Hydrogen flash cards

1. 1.Why is the position of hydrogen in the periodic table anomalous?

   Hydrogen resembles both alkali metals (group 1: $\text{ns}^1$ config, forms $\text{H}^+$, unipositive ion) and halogens (group 17: needs 1 electron to complete duplet, forms $\text{H}^-$, diatomic $\text{H}_2$). Hence it fits no single group perfectly.

   Hint: 1 valence electron but also 1 short of noble gas He.

2. 2.Give the electronic configuration of hydrogen and why it can be placed with alkali metals.

   $1\text{s}^1$. Like alkali metals it has one valence electron, is electropositive, forms unipositive $\text{H}^+$, and shows +1 oxidation state.

   Hint: $\text{ns}^1$ resemblance.

3. 3.List ways hydrogen resembles halogens.

   Diatomic molecule $\text{H}_2$; needs one electron for stable configuration; forms $\text{H}^-$ (hydride) like $\text{X}^-$; ionisation enthalpy comparable; forms covalent compounds with non-metals.

   Hint: One electron short of noble gas.

4. 4.How does hydrogen differ from alkali metals?

   Much higher ionisation enthalpy ($1312\ \text{kJ/mol}$), non-metallic, $\text{H}^+$ is a bare proton that never exists free (always solvated), forms mostly covalent compounds, and is a gas not a metal.

   Hint: Bare proton has huge polarising power.

5. 5.Why is $\text{H}^+$ (bare proton) never found free in condensed phases?

   Its radius ($\sim 1.5\times10^{-3}\ \text{pm}$) is extremely small giving enormous charge density and polarising power, so it always associates with other atoms/molecules (e.g. $\text{H}_3\text{O}^+$ in water).

   Hint: Compare with normal ionic radii $\sim$50–200 pm.

6. 6.Name the three isotopes of hydrogen with symbols and mass numbers.

   Protium $^1_1\text{H}$, Deuterium $^2_1\text{H}$ (D), Tritium $^3_1\text{H}$ (T). They have 1 proton each with 0, 1, 2 neutrons respectively.

   Hint: Same protons, differ in neutrons.

7. 7.What is the relative natural abundance of the hydrogen isotopes?

   Protium $\approx 99.98\%$, Deuterium $\approx 0.0156\%$, Tritium is radioactive and present in traces ($\sim 1$ atom per $10^{18}$ H atoms).

   Hint: Protium dominates.

8. 8.Which hydrogen isotope is radioactive and what is its emission?

   Tritium ($^3_1\text{T}$) is radioactive, a $\beta^-$ emitter with half-life $\approx 12.3$ years.

   Hint: 3 mass number, unstable.

9. 9.Why do isotopes of hydrogen show large differences in physical properties compared to isotopes of heavier elements?

   Because the relative mass difference is huge (D is 2×, T is 3× protium), significantly affecting rates, bond energies and physical constants (kinetic isotope effect is largest for H/D/T).

   Hint: Mass ratio 1:2:3 is unusually large.

10. 10.What is the difference between ortho- and para-hydrogen?

    They differ in nuclear spin: ortho-hydrogen has the two proton spins parallel, para-hydrogen has them antiparallel. At room temperature $\text{H}_2$ is $\sim 75\%$ ortho and $25\%$ para.

    Hint: Nuclear spin alignment.

11. 11.On the lab scale, how is dihydrogen usually prepared from a metal and acid?

    By reacting a reactive metal (e.g. Zn) with dilute acid: $\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2$.

    Hint: Metal + dilute acid → salt + H2.

12. 12.How can dihydrogen be prepared using zinc and an alkali?

    Zinc (amphoteric) reacts with NaOH: $\text{Zn} + 2\text{NaOH} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2$ (sodium zincate).

    Hint: Amphoteric metal + strong base.

13. 13.What is the electrolytic method of preparing very pure dihydrogen?

    Electrolysis of acidified/warm water using platinum electrodes liberates $\text{H}_2$ at the cathode: $2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2$. Traces of acid/base act as electrolyte.

    Hint: Water splitting; cathode gas.

14. 14.What is electrolysis of warm aqueous barium hydroxide used for?

    To obtain high-purity dihydrogen using nickel electrodes.

    Hint: Pure H2, $\text{Ba(OH)}_2$ electrolyte.

15. 15.What is 'water gas' and how is it made?

    Water gas is a mixture of CO and $\text{H}_2$ ($\text{CO} + \text{H}_2$). It is produced by passing steam over red-hot coke at $\sim 1270\ \text{K}$: $\text{C} + \text{H}_2\text{O} \rightarrow \text{CO} + \text{H}_2$.

    Hint: Steam + hot coke; also called syngas.

16. 16.Why is water gas / CO+H2 also called 'syngas' or 'synthesis gas'?

    Because it is used to synthesise methanol and a large number of hydrocarbons; it is a feedstock for chemical synthesis.

    Hint: Feedstock for methanol/hydrocarbons.

17. 17.What is the 'coal gasification' process?

    Production of syngas by treating coal/coke with steam: $\text{C} + \text{H}_2\text{O} \xrightarrow{1270\text{ K}} \text{CO} + \text{H}_2$. It converts solid coal into gaseous fuel/feedstock.

    Hint: Coal → CO + H2.

18. 18.What is the water-gas shift reaction and its purpose?

    $\text{CO} + \text{H}_2\text{O} \xrightarrow{673\text{ K, catalyst}} \text{CO}_2 + \text{H}_2$. It increases the yield of $\text{H}_2$ by converting CO with more steam; $\text{CO}_2$ is removed by scrubbing in water under pressure.

    Hint: CO converted to CO2 to enrich H2.

19. 19.Which catalyst is used in the water-gas shift reaction?

    Iron chromate ($\text{FeCrO}_4$) catalyst.

    Hint: Fe–Cr oxide.

20. 20.How is dihydrogen obtained commercially from hydrocarbons (steam reforming)?

    Steam reforming of methane/natural gas over Ni at $\sim 1270\ \text{K}$: $\text{CH}_4 + \text{H}_2\text{O} \rightarrow \text{CO} + 3\text{H}_2$.

    Hint: Natural gas + steam over Ni.

21. 21.What is the largest single commercial use of dihydrogen?

    Synthesis of ammonia via the Haber process ($\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$), used for fertilizers.

    Hint: Haber process.

22. 22.How is vanaspati ghee manufactured using dihydrogen?

    By catalytic hydrogenation of unsaturated vegetable oils (polyunsaturated → saturated) using finely divided nickel catalyst, converting liquid oils to solid fat.

    Hint: Hydrogenation of oils, Ni catalyst.

23. 23.Why is dihydrogen not very reactive at room temperature despite a high bond enthalpy?

    The H–H bond enthalpy is very high ($435.9\ \text{kJ/mol}$), the highest for a single bond between two atoms of any element, so the molecule is kinetically inert unless activated by heat/catalyst.

    Hint: Strong H–H bond resists dissociation.

24. 24.Write the reaction of dihydrogen with dioxygen.

    $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$; highly exothermic and explosive when ignited, releasing large energy.

    Hint: Combustion of H2.

25. 25.How does dihydrogen react with dinitrogen and with halogens?

    With $\text{N}_2$: $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$ (Haber). With halogens: $\text{H}_2 + \text{X}_2 \rightarrow 2\text{HX}$ (reactivity decreases $\text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2$).

    Hint: Ammonia; hydrogen halides.

26. 26.How does dihydrogen react with metals to form hydrides?

    With strongly electropositive s-block metals it forms ionic hydrides: $2\text{Na} + \text{H}_2 \rightarrow 2\text{NaH}$; here hydrogen is in $-1$ oxidation state.

    Hint: H acts as oxidising agent, forms H⁻.

27. 27.Define hydrides and give their three main classes.

    Hydrides are binary compounds of hydrogen with another element. Classes: (1) ionic/saline, (2) covalent/molecular, (3) metallic/interstitial (non-stoichiometric).

    Hint: Ionic, covalent, metallic.

28. 28.Which elements form ionic (saline) hydrides and give examples?

    Highly electropositive s-block metals (group 1 and heavier group 2: Ca, Sr, Ba) form ionic hydrides, e.g. NaH, KH, $\text{CaH}_2$, containing the $\text{H}^-$ ion.

    Hint: Alkali + heavier alkaline earth metals.

29. 29.What happens when ionic hydrides react with water?

    They react violently, liberating $\text{H}_2$: $\text{NaH} + \text{H}_2\text{O} \rightarrow \text{NaOH} + \text{H}_2$. $\text{H}^-$ is a strong base and reducing agent.

    Hint: H⁻ + H2O → OH⁻ + H2.

30. 30.Why do ionic hydrides conduct electricity in the molten state, and what forms at the anode?

    They contain mobile $\text{H}^-$ ions; on electrolysis of molten hydride, $\text{H}_2$ is liberated at the anode, proving hydride ion carries negative charge.

    Hint: H⁻ migrates to anode.

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