Chapter 8 The d and f Block Elements Solutions
Question - 31 : - Compare the chemistry of actinoids with that of the lanthanoids withspecial reference to
Answer - 31 : - (i) electronic configuration
(ii) atomic and ionic sizes
(iii) oxidation state and
(iv) chemical reactivity.
Answer
(i)Electronicconfiguration: Lanthanoids have general electronicconfiguration of [Xe] 4f1-14 5d0-1 6s2 andactinoids have general electronic configuration of [Rn]5f1-14 6d0-1 7s2.Thus, lanthanoids belong to 4 f-series whereas actinoids belong to 5 f-series.
(ii) Atomic andionic sizes : The atomic size of lanthanoids decreasesfrom lanthanum to lutetium. Though the decrease is not regular, in case ofatomic radii, the decrease in the ionic size (M3+) is regular.Decrease in size between two successive elements is higher in actinoids due topoor screening by 5f electrons.
(iii) Oxidationstate : The most common oxidation state oflanthanoids is +3 while actinoids show more variable oxidation states thanlanthanoids ranging from +3 to +7. The tendency of showing greater range ofoxidation states can be attributed to the fact that the 5f 6d and 7s levels areof comparable energies and larger distance of 5fas compared to 4f from thenucleus.
(iv) Chemicalreactivity : Actinoids are far more reactive thanlanthanoids. They react with non-metals at moderate temperatures whereaslanthanoids react at high temperatures. Most actinoids are attacked by HCl butare slightly affected by HNO3 due to formation of a protectivelayer of oxide and alkalies give no reaction.
Answer - 32 : - (i) Of the d_4 species, Cr_2+ is strongly reducing while manganese(lll) is strongly oxidising.
(ii) Cobalt(ll) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised.
(iii) The d_1 configuration is very unstable in ions
Answer
(i) Both Cr2+ and Mn3+ have d4 configuration, Cr2+ is reducingsince its configuration is converted to d3 from d4. d3 has half filled t28,configuration with higher stability. Mn3+ is oxidising since in changing fromd4 to d5 the configuration becomes half filled which has extra stability,
(ii) Co(II) gets oxidised to Co(III) in presence of complexingagent because Co(III) is more stable than Co(II). Most of the strong fieldligands cause pairing of electrons forming diamagnetic octahedral complexeswhich are very stable due to very large crystal field stabilization energy.
(iii) d1 configurationis very unstable in ions because after losing one more electron it will becomemore stable due to vacant d-orbital. All elements with d1 configurationare either reduced or undergo disproportionation, e.g.,
Answer - 33 : -
Disproportionation reaction involves theoxidation and reduction of the same substance. The two examples ofdisproportionation reaction are
1. 2Cu+ →Cu2+ + Cu
2. 3MnO42- +4H+ → 2MnO4– + MnO2 +2H2O
Question - 34 : - Which metal in the first series of transition metals exhibits +1oxidation state most frequently and why?
Answer - 34 : -
Copper exhibits +1 oxidation state in thefirst series of transition metals because when one electron is lost, theconfiguration becomes stable due to fully filled d10 configuration.
Question - 35 : - Calculate the number of unpaired electrons in the following gaseous ions: Mn3+, Cr3+, V3+ and Ti3+.Which ono of these is the most stable in aqueous solution ?
Answer - 35 : - Which ono of these is the most stable in aqueous solution ?
Answer
Question - 36 : - Give examples and suggest reasons for the following features of thetransition metal chemistry :
Answer - 36 : - (i) The lowest, oxide of transition metal is basic, the highest is amphoteric/acidic.
(ii) A transition metal exhibits highest oxidation state in oxides and fluorides.
(iii) The highest oxidation state is exhibited in oxoanions of a metal.
Answer
(i) Lowest oxidation compounds of transition metals are basicdue to their ability to get oxidised to higher oxidation states. Whereas thehigher oxidation state of metal and compounds gets reduced to lower ones andhence acts as acidic in nature.
e.g. MnO is basic whereas Mn2O7 is acidic.
(ii) Due to high electronegativities of oxygen and fluorine,the oxides and fluorides of transition metals exhibit highest oxidation state.
e.g. OSF6,V2O5
(iii) In oxoanions of metals, the metals from bonds with oxygenand hence are present in their highest oxidation states. For example : Cr formsCrO42- and Cr2O72-, bothcontain chromium in +6 oxidation state.
Permanganate ion, MnO4– containsMn in its highest oxidation state of +7.
Indicate the steps in the preparation of
Answer - 37 : -
1. K_2Cr_2O_7 from chromite ore
2. KMnO_4 from pyrolusite ore.
Answer
Question - 38 : - What are alloys? Name an important alloy which contains some of thelanthanoid metals. Mention its uses.
Answer - 38 : -
An allov is a blend of metals prepared bymixing the components. Alloys are homogeneous solid solutions in which theatoms of one metal are distributed randomly among the atoms of other. Mischmetal is an alloy which contains some of the lanthanoid metals. It contains 95%lanthanoid metals, 5% iron and traces of S, C, Ca and Al. Mischmetall is usedin Mg-based alloy to produce bullets, shell and lighter flint.
Question - 39 : - What are inner transition elements? Decide which of the following atomicnumbers are the atomic numbers oftheinnertransition element: 29, 59, 74,95,102,104.
Answer - 39 : -
Lanthanoids and actinoids are called innertransition elements because inner f-orbitals are progressively filled and thelast electron goes to anti penultimate f orbital.
Elements with atomic number59, 95, 102 are inner transition metals because they belong to lanthanoids andactinoids.
Question - 40 : - The chemistry of the actinoid elements is not so smooth as that of thelanthanoids. Justify this statement by giving some examples from the oxidationstate of these elements.
Answer - 40 : -
The actinoids are radioactive elements and theearlier members have relatively long half-lives, the latter ones have half-lifevalues ranging from a day to 3 minutes for lawrencium (Z = 103). The lattermembers could be prepared only in nanogram quantities. These facts render theirstudy more difficult.
There is a greater range of oxidation states,which is attributed to the fact that the 5f, 6d and 7s levels are of comparableenergies. The actinoids show in general +3 oxidation state. The elements, inthe first half of the series frequently exhibit higher oxidation states. Forexample, the maximum oxidation state increases from +4 in Th to +5, +6 and +7respectively in Pa, U and Np but decreases in succeeding elements, Theactinoids resemble the lanthanoids in having more compounds in +3 state than inthe +4 state. However, +3 and +4 ions tend to hydrolyse. Because thedistribution of oxidation states among the actinoids is so uneven and sodifferent for the earlier and latter elements. It is unsatisfactory to reviewtheir chemistry in terms of oxidation states.