7.2.3: How to Spot an Acid

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Moving on from water, can we predict whether a compound will be an acid, a base, or neither? We have learned that we can predict many properties of materials by considering their molecular structure. When acids are written in their simplified form (for example HNO₃ or H₂SO₄) it can be very difficult to see any similarities, but if we draw out the Lewis structures some commonalities emerge. Let us take a look at the Lewis structures for several strong acids, such as hydrochloric acid HCl_(aq), nitric acid HNO₃_(aq), and sulfuric acid H₂SO₄_(aq).¹²⁹ What structural feature do these substances have in common? Well, from their formulae it is clear that they all contain hydrogen, but there are many compounds that contain hydrogen that are not acidic. For example, methane (CH4) and other hydrocarbons are not acidic; they do not donate protons to other molecules.

One common feature of acids is that the proton that gets donated (or picked off) is bonded to a highly electronegative atom. This atom is often either an oxygen or a halogen such as chlorine (Cl), bromine (Br), or iodine (I). Once you know what to look for, it is quite easy to spot the potentially acidic sites in a molecule. For example, in the previous figure, you could circle the “vulnerable” hydrogens. The ability to spot donatable hydrogens is a useful skill that allows you to predict properties of more complex molecules. But why is a hydrogen that is covalently bonded to an electronegative element potentially acidic and donatable?

First, let us consider the O—H bond. Based on our discussion of water molecules, we can predict that it is polarized, with a partial positive charge on the H and a partial negative on the O. In water, the H is (on average) also part of a hydrogen bonding interaction with the oxygen of another water molecule. It turns out that it does not take much energy to break the original O—H bond. Remember that H⁺ does not just “drop off” the acid, but at the same time forms a bond with the base molecule. In fact, strong acid–base reactions are typically exothermic, meaning that the new bond formed between the proton (H⁺) and the base is stronger than the bond that was broken to release the H⁺. The released energy raises the temperature of the surroundings. In an aqueous solution of a strong acid, hydrogen ions are moving rapidly and randomly from one oxygen to another. The energy for all this bond-breaking comes from the thermal motion of water molecules.

We must also consider what happens to the oxygen that gets left behind. When the acidic hydrogen is transferred, it leaves behind the electrons that were in the bond, giving that atom more electrons than it started with. The species left behind must be stable even with those extra electrons (the negative charge). In the example below, chloride ion Cl^–_(aq) is left behind when the proton gets transferred away. We know chloride is stable and common. It is not surprising that it is one of the products of the reaction.

HCl _(g) + H₂O _(l) ⇄ H₃O⁺_(aq) + Cl^– _(aq)

acid base acid conjugate base

conjugate

If you recall, electronegativity is a measure of the ability to attract (and retain) electrons¹³⁰. Therefore, it makes sense that a negatively-charged, electronegative atom (like chlorine or oxygen) will be more stable than a negatively-charged, less electronegative atom (like carbon).

Questions to Answer

What other atoms besides chlorine or oxygen are electronegative enough to stabilize those extra electrons?
Draw the reactions of each of the strong acids with water: (HCl_(aq)), nitric acid (HNO₃ _(aq)), sulfuric acid (H₂SO_{4 (aq)}), hydrogen bromide (HBr_(aq)), and hydrogen iodide (HI_(aq)). What are the commonalities? What are the differences?
Draw the structures of methanol (CH3OH), acetic acid (CH3COOH), and methane (CH4) and write a potential reaction with water. Label the conjugate acid–base pairs.
Which reactions do you think are likely to occur? Why?

Questions for Later

What other methods (besides having a strongly electronegative atom) might be available to stabilize the electrons (recall that one model of bonding allows for molecular orbitals that extend over more than two atoms)? We will return to this idea later.

References

¹²⁹ In strong acids, the proton is completely donated to water in aqueous solution (i.e., there is no detectable amount of un-ionized acid in the water).

¹³⁰ Recall also that electronegativity stems directly from the effective nuclear charge on a particular atom. If you don’t remember why, go back to chapter 2 and review this important idea.