LaRita holds a master's degree and is currently an adjunct professor of Chemistry.
In this lesson, we will define polyprotic acids and diprotic acids. We will take a closer look at the mechanism of diprotic acids by studying the dissociation of carbonic acid.
Polyprotic and Diprotic
What's your favorite snack? Cookies? Imagine there exists a machine designed to give you one cookie whenever you turn it on. Let's call it a Cookie Machine. Now imagine a similar machine that is designed to give you not just one, but several cookies when it's activated. We'll call this one a PolyCookie Machine. That may sound like an odd name for our made-up invention, but in chemistry, it's almost exactly the sort of terminology we use to classify certain types of acids. Acids, rather than giving away cookies, give away protons.
A proton is a positively charged hydrogen atom, while an acid is a compound that releases a proton. Acids that are able to release more than a single proton are known as polyprotic acids. Let's take a moment to break down that term: The root '-prot-' is derived from the word 'proton,' and the prefix 'poly-' is Greek for 'many.' Thus, polyprotic acids have - you guessed it - many protons!
One particular kind of polyprotic acid is diprotic acid. If we break down the term 'diprotic' the same as before, the prefix '-di' is Greek for 'two.' Thus, just as the prefix suggests, diprotic acids have two protons to donate. Sulfuric acid (Hsub2SOsub4), hydrogen sulfide (Hsub2S), carbonic acid (Hsub2COsub3), chromic acid (Hsub2CrOsub4), and oxalic acid (Hsub2Csub2Osub4) are all examples of diprotic acids.
While each of the acids listed above has two protons available, they will NOT release them both at the same time. Instead, when diprotic acids dissociate in aqueous solution or react with a base, they give up one proton at a time in what's called a stepwise dissociation.
To get a better understanding of stepwise dissociation, we can study the mechanism of carbonic acid (Hsub2COsub3) in aqueous solution. After Hsub2COsub3 donates one proton, it becomes HCOsub3^- (Equation 1). While HCOsub3^- is the product of Hsub2COsub3 in the first dissociation, it goes on to act as a reactant in the second step. In the second dissociation, HCOsub3^- acid releases the second proton and becomes COsub3^2- (Equation 2).
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I know what you're saying: When the PolyCookie Machine dispenses a second cookie, is it just as good as the first? Better yet, When an acid produces a second proton, is it just as acidic as the first?
Well, for the stepwise dissociation of polyprotic acids, each proton release is considered a separate reaction and therefore, reaches its own equilibrium and has its own acid equilibrium constant, Ksuba. The value of Ksuba for a reaction is determined experimentally and provides us with a general idea about the strength of the acid reacting.
Notice that the Ksuba of the first-step reaction in Equation 1 is larger than the Ksuba of the second-step reaction in Equation 2. This tells us that the acid reacting in step one (Hsub2COsub3) is significantly stronger than the acid reacting in step two (HCOsub3^-), which is characteristic of all diprotic acids. In fact, for any typical diprotic or polyprotic acid, the acid involved in each successive step of the dissociation will be weaker than the acid in the previous step. Since stronger acids are better proton donors, we can conclude that the first proton in a diprotic acid dissociates more easily than the second.
A proton is a positively charged hydrogen atom, while an acid is a compound that releases a proton. Acids that are able to release more than a single proton are known as polyprotic acids. Diprotic acids have the ability to supply exactly two protons, and they do so in a stepwise manner. The first step of the dissociation produces a conjugate base that goes on to act as a weaker acid in the second step.
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