Chemical Thermodynamics · JEE Main previous year questions

2009 3 questions

1. Q652009 · 27 Apr 2009 · Shift 1

   On the basis of the following thermochemical data : $({\Delta }_f{G}^o{H}_{(aq)}^{+}=0)$ ${\mathrm{H}}_2\mathrm{O}(\mathrm{l})\to {\mathrm{H}}^{+}(\mathrm{aq})+\mathrm{O}{\mathrm{H}}^{-}(\mathrm{aq});\mathrm{\Delta H}=57.32\mathrm{kJ}$ ${\mathrm{H}}_2(\mathrm{g})+\frac{1}{2}{\mathrm{O}}_2(\mathrm{g})\to {\mathrm{H}}_2\mathrm{O}(\mathrm{l});\mathrm{\Delta H}=-286.20\mathrm{kJ}$ The value of enthalpy of formation of $\mathrm{O}{\mathrm{H}}^{-}$ ion at $25^{\circ }\mathrm{C}$ is :

   1. A.$-22.88\mathrm{kJ}$
   2. B.$-228.88\mathrm{kJ}$
   3. C.$+228.88\mathrm{kJ}$
   4. D.$-343.52\mathrm{kJ}$
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   Answer: B

2. Q732009 · 27 Apr 2009 · Shift 1

   In a fuel cell methanol is used as fuel and oxygen gas is used as an oxidizer. The reaction is $\mathrm{C}{\mathrm{H}}_3\mathrm{OH}(\mathrm{l})+3/2{\mathrm{O}}_2\to \mathrm{C}{\mathrm{O}}_2(\mathrm{g})+2{\mathrm{H}}_2\mathrm{O}(\mathrm{l})$ At 298K standard Gibb's energies of formation for $\mathrm{C}{\mathrm{H}}_3\mathrm{OH}(\mathrm{l})$, ${\mathrm{H}}_2\mathrm{O}(\mathrm{l})$ and $\mathrm{C}{\mathrm{O}}_2(\mathrm{g})$ are -166.2, -237.2 and $-394.4\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ respectively. If standard enthalpy of combustion of methanol is $-726\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$, efficiency of the fuel cell will be

   1. A.$87\%$
   2. B.$90\%$
   3. C.$97\%$
   4. D.$80\%$
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   Answer: C

3. Q862009 · 27 Apr 2009 · Shift 1

   The bond dissociation energy of $\mathrm{B}-\mathrm{F}$ in $\mathrm{B}{\mathrm{F}}_3$ is $646\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ whereas that of $\mathrm{C}-\mathrm{F}$ in $\mathrm{C}{\mathrm{F}}_4$ is $515\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$. The correct reason for higher $\mathrm{B}-\mathrm{F}$ bond dissociation energy as compared to that of $\mathrm{C}-\mathrm{F}$ is :

   1. A.smaller size of $\mathrm{B}$-atom as compared to that of $\mathrm{C}$-atom
   2. B.stronger $\sigma$ bond between $\mathrm{B}$ and $\mathrm{F}$ in $\mathrm{B}{\mathrm{F}}_3$ as compared to that between $\mathrm{C}$ and $\mathrm{F}$ in $\mathrm{C}{\mathrm{F}}_4$
   3. C.significant $p\pi -p\pi$ interaction between $\mathrm{B}$ and $\mathrm{F}$ in $\mathrm{B}{\mathrm{F}}_3$ whereas there is no possibility of such interaction between $\mathrm{C}$ and $\mathrm{F}$ in $\mathrm{C}{\mathrm{F}}_4$.
   4. D.lower degree of $p\pi -p\pi$ interaction between $\mathrm{B}$ and $\mathrm{F}$ in $\mathrm{B}{\mathrm{F}}_3$ than that between $\mathrm{C}$ and $\mathrm{F}$ in $\mathrm{C}{\mathrm{F}}_4$.
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   Answer: C