How to predict the strength of acids and bases

Here is a general guide to see if we are faced with strong or weak an acid.

They are strong acids:

  • All the hydrogen halides (compounds in which a hydrogen atom is bonded to a halogen) except HF, hydrofluoric acid: HI, HBr, HCl → strong acids
  • All oxyacids (compounds with the general formula H x NmeO y) in which the number of oxygen atoms exceeds by two or more the number of acidic protons. For example: H 2 SO 4 4 = number of oxygen atoms; number of protons ionizable = 2; difference (nO - nH) = 2

      For example: HNO 3, H 2 SO 4, HClO 4 → strong acids

They are weak acids:

  • Hydrofluoric acid, HF
  • All oxyacids in which the number of oxygen atoms is equal to or exceeds by 1 the number of acidic protons.

For example: H 3 PO 4, HClO, HClO 2, H 3 BO 3 → weak acids

  • Much of carboxylic acids (organic acids with the general formula RCOOH)

For example: HCOOH (formic acid), CH 3 COOH (acetic acid), C 6 H 5 COOH (benzoic acid) weak acids

They have strong bases:

  • soluble oxides and hydroxides (compounds containing O 2- or OH -)

This category includes two classes of elements:

The alkali metals, which are the elements of the first group of the periodic table. Damage soluble hydroxides of the general formula Me 2 O, and hydroxides of the formula Me (OH). Oxides and hydroxides of lithium, sodium, potassium, rubidium, cesium → strong bases

The alkaline earth metals, which are the elements of the second group of the periodic table. Damage MeO soluble oxides and hydroxides Me (OH) 2 only strontium, barium and calcium → strong bases

Are weak bases:

  • The ammonia and amines.

In general those compounds in which a nitrogen atom (N) exhibits a doublet of not sharing (lone pair) that can accept a proton.

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