The enthalpy of the formation of $CO_2$ and $H_2O$
Question:

The enthalpy of the formation of CO2CO_2 and H2OH_2O are - 395kJ395\, kJ and - 285kJ285\, kJ respectively and the enthalpy of combustion of acetic acid is 869kJ.869\, kJ. The enthalpy of formation of acetic acid is:

Updated On: Jun 20, 2022
  • 235 kJ
  • 340 kJ
  • 420 kJ
  • 491 kJ
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The Correct Option is D

Solution and Explanation

Combustion reaction for acetic acid is :

CH3COOH+2O22CO2+2H2OCH _{3} COOH +2 O _{2} \longrightarrow 2 CO _{2}+2 H _{2} O

Thus,

HR=HfH _{R}=\sum H _{f}^{\circ} (product) Hf-\sum H _{f}^{\circ} (reactant)

869=[2×Hf(CO2)-869=\left[2 \times H _{f\left( CO _{2}\right)}^{\circ}\right.
+2×Hf(H2O)](Hf(CH3COOH)+0)\left.+2 \times H _{f\left( H _{2} O \right)}^{\circ}\right]\left( H _{ f \left( CH _{3} COOH \right)}^{\circ}+0\right)
869=[2×(395)+2×(285)]-869=[2 \times(-395)+2 \times(-285)]
[Hf(CH3COOH)]-\left[ H _{f\left( CH _{3} COOH \right)}^{\circ}\right]
869=790570Hf(CH3COOH)-869=-790-570- H _{f\left( CH _{3} COOH \right)}^{\circ}
Hf(CH3COOH)=1360+869=491kJ\therefore H _{f\left( CH _{3} COOH \right)}^{\circ}=-1360+869=-491\, kJ
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Thermodynamics in physics is a branch that deals with heat, work and temperature, and their relation to energy, radiation and physical properties of matter.

Important Terms

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Thermodynamic Process

A system undergoes a thermodynamic process when there is some energetic change within the system that is associated with changes in pressure, volume and internal energy.

There are four types of thermodynamic process that have their unique properties, and they are:

  • Adiabatic Process – A process in which no heat transfer takes place.
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Zeroth Law of Thermodynamics

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First Law of Thermodynamics 

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Second Law of Thermodynamics 

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Third Law of Thermodynamics

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