Bond Enthalpy and Mean Bond Enthalpy

Bond Enthalpy and Mean Bond Enthalpy

What is bond enthalpy?

The enthalpy change when one mole of a bond in the gaseous state is broken.

What symbol is used for bond enthalpy?

ΔbH.

What is the unit of bond enthalpy?

kJ mol⁻¹.

What physical state must substances be in when measuring bond enthalpy?

The gaseous state.

Is bond breaking endothermic or exothermic?

Endothermic.

Why is bond breaking endothermic?

Energy must be supplied to overcome the attraction between the bonded atoms.

What sign do bond enthalpies have?

Positive (+).

What general equation represents the bond enthalpy of a diatomic molecule XY?

XY(g) → X(g) + Y(g).

What is meant by the H–H bond enthalpy?

The enthalpy change when one mole of H–H bonds in gaseous hydrogen molecules is broken to form gaseous hydrogen atoms.

What equation represents the H–H bond enthalpy?

H₂(g) → 2H(g).

Why must each bond in a polyatomic molecule be considered separately?

The energy required to break a particular type of bond varies depending on the chemical environment surrounding it.

Why can successive C–H bonds in methane have different bond enthalpies?

After each C–H bond is broken, the chemical environment of the remaining C–H bonds changes.

What is mean bond enthalpy?

The mean enthalpy change when one mole of a specified type of bond, averaged over many different molecules, is broken in the gaseous state.

Why are mean bond enthalpies used?

The bond enthalpy of the same type of bond varies between different molecules.

How is a mean bond enthalpy calculated?

By averaging the bond enthalpies for that type of bond across different bonds or molecules.

What is the approximate mean C–H bond enthalpy in methane?

+416 kJ mol⁻¹.

What is the approximate mean C–H bond enthalpy across a large number of organic compounds?

+413 kJ mol⁻¹.

Why might the bond enthalpy for a particular bond differ from its mean bond enthalpy?

Mean bond enthalpy is an average from many different molecules, while the actual bond strength depends on the bond's chemical environment.

What is the mean C–H bond enthalpy in ethane?

Approximately +420 kJ mol⁻¹.

What does a larger bond enthalpy indicate?

A stronger bond that requires more energy to break.

What does a smaller bond enthalpy indicate?

A weaker bond that requires less energy to break.

Which is generally stronger: a C–C single bond or C=C double bond?

A C=C double bond.

Which is generally stronger: a C=C double bond or C≡C triple bond?

A C≡C triple bond.

What is the approximate mean bond enthalpy of a C–C bond?

+347 kJ mol⁻¹.

What is the approximate mean bond enthalpy of a C=C bond?

+612 kJ mol⁻¹.

What is the approximate mean bond enthalpy of a C≡C bond?

+838 kJ mol⁻¹.

What is the approximate mean bond enthalpy of a C–O bond?

+358 kJ mol⁻¹.

What is the approximate mean bond enthalpy of a C=O bond?

+743 kJ mol⁻¹.

What is the approximate mean bond enthalpy of an O–H bond?

+464 kJ mol⁻¹.

What is the approximate mean bond enthalpy of a C–F bond?

+467 kJ mol⁻¹.

What is the approximate mean bond enthalpy of a C–Cl bond?

+346 kJ mol⁻¹.

What is the approximate mean bond enthalpy of a C–Br bond?

+290 kJ mol⁻¹.

What is the approximate mean bond enthalpy of a C–I bond?

+228 kJ mol⁻¹.

How are mean bond enthalpies written in data tables?

E followed by the bond in brackets, for example E(C–C) = +347 kJ mol⁻¹.

Why is bond enthalpy always quoted for gaseous species?

This provides a consistent standard for comparing bond strengths without intermolecular forces or changes of state affecting the value.

Define bond enthalpy.

The enthalpy change when one mole of a bond in the gaseous state is broken.

Define mean bond enthalpy.

The mean enthalpy change when one mole of a specified type of bond, averaged over many different molecules, is broken in the gaseous state.