# How to predict whether a salt will act as an acid or as a base in aqueous medium? [closed]

How to would we know if a salt such as $$\ce{CO_2}$$ reacts acidic or basic in water.

The explanation I see everywhere is that you have to see the acid and base that makes up the salt, in the case of $$\ce{NH_4Cl}$$ it would be a weak base and strong acid making the solution acidic, but is there a better theory for that?

(This method would fall apart too easily. If the acid, base or strength of both is unknown, this method doesn't help.)

• On adding $\ce{CO2}$ in water $\ce{H2CO3}$ is formed which is a weak acid thus the solution will be acidic. – Harsh Wasnik Jul 16 '19 at 18:36
• Generally the oxides of non metals are basic in nature like $\ce{SO3 , SO2, NO2 , NO3, CO2}$ thus on hydrolysis the give acidic solution. – Harsh Wasnik Jul 16 '19 at 18:38
• Similarly oxides and hydroxides are basic in nature – Harsh Wasnik Jul 16 '19 at 18:41
• More advanced theory won't help you with fewer data. Quite the opposite, in fact. – Ivan Neretin Jul 16 '19 at 21:59

You mentioned the strengths of the acids and bases can be used to determine whether the salt is acidic or basic. This is largely correct because a strong acid has a very weak conjugate base, while a weak acid has a relatively strong conjugate base; the same applies to bases and their conjugate acids. There are a few other rules of thumb though that can be used to predict whether a salt is acidic or basic.

## Oxides

In general, metallic oxides (base anhydrides) are basic in aqueous solution. For example, calcium oxide, $$\ce{CaO}$$, produces a basic solution when dissolved in water: $$\ce{CaO + H2O -> Ca(OH)2}$$

Nonmetal oxides (acid anhydrides) are acidic in aqueous solution. Since carbon dioxide, $$\ce{CO2}$$, is a nonmetal oxide, it is acidic in solution: $$\ce{CO2 + H2O <=> H2CO3}$$

## Cations

The conjugate acids of weak bases will act as proton donors. For example, $$\ce{NH4+}$$ can donate a proton to water: $$\ce{NH4+ + H2O <=> NH3 + H3O+}$$

Additionally, small, highly charged metal cations are also acidic because they can act as Lewis acids, with water acting as a Lewis base. For example, $$\ce{Fe^3+}$$ ions can form the $$\ce{[Fe(H2O)6]^3+}$$ ion. The electrons in the water molecules are pulled toward the metal ion, making the $$\ce{O-H}$$ bonds more polar. Thus, water molecules are more likely to become deprotonated, donating a hydrogen ion to nearby water molecules. For example: $$\ce{[Fe(H2O)6]^3+ + H2O -> [Fe(H2O)5(OH)]^2+ + H3O+}$$

Cations of group 1 and 2, as well as those with a charge of $$+1$$, are neutral in aqueous solution. They are too large or have too little charge to polarize the nearby water molecules to any significant extent.

There are no basic cations because protons are repelled by the positive charge.

## Anions

There are very few acidic anions, because protons are attraced to the negative charge. However, hydrogen sulfate ($$\ce{HSO4^2-}$$) and dihydrogen phosphate ($$\ce{H2PO4-}$$) are among the few exceptions.

The conjugate bases of strong acids are neutral. Because the acids are so strong, their conjugates are very weak. Examples include $$\ce{Cl-}$$ and $$\ce{NO3-}$$.

The conjugate bases of weak acids are basic. Because the acids are weak, their conjugates will be stronger. Examples here include $$\ce{F-}$$ and $$\ce{NO2-}$$.

## The Acidity or Alkalinity of the Salt

To predict whether a salt will produce an acidic or basic solution when dissolved in water, we have to consider both the cation and the anion. In some cases, these rules of thumb are insufficient, such as with ammonium nitrite, $$\ce{NH4NO2}$$; the cation is acidic while the anion is basic. In this case, it's necessary to just carry out the equilibrium calculations.

Source: Atkins, Peter and Loretta Jones. Chemical Principles: The Quest for Insight, 5th edition.