Write the structure and hybridisation of ethyne

Ethyne is $\mathrm{HC\equiv CH}$.

In ethyne, each carbon atom is $sp$-hybridised. Therefore, the molecule is linear with bond angle $180^\circ$.

Check each statement

Option A:

"The carbon-carbon bond in ethyne is weaker than that in ethene"

• Ethyne has a triple bond $\mathrm{C\equiv C}$.
• Ethene has a double bond $\mathrm{C=C}$.
• A triple bond has higher bond dissociation energy than a double bond, so it is stronger, not weaker.

Hence, Option A is incorrect.

Option B:

"Both carbons are $sp$ hybridised"

This is true for ethyne.

Hence, Option B is correct.

Option C:

"The $\mathrm{C-C}$ bond in ethyne is shorter than that in ethene"

• Triple bond length $<$ double bond length.
• So the $\mathrm{C\equiv C}$ bond in ethyne is shorter than the $\mathrm{C=C}$ bond in ethene.

Hence, Option C is correct.

Option D:

"Ethyne is linear"

Since each carbon is $sp$ hybridised, the geometry is linear.

Hence, Option D is correct.

Conclusion

The incorrect statement is: $\boxed{A}$

Comparison with stored correct answer

Stored correct answer: $A$

My derived answer matches the stored correct answer.