Acid and Base Strength

From here on out we will be discussing and describing Brønsted-Lowry acids and bases. As a reminder acids under this definition are proton donors or they give up their H+ ions to the environment or other species. While Brønsted-Lowry bases are protons acceptors or they take up H+ ions from the environment or other species.

What is Strength?

Not all acids and bases are created equally though. Some give up their protons easily while others hold onto them more tightly. When an acid gives up its protons super easily we say it dissociates completely into solution because the protons just fall off. Since the acidity of a substance is determined by its ability to produce H+ ions, acids that freely dissociate are strong acids. In contrast, weak acids only dissociate partially because these acids hold onto their protons more strongly.

The same is also true of bases but here we are measuring a species ability to accept a hydrogen rather than lose one. In this case, bases that readily accept hydrogens are considered strong while those who don’t are classified as weak.

Note: We have to be careful here when distinguishing between the terms dissolve and dissociate. While an acid or base might readily dissolve into a solution based on its solubility it might not easily dissociate because it is quite weak.

Both HCl and butanol are polar so they will dissolve in water. However, only HCl will dissociate since butanol is too weak to lose its hydrogen.

Acid Strength Effects

With this in mind, multiple factors can be used to determine whether an acid is weak or strong before observing its behavior in solution. The most fundamental is the stability of a compound and its conjugate. A conjugate is the resulting chemical species produced when an acid or base undergoes an acid-base reaction.

Therefore bases produce conjugate acids since they gain a hydrogen and now have one to donate converting them into their acidic form.

The green highlight shows the acidic and basic spot on acetic acid. Here we can see that acetate on the reactants side (left) is missing an H so it is a base. Once it has gained an H it is converted into its acidic form.

While acids form conjugate bases since they will now have an open space to accept a hydrogen converting them into their basic form.

Here we are looking at the reverse reaction of the one shown above. In this case we transition from the acid form to the conjugate base form since a hydrogen is lost in this reaction.

If the conjugate base of an acid is particularly stable that acid is said to be strong since losing its hydrogen often results in a more stable compound. In contrast, the opposite is often true for weak acids whose conjugates are significantly more unstable.

HCl is a strong acid and forms Cl- a far more stable conjugate base this means that HCl spontaneously loses its H+. Ethanol on the other hand produces a really unstable base making the reaction non-spontaneous.

When evaluating basicity it is easiest to look at the base and assess its stability. Here unstable bases want to find a hydrogen and reach a more stable state making them stronger while weaker bases are fairly stable without a hydrogen.

Cl- the conjugate base of HCl is super stable and is a really weak base the butanol ion on the other hand is super unstable and makes it an excellent base

Electron Withdrawing Groups

In both organic and inorganic acids, electron-withdrawing and donating groups alter the acidity and basicity of compounds by stabilizing or destabilizing different chemical species.

When an acid loses an H+ ion it will be left with a negative charge on its conjugate. Electron-withdrawing groups help stabilize this negative charge making the conjugate base more stable and increasing the acidity of the compound. In contrast, an electron-donating group would destabilize the negative charge and decrease the acidity of the compound.

For acids greater conjugate base stability equal greater acidity and vice versa.

When looking at the strength of a base the same applies except it is easiest to look at the relative stability of the base itself rather than the conjugate acid. Since unstable bases quickly grab up hydrogen ions to form more stable conjugate acids strong bases have electron-donating groups. While the opposite occurs with electron-withdrawing groups resulting in a weaker base.

In either case, the closer the electron-donating or withdrawing group is to the acidic or basic spot on the chemical species the greater the effect.

Cl is an electron withdrawing group and here we can see that as the Cl gets closer to the acidic OH the acid strength increases.

How can we tell if something is an electron withdrawing or donating group though? Electronegativity. Groups that are more electronegative than the atom the H is bonded to will be electron withdrawing while less electronegative ones will be electron donating.


Resonance also affects the acidity and basicity of a reactant. Here acids with resonance are more acidic while bases with resonance are less basic. This occurs as electrons are withdrawn into a partial double bond stabilizing the molecule. This is why carboxylic acids are particularly strong for an organic acid anyway and why amides are particularly weak bases.

Must Know Acids and Bases

While it is helpful to be able to look at an acid or base and determine its relative strength on the basis of its chemical formula there are a handful of acids and bases you need to memorize. The list includes the six strongest acids and the six strongest bases.

Strong AcidsStrong Bases
HCl, HBr, HI, H2SO4, HNO3, HClO4LiOH, NaOH, KOH, Ca(OH)2, Sr(OH)2,Ba(OH)2

Although we are only memorizing twelve compounds this list also includes information about weak acids and bases. Basically, if it isn’t one of the twelve it’s weak.


Acids and bases can also act as electrolytes when they dissociate into their component parts since they generally produce charged species. Let’s look at both a strong acid and a strong base to see how this works.

\[HCl\;(aq)\rightleftharpoons H^+\; (aq)\; + \; Cl^-\; (aq)\]
\[NaOH\;(aq)\rightleftharpoons Na^+\; (aq)\; + \; OH^-\; (aq)\]

Since both HCl and NaOH will completely dissociate into their charged products they will act as strong electrolytes with the ability to conduct electricity.

In contrast, weak acids and weak bases barely dissociate so the number of charged molecules in solution is quite low. As a result, solutions made from weak acids and bases are poor conductors of electricity.

We have to be careful here though since we can be tricked into thinking that a weak acid will make an excellent conductor. How you ask? Dissolving and dissociation are two very different things. Plenty of compounds will dissolve because they are polar. This doesn’t mean they will break apart into ions that conduct electricity. Let’s take a look at the previous example to see why.

Only the left solution will conduct electricity even though both compounds dissolve in water.

This is why it is much safer to swim in the ocean during a thunderstorm as compared to the community pool. In the ocean, the high salt content of the water makes it an excellent conductor. Since electricity takes the path of least resistance it is more likely to flow through the water and not you. In contrast in the pool, you are the most conductive material and the lighting strike prefers to course through your body and not the water.

Concept Check: Acid-Base Strength