thanks Dr.Bob, I got that question. I also have another question that I am having trouble on.
I am being asked to arrange the oxides in each of the following groups in order of increasing basicity:
(a) K2O, Al2O3, BaO
(b) CrO3, CrO, Cr2O3
I know the answers are (a)Al2O3 < BaO < K2O and (b) CrO3 < Cr2O3 < CrO but I do not understand why these are the answers. How do you get these answers?
Thanks
I look at the acid strengths. Two rules you can remember is that acidity increases from left to right in the periodic table (due to electronegativity) in the same row and from top to bottom in the same column (due to size). So the left side elements (oxides in this case) form stronger bases and the right side oxides form stronger acids. For oxoacids, the stronger acid belongs to the compound (same central element) with more oxygen atoms; i.e., HClO < HClO2 < HClO3 < HClO4.
H2SO3 < H2SO4
HNO2 < HNO3
etc.
To determine the order of increasing basicity for the oxides, you need to consider their chemical properties and reactions with water.
In general, basicity refers to the ability of a substance to donate or accept protons (H+ ions). Basic oxides react with water to form hydroxide (OH-) ions, while acidic oxides react with water to form hydronium (H3O+) ions.
For the given groups of oxides:
(a) K2O, Al2O3, BaO
To determine their basicity, we need to consider how they react with water.
K2O: It reacts with water to form KOH, which is a strong base and dissociates completely in water, producing OH- ions. Therefore, K2O is a basic oxide.
Al2O3: It does not react with water to form OH- or H3O+ ions. Hence, it is considered as amphoteric oxide, which can act as both acidic and basic depending on the circumstances.
BaO: It reacts with water to form Ba(OH)2, which is a strong base. Similar to K2O, BaO is a basic oxide.
Therefore, the order of increasing basicity for (a) is Al2O3 < BaO < K2O, where Al2O3 is the least basic and K2O is the most basic.
(b) CrO3, CrO, Cr2O3
Again, we need to consider their reaction with water to determine their basicity.
CrO3: It reacts with water to form H2CrO4, which is an acid. Therefore, CrO3 is an acidic oxide.
CrO: It reacts with water to form H2CrO3, which is also an acidic oxide. Similar to CrO3, CrO is an acidic oxide.
Cr2O3: It does not react with water to form OH- or H3O+ ions. Hence, it is considered as an amphoteric oxide, which can act as both acidic and basic depending on the circumstances.
Therefore, the order of increasing basicity for (b) is CrO3 < Cr2O3 < CrO, where CrO3 is the most acidic and CrO is the least acidic.
I hope this helps clarify the reasons for the order of increasing basicity in the given oxides.
To determine the order of increasing basicity for the given oxides, you need to consider their chemical properties and how they react with water.
Basicity refers to the ability of a substance to accept protons (H+ ions) or donate electron pairs. In the case of oxides, basicity can be understood as their ability to react with water to form a base.
Let's analyze each group separately:
(a) K2O, Al2O3, BaO
To determine the basicity of these oxides, we need to consider the nature of the cations (positively charged ions) present in them. The more electronegative the cation, the more it attracts electron density towards itself, reducing the basicity of the oxide.
In this case:
- K2O contains potassium cations (K+), which are relatively less electronegative compared to other cations present in the group.
- BaO contains barium cations (Ba2+), which are more electronegative than potassium cations but less electronegative than aluminum cations.
- Al2O3 contains aluminum cations (Al3+), which are the most electronegative among the three cations present in this group.
Since the basicity of an oxide increases with the electronegativity of the cation, the order of increasing basicity is Al2O3 < BaO < K2O.
(b) CrO3, CrO, Cr2O3
Similarly, to determine the basicity order of these oxides, we need to consider the cations present in them.
In this case:
- CrO3 contains chromium cations (Cr6+), which are highly electronegative.
- CrO contains chromium cations (Cr6+) in a lower oxidation state.
- Cr2O3 contains chromium cations (Cr6+) in an even lower oxidation state.
Again, based on the electronegativity of the cations, the basicity order can be determined as CrO3 < Cr2O3 < CrO.
Overall, when comparing oxides within a group, you need to consider the electronegativity of the cations present in them. The more electronegative the cation, the lower the basicity of the oxide.