The standard heat of formation of N2H4(g) is 62 kJ mol−1. Then the bond energy of N≡N is _____Given: The bond dissociation energies of N−N,H−H and N−H bonds are 159 kJ mol−1,436 kJ mol−1,398 kJ mol−1 respectively.
Question
The standard heat of formation of N2H4(g) is 62 kJ mol−1. Then the bond energy of N≡N is _____Given: The bond dissociation energies of N−N,H−H and N−H bonds are 159 kJ mol−1,436 kJ mol−1,398 kJ mol−1 respectively.
Solution
To find the bond energy of N≡N, we first need to understand the structure of N2H4. It consists of two N-H bonds and one N-N bond.
The total energy required to break all the bonds in one mole of N2H4 is calculated as follows:
Energy to break N-H bonds = 4 * Energy of one N-H bond = 4 * 398 kJ/mol = 1592 kJ/mol Energy to break N-N bond = 1 * Energy of one N-N bond = 1 * 159 kJ/mol = 159 kJ/mol
Total energy to break all bonds = 1592 kJ/mol + 159 kJ/mol = 1751 kJ/mol
The heat of formation of N2H4 is the energy released when one mole of N2H4 is formed from its elements in their standard states. In this case, it's given as 62 kJ/mol.
The energy required to break the N≡N bond can be calculated by subtracting the heat of formation of N2H4 and the energy required to break the H-H bonds from the total energy required to break all bonds in N2H4.
Energy to break H-H bonds = 4 * Energy of one H-H bond = 4 * 436 kJ/mol = 1744 kJ/mol
Energy to break N≡N bond = Total energy to break all bonds - Heat of formation of N2H4 - Energy to break H-H bonds = 1751 kJ/mol - 62 kJ/mol - 1744 kJ/mol = -55 kJ/mol
The negative sign indicates that energy is released when the N≡N bond is formed. Therefore, the bond energy of N≡N is 55 kJ/mol.
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