Potential questions on shapes of molecules (excluding carbon). Edexcel Unit 2 AS Chemistry

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  • Created by: Katherine
  • Created on: 28-05-14 10:35

Created using a combination of Edexcel textbook, CGP revision guide, George Facer Unit Guide, and Edexcel revision guide and some past papers

 

They will ask you to draw the shape, state the bond angle(s), name the type of shape for the most basic questions. You will need to also be able to compare bond angles, shapes, and explain why they are different.

*These are slightly more difficult questions that may come up in Sections B/C.

Q: What shape and how many lone pairs on H2S?

H2S shape – bent, two lp. Has 6 valence electrons, 2 are used to form two single bonds with hydrogen, leaving four electrons, thus two bonding pairs.

REMEMBER: Explain how you figured out the number of lp’s by stating number of valence electrons, bp’s AND lp’s

Q: Explain how electron pair repulsion theory predicts the shape of the planar SO3 molecule.

According to VSEPRT, the strength of repulsion between types of electrons is as follows: lp/lp>lp/bp>bp/bp. As the SO3 molecule contains three bonding pairs only, they will distance themselves equally around the central atom to minimise repulsion, and the best way is to arrange by the corners of an equilateral triangle, creating the trigonal planar shape and bond angle of 120.

(NB: The shapes of molecules is predicted from the number of electrons around the central atom)

Q: Why ICl4- is not tetrahedral?

It is square planar because there are both four chlorine atoms, and a further two lp’s of electrons, and according to VSEPRT, lp/lp has the greatest repulsion, so will be distanced furthest apart, hence on opposite sides of the iodine (both above and below), decreasing the bond angle between chlorine atoms

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