Would you give marks for this question? Watch
The question in the booklet sates "Explain using diagrams, the difference between the sigma and pie bonds in the carbon-carbon double bond of an alkene"
They expect a diagram and explanation for full marks
I've drawn a diagram representing the bonds similar to this diagram
However the mark scheme shows the bonds seperately though
My main issue though is the explanation, I'm not sure if I should give myself a mark or not.
I mentioned that "Pie bonds are weaker covalent bonds than sigma bonds as there is a poor overlap of p orbitals in the pie bond unlike the good overlap of s orbitals in the sigma bond"
The mark scheme is quite generic as it says "any written mention, or clear evidence from the diagram, of the resultant electron density between the two nuclei"
How many marks would you give for that answer?
The main point to raise when talking about relative strengths of pi and sigma bonds is the exposure of the nuclei to each other. Consider a sigma bond, the electron density resides mostly in between the nuclei and around it. However, pi bonds have what is called a 'Nodal Plane' that is a region where the electron density due to the contribution of the pi electrons is zero. This is between the nuclei in pi bonding, hence the nuclei are relatively more 'exposed' to each other and so there is some mutual repulsion going on between the positively charged nuclei. I'm not quite sure what the mark scheme wants, but this is a rough outline on what the reasoning is.
I'm guessing my answer's not correct then right?
It really depends on what your mark scheme is looking for. And also it depends on what exam board you are on as to what you are expected to know. It isn't incorrect what you have said, since there is a greater overlap for sigma in comparison to Pi.
I'll ask one of my chemistry teachers tomorrow
I'm on Edexcel