1. Acid & base and redox properties of elements
Basic acid & base concepts
ARRHENIUS, BRØNSTED-LOWRY AND LEWIS THEORIES
FACTORS INFLUENCING ACID-BASE STRENGTH
ACID-BASE TRENDS IN THE PERIODIC TABLE
Carefully analysing all trends and behaviors described in the previous sections we can easily observe clear trends in acidity and basicity based upon where the elements lie in the Periodic Table.
It is clear that when we move along a row from left to right in the Periodic Table there is a transition from basic to acid properties. Let’s examine what happens in period 3 which is in agreement with this behavior.
Sodium, the first element of the row, is present as sodium ion, Na+ , for all pH intervals while calcium, the next element in the row, is forming calcium hydroxide at higher pH values. Aluminium exhibits amphoteric properties, due to the fact is present in solution as Al3+ for low pH values and as the aluminate ion [Al(OH)4]− ,at higher pH values. When we move at intermediates pH, aluminium is in the form of the insoluble hydroxide, Al(OH)3 . In the case of silicon, cationic solutions are not formed even at low pH values, instead forming hydrated SiO2 . When we move at strongly alkaline pH it dissolves to form different ions such as [SiO(OH)3]− ; and [SiO2(OH)2]− .The next elements of the row, phosphorus, Sulphur and chlorine are all non-metals and none of them are found in solution as a free cation. Their oxides react with water to form the corresponding oxoacid. If we compare oxoacids of much higher oxidation state, e.g. phosphoric acid, sulphuric acid and perchloric acid, the strength of this oxoacids increases along the row as was discussed in the previous section.
We can take a similar approach about how the acid and base properties vary along a column or group in the Periodic Table. The general behaviour is that descending the column elements turn more basic. We see this pattern if we examine the pka values of the oxoacids with the highest oxidation state of elements of group 15. While nitric acid is a strong acid, phosphoric and arsenic acids are weak with similar values of pka , antimonic acid is amphoteric whilst the corresponding bismuth compound, Bi(OH)3 is weakly basic.
Another important correlation can be established between the oxidation state and the acid or base properties. The higher the oxidation state the higher their acidic properties. This effect is highly notorious for elements of the transition series not covered in this course.
ACID-BASE PROPERTIES OF OXIDES
The oxides of the elements can be classified as: acidic, basic, amphoteric or neutral.
Acidic oxides are those oxides which dissolve in water to give an acidic solution (e.g. P2O5 , N2O5 , Cl2O7 , etc.) or an oxide insoluble in water but which dissolve in excess alkali (e.g. SiO2 ).
Basic Oxides are those which dissolved in water give basic solutions (e.g. Na2O , CaO , etc.) or insoluble in water and which dissolve in excess acid (e.g. MgO )
Amphoteric oxides are usually insoluble in water but dissolve in both alkali and acids. (e.g. Al2O3 , BeO , Bi2O3 )
Neutral Oxides which do not interact with any other oxide, dissolve in water to give neutral solution or are insoluble at any pH (e.g CO , NO )
Basic oxides are the anhydrides of strong bases as illustrated by the following equilibria:
These basic oxides are combinations of oxygen with electropositive metals of group 1 and 2. They are ionic compounds while covalent oxides are the combinations of oxygen with most of the nonmetallic elements of the Periodic Table. These covalent oxides are acidic and they are the anhydride of the corresponding acid, as the following equations show
One of the most typical reactions of the oxides is the reaction of an acid and a base oxide to form a salt. For example the following reactions to produce two important compounds involve this type of reaction:
In the case of an amphoteric oxide such as Al2O3 or ZnO , they will react with both acids and base, as the for example:
It is remarkable to note that if we compare the oxides of the elements of the second row, there is a transition from the more basic oxides on the left to the more basic oxides to the right. The acidic character of non-metallic oxides increase with the increase in oxidation number of the elements. It is also interesting to consider a similar trend along a column, as can be illustrated in group 14. CO2 is a weak acid but down the group the oxide becomes increasingly ionic and less acid and PbO2 is amphoteric.
THE HARD AND SOFT ACIDS AND BASES: THE HSAB CONCEPT
It seems reasonable and useful to establish an unambiguous way to order the basic and acid character of Lewis acids and bases in a similar way as we did with BrØnsted-Lowry species in the previous section. According to the Lewis theory of acid and bases, they react as shown in the following equation to form the corresponding complex
Thus, we can establish a similar scale to that presented in the previous section if we can rationalise the interaction between Lewis acids and bases.
The more favorable this interaction the more stable the complex will be. However, it turns out to be an extremely difficult and challenging task because the coordination between Lewis acids and bases can vary widely. For example, BCI3 shows greater acidity than BF3 with respect to bases such as NH3 while toward weaker bases such CO , BF3 is a stronger acid than BCI3 . In order to deal with this situation a number of qualitative relationships were developed to categorise Lewis acids and bases. In 1963 American chemist R. G. Pearson suggested a systematic form to approach this situation based on the concepts of hardness and softness. According to the descriptions given by Pearson, hard acids are those having high positive charge, small size and the absence of outer electrons. They have a low polarizability (the ability to distort the electron cloud of a specie), while the soft acids have the opposite characteristics. A hard base usually has high electronegativity, low polarizability, and are difficult to oxidize while soft bases have the opposite characteristics. According this classification Pearson establishes that the most favorable interactions occur when the acid and the base have similar electronic character. This is the HSAB principle, and can be summarised as hard acids preferring hard bases and soft acids preferring soft bases.
A number of Lewis acids and bases have been assigned to hard or soft groups according what we have explained above (see table 1), although it is hard to determine the exact line of separation of both categories and therefore some species are considered as intermediate or borderline cases
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