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Chemistry Question 129 – AP-EAMCET 2025

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The enthalpy of atomization of CH3NH2(g) is 2313 kJ mol−1. If ΔC−HH⊖ and ΔN−HH⊖ are 414 and 389 kJ mol−1 respectively, then ΔC−NH⊖ (in kJ mol−1) will be

The enthalpy of atomization is the total energy required to break all the bonds in one mole of a gaseous substance to form individual gaseous atoms.

Step 1: Identify Bonds and Their Counts in Methylamine✦ Active

Methylamine (CH3NH2) has the following structure: one carbon atom bonded to three hydrogen atoms and one nitrogen atom, and the nitrogen atom is bonded to two hydrogen atoms. Therefore, the molecule contains:

- Three C−H bonds

- Two N−H bonds

- One C−N bond

💡 Teacher's Secret Hint

Visualizing the Lewis structure or structural formula of the molecule is crucial to correctly count the number of each type of bond.

Step 2: Formulate the Enthalpy of Atomization Equation○ Expand

The enthalpy of atomization (ΔatomizationH⊖) is the sum of the average bond enthalpies of all the bonds in the molecule.

ΔatomizationH⊖=(3×ΔC−HH⊖)+(2×ΔN−HH⊖)+(1×ΔC−NH⊖)
💡 Teacher's Secret Hint

Remember that bond enthalpies are always positive values, as energy is required to break bonds.

Step 3: Substitute Known Values○ Expand

Given:

- ΔatomizationH⊖=2313 kJ mol−1

- ΔC−HH⊖=414 kJ mol−1

- ΔN−HH⊖=389 kJ mol−1

Substitute these values into the equation:

2313=(3×414)+(2×389)+ΔC−NH⊖
💡 Teacher's Secret Hint

Pay close attention to the stoichiometry (number of each type of bond) when multiplying bond enthalpies.

Step 4: Solve for the C-N Bond Enthalpy○ Expand

Perform the multiplications and then solve for ΔC−NH⊖:

2313=1242+778+ΔC−NH⊖
2313=2020+ΔC−NH⊖
ΔC−NH⊖=2313−2020
ΔC−NH⊖=293 kJ mol−1
💡 Teacher's Secret Hint

Double-check your arithmetic, especially when dealing with multiple additions and subtractions. The units for bond enthalpy are typically kJ mol−1.

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