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Consider the following oxides of chromium: CrO2, CrO3, Cr2O3 a. Calculate the oxidation state of cr in each of the above-mentioned oxides. b. Fully explain how changes in oxidation number of transition metal oxides can affect both their covalent character as well as their acidity in aqueous solution.
Accepted Answer
a. The oxidation states of Cr in the given oxides are:
CrO2: +4
CrO3: +6
Cr2O3: +3
b. The oxidation state of a transition metal significantly influences the covalent character and acidity of its oxides. Here's why:
Covalent Character: Higher oxidation states lead to a greater polarization of the metal-oxygen bond due to the higher positive charge on the metal. This increased polarization enhances the covalent character of the bond. For instance, CrO3 (Cr+6) is more covalent than Cr2O3 (Cr+3).
Acidity in Aqueous Solution: Metal oxides generally exhibit acidic behavior in aqueous solutions. This acidity increases with the oxidation state of the metal. Higher oxidation states result in a stronger pull of electrons towards the metal, weakening the O-H bond in the hydrated oxide and facilitating the release of H+ ions, leading to increased acidity. For example, CrO3 is a strong oxidizing agent and forms chromic acid (H2CrO4) in water, whereas Cr2O3 is relatively less acidic.
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