Q.Standard molar enthalpy of formation, ΔfH° is just a special case of enthalpy of reaction, ΔrH°. Is the ΔrH° for the following reaction same as ΔfH°? Give reason for your answer.
CaO(s) + CO2(g) → CaCO3(s); ΔfH° = -178.3 kJ mol^-1

Showing the 12 most recent of 13 on this concept.

1. COMEDK 2026Set 2026-A1 markMCQ
   Q.The standard enthalpies of formation of $\mathrm{CH}_4(\mathrm{~g}), \mathrm{CO}_2(\mathrm{~g})$ and $\mathrm{H}_2 \mathrm{O}(\mathrm{l})$ are $-74.8 \mathrm{~kJ} \mathrm{~mol}^{-1},-393.5 \mathrm{~kJ} \mathrm{~mol}^{-1}$ and $-285.8 \mathrm{~kJ} \mathrm{~mol}^{-1}$ respectively. Then the enthalpy change for the given reaction in $\mathrm{kJ} \mathrm{mol}^{-1}$ will be: $2 \mathrm{CH}_4(\mathrm{~g})+4 \mathrm{O}_2(\mathrm{~g}) \rightarrow 2 \mathrm{CO}_2(\mathrm{~g})+4 \mathrm{H}_2 \mathrm{O}(\mathrm{l})$ (A) $-890.3$ (B) $+890.3$ (C) $+1780.6$ (D) $-1780.6$

   ›Reveal solutionSolution

   The enthalpy change for the reaction is found using Hess’s law: $\Delta H^\circ = \sum \Delta H_f^\circ(\text{products}) - \sum \Delta H_f^\circ(\text{reactants})$, giving $-1780.6\ \mathrm{kJ\ mol^{-1}}$, so the correct option is (D).

   The key idea here is Hess’s law: the enthalpy change of a reaction depends only on the initial and final states, not the path. Since we are given standard enthalpies of formation ($\Delta H_f^\circ$) for each compound, we can compute the reaction enthalpy directly. The enthalpy of formation of an element in its standard state (like $\mathrm{O}_2(g)$) is zero by definition, which simplifies the calculation.

   We proceed step by step:

   1. Write the general formula For any reaction, the standard enthalpy change is:

   $\Delta H^\circ = \sum \nu_p \Delta H_f^\circ(\text{products}) - \sum \nu_r \Delta H_f^\circ(\text{reactants})$

   where $\nu$ are the stoichiometric coefficients.

   2. Identify the given data

      • $\Delta H_f^\circ(\mathrm{CH}_4(g)) = -74.8\ \mathrm{kJ\ mol^{-1}}$
      • $\Delta H_f^\circ(\mathrm{CO}_2(g)) = -393.5\ \mathrm{kJ\ mol^{-1}}$
      • $\Delta H_f^\circ(\mathrm{H}_2\mathrm{O}(l)) = -285.8\ \mathrm{kJ\ mol^{-1}}$
      • $\Delta H_f^\circ(\mathrm{O}_2(g)) = 0$ (element in standard state)

   3. Sum the enthalpies of formation of products

      Products: $2\ \mathrm{CO}_2(g)$ and $4\ \mathrm{H}_2\mathrm{O}(l)$

   $\sum \Delta H_f^\circ(\text{products}) = 2(-393.5) + 4(-285.8)$

   Calculate:

   $2(-393.5) = -787.0$

   $4(-285.8) = -1143.2$

   Sum:

   $-787.0 + (-1143.2) = -1930.2\ \mathrm{kJ}$

   4. Sum the enthalpies of formation of reactants Reactants: $2\ \mathrm{CH}_4(g)$ and $4\ \mathrm{O}_2(g)$

   $\sum \Delta H_f^\circ(\text{reactants}) = 2(-74.8) + 4(0)$

   $2(-74.8) = -149.6$ …

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2. COMEDK 2026Set 2026-M1 markMCQ
   Q.Ozone is formed by the reaction $\mathrm{O}_{2(g)}+\mathrm{O}_{(g)} \rightarrow \mathrm{O}_{3(g)}, \Delta \mathrm{H}=-107.2 \mathrm{~kJ} .$ Given $\mathrm{O}=0$ bond energy is $498.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$, the average bond energy of ozone is: (A) $302.6 \mathrm{~kJ} \mathrm{~mol}^{-1}$ (B) $520.6 \mathrm{~kJ} \mathrm{~mol}^{-1}$ (C) $120.5 \mathrm{~kJ} \mathrm{~mol}^{-1}$ (D) $201.8 \mathrm{~kJ} \mathrm{~mol}^{-1}$

   ›Reveal solutionSolution

   The average O–O bond energy in ozone is found by applying Hess’s law to the formation reaction: the enthalpy change equals the bond broken (O₂) minus the bonds formed (two O–O bonds in O₃). Solving gives 302.6 kJ mol⁻¹, which is option (A).

   Concept & Intuition

   Bond energies are always positive (energy required to break a bond) and endothermic. When bonds form, energy is released (exothermic). The enthalpy change of a reaction, ΔH, can be estimated as:

   ΔH = Σ(bond energies of bonds broken) – Σ(bond energies of bonds formed).

   Here, we form ozone from O₂ and an oxygen atom. We know the O=O double bond energy in O₂, and we know the overall ΔH. The ozone molecule has two equivalent O–O bonds (it’s bent, with bond order ~1.5, but we treat them as identical for an average). So we can solve for the average bond energy in O₃.

   Step-by-step

   1. Write the reaction with bond changes

   $\mathrm{O_2} + \mathrm{O} \rightarrow \mathrm{O_3}$

   In O₂, one O=O double bond is broken. In O₃, two O–O bonds are formed (since O₃ is O–O–O with two bonds).

   2. Apply the bond-energy formula

   $\Delta H = \text{(bonds broken)} - \text{(bonds formed)}$

   Bonds broken: 1 mol of O=O bonds, energy = 498.0 kJ.

   Bonds formed: 2 mol of O–O bonds (average), each of unknown energy $E$.

   So:

   $\Delta H = 498.0 - 2E$

   3. Insert the given ΔH The reaction is exothermic: $\Delta H = -107.2$ kJ. $$$$

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3. COMEDK 2026Set 2026-M1 markMCQ
   Q.The heat of combustion of carbon to $\mathrm{CO}_2$ is $-393.5 \mathrm{~kJ} \mathrm{~mol}^{-1}$. The heat released on the formation of 35.2 g of $\mathrm{CO}_2$ by combustion of C is: (A) +215 kJ (B) -315 kJ (C) -325 kJ (D) +620 kJ

   ›Reveal solutionSolution

   The heat of combustion is given per mole of CO₂; we find the moles in 35.2 g, then multiply by the enthalpy change. The result is –315 kJ, so option (B) is correct.

   The key idea is that the enthalpy change for a reaction is proportional to the amount of substance that reacts. Here, the combustion of carbon to CO₂ releases –393.5 kJ per mole of CO₂ formed. So if we know how many moles of CO₂ are produced from 35.2 g, we can directly scale the given enthalpy.

   1. Find the molar mass of CO₂. Carbon has atomic mass 12.0 g mol⁻¹, oxygen 16.0 g mol⁻¹.

   $M_{\mathrm{CO_2}} = 12.0 + 2 \times 16.0 = 44.0\ \mathrm{g\ mol^{-1}}$

   2. Calculate the number of moles in 35.2 g of CO₂.

   $n = \frac{\text{mass}}{\text{molar mass}} = \frac{35.2\ \mathrm{g}}{44.0\ \mathrm{g\ mol^{-1}}} = 0.800\ \mathrm{mol}$

   3. Relate the heat released to the moles of CO₂ formed. The combustion reaction is:

   $\mathrm{C(s) + O_2(g) \rightarrow CO_2(g)} \quad \Delta H = -393.5\ \mathrm{kJ\ mol^{-1}}$

   This means that for every 1 mol of CO₂ produced, 393.5 kJ of heat is released (hence the negative sign). For 0.800 mol, the heat released is:

   $$$$

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4. COMEDK 2025Set 2025-A1 markMCQ
   Q.For the reaction $\mathrm{A}_2+\mathrm{B}_2 \cdots \cdots>2 \mathrm{AB}, \Delta \mathrm{H}_{\mathrm{f}}=-400 \mathrm{~kJ} / \mathrm{mol}$. The bond dissociation enthalpies of $\mathrm{A}_2$, $\mathrm{B}_2$ and AB are in the ratio $1: 0.75: 1$. What is the bond dissociation enthalpy of $\mathrm{B}_2$ in $\mathrm{kJ} / \mathrm{mol}$ ? (A) 1600 (B) 2400 (C) 3200 (D) 800

   ›Reveal solutionSolution

   $\Delta H_f$ is the enthalpy of forming 1 mol of AB: $\tfrac12\mathrm{A}_2+\tfrac12\mathrm{B}_2\rightarrow\mathrm{AB}$. Applying bonds-broken minus bonds-formed with the ratio $1:0.75:1$ gives $\text{BDE}(\mathrm{B}_2)=2400\text{ kJ/mol}$ — option (B).

   Set up the bond energies from the ratio

   Let $\text{BDE}(\mathrm{A}_2)=x$. From the ratio $\mathrm{A}_2:\mathrm{B}_2:\mathrm{AB}=1:0.75:1$:

   $\text{BDE}(\mathrm{A}_2)=x,\qquad \text{BDE}(\mathrm{B}_2)=0.75x,\qquad \text{BDE}(\mathrm{AB})=x$

   Write the formation reaction

   The enthalpy of formation is defined per mole of product, so the balanced formation reaction of AB is

   $\tfrac12\,\mathrm{A}_2+\tfrac12\,\mathrm{B}_2\rightarrow \mathrm{AB},\qquad \Delta H_f=-400\text{ kJ/mol}$

   Apply Hess's law ($\Delta H=\text{bonds broken}-\text{bonds formed}$): …

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5. COMEDK 2025Set 2025-E1 markMCQ
   Q.The enthalpies of combustion of $\mathrm{H}_2, \mathrm{C}$ (graphite) and $\mathrm{C}_2 \mathrm{H}_6(\mathrm{~g})$ are $-286.0,-394.0$ and $-1560.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ at $25^{\circ} \mathrm{C}$ and 1 atm pressure. The enthalpy of formation of ethane is : (A) $-97.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ (B) $-86.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ (C) $-92.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ (D) $-78.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$

   ›Reveal solutionSolution

   Using Hess’s law, the enthalpy of formation of ethane is found by combining the combustion enthalpies of its elements and the combustion enthalpy of ethane. The result is –86.0 kJ mol⁻¹, which corresponds to option (B).

   Concept & Intuition

   The enthalpy of formation of a compound is the heat change when one mole of it is formed from its elements in their standard states. We are given combustion enthalpies, not formation enthalpies directly. But combustion is just a chemical reaction, and Hess’s law tells us that enthalpy change for a reaction is the same whether it happens in one step or many. So we can construct a thermochemical cycle: the combustion of the elements (H₂ and C) to CO₂ and H₂O, minus the combustion of the product (ethane), gives the formation reaction of ethane. This works because the combustion products are the same for all paths.

   Step-by-step reasoning

   1. Write the target formation reaction Formation of ethane from its elements in standard states:

   $2\,\mathrm{C(s)} + 3\,\mathrm{H_2(g)} \rightarrow \mathrm{C_2H_6(g)} \quad \Delta H_f = ?$

   2. Write the given combustion reactions For hydrogen:

   $\mathrm{H_2(g)} + \tfrac12\,\mathrm{O_2(g)} \rightarrow \mathrm{H_2O(l)} \quad \Delta H = -286.0\ \mathrm{kJ\,mol^{-1}}$

   For carbon (graphite):

   $\mathrm{C(s)} + \mathrm{O_2(g)} \rightarrow \mathrm{CO_2(g)} \quad \Delta H = -394.0\ \mathrm{kJ\,mol^{-1}}$

   For ethane:

   $\mathrm{C_2H_6(g)} + \tfrac72\,\mathrm{O_2(g)} \rightarrow 2\,\mathrm{CO_2(g)} + 3\,\mathrm{H_2O(l)} \quad \Delta H = -1560.0\ \mathrm{kJ\,mol^{-1}}$

   3. Use Hess’s law: formation = combustion of elements – combustion of compound Imagine forming ethane from its elements, then burning the ethane to CO₂ and H₂O. Alternatively, burn the elements directly to the same products. The enthalpy of the direct combustion of elements minus the combustion of ethane gives the formation enthalpy. Mathematically: $$$$

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6. COMEDK 2025Set 2025-M1 markMCQ
   Q.Given that the standard enthalpy of combustion of $\mathrm{C}_{(\mathrm{S})}$ and $\mathrm{CS}_2(\mathrm{l})$ are -393.3 and $-1108.76 \mathrm{~kJ} / \mathrm{mol}$ respectively and the standard enthalpy of formation of $\mathrm{CS}_2$ is $128.02 \mathrm{~kJ} / \mathrm{mol}$. What is $\Delta H_f^0$ of $\mathrm{SO}_2$ ? (A) $-510.6 \mathrm{~kJ} / \mathrm{mol}$ (B) $-293.72 \mathrm{~kJ} / \mathrm{mol}$ (C) $-321.2 \mathrm{~kJ} / \mathrm{mol}$ (D) $-587 \mathrm{~kJ} / \mathrm{mol}$

   ›Reveal solutionSolution

   We use Hess’s law: combine the combustion reactions of C(s) and CS₂(l) with the formation of CS₂ to isolate the formation reaction of SO₂. The result is ΔH_f°(SO₂) = -293.72 kJ/mol, which corresponds to option (B).

   Concept & Intuition

   We are given enthalpies of combustion and formation, but we need the enthalpy of formation of SO₂. The key is that formation reactions are defined from elements in their standard states. Here, the formation of SO₂ is:

   $\frac{1}{2} S_2(s) + O_2(g) \rightarrow SO_2(g)$

   (We use S₂(s) as the standard state for sulfur, but the data involves CS₂ and C(s), so we must build a thermochemical cycle using the given reactions.)

   Step-by-step reasoning

   1. Write the given reactions with their enthalpies
      • Combustion of C(s):

   $C(s) + O_2(g) \rightarrow CO_2(g) \quad \Delta H = -393.3\ \text{kJ/mol}$

   • Combustion of CS₂(l):

   $CS_2(l) + 3O_2(g) \rightarrow CO_2(g) + 2SO_2(g) \quad \Delta H = -1108.76\ \text{kJ/mol}$

   • Formation of CS₂(l) from elements:

   $C(s) + 2S(s) \rightarrow CS_2(l) \quad \Delta H = +128.02\ \text{kJ/mol}$

    (Note: The standard state of sulfur is S(s), often taken as rhombic; we treat it as S(s) here.)
   

   2. Target reaction

   We want the standard enthalpy of formation of SO₂:

   $\frac{1}{2} S_2(s) + O_2(g) \rightarrow SO_2(g)$

   But since S(s) is the standard state, we can equivalently write:

   $S(s) + O_2(g) \rightarrow SO_2(g) \quad \Delta H_f^\circ(SO_2)$

   (We’ll find this value; note that if the problem uses S₂, the answer per mole of SO₂ is the same.)

   3. Use Hess’s law to combine reactions We need to eliminate C(s), CS₂(l), and CO₂(g) to leave only S(s) and O₂(g) forming SO₂.
      • Start with the combustion of CS₂:

   $CS_2(l) + 3O_2(g) \rightarrow CO_2(g) + 2SO_2(g) \quad \Delta H = -1108.76$

   • Subtract the combustion of C(s) (to remove CO₂ and C):

   $[CS_2(l) + 3O_2(g) \rightarrow CO_2(g) + 2SO_2(g)] - [C(s) + O_2(g) \rightarrow CO_2(g)]$

    Gives:  
   

   $CS_2(l) + 2O_2(g) \rightarrow C(s) + 2SO_2(g) \quad \Delta H = -1108.76 - (-393.3) = -715.46\ \text{kJ}$

   • Now subtract the formation of CS₂ (reversed) to replace CS₂(l) with C(s) and S(s): The formation reaction is:

   $C(s) + 2S(s) \rightarrow CS_2(l) \quad \Delta H = +128.02$

    Reversing it:  
   

   $CS_2(l) \rightarrow C(s) + 2S(s) \quad \Delta H = -128.02$

    Add this to the previous step:  
   

   $[CS_2(l) + 2O_2(g) \rightarrow C(s) + 2SO_2(g)] + [CS_2(l) \rightarrow C(s) + 2S(s)]$

    Cancel CS₂(l) on left and C(s) on right? Wait carefully:  
    Actually, we want to replace CS₂(l) with elements. So we add the reversed formation to the equation from step 3:  
   

   $(CS_2(l) + 2O_2(g) \rightarrow C(s) + 2SO_2(g)) \quad \Delta H = -715.46$

   $+ (CS_2(l) \rightarrow C(s) + 2S(s)) \quad \Delta H = -128.02$

    Sum:  
   

   $2CS_2(l) + 2O_2(g) \rightarrow 2C(s) + 2SO_2(g) + 2S(s)$

    That’s not clean — we have extra CS₂. Better approach: Instead, subtract the formation reaction directly.
   

   4. Correct combination

   We want to get from elements to SO₂. Let’s write the target as:

   $2S(s) + 2O_2(g) \rightarrow 2SO_2(g) \quad \Delta H = 2 \times \Delta H_f^\circ(SO_2)$

   From step 3 we had:

   $CS_2(l) + 2O_2(g) \rightarrow C(s) + 2SO_2(g) \quad \Delta H = -715.46$

   Now, if we subtract the formation of CS₂ (i.e., use its reverse):

   $C(s) + 2S(s) \rightarrow CS_2(l) \quad \Delta H = +128.02$

   Reverse:

   $CS_2(l) \rightarrow C(s) + 2S(s) \quad \Delta H = -128.02$

   Add this to the equation from step 3:

   $[CS_2(l) + 2O_2(g) \rightarrow C(s) + 2SO_2(g)] + [CS_2(l) \rightarrow C(s) + 2S(s)]$

   Sum:

   $2CS_2(l) + 2O_2(g) \rightarrow 2C(s) + 2SO_2(g) + 2S(s)$

   That’s not right — we have doubled CS₂. The mistake: we should subtract the formation reaction, not add its reverse. Let’s do it properly:

   We have:

   (1) C(s) + O₂(g) → CO₂(g) ΔH = -393.3

   (2) CS₂(l) + 3O₂(g) → CO₂(g) + 2SO₂(g) ΔH = -1108.76

   (3) C(s) + 2S(s) → CS₂(l) ΔH = +128.02

   Target: S(s) + O₂(g) → SO₂(g)

   Manipulation:

   • Reverse (3): CS₂(l) → C(s) + 2S(s) ΔH = -128.02
   • Add to (2): (2) + reversed (3): CS₂(l) + 3O₂(g) + CS₂(l) → CO₂(g) + 2SO₂(g) + C(s) + 2S(s) That gives 2CS₂(l) on left — still messy.

   Better: Use (2) minus (1) to eliminate CO₂ and C:

   (2) - (1):

   $CS_2(l) + 3O_2(g) - C(s) - O_2(g) \rightarrow CO_2(g) + 2SO_2(g) - CO_2(g)$

   Simplify:

   $CS_2(l) + 2O_2(g) - C(s) \rightarrow 2SO_2(g)$

   Or:

   $CS_2(l) + 2O_2(g) \rightarrow C(s) + 2SO_2(g) \quad \Delta H = -1108.76 - (-393.3) = -715.46$

   Now subtract (3) from this (i.e., subtract the formation of CS₂):

   $[CS_2(l) + 2O_2(g) \rightarrow C(s) + 2SO_2(g)] - [C(s) + 2S(s) \rightarrow CS_2(l)]$

   Subtract means reverse the second and add:

   $CS_2(l) + 2O_2(g) \rightarrow C(s) + 2SO_2(g) \quad \Delta H = -715.46$

   $+ \quad CS_2(l) \rightarrow C(s) + 2S(s) \quad \Delta H = -128.02$

   Sum:

   $2CS_2(l) + 2O_2(g) \rightarrow 2C(s) + 2SO_2(g) + 2S(s)$

   Still double. The issue: we need to cancel CS₂(l) and C(s) properly. Instead, do this:

   From (2) - (1) we got:

   $CS_2(l) + 2O_2(g) \rightarrow C(s) + 2SO_2(g) \quad \Delta H = -715.46$

   Now subtract (3) but in a way that cancels CS₂ and C:

   Actually, we want to replace CS₂(l) and C(s) with elements. So take the equation above and add the reverse of (3):

   Reverse of (3): CS₂(l) → C(s) + 2S(s) ΔH = -128.02

   Add:

   $[CS_2(l) + 2O_2(g) \rightarrow C(s) + 2SO_2(g)] + [CS_2(l) \rightarrow C(s) + 2S(s)]$

   =

   $2CS_2(l) + 2O_2(g) \rightarrow 2C(s) + 2SO_2(g) + 2S(s)$

   This still has 2CS₂ and 2C. The correct trick: Instead, subtract (3) from the equation (2)-(1) directly:

   (2)-(1): CS₂(l) + 2O₂(g) → C(s) + 2SO₂(g) ΔH = -715.46

   Subtract (3): C(s) + 2S(s) → CS₂(l) ΔH = +128.02

   That means:

   $[CS_2(l) + 2O_2(g) \rightarrow C(s) + 2SO_2(g)] - [C(s) + 2S(s) \rightarrow CS_2(l)]$

   =

   $$$$

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7. KCET 2024Set B-21 markMCQ
   Q.The energy associated with first orbit is He$^+$ is (A) $0\,\text{J}$ (B) $-8.72 \times 10^{-18}\,\text{J}$ (C) $-4.58 \times 10^{-18}\,\text{J}$ (D) $-0.545 \times 10^{-18}\,\text{J}$

   ›Reveal solutionSolution

   The energy of the first orbit in a hydrogen-like ion is given by $E_n = -13.6 \frac{Z^2}{n^2}$ eV. For He$^+$ ($Z=2$, $n=1$), this is $-54.4$ eV, which converts to $-8.72 \times 10^{-18}$ J. The correct option is (B).

   The key here is recognizing that He$^+$ is a hydrogen-like ion — it has only one electron, just like hydrogen, but its nucleus has a charge of $+2e$ (since helium has atomic number $Z=2$). The Bohr model applies directly to any one-electron system, with the energy scaling as $Z^2$.

   Why does the energy scale with $Z^2$? In the Bohr model, the electron's total energy is the sum of its kinetic energy and electrostatic potential energy. The Coulomb attraction between the electron and nucleus is proportional to $Z$ (stronger for higher $Z$), which pulls the electron into a tighter orbit. This increases both the kinetic energy (in magnitude) and the potential energy (negative, larger in magnitude), resulting in a total energy that scales as $Z^2$. For hydrogen ($Z=1$), the ground state energy is $-13.6$ eV; for He$^+$, it's four times that.

   Now let's work through the calculation step by step.

   1. Write the general formula for energy of a hydrogen-like ion. The energy of the $n$-th orbit in a hydrogen-like atom is:

   $E_n = -13.6 \frac{Z^2}{n^2} \text{ eV}$

   This comes from the Bohr model, where $13.6$ eV is the Rydberg energy for hydrogen.

   2. Plug in the values for He$^+$. For He$^+$, $Z = 2$ and the first orbit means $n = 1$. So:

   $E_1 = -13.6 \times \frac{2^2}{1^2} = -13.6 \times 4 = -54.4 \text{ eV}$

   3. Convert electron volts to joules. The conversion factor is $1 \text{ eV} = 1.602 \times 10^{-19} \text{ J}$. Therefore:

   $E_1 = -54.4 \times 1.602 \times 10^{-19} \text{ J}$

   4. Perform the multiplication. First, $54.4 \times 1.602 = 54.4 \times (1.6 + 0.002) = 87.04 + 0.1088 = 87.1488$. More precisely:

   $54.4 \times 1.602 = 87.1488$

   So:

   $E_1 = -87.1488 \times 10^{-19} \text{ J} = -8.71488 \times 10^{-18} \text{ J}$

   5. Round to match the given options. …

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8. COMEDK 2024Set 2024-A1 markMCQ
   Q.The $\Delta \mathrm{H}_{(\mathrm{f})}^{\mathrm{o}}$ of $\mathrm{NO}_2(\mathrm{~g})$ and $\mathrm{N}_2 \mathrm{O}_4(\mathrm{~g})$ are 16.0 and $4.0 \mathrm{k} \mathrm{cal} \mathrm{mol}^{-1}$ respectively. The heat of dimerisation of $\mathrm{NO}_2$ in $\mathrm{k}$ cal is : (A) $-16$ k cal (B) $-8$ k cal (C) $-28$ k cal (D) $-14$ k cal

   ›Reveal solutionSolution

   Dimerisation is $2\,\mathrm{NO_2(g) \to N_2O_4(g)}$, so $\Delta H = \Delta H_f(\mathrm{N_2O_4}) - 2\,\Delta H_f(\mathrm{NO_2}) = 4.0 - 32.0 = -28$ kcal.

   The dimerisation reaction:

   $2\,\mathrm{NO_2(g) \rightarrow N_2O_4(g)}$

   Using $\Delta H = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$: …

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9. COMEDK 2024Set 2024-E1 markMCQ
   Q.Given : $\begin{gathered} \Delta \mathrm{H}^0 \mathrm{f}_{\text {of }} \mathrm{CO}_2(\mathrm{~g})=-393.5 \mathrm{~kJ} / \mathrm{mol} \\ \Delta \mathrm{H}^0{ }_{\mathrm{f}} \text { of } \mathrm{H}_2 \mathrm{O}(\mathrm{l})=-286 \mathrm{~kJ} / \mathrm{mol} \\ \Delta \mathrm{H}^0{ }_{\mathrm{f}} \text { of } \mathrm{C}_3 \mathrm{H}_6(\mathrm{~g})=+20.6 \mathrm{~kJ} / \mathrm{mol} \end{gathered}$ $\Delta \mathrm{H}^0$ isomerisation of Cyclopropane to Propene $=-33 \mathrm{~kJ} / \mathrm{mol}$ What is the standard enthalpy of combustion of Cyclopropane? (A) $-2092$ kJ/mol (B) $-1985$ kJ/mol (C) $+2384$ kJ/mol (D) $-2051$ kJ/mol

   ›Reveal solutionSolution

   The key idea is to use Hess’s law: combine the combustion of propene with the isomerisation enthalpy to find the combustion enthalpy of cyclopropane. The result is $-2092\ \mathrm{kJ/mol}$, which corresponds to option (A).

   We are given the standard enthalpies of formation for $\mathrm{CO_2(g)}$, $\mathrm{H_2O(l)}$, and $\mathrm{C_3H_6(g)}$ (propene), plus the enthalpy change for the isomerisation of cyclopropane to propene:

   $\text{cyclopropane} \rightarrow \text{propene}, \quad \Delta H^\circ = -33\ \mathrm{kJ/mol}$

   We need the standard enthalpy of combustion of cyclopropane. Combustion means burning in oxygen to produce $\mathrm{CO_2}$ and $\mathrm{H_2O}$.

   Concept & Intuition

   We don’t have the formation enthalpy of cyclopropane directly, but we can get it from the isomerisation data and the formation enthalpy of propene. Then, using the formation enthalpies of the products, we can compute the combustion enthalpy via Hess’s law:

   $\Delta H^\circ_{\text{comb}} = \sum \Delta H^\circ_f(\text{products}) - \sum \Delta H^\circ_f(\text{reactants})$

   This avoids needing to measure the combustion directly.

   ---

   Step-by-step solution

   1. Find the standard enthalpy of formation of cyclopropane The isomerisation reaction is:

   $\mathrm{C_3H_6(cyclopropane)} \rightarrow \mathrm{C_3H_6(propene)}$

   Given $\Delta H^\circ_{\text{isom}} = -33\ \mathrm{kJ/mol}$.

   By definition:

   $\Delta H^\circ_{\text{isom}} = \Delta H^\circ_f(\text{propene}) - \Delta H^\circ_f(\text{cyclopropane})$

   So:

   $-33 = (+20.6) - \Delta H^\circ_f(\text{cyclopropane})$

   $\Delta H^\circ_f(\text{cyclopropane}) = 20.6 + 33 = 53.6\ \mathrm{kJ/mol}$

   2. Write the combustion reaction for cyclopropane Cyclopropane is $\mathrm{C_3H_6}$. Complete combustion:

   $\mathrm{C_3H_6(g)} + \frac{9}{2} \mathrm{O_2(g)} \rightarrow 3\mathrm{CO_2(g)} + 3\mathrm{H_2O(l)}$

   (Balanced: 3 C → 3 CO₂, 6 H → 3 H₂O, oxygen: $3\times2 + 3\times1 = 9$ O atoms → $\frac{9}{2}$ O₂.)

   3. Apply Hess’s law for combustion enthalpy

   $\Delta H^\circ_{\text{comb}} = \left[3 \Delta H^\circ_f(\mathrm{CO_2}) + 3 \Delta H^\circ_f(\mathrm{H_2O})\right] - \left[\Delta H^\circ_f(\mathrm{cyclopropane}) + \frac{9}{2} \Delta H^\circ_f(\mathrm{O_2})\right]$

   The standard enthalpy of formation of $\mathrm{O_2(g)}$ is zero.

   Substitute values:

   $$$$

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10. COMEDK 2024Set 2024-M1 markMCQ
    Q.The standard enthalpy of formation of $\mathrm{CH}_4$, the standard enthalpy of sublimation of Carbon and the bond dissociation enthalpy of Hydrogen gas are $-74.8,+719.6$ and $436 \mathrm{~kJ} / \mathrm{mol}$ respectively. What is the bond enthalpy of $\mathrm{C}-\mathrm{H}$ bond in Methane? (A) 18.7 kJ (B) 416.6 kJ (C) 74.8 kJ (D) 1666.4 kJ

    ›Reveal solutionSolution

    The bond enthalpy of C–H in methane is found by applying Hess’s law: the enthalpy of formation of CH₄ equals the enthalpy to sublime carbon plus twice the H–H bond dissociation enthalpy minus four times the C–H bond enthalpy. Solving gives 416.6 kJ/mol, so the correct option is (B).

    Concept & Intuition

    Bond enthalpy is the energy required to break one mole of a specific bond in the gas phase. For methane (CH₄), we want the average C–H bond enthalpy. We cannot measure it directly, but we can use Hess’s law: the enthalpy change for a reaction is the same whether it happens in one step or many.

    The formation of CH₄ from its elements in their standard states (C(s, graphite) and H₂(g)) is given. We can imagine breaking the reactants into atoms (which costs energy) and then forming C–H bonds (which releases energy). The net enthalpy change is the standard enthalpy of formation. This lets us solve for the unknown C–H bond enthalpy.

    Step-by-step reasoning

    1. Write the target reaction The formation of methane:

    $\text{C(s, graphite)} + 2\,\text{H}_2(g) \rightarrow \text{CH}_4(g) \quad \Delta H_f^\circ = -74.8\ \text{kJ/mol}$

    2. Break the process into atomization steps
       • Sublimation of carbon:

    $\text{C(s)} \rightarrow \text{C(g)} \quad \Delta H = +719.6\ \text{kJ/mol}$

    • Dissociation of hydrogen molecules:

    $2\,\text{H}_2(g) \rightarrow 4\,\text{H}(g) \quad \Delta H = 2 \times 436 = +872\ \text{kJ/mol}$

     (Bond dissociation enthalpy of H–H is 436 kJ/mol, and we need to break two moles of H₂.)
    

    3. Form C–H bonds

    The carbon atom and four hydrogen atoms combine to form methane:

    $\text{C(g)} + 4\,\text{H}(g) \rightarrow \text{CH}_4(g)$

    This step releases energy equal to four times the C–H bond enthalpy (since four bonds form). Let the C–H bond enthalpy be $x$ kJ/mol. Then the enthalpy change for this step is $-4x$ (negative because bond formation is exothermic). …

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11. COMEDK 2024Set 2024-M1 markMCQ
    Q.If the enthalpy of formation of a diatomic molecule $\mathrm{AB}$ is $-400 \mathrm{~kJ} / \mathrm{mol}$ and the bond dissociation energies of $\mathrm{A}_2$ and $\mathrm{B}_2$ and $\mathrm{AB}$ are in the ratio $2: 1: 2$, what is the bond dissociation enthalpy of $\mathrm{B}_2$ ? (A) 800 kJ/mol (B) 600 kJ/mol (C) 1600 kJ/mol (D) 400 kJ/mol

    ›Reveal solutionSolution

    Using $\Delta H_f = \tfrac12\,\text{BDE}(A_2) + \tfrac12\,\text{BDE}(B_2) - \text{BDE}(AB)$ with the ratio $2:1:2$ gives $\text{BDE}(B_2) = 800\ \text{kJ/mol}$ — option (A).

    Concept

    The standard formation reaction of the diatomic $AB$ from its elements is

    $\tfrac12 A_2 + \tfrac12 B_2 \rightarrow AB$

    Its enthalpy equals the energy to break half a mole of $A_2$ bonds and half a mole of $B_2$ bonds (endothermic, $+$) minus the energy released on forming one mole of $A\text{--}B$ bonds (exothermic, $-$).

    Solution

    1. Enthalpy balance in terms of bond dissociation enthalpies (BDE):

    $\Delta H_f = \tfrac12\,\text{BDE}(A_2) + \tfrac12\,\text{BDE}(B_2) - \text{BDE}(AB)$

    2. Apply $\text{BDE}(A_2):\text{BDE}(B_2):\text{BDE}(AB) = 2:1:2$. Let $\text{BDE}(B_2)=x$, so $\text{BDE}(A_2)=2x$ and $\text{BDE}(AB)=2x$. …

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12. COMEDK 2023Set 2023-M1 markMCQ
    Q.If 2 moles of $\mathrm{C}_6 \mathrm{H}_6(\mathrm{~g})$ are completely burnt $4100 \mathrm{~kJ}$ of heat is liberated. If $\Delta H^{\circ}$ for $\mathrm{CO}_2(\mathrm{~g})$ and $\mathrm{H}_2 \mathrm{O}(l)$ are $-410$ and $-285 \mathrm{~kJ}$ per mole respectively then the heat of formation of $\mathrm{C}_2 \mathrm{H}_6(g)$ is (A) $-116 \mathrm{~kJ}$ (B) $-375 \mathrm{~kJ}$ (C) $-775 \mathrm{~kJ}$ (D) $-885 \mathrm{~kJ}$

    ›Reveal solutionSolution

    Using Hess's law on the combustion ($\Delta H_c=-2050$ kJ/mol) with $\Delta H_f(CO_2)=-410$ and $\Delta H_f(H_2O)=-285$ gives a heat of formation of magnitude 375 kJ, matching option (B).

    The data are self-consistent only for the hydrocarbon $C_2H_6$ (ethane), whose combustion is

    $C_2H_6+\tfrac{7}{2}O_2\rightarrow 2CO_2+3H_2O.$

    Heat liberated for 2 mol is 4100 kJ, so per mole $\Delta H_c=-2050$ kJ.

    Applying $\Delta H_c=\sum\Delta H_f(\text{products})-\Delta H_f(\text{reactant})$:

    $-2050=[2(-410)+3(-285)]-\Delta H_f(C_2H_6)$ …

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