PROPERTIES OF MATTER 39 



lively strong tendencies are strong acids, and those with relatively less 

 ability to lose protons are weak. Hydrogen chloride is a strong acid 

 because it transfers essentially all its protons when placed in \vater. 



HCl + H,0 -^ H3O+ + Cl- 



Hydrochloric acid becomes a strong acid because of the high propor- 

 tion of hydrogen ion present, this ion in turn being a powerful proton 

 donor. In aqueous systems, the strong acids transfer most of the avail- 

 able protons to water, forming a high proportion of hydrogen ions. 

 Weak acids fail to do this, and their aqueous dissociations are written 

 as reversible equilibria. 



CH3COOH + H.O ±^ H3O+ + CH3COO- 



In this typical case only about 1 per cent (depending on the concen- 

 tration and temperature) of the molecides of acetic acid has reacted 

 with water. Such behavior is characteristic of weak acids, many being 

 far weaker than even acetic acid. 



In a similar way there are weak and strong bases. Ordinarily the 

 strong bases always exist in ionic forms even in the absence of water, 

 and only one of the ions present is the actual base. For example, 

 sodium hydroxide in all ordinary states is composed of sodium and 

 hydroxyl ions, the latter actually functioning as the base although the 

 compound as a whole is commonly called a base. In fact, hydroxyl ion 

 is a very powerfid acceptor of protons and thus a strong base. Weak 

 bases have a relatively limited affinity for protons, as illustrated by 

 chloride ion, which cannot compete effectively with water for protons 

 in hydrochloric acid solutions. Aqueous ammonia is a weak base 

 because only about 1 per cent of the ammonia molecules is capable 

 of taking protons from water. 



H2O + NH3 ±^ NH4+ 4- OH- 



Some acids can donate two or more protons, and some bases can 

 take up two or more. Phosphoric acid and the corresponding phos- 

 jDhates serve as examples. 



Acid Base Acid Base 



H3PO4 + H2O ±^ H3O+ + H2PO4- 

 H2PO4- + H2O ±^ H3O+ + HP04= 

 HP04= + HoO ±^ H3O+ + P04= 



Thus H3PO4 may yield up to three protons, depending on the acceptor 

 present, and P04= may acquire up to three protons, depending on the 



