Monoprotic and Polyprotic Acids

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  • 0:04 Definition and Examples
  • 1:21 Monoprotic Acids in Action
  • 3:20 Polyprotic Acids in Action
  • 4:36 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Deduce the difference between monoprotic and polyprotic acids, then learn about how acids break apart and what that means for acid strength. Then, assess your new knowledge with a quiz.

Definition and Examples

According to both the Arrhenius and Bronsted-Lowry definitions, acids are compounds that contain hydrogen ions. When these compounds are added to a solution, the hydrogen ions break free or dissociate. The easier the hydrogens pop off, the stronger the acid is considered to be.

In this table are common monoprotic and polyprotic acids.

Monoprotic Acid Examples Polyprotic Acid Examples
HCl H2 SO4
HBr H3 PO4

Monoprotic and polyprotic acids are types of acids that contain one or more hydrogen ions, which are often referred to as protons. Can you figure out the differences between monoprotic and polyprotic acids? If you've been studying chemistry for a while, you've probably encountered the prefixes mono- and poly- a few times. You might even recognize that mono- means one and poly- means many. Does this give you a clue?

Monoprotic acids, like HCl and HBr, contain just one hydrogen ion. Polyprotic acids, like H2 SO4 and H3 PO4, contain two or three hydrogen ions.

It's tempting to think that polyprotic acid are stronger than monoprotic acids because they contain multiple hydrogen ions, but that's actually not true. Sometimes a polyprotic acid only lets go of one hydrogen and keeps its other hydrogen ions attached. Keep going to find out why.

Monoprotic Acids in Action

In water, monoprotic acids lose their hydrogen to the surrounding water molecules, making a hydronium ion (H3 O+). What's left of the acid after the hydrogen is lost is called the conjugate base. You can see how HBr (aka hydrobromic acid) reacts with water.

Hydrobromic acid (HBr) dissociation.
HBr Dissociation

The hydrogen in HBr joins with water making H3 O+ and Br- is left behind. HBr is a monoprotic acid that's considered to be a strong acid because it readily loses its hydrogen ion in solution. Acids that do not readily lose their hydrogens in solution are considered to be weak acids.

In general, the extent that a monoprotic acid loses its hydrogen depends on how attracted the hydrogen is to the rest of the compound. A monoprotic acid like HBr loses its hydrogen quite easily because the bromide ion (Br-) and the hydrogen ion are not that attracted to each other. Meanwhile, a monoprotic acid like HF (hydrofluoric acid) doesn't like to break apart at all. The hydrogen and the fluoride ion (F-) are very attracted to one another. Thus, HF is considered to be a weak acid.

We measure the likelihood of hydrogens breaking off using something called the acid dissociation constant (Ka). This value is both unique and constant for every acid. In general, the higher the Ka value, the higher the likelihood of hydrogen ions breaking off in solution. Since more hydrogen ions in solution means a stronger acid, higher Ka values mean stronger acids. Lower Ka values mean weaker acids.

The dissociation constant for a strong acid like HBr is so high it's often just referred to as 'large.' This means nearly 100% of the acid breaks apart in solution. The dissociation constant for a weak acid like HF is 7.2 x 10-4. That means hardly any of the acid is breaking apart in solution.

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