Question

It is well known that the emission of greenhouse gases such as carbon dioxide presents a very
grave threat to the environment. Attempts are ongoing to absorb (sequester) this gas by several
means. One such approach involves the use of vegetation, as plants utilize the gas by means of
photosynthesis.
One such plant, which has shown great promise, is the “Spekboom” (Portulacaria Afra) which
is endemic to the semi-arid Karoo region of the Eastern Cape.
 It has been estimated that fully-grown Spekboom can absorb 3.8 tonnes of carbon per
hectare of plants per year.
 If the system is fully functional, South Africa can generate 234.5 × 10ଽ ܹ݇ℎ of electricity
per year.
 Assume that 1559 pounds of CO2 is emitted per year per Megawatt-hour (MWh) of
electricity generated in the country.
Determine, by means of the use of conversion factors, how many square kilometres of
Spekboom could theoretically absorb all of this carbon dioxide?
Given: 1 hectare = exactly 10000. m2
; 1 pound = 453.59237 g; 1 tonne = exactly 1000 kg
[9]
1.2 Consider the molecule, CℓPO2. (The phosphorus atom is the central atom)

1.3 Draw the 3-D Lewis dot structure of the molecule, including all possible resonance structures.
Your structures must not show any hyper-valency. Use different symbols for the valence
electrons of the different atoms. Do not use dashes to represent bonds. Indicate all lone electron
pairs.
[4]
1.2.2 Calculate the formal charges of all atoms in all the Lewis structures. Write these values on the
structures you have drawn in question 1.2.1.
[2]
1.2.3 Calculate the bond order of the P–O bond(s). Briefly explain the meaning of your answer.
[2]

Answer 1

(Part 1)

Note : In the question it is not clear about the electricity generated by South Africa due to some written description. However it can be verified with original data that the unit meant to be placed there is TWh or 10⁶ MWh. Hence the electricity generated by South Africa would be taken as 234.5 TWh or 234.5 *10⁶ MWh per year. If there is any change in this, student is advised to make the change and do the calculation as per the change in the below described way.

Electricity generated by South Africa =

It is given that

hence

Therefore

We know that

Hence

therefore

This can be written as

produced every year

It has been given that  

1 tonne = 1000kg

Hence our CO₂ produced per year can be further written as

produced every year.

It is also mentioned that (right arrow indicates absorbs)

Hence

Therefore

has to be planted to absorb the carbon dioxide produced.

We have to find the answer in square kilometers.

Hence

---------------> (Final Answer in km² of Speckboom)

Part 2 (Lewis Dot Structure)

(1.2)

Molecule :

The phosphorous molecule has 5 valence electron in its outermost shell. Oxygen has 6 valence electron in its outermost shell while Chlorine has 7 electrons in its valence shell. Phosphorous is known to exhibit +3 or +5 valencey and chlorine exhibits -1 and oxygen -2 in almost all cases. Using this as our initial assumption,

As the net charge on the molecule is 0

Solving for

Hence Phosphorous donates all its valence electrons and attains a valencey of +5.

As it is mentioned to represent all the atoms with different valence electron we will follow the below given legend key.

As phosphorous is the central atom and 1 oxygen requires two electrons (hence two oxygen atoms will take up 4 electrons) and chlorine atom will take up the remaining 1 electron. Inorder to get a rough idea about the structure one needs to have an idea about the lone pair interactions that would come in to play. Chlorine with 3 lone pairs will try to occupy a place where it has maximum space while each oxygen with two lone pair will occupy the space where it has minimum interaction with the lone pairs of both the chlorine and neighbouring oxygen atom. This can be best visualized when they are at the corners of a triangle with phosphorous as the centre atom.

Now joining the bonds we get the following structure. There is only one resonance structure as all the valencies are satisfied and both oxygen atoms are double bonded. The structre is also trigonal planar and hence no possibility of different structure. Hence only one resonance structure

Some additional information about the structure

Hybridisation = sp²

Bond angle = 120⁰

Bond structure = Trigonal planar

(1.2.2)

Formal charge calculation is used to determine the best posible resonant structure/ Lewis structure in such a way that when we calculate formal charge for every atom it needs to be values which are close to zero. Here we only have one resonant strucutre and hence we calculate formal charge for each atom in this strucutre. Formal charge is given by:

Writting in short

For each atoms we have to calculate it. One has to be carefull while writing the bonding electrons. You have to include electrons used to make up that bond even if it belongs to other atoms. (Bonding electrons/2) can also be treated as no of bonds for verification.

Phosphorous

Oxygen atom 1

Oxygen atom 2

Chlorine atom

Now to represent this in the structure near to the atoms we write the formal charge in square brackets.

(1.2.3)

Bond order is the number of electron pairs between two atoms forming bonds in a Lewis structure. Hence the bond order of P-O bond can be understood as follows

No of bonding electron pairs = 2 (that is 4 electrons)

Hence bond order of P-O bond is 2

Significance of bond order is that as the bond order increases the strength of the bond increases. It would require more energy to break the bond. Notice that the bond order of P-Cl is one and hence when we supply energy to the molecule the first bond that breaks is CL^- and hence the molecule will release a Cl^- ion on disassociation resulting in a PO₂⁺ ion. Hence bond order gives us an idea about the bond strength and disassociation products also. We can also further extend our reasoning to the fact that as the bond order increases the bond length decreases. Hence P-O bond will be shorter than P-Cl bond.