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.
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:
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
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
Carbon dioxide has 2 double bonds hence 2 pi bonds.
3. Ethanal, CH3CHO
Ethanal has only 1 double bond hence only 1 pi bond.
4. Propene, CH2CHCH3
Propene has only 1 double bond hence only 1 pi bond.
We can finally compare the options.
Only species 2 has two pi bonds hence the answer to this question is C.
Topic: Chemical Bonding, Physical Chemistry, A Level Chemistry, Singapore
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