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The d-And-f-Block Elements
Question
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Which one of the following compounds is a peroxide? 

  • KO2

  • BaO2

  • MnO2

  • NO2

Answer

B.

BaO2

In peroxides, the oxidation state of O is -1 and they give H2O2, with dilute acids, and have peroxide linkage.
In KO2, 
+1 + (x  X 2) = 0
x = -1/2 (thus, it is a superoxide, not a peroxide.)
In BaO2, + 2 + (x  X 2) = 0
x = -1
Thus, it is a peroxide. Only it gives H2O2 when reacts with dilute acids and has peroxide linkage as 
Ba2+ [ O- O]2-
  Peroxide linkage
In MnO2 and NO2, Mn and N exhibit variable oxidation states, thus, the oxidation state of O in these is -2. Hence, these are not peroxides. Thus, it is clear that among the given molecules only BaO2 is a peroxide.

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Some More Questions From The d-And-f-Block Elements Chapter

In the series Sc (Z = 21) to Zn(Z = 30), the enthalpy of atomisation of zinc is the lowest i.e., 26, kJ mol. Why?

The E° (M2+/M) value for copper is positive (+ 0.34 V). What is possibility the reason for this? 

How would you account for the irregular variation of ionisation enthalpies (first and second) in the first series of the transition elements?

Why is the highest oxidation state of a metal exhibited in its oxide or fluoride only?

Which is a stronger reducing agent Cr2+ or Fe2+ and why?

Explain why Cu+ ion is not stable in aqueous solutions?

Actinoid contraction is greater from element to element than lanthanoid contraction. Why?

On what ground can you say that scandium (Z = 21) is a transition element but zinc (Z = 30) is not?

The outer electronic configuration of copper is 3d10 4s1, yet it is considered transition element. Why?