The s-Block Elements

  • Question 161
    CBSEENCH11007139

    Alkaline earth metals are less electropositive than the alkali metals. Explain. 

    Solution

    Alkaline earth metals are less electropositive because of the smaller size and higher ionisation enthalpies of alkaline earth metals as compared to alkali metals.

    Question 162
    CBSEENCH11007140

    Beryllium and magnesium do not give colour to flame whereas other alkaline earth metals do so. Why?

    Solution
    Beryllium and magnesium do not give colour to flame because they (Be and Mg) have a small size and very high ionisation enthalpies. The enthalpy of the flame is not sufficient to excite their electrons to higher energy levels. Other alkaline earth metals impart colour to the flame because they have fairly low ionisation enthalpies, so electrons in their atoms can be easily excited by the enthalpy of the flame. The excited electrons on returning to their original positions lose enthalpy in the form of coloured light.
    Question 163
    CBSEENCH11007141

    How will you explain the reducing character of alkaline earth metals?

    Solution

    Except beryllium, the alkaline earth metals have a fairly strong tendency to lose two electrons to form dipositive ions because of their low ionisation enthalpies and high negative value of standard electrode potentials. Therefore, they act as reducing agents.
    The reducing character of alkaline earth metals increases as we move down the group from Be to Ba because the ionisation enthalpies increase and electrode potentials become more and more negative with increasing atomic number from Be to Ba. 

    Question 164
    CBSEENCH11007142

    Comment on each of the following observations:
    straight E to the power of circled dash space for space straight M to the power of 2 plus end exponent left parenthesis aq right parenthesis space plus 2 straight e to the power of minus space space rightwards arrow space space straight M left parenthesis straight s right parenthesis.
      (where M = Ca,  Sr or Ba) is nearly constant.

    Solution
    straight E to the power of circled dash space of space any space straight M to the power of 2 plus end exponent vertical line space straight M space electrode space depends space upon space three space factors colon(i) Ionisation enthalpy
    (ii) Enthalpy of hydration
    (iii) Enthalpy of vaporisation
    The combined effect of these factors is approximately the same for Ca, Sr, and Ba. Hence, their electrode potentials are nearly constant.

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