Question

VSEPR

Objective :

This lab is to study the shape and polarity of molecules using VSEPR theory with emphasis on sigma bonds (Part 1) and pi bonds and resonance (Part 2)

Procedures:

Nothing to do before the lab. We are making the models together in class.

        

Prelab Questions :

PART 1:  

(PL1) Draw the Lewis dot structure of each compound (as having covalent bonds).

(a) BeH2 (b) BH3 (c) GeH2 (d) CH4 (e) NH3 (f) H2O (g) PCl5 (h) SF4 (I) ClF3 (j) XeF2 (k) SF6 (l) BrF5 (m) XeF4

(PL2) If you are using the ball-and-stick model, what does each ‘ball’ represent? Each “stick”? Be very specific.

(PL3) What does the word “VSEPR” stand for?

(PL4) When and why do we want to discuss VSEPR of a compound?

(PL5) Are all single bonds considered sigma bonds? Explain.

(PL6) For the lab, you were told to draw all compounds as having covalent bonds. Draw the actual (the correct one!) Lewis Dot structure of BeH2 which is ionic.

PART 2:

(PL7) List at least three differences between sigma bonds and pi bonds.

(PL8) Draw the Lewis dot structure of each compound. If resonance forms are present, you must draw all.

1. CH3CH3 (b) CH2CH2 (c) CHCH (d) CO2 (e) NO3-1 (f) C6H6 (this has 6 C’s in a ‘ring’)

Questions :

(Q1) Draw the Lewis Dot structure of CO2 and discuss VSEPR for C.

(Q2) Explain why lone pairs might take up more space than the bonding pair.

(Q3) What is not clear about a question that states ‘discuss the VSEPR of acetic acid, CH3COOH’ ?

(Q4) Draw the Lewis Dot structure of O3 with a linear (I don’t mean the shape is linear) backbone ( O-O-O backbone) and with a cyclic backbone (three oxygen atoms in a ring). Discuss VSEPR of both structures. Can you tell which structure is more stable? HINT: Compare the desired angles vs. actual angles.

(Q5) Draw energy diagram (before and after) for C in each compound in CH3CH3, Ch2CH2, and CHCH.  

(Q6) Draw energy diagram (before and after) for N in NO3-1. Count the electrons carefully. HINT: formal charge!

(Q7) What is the name of the hybrid orbital for C in CO2? How many p-orbitals left un-hybridized?

(Q8) What type of orbitals overlap to form a sigma bond?

(Q9) What type of orbitals overlap to form a pi bond?

(Q10) Which is stronger, sigma or pi? Explain.

    

Summary :

Part 1.

A separate summary sheet will be given after the lab. Try to fill it out without looking at other references. Tape/staple it to the notebook.

Part 2

Sketch each of 6 compounds and ions given in (PL8) showing all sigma bonds (as a line) and pi bonds (must show two ‘lobes’ for each p orbital and “rubber bands” as pi bonds). Shape matters!

If the compound has resonance, draw ONE sketch of what it looks like at all times.

Abstract:

No more than 100 words stating why the lab was done, what was done, the major result(s), and conclusion(s).

Reflection Statement:

One thing I might do differently next time is …… because…….

Answer 1

Part1:

(PL2) 'Ball' refers to atom or group and 'stick' refers to either sigma or pi bond.

(PL3).VSEPR means Valence Shell Electron Pair Repulsion

(PL4) To discuss the molecular geometry of the molecule when lone pair of electrons  are present

(PL5) Yes, all single bonds are sigma bonds.

(PL6)

H:Be:H or H-Be-H Lewis structure

(PL7)  

Sigma bond	Pi Bond
Formed by the head-on overlap of the atomic orbitals	Formed after the formation of sigma bond, by the sidewise overlap of atomic orbitals
Extent of overlap is more, the bond formed is strong	Extent of overlap is less, the bond formed is weak
Free rotation around sigma bond	Restricted rotation around pi bond

(PL8)

Lew's X 0 ССССС tot 37 H, H' CH (a) cң ең С а = си 1. - И І @ HC CH H-C= _ H

19 HCH 0-c=0 7 co :8=cfo Ö= -0 ... Resonating structures - Q: NO x-1