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Discuss the consequence of high enthalpy of H-H bond in terms of chemical reactivity of dihydrogen. |
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Answer» Solution : The chemical BEHAVIOUR of dihydrogen is determined, to a large extent, by bond dissociation enthalpy. The H-H bond dissociation enthalpy is the highest for a SINGLE bond between two atoms of any element. The dissociation of dihydrogen into its atoms is only ~ 0.081% around 2000K which increases to 95.5% at 5000K. It is relatively INERT at room temperature due to the high H-H bond enthalpy. Thus, the atomic hydrogen is produced at a high temperature in an electric arc or under ultraviolet RADIATIONS. Since its orbital is incomplete with `1s^1` electronic configuration, it does combine with almost all the elements. It accomplishes reactions by (i) loss of the only electron to give `H^+`. (ii) gain of an electron to form `H^(+)`, and (iii) sharing electrons to form a single covalent bond. Reaction with halogens: It reacts with halogens, `X_2` to give hydrogen halides, HX, `H_(2(g)) + X_(2(g)) to 2HX_((g))`[X=F , Cl, Br,I] While the reaction with fluorine occurs even in the dark, with iodine it requires a catalyst. Reaction with dioxygen : It reacts with dioxygen to form water. The reaction is highly exothermic. `2H_(2(g)) +O_(2(g)) underset"heating"overset"Cytalyst"to 2H_2O_((l)) DeltaH^(ө)=-285.9 "kJ mol"^(-1)` Reaction with dinitrogen : With dinitrogen it forms ammonia. `3H_(2(g)) + N_(2(g)) underset"Fe"overset"673K, 200atm"to 2NH_(3(g)) DeltaH^(ө)=-92.6 "kJ mol"^(-1)` This is the method for the MANUFACTURE of ammonia by the Haber process. Reactions with metals : With many metals it combines at a high temperature to yield the corresponding hydrides. `H_(2(g)) + 2M_((g)) to 2MH_((g))`, where M is an alkali metal Reactions with metal ions and metal oxides : It reduces some metal ions in aqueous solution and oxides of metals (less active than iron) into corresponding metals. `H_(2(g)) + Pd_((aq))^(2+) to Pd_((s)) + 2H_((aq))^(+)` `yH_(2(g)) + M_x O_(y(s)) to xM_((s)) + yH_2O_((l))` Reactions with organic compounds : It reacts with many organic compounds in the presence of catalysts to give useful hydrogenated products of commercial importance . For example : (i) Hydrogenation of vegetable oils using nickel as catalyst gives edible fats (margarine and vanaspati ghee) (ii) Hydroformylation of olefins yields aldehydes which further undergo reduction to give alcohols. `H_2+CO+RCH=CH_2 to RCH_2CH_2CHO` `H_2+RCH_2CH_2CHO to RCH_2CH_2CH_2OH` |
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