Acid strengths and the role of water

Acid-base reactions are essentially competitions of two acceptors (bases) for a proton. Thus when we say that HCl acts as a strong acid in water, we mean that H2O is a much stronger base than is Cl . Similarly, HCN is a weak acid in water because the proton is is able to share the lone pair electrons of the cyanide ion CN more effectively than it can with H2O . Thus the reaction 

Notice that H2O is shown in two places on this diagram. Near the top it is the conjugate base of H3O+, while near the bottom it is an acid whose conjugate base is OH . HCl acts as a strong acid in water because protons can fall from HCl to H2O, producing H3O+. This fall is so favorable energetically that we say HCl is a strong acid a 1 M solution of HCl is really a 1 M solution of H3O+. 

Contrast the case of HCl with that of HCN. The HCN/CN pair lies below the H2O/H3O+ line. If HCN is to donate a proton to water, in input of energy is needed to boost the proton up to the level of 

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