2019 P1 Q5 - Number of Sigma and Pi Bonds in Molecules

In this video we want to discuss 2019 A Level H2 Chemistry Paper 1 Question 5.

2019 p1 q5 question

We need to deduce the number of pi bonds for each of the 4 above-mentioned molecules and determine which ones have 2 pi bonds.

Deduce Sigma Bond and Pi Bond

For covalent bonds between 2 atoms X and Y, there are usually 3 possibilities:

2019 p1 q5 sigma and pi bond

single bond = sigma bond
double bond = sigma bond and pi bond
triple bond = sigma bond and 2 pi bonds

Hence we just need to draw the Lewis structure for the molecules to count the number of pi bonds.

1. Aluminium chloride dimer, Al2Cl6

2019 p1 q5 Al2Cl6

Aluminium chloride dimer has only single bonds and no pi bonds.

Take note the dative bonds from Cl to Al are still single bonds and hence considered as sigma bonds.

2. Carbon dioxide, CO2

2019 p1 q5 CO2

Carbon dioxide has 2 double bonds hence 2 pi bonds.

3. Ethanal, CH3CHO

2019 p1 q5 CH3CHO

Ethanal has only 1 double bond hence only 1 pi bond.

4. Propene, CH2CHCH3

2019 p1 q5 CH2CHCH3

Propene has only 1 double bond hence only 1 pi bond.

We can finally compare the options.

2019 p1 q5 answer

Only species 2 has two pi bonds hence the answer to this question is C.

Topic: Chemical Bonding, Physical Chemistry, A Level Chemistry, Singapore

Back to list of questions for 2019 A Level H2 Chemistry Paper 1

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