Chemistry · Thermodynamics · NEET
Bond dissociation enthalpy is the enthalpy change when ONE specific mole of bonds is broken in the gas phase, for one exact bond in one exact molecule. Mean bond enthalpy is used when a molecule has several bonds of the same kind. You break ALL of them, add up the total energy, then divide by the number of bonds to get an average. So dissociation enthalpy is exact for one bond; mean bond enthalpy is an average over many identical bonds. This matters for NEET because questions on methane (CH4) or water (H2O) expect the AVERAGE value, while questions on H2 or Cl2 give an exact single value.
In a molecule like methane, CH4, all four C-H bonds LOOK the same. But when you remove them one at a time, each step needs a different amount of energy (because the leftover fragment changes). So there is no single true C-H value. To make bond-energy calculations simple, chemists take the total energy to break all 4 bonds and divide by 4. That average is the mean bond enthalpy. For methane, the total atomization energy is about 1665 kJ/mol, so the mean C-H bond enthalpy is 1665/4 = about 416 kJ/mol.
For a DIATOMIC molecule (only 2 atoms, one bond), YES they are equal. Example: H2(g) -> 2H(g), the H-H bond dissociation enthalpy is 435 kJ/mol, which is also the enthalpy of atomization of H2. But for a POLYATOMIC molecule they are NOT equal. Atomization enthalpy breaks the whole molecule into all its separate atoms at once, so it equals the SUM of all bond energies, not a single bond. This diatomic-only equality is a favourite NEET trap.
Every time you pull off one hydrogen, the fragment left behind is different (CH3, then CH2, then CH, then C). Each fragment has different stability, so each next C-H bond needs a different amount of energy to break. The four values are roughly 427, 439, 452, and 347 kJ/mol. They are all different, but their sum is fixed. Divide the sum by 4 and you get the mean C-H bond enthalpy (~416 kJ/mol). This is exactly why the 'mean' idea exists.
Breaking a bond ALWAYS needs energy, so bond dissociation enthalpy is always positive (endothermic, +sign). Forming a bond ALWAYS releases energy, so bond formation is negative (exothermic, -sign). NEET uses this in the formula: enthalpy of reaction = (sum of bond enthalpies of bonds BROKEN in reactants) - (sum of bond enthalpies of bonds FORMED in products). Remember the direction and you will not lose the sign in numerical questions.
In NEET numerical problems you almost always use MEAN bond enthalpy values from a table, because those are the general averages that work across many molecules. You use exact bond dissociation enthalpy only when the question specifically gives you one bond's exact value (like H2 or Cl2, which are diatomic anyway). If a table just says 'C-H bond enthalpy = 414 kJ/mol', that is a mean value.
The bond dissociation energies of X2, Y2 and XY are in the ratio of 1 : 0.5 : 1. ΔH for the formation of XY is -200 kJ/mol. The bond dissociation energy of X2 will be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Both bond dissociation enthalpy and mean bond enthalpy are measured in kJ/mol (kilojoules per mole of bonds).
Yes. It represents energy needed to break bonds, and bond breaking always absorbs energy, so the value is always positive.
Yes. H2 is diatomic with one bond, so breaking that bond (435 kJ/mol) is the same as fully atomizing it into 2 H atoms.
Take the total energy to atomize CH4 into C(g) + 4H(g), about 1665 kJ/mol, and divide by 4 to get roughly 416 kJ/mol per C-H bond.
NEET numerical questions on reaction enthalpy use tabulated mean bond enthalpies. Knowing it is an average (not exact per bond) prevents sign and value mistakes.
