How Acid & Base Structure Affect pH & pKa Values

Instructor: Alexandra Carpenter

Alexandra has an MA in English and a BA in history and film studies.

This lesson covers how molecular structure affects the pH and pKa of a compound. Resonance, atomic size, electronegativity, and inductive effects are covered, with examples of each.

Acid Review

Ever wondered why you can drink the acid found in orange juice, but not the acid of your car battery?

These two acids have different strengths. It turns out that the molecular structure of the compound can influence the strength of the acid. Let's find out more about how structure affects the strength of the acid.

To determine how acidic a substance is, a chemist can measure the pH, which is based on how much of the acid is in the solution. The lower the pH, the more acidic the solution.

If you have just a little bit of a strong acid, like sulfuric acid, in solution, it can have a higher pH (be less acidic) than if you have lots of a weak acid like citric acid in solution. The amount of the acid matters.

Chemists can also use pKa to evaluate acidity, which is not based on the amount of acid. The pKa value describes how many acidic ions a substance will produce in a solution.

For example, say you have two solutions with the same number of molecules of sulfuric acid in one and citric acid in the other. The sulfuric acid solution will be more acidic because the sulfuric acid loses more protons to produce more hydronium (H3 O+ ) ions.

Similar to pH, the lower the pKa, the stronger the acid. So, sulfuric acid has a lower pKa than citric acid (-3 vs 3.1).

There are millions of chemical substances in the world, and it would be a real pain to measure all the pKa values for each one. So, we look at similarities in the structure to help us predict these values.

Resonance

Acids exist in an equilibrium with their conjugate base. The strength of the acid (pKa) depends on the stability of the base. When the proton leaves the acid, it leaves behind its electrons. Those are super negative, and there is a big negative charge on the conjugate base.

You know that if there is a concentrated negative charge, the base is not very stable. However, if the charge can be spread out, then the base is more stable, which means we have a stronger acid. If a molecule has electrons that are spread over more than one bond, it has resonance.

So, which do you think has the lower pKa: propanol or propanoic acid?


Which molecule has a lower pKa?
n-propanol versus propanoic acid


If you chose propanoic acid, you are right! When propanoic acid loses a proton, two oxygen molecules share the negative charge, giving it resonance and, therefore, stabilizing the base and making for a strong acid. When propanol loses a proton, there is no resonance.


When the proton is removed from propanoic acid, the negative charge is shared between two oxygen atoms, so it is a stronger acid than propanol
proton loss


Atomic Size

Remember, the stronger acid has a more stable base. One thing that influences base stability is the size of the ion. A larger ion can accommodate a negative charge better. Imagine that the charge has more 'room' to spread out.

Luckily, we can use periodic trends to predict the size of the ion. This means we can predict the more stable base and therefore, the stronger acid. Ion size increases as we go from top to bottom of a column on the periodic table. (Ion size also increases from right to left, but that does not influence base stability due to electronegativity, which we'll talk about in a moment).


Atom size increases as you go down a column on the periodic table
periodic table


So, which has the lower pka, hydrochloric acid (HCl) or hydrobromic acid (HBr)?

If you answered hydrobromic acid you are right! Br- is larger than Cl-. This means Br- is a more stable base, so HBr is a stronger acid. The pKa of HBr is -9, while the pKa of HCl is -7.

Electronegativity

Remember that electronegativity is the ability of an atom to attract electrons. Electronegativity increases as you go from left to right across a row of the periodic table. It also increases as you go up a column, but the size of the atom influences acidity more. Imagine it like a fight. Electronegativity 'wins' horizontally on the periodic table, while atom size 'wins' vertically.

If an atom is very electronegative, it can accommodate the extra electrons that are left behind when the acid loses a proton. Therefore, the base is more stable.


Acidity of a molecule increases with increasing electronegativity as you go from left to right on the periodic table. As the size increases down a column, the acidity also increases.
trend


So, which is a stronger acid HCl or H2 S?

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