PHET - Molecular Shape & Polarity PDF

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PHET - Molecular Shape & Polarity...

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PHET Simulation – MOLECULAR SHAPE & POLARITY Atoms bond to satisfy their need for more electrons. If both atoms have high electronegativities (e.g. nonmetals), atoms will share electrons to satisfy the Octet Rule – every atom wants 8 electrons to fill the s and p orbitals in the outer energy level. Because electrons have a negative charge and atoms occupy space, bonds and electrons will spread out as much as possible. Since we write in a two-dimensional plane on paper, it is difficult to visualize the true geometry of these molecules. This activity allows us to visualize on a more 3-dimensional scale.

Part 1 – MOLECULAR SHAPE   

[24 Comm marks]

https://phet.colorado.edu/sims/html/molecule-shapes/latest/molecule-shapes_en.html Choose MODEL mode Check the boxes for “Molecule Geometry”, “Electron Geometry”, “Show Lone Pairs” and “Show Bond Angles”

1. The phrase ‘electron domain’ is used in discussions of molecular geometry to mean either a lone pair or a bond on the central atom of a molecule. Using only single-bonded atoms, fill in the following chart below for different domains [2.5m] # domain

# of bonds

# lone pair

Molecule Geometry

Electron Geometry

Bond Angle

Drawing

2

2

0

Linear

Linear

180°

●─○─●

3

0

Trigonal Planar

Trigonal Planar

120°

2

1

Bent

Trigonal Planar

120°

4

0

Tetrahedral

Tetrahedral

109.5°

3

1

Trigonal Pyramid

Tetrahedral

109.5°

2

2

Bent

Tetrahedral

109.5°

3

4

2. Do any of the geometry names change if you use double or triple bonds instead of single bonds? [1m]

3. If you replace a bond with a lone pair of electrons

[1m]

a) How does the electron geometry change? The electron geometry does not change.

b) How does the molecule geometry change?

1

Since the number of bonds decreases by 1 when replaced with a lone pair of electrons, the molecule geometry will change. For example, tetrahedral (4 bonds) will turn into trigonal planar (3 bonds).

4. Can you change the shape of a molecule by twisting it around? What happens to the molecule when you stop twisting it? [0.5m]

No you cannot. When you stop twisting it, the molecule reverts back to its original shape.

5. Draw the 3-D Lewis Structure for the molecules in the table below

[3m]

a) List the bond angles in each molecule using the values determined in Table 1*** Molecul e

3-D LEWIS STRUCTURE

Shapes

Model Bond Angle

Real Bond Angle

Molecule: Tetrahedral CH4 Electron: Tetrahedral Molecule: NH3 Electron: Molecule: H2O Electron: Molecule: BF3 Electron: Molecule: SO2 Electron: Molecule: CO2 Electron:

b) Now switch to the REAL MOLECULES mode https://phet.colorado.edu/sims/html/molecule-shapes/latest/molecule-shapes_en.html



Check the boxes for “Molecule Geometry”, “Electron Geometry”, “Show Lone Pairs” and “Show Bond Angles”



Record the real bond angle for each molecule in Table 2 above.

2

6. Which of the molecules in Table 2 violate the OCTET RULE? [1m] 7. Run through the molecules quickly and list any other molecules that also violate the Octet Rule [6m] Molecule

# bonds + Lone pairs

XeF2

2+3=5

Shape

Drawing

M: linear E: Trigonal bipyramidal

M: E: M: E: M: E: M: E: M: E: M: E:

POST-LAB QUESTIONS (to be done at home later) 8. Refer to the Table in Q5

[4m]

a) Explain why in some cases the model bond angles are different from the real bond angles? Give an example from Table 2

b) Explain why water has a larger difference than ammonia when comparing the real bond angle to the model bond angle.

3

c) Ammonia (NH3) and BF3 have similar empirical formulas, yet they have different 3-D structures. Use the data from Table 1 to explain this observation.

d) Carbon dioxide (CO2) and SO2 have similar empirical formulas, yet they have different 3-D structures. Use the data from Table 1 to explain this observation.

9. In this simulation a) How many lone pairs and bonds can you add to a central atom in total? Why do you think you cannot get more than this number around the central atom? [1m]

b) What is the electron geometry of this molecule?

[1m]

Draw the structure

10. Imagine a molecule with 7 electron domains. The geometry is called pentagonal bipyramidal. This has a lot of similarities to a 6-coordinate molecule in Question 9 except there are 5 domains in the horizonal plane and 2 domains in the vertical plane. Predict the following based on this information: a) Draw the structure of a molecule with 7 bonding domains. List all the bond angles possible between adjacent atoms on the diagram. [1m]

4

b) Predict the electron and molecular geometry for a molecule with 6 bonding domains and a single lone pair. Draw this molecule and explain why you choose to draw the molecule that way. [1m]

c) Predict the electron and molecular geometry for a molecule with 5 bonding domains and two lone pairs. Draw this molecule and explain why you choose to draw the molecule that way. [1m]

5

Part 2 – MOLECULE POLARITY

[16 TI marks]

https://phet.colorado.edu/sims/html/molecule-polarity/latest/molecule-polarity_en.html Two-Atom Investigation 

Check the boxes for “Bond Dipole”, “Partial Charges” and “Bond Character”. Turn on the Electric Field.

1. Keep the electronegativity of A at low and increase the electronegativity of B

[1.5m]

As the electronegativity of atom B increases the... 

The arrow



The partial charges



The bond character

2. Change the electronegativity of A to the middle and vary the electronegativity of B [1.5m] 

The polarity arrow always points to the

electronegative atom.



The partial positive charge is always on the



The larger the electronegativity difference the more

electronegative atom. the bond character.

Three-Atom Investigation 3. Keep the original “bent” arrangement of atoms and set the electronegativities of A and C to low and B to middle. Click to show the bond dipoles, molecular dipoles and partial charges. [2m] a) On the diagram on the right, draw



the bond dipoles on the bonds



the overall molecular dipole (in a different colour)



the partial charges

b) Click the electric field on. Describe what happened to the molecule – explain your observation.

4. Reset! Keep the “bent” arrangement of atoms and set the electronegativities of A and C to high and B to middle. Click to show the bond dipoles, molecular dipoles and partial charges. [2m] a) On the diagram on the right, draw



the bond dipoles on the bonds



the overall molecular dipole (in a different colour)



the partial charges

6

b) Click the electric field on. Describe what happened to the molecule – explain your observation.

5. Reset! Change the arrangement of atoms to a HORIZONAL “linear” arrangement (see below) and set the electronegativities of A and C to high and keep B to middle. Click to show the bond dipoles, molecular dipoles and partial charges. [2m] a) On the diagram on the right, draw



the bond dipoles on the bonds



the overall molecular dipole (in a different colour)



the partial charges

b) Click the electric field on. Describe what happened to the molecule – explain your observation.

6. Take off the electric field. Rotate the molecule to keep the HORIZONTAL “linear” arrangement (see below) and set the electronegativities of A to high, keep B to middle and set C to low. Click to show the bond dipoles, molecular dipoles and partial charges. [2m] a) On the diagram on the right, draw



the bond dipoles on the bonds



the overall molecular dipole (in a different colour)



the partial charges

b) Click the electric field on. Describe what happened to the molecule – explain your observation.

7

POST – LAB QUESTIONS (to be done at home later) 7. Predict how the linear A-B-C molecule would behave in the electric field if the electronegativities of A and C are set to low, and the electronegativity of B set to high. Use the diagram below to explain [1m]

8. Predict how the linear A-B-C molecule would behave in the electric field if the electronegativities of A B and C are set to the middle. Use the diagram below to explain [1m]

9. You will now apply what you learned to real molecules a) How would HF move in an electric field? Explain and Draw it in the field provided.

b) How would H2O move in an electric field? Explain and Draw it in the field provided

c) How would CO2 move in an electric field? Explain and Draw it in the field provided

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[3m]...