Hydrogen and s-Block

Q1. (4 marks) Any two of the following (2 marks each):

• Resembles Group 1: has $1s^1$ configuration, forms $\text{H}^+$ (unipositive ion), is a strong reducing agent, combines with halogens/oxygen like alkali metals. (2)
• Resembles Group 17: needs one electron to complete its shell, forms $\text{H}^-$ (hydride ion), exists as a diatomic gas $\text{H}_2$, high ionisation enthalpy like halogens. (2) Why: $\text{H}$ has a dual character since $1s^1$ is both "one valence electron" (like Na) and "one short of a full shell" (like F).

Q2. (4 marks)

Q3. (4 marks) (a) $\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\uparrow$ — correct reactants (1) + balanced (1). (Also acceptable: $\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$.) (b) Two uses (1 each): manufacture of ammonia (Haber process); hydrogenation of oils (vanaspati); rocket fuel; methanol synthesis; welding torches.

Q4. (4 marks)

• Ionic (saline): e.g. $\text{NaH}$, $\text{CaH}_2$ (1)
• Covalent (molecular): e.g. $\text{CH}_4$, $\text{NH}_3$, $\text{HCl}$ (1)
• Interstitial (metallic): e.g. $\text{TiH}_{1.7}$, $\text{PdH}_x$ (1) $\text{NaH}$ is ionic — Na (highly electropositive) transfers its electron to H giving $\text{Na}^+\text{H}^-$. (1)

Q5. (4 marks) (a) On cooling, water molecules form an open hydrogen-bonded (cage-like) structure below $4^\circ\text{C}$; this open lattice occupies more volume, so density decreases below $4^\circ$. Maximum packing (minimum volume, max density) occurs at $4^\circ\text{C}$. (2) (b) Oxygen is $sp^3$ hybridised; the two lone pairs on O exert greater repulsion (lp–lp > lp–bp > bp–bp), compressing the H–O–H angle from $109.5^\circ$ to $104.5^\circ$. (2)

Q6. (5 marks)

• Temporary hardness: due to bicarbonates $\text{Ca(HCO}_3)_2$, $\text{Mg(HCO}_3)_2$. (1)
• Permanent hardness: due to chlorides/sulphates $\text{CaCl}_2, \text{MgCl}_2, \text{CaSO}_4, \text{MgSO}_4$. (1)
• Removal of temporary hardness by boiling: (2) $\text{Ca(HCO}_3)_2 \xrightarrow{\Delta} \text{CaCO}_3\downarrow + \text{H}_2\text{O} + \text{CO}_2\uparrow$ (Clark's method with $\text{Ca(OH)}_2$ also accepted.)
• Removal of permanent hardness: washing soda ($\text{Na}_2\text{CO}_3$) / ion-exchange (zeolite / resin) method. (1)

Q7. (5 marks) (a) $\text{H}_2\text{O}_2$ has a non-planar open-book (skew) structure: the two O–H bonds lie in different planes; O–O bond ~$1.48$ Å, dihedral angle ~$111^\circ$ (gas) / $90.2^\circ$ (solid). (2) (b) Oxidising agent (accept any): $\text{PbS} + 4\text{H}_2\text{O}_2 \rightarrow \text{PbSO}_4 + 4\text{H}_2\text{O}$; or $2\text{Fe}^{2+} + \text{H}_2\text{O}_2 + 2\text{H}^+ \rightarrow 2\text{Fe}^{3+} + 2\text{H}_2\text{O}$. (1.5) Reducing agent: $2\text{MnO}_4^- + 5\text{H}_2\text{O}_2 + 6\text{H}^+ \rightarrow 2\text{Mn}^{2+} + 5\text{O}_2 + 8\text{H}_2\text{O}$; or $\text{Cl}_2 + \text{H}_2\text{O}_2 \rightarrow 2\text{HCl} + \text{O}_2$. (1.5)

Q8. (4 marks) (a) Two of (1 each): small size & high charge density of $\text{Li}^+$; strong polarising power (Fajans) → more covalent compounds; $\text{Li}$ is hardest, highest m.p. of Group 1; $\text{LiF}$, $\text{Li}_2\text{CO}_3$ sparingly soluble; forms only normal oxide $\text{Li}_2\text{O}$ (not peroxide/superoxide); reacts directly with $\text{N}_2$ giving $\text{Li}_3\text{N}$. (b) $\text{Li}$ resembles $\text{Mg}$; $\text{Be}$ resembles $\text{Al}$ (diagonal relationship). (2)

Q9. (4 marks) $\text{NaCl} + \text{NH}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{NaHCO}_3\downarrow + \text{NH}_4\text{Cl}$ (1.5) $2\text{NaHCO}_3 \xrightarrow{\Delta} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2\uparrow$ (1.5) Fails for $\text{K}_2\text{CO}_3$ because $\text{KHCO}_3$ is too soluble and does not precipitate out from solution. (1)

Q10. (2 marks, ½ each)

• Na⁺: transmission of nerve impulses / regulation of fluid & blood pressure (osmotic balance in extracellular fluid).
• K⁺: intracellular cation; nerve impulse transmission; activation of enzymes.
• Ca²⁺: formation of bones & teeth; blood clotting; muscle contraction.
• Mg²⁺: central atom in chlorophyll; enzyme activator (ATP reactions).

[
  {"claim":"Zn + H2SO4 -> ZnSO4 + H2 is balanced (atoms conserved)","code":"Zn_l=1; H_l=2; S_l=1; O_l=4; Zn_r=1; H_r=2; S_r=1; O_r=4; result=(Zn_l==Zn_r and H_l==H_r and S_l==S_r and O_l==O_r)"},
  {"claim":"Decomposition 2 NaHCO3 -> Na2CO3 + H2O + CO2 balances Na,H,C,O","code":"Na_l=2; H_l=2*1; C_l=2*1; O_l=2*3; Na_r=2; H_r=2; C_r=1+1; O_r=3+1+2; result=(Na_l==Na_r and H_l==H_r and C_l==C_r and O_l==O_r)"},
  {"claim":"MnO4- + H2O2 reaction: 2 MnO4- +5 H2O2 +6 H+ -> 2 Mn2+ +5 O2 +8 H2O conserves charge","code":"left_charge=2*(-1)+6*(1); right_charge=2*(2); result=(left_charge==right_charge)"},
  {"claim":"Isotope neutron counts: protium 0, deuterium 1, tritium 2 (mass - proton)","code":"result=(1-1==0 and 2-1==1 and 3-1==2)"}
]