears at the end. Only the single hydrogen at the end is acidic.
Examples of Polyprotic Acids
The most common polyprotic acids are H2SO4 and H3PO4. In today's post I'll be using H2SO4, or sulfuric acid, to demonstrate the typical behavior of a polyprotic acid.
To the left, I've shown the dissociation of H2SO4. Sulfuric acid is a strong acid that is capable of donating two hydrogen ions when placed in water. The top reaction shows the first dissociation of sulfuric acid in water.
Since sulfuric acid is a strong acid, the first dissociation is considered "strong". Meaning, the first dissociation goes to completion and no sulfuric acid remains undissociated in solution. However, the HSO4- ion is a weak acid. This means the second dissociation does not go to completion.
Ka's of different Polyprotic Acid dissociations
As I stated in the previous section, the first dissociation is strong. This makes it very easy for a base to "pluck off" or remove a hydrogen ion from the acid.
The second dissociation, however, is weak. This means it is more difficult to remove the second hydrogen from sulfuric acid. Due to the negative charge on the HSO4- ion, the second hydrogen is more tightly bound to the ion because it is a positive charge which partially balances out the large negative charge.
Equilibrium expressions are a ratio of products to reactants. Given the first dissociation is strong, the products are heavily favored. This means that the Ka value for the first dissociation is very large.
Each dissociation after has a Ka value which gets progressively smaller. This is because the intermediate reactants hold onto their hydrogens much more tightly. Therefore fewer products are produced and the ratio of products to reactants becomes smaller.
ICE Charts and Polyprotic Acids
Below is an ICE chart set up for sulfuric acid. Because sulfuric acid is a polyprotic acid, an ICE chart is required for each dissociation. However, the first ICE chart is rather simplistic because the entire concentration of sulfuric acid dissociates. Meaning the x value in the first ICE chart is equivalent to the initial concentration of sulfuric acid.
The second ICE chart is the more typical version of an ICE chart. HSO4- is a weak acid which means it only partially dissociates. The concentration of HSO4- from the first ICE chart is carried through to the second ICE chart. To find the total concentration of H+, the second Ka expression must be solved for x, then the two concentrations of H+ from each ICE chart are added together. Alternatively, the hydrogen ion concentration from the first ICE chart can be carried over to the H+ initial concentration in the second ICE chart.
Summary
- A polyprotic acid is an acid that is capable of donating multiple hydrogen ions.
- Sulfuric acid is a strong acid that is capable of donating two hydrogen ions when placed in water.
- The first dissociation is strong. This makes it very easy for a base to "pluck off" or remove a hydrogen ion from the acid.
- The second dissociation, however, is weak. This means it is more difficult to remove the second hydrogen from sulfuric acid.
- This means that the Ka value for the first dissociation is very large. Each dissociation after has a Ka value which gets progressively smaller.
I hope you've found this post helpful. If you have any questions, feel free to comment on this post or message me. You could also comment on my youtube video description of polyprotic acids, which I've linked below.
- Saren
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