Video: Identifying the Acid-Base Conjugate Pair in a Set of Pairs of Chemical Formulas

Which of the following is an acid–base conjugate pair? [A] Na₂CO₃ and CO₃²⁻ [B] C₂H₄ and C₂H₆ [C] NO₃⁻ and HNO₃ [D] K⁺ and Br⁻ [E] H₂ and H⁺

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Video Transcript

Which of the following is an acid–base conjugate pair? (A) Na2CO3 and CO32−, (B) C2H4 and C2H6, (C) NO3− and HNO3, (D) K+ and Br−, (E) H2 and H+.

When you have an acid or base that’s in water, the acid or base can react with the water. In this case, we have hydrofluoric acid, which would react with water to form H3O+, or the hydronium ion, and F−, or the fluoride ion. Sometimes you’ll see this type of reaction simplified like this, where we just have the hydrofluoric acid dissociating to form the H+ ion and the F− ion. But this is just a simplification as the H+ ion would immediately react with water to form hydronium. And there wouldn’t be any H+ ions remaining in the solution after a very short period of time.

If we look in detail at this reaction, we’ll notice that hydrofluoric acid loses a proton, or the H+ ion, over the course of the reaction. This proton that the hydrofluoric acid loses is gained by the water when it forms hydronium. Since hydrofluoric acid loses a proton over the course of the reaction, it’s a Brønsted–Lowry acid, which is a chemical species that donates a proton.

Similarly, water in this reaction would be a Brønsted–Lowry base since it’s a chemical species that accepts a proton over the course of the reaction. So when a Brønsted–Lowry acid or a Brønsted–Lowry base reacts, we’ll end up with a pair of chemical species that differ from each other by a proton, which we call an acid–base conjugate pair.

So in our reaction, our hydrofluoric acid and the fluoride ion are an acid and a base that differ from each other by a proton. The same is true for water and the hydronium ion. Again, they are an acid and a base that differ from each other by a proton. So they’re also an acid–base conjugate pair.

So we can identify an acid–base conjugate pair by finding an acid and a base that differ from each other by a proton or the H+ ion. We can see the same thing in the reaction of the weak base ammonia with water, which would form hydroxide and the weak acid ammonium. Ammonia and ammonium differ from each other by a proton. So they’re an acid–base conjugate pair. Same would be true for water and the hydroxide ion.

So now that we understand what an acid–base conjugate pair is, let’s look through our answer choices. Our first answer choice is Na2CO3, or sodium carbonate, and CO32−, which is the polyatomic anion carbonate. Though both of these have the carbonate ion in common, and carbonate can act as a base, these two chemical species clearly do not differ by a proton. So they’re not an acid–base conjugate pair.

Our next answer choice is C2H4, which is called ethene, and C2H6, which is called ethane. Both are organic molecules. Though these two molecules are similar and differ from each other by some hydrogens, neither of them will act as an acid or a base in water. So this is not the answer choice that we’re looking for either.

Our next answer choice is NO3–, which is the polyatomic anion nitrate, and HNO3, which is called nitric acid. Nitric acid is a strong acid that can react with water to form the hydronium ion and nitrate. Nitric acid is definitely an acid. And HNO3 and NO3– differ from each other by a proton. This makes them an acid–base conjugate pair, which means that answer choice (C) is the answer that we’re looking for.

It’s worth mentioning that since nitric acid is a strong acid, its conjugate base, NO3–, won’t be a very good base since the conjugate bases of strong acids are very weak bases. This means that the nitrate ion will react much less than the conjugate bases of weaker acids, like the conjugate base of the hydrofluoric acid that we were looking at earlier, the fluoride ion.

So we’ve identified our correct answer. But let’s quickly look through our two remaining answer choices just so we understand why they’re not an acid–base conjugate pair. Answer choice (D) features the potassium cation and the bromide anion. These two chemical species clearly do not differ by a proton, which is why answer choice (D) is incorrect.

Our final answer choice is H2, or hydrogen gas, and H+, or the hydrogen cation. Though this answer choice may look tempting, since the difference between H2 and H+ is a hydrogen, it would in fact be the hydrogen anion, not a proton. Additionally, H2 gas is not an acid or a base. When dissolved in water, no reaction occurs. So H2 and H+ are not an acid–base conjugate pair.

Of our answer choices, the only one that was an acid–base conjugate pair was NO3– and HNO3, and answer choice (C).

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