Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Thermodynamics question

2022 · 25 Jun · Shift 2 · Q2
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Main
  3. /Chemistry
  4. /Thermodynamics
  5. /2022 · 25 Jun · Shift 2 · Q2

Thermodynamics question

2022 · 25 Jun · Shift 2 · Q2

JEE MainChemistryThermodynamicsMCQ+4 / −1
At 25 ∘^\circ∘ C and 1 atm pressure, the enthalpy of combustion of benzene (I) and acetylene (g) are −-− 3268 kJ mol −-− 1 and −-− 1300 kJ mol −-− 1, respectively. The change in enthalpy for the reaction 3 C2H2C_2H_2C2​H2​(g) →\to→ C6H6C_6H_6C6​H6​ (I), is :
  1. A
    +324 kJ mol −-− 1
  2. B
    +632 kJ mol −-− 1
  3. C
    −-− 632 kJ mol −-− 1
  4. D
    −-− 732 kJ mol −-− 1
View written solutionFree

Correct answer: C

  1. Write the combustion reactions

For acetylene:

C2H2(g)+52O2(g)→2CO2(g)+H2O(l)\mathrm{C_2H_2(g) + \frac{5}{2}O_2(g) \to 2CO_2(g) + H_2O(l)}C2​H2​(g)+25​O2​(g)→2CO2​(g)+H2​O(l)

Given:

ΔHc(C2H2)=−1300 kJ mol−1\Delta H_c(\mathrm{C_2H_2}) = -1300\ \text{kJ mol}^{-1}ΔHc​(C2​H2​)=−1300 kJ mol−1

For 3 moles of acetylene:

3C2H2(g)+152O2(g)→6CO2(g)+3H2O(l)\mathrm{3C_2H_2(g) + \frac{15}{2}O_2(g) \to 6CO_2(g) + 3H_2O(l)}3C2​H2​(g)+215​O2​(g)→6CO2​(g)+3H2​O(l)

So,

ΔH=3(−1300)=−3900 kJ\Delta H = 3(-1300) = -3900\ \text{kJ}ΔH=3(−1300)=−3900 kJ

For benzene:

C6H6(l)+152O2(g)→6CO2(g)+3H2O(l)\mathrm{C_6H_6(l) + \frac{15}{2}O_2(g) \to 6CO_2(g) + 3H_2O(l)}C6​H6​(l)+215​O2​(g)→6CO2​(g)+3H2​O(l)

Given:

ΔHc(C6H6)=−3268 kJ mol−1\Delta H_c(\mathrm{C_6H_6}) = -3268\ \text{kJ mol}^{-1}ΔHc​(C6​H6​)=−3268 kJ mol−1
  1. Target reaction

We need:

3C2H2(g)→C6H6(l)\mathrm{3C_2H_2(g) \to C_6H_6(l)}3C2​H2​(g)→C6​H6​(l)

Let its enthalpy change be ΔH\Delta HΔH.

  1. Apply Hess's law

If we add the target reaction

3C2H2(g)→C6H6(l)(ΔH=?)\mathrm{3C_2H_2(g) \to C_6H_6(l)} \qquad (\Delta H = ?)3C2​H2​(g)→C6​H6​(l)(ΔH=?)

and then combust benzene:

C6H6(l)+152O2(g)→6CO2(g)+3H2O(l)(ΔH=−3268)\mathrm{C_6H_6(l) + \frac{15}{2}O_2(g) \to 6CO_2(g) + 3H_2O(l)} \qquad (\Delta H = -3268)C6​H6​(l)+215​O2​(g)→6CO2​(g)+3H2​O(l)(ΔH=−3268)

we get the combustion of 3 moles of acetylene:

3C2H2(g)+152O2(g)→6CO2(g)+3H2O(l)(ΔH=−3900)\mathrm{3C_2H_2(g) + \frac{15}{2}O_2(g) \to 6CO_2(g) + 3H_2O(l)} \qquad (\Delta H = -3900)3C2​H2​(g)+215​O2​(g)→6CO2​(g)+3H2​O(l)(ΔH=−3900)

Therefore,

ΔH+(−3268)=−3900\Delta H + (-3268) = -3900ΔH+(−3268)=−3900 ΔH=−3900+3268=−632 kJ mol−1\Delta H = -3900 + 3268 = -632\ \text{kJ mol}^{-1}ΔH=−3900+3268=−632 kJ mol−1
  1. Check options
  • A: +324+324+324 ❌
  • B: +632+632+632 ❌
  • C: −632-632−632 ✅
  • D: −732-732−732 ❌

Hence, the correct answer is:

−632 kJ mol−1\boxed{-632\ \text{kJ mol}^{-1}}−632 kJ mol−1​
PreviousNext

More from Thermodynamics

  • 2.4 g coal is burnt in a bomb calorimeter in excess of oxygen at 298 K and 1 atm pressure. The temperature of the calorimeter rises from 298 K to 300 K. The enthalpy change during…2022 · Numerical
  • For the reaction H2​F2​( g)→H2​( g)+F2​( g)ΔU=−59.6 kJ mol−1 at 27∘C. The enthalpy change for the…2022 · Numerical
  • For complete combustion of methanol CH3​OH(I) + 23​ O2​(g) → CO2​(g) + 2H2​O(I) the amount of heat produced as measured by bomb calorimeter is 726 kJ mol − 1 at 27 ∘ C. The enthalpy of combustion for the…2022 · Numerical
  • A fish swimming in water body when taken out from the water body is covered with a film of water of weight 36 g. When it is subjected to cooking at 100 ∘ C, then the internal energy for vaporization in kJ mol − 1 is ​…2022 · Numerical
  • The molar heat capacity for an ideal gas at constant pressure is 20.785 J K−1 mol−1. The change in internal energy is 5000 J upon heating it from 300 K to 500 K.…2022 · Numerical
  • A gas (Molar mass = 280  g mol−1) was burnt in excess O2​ in a constant volume calorimeter and during combustion the temperature of calorimeter increased from 298.0 K to 298.45K.…2022 · Numerical
  • Match List-I with List-II. Choose the correct answer from the options given below : Includes table2022 · MCQ
  • When 5 moles of He gas expand isothermally and reversibly at 300 K from 10 litre to 20 litre, the magnitude of the maximum work obtained is ​ J. [nearest integer] (Given : R = 8.3 J K − 1 mol − 1 and log 2 =…2022 · Numerical