Q.Calculate the enthalpy change for the process and calculate the bond enthalpy of C–Cl in . kJ mol; kJ mol; kJ mol (enthalpy of atomisation); kJ mol.
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Start your 14-day free trial to unlock the full solution →To break into gaseous atoms, we reverse its formation and add atomisation energies for the elements. The total enthalpy change is kJ mol, giving a C–Cl bond enthalpy of kJ mol.
Why this approach works
Bond enthalpy measures the energy needed to break one mole of a particular bond in the gas phase, producing gaseous atoms. For , we have four C–Cl bonds, so the process requires breaking all four. The challenge is that we're not given this dissociation energy directly—instead, we have formation data and atomisation energies.
The key insight: we can construct any thermochemical process by combining others through Hess's Law. We'll build a cycle that takes back to its elements in their standard states, then atomises those elements into gaseous atoms.
Step-by-step construction
1. Reverse the formation of
The standard enthalpy of formation tells us:
Reversing this:
2. Vaporise to get the gaseous molecule
We need , not the liquid. The vaporisation enthalpy is given:
Combining steps 1 and 2, we can write:
A common mistake is forgetting to account for the phase of . The formation enthalpy given is for the liquid, so we must subtract the vaporisation energy to work with the gas.
3. Atomise carbon
Now we break the solid carbon into gaseous atoms:
This is the enthalpy of atomisation (or sublimation energy) of carbon.
4. Atomise chlorine
We need four moles of , which means breaking two moles of bonds:
Each molecule requires kJ mol to dissociate.
5. Sum the entire cycle …
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