Molecular Shapes and Intermolecular Forces

Question 1

What is the VSEPR Theory, and explain it

Answer 1

• Electron pairs around a central atom will position themselfs as FAR APART as possible

Question 2

Name examples for Linear , Trigonal Planar, Tetrahedral , Trigonal Bipyramidal

Answer 2

1. Linear = CO2, BeCl2, BeH2
2. Trigonal Planar = AlCl3, BCl3,
3. Tetrahedral = CH4, P4
4. Trigonal Bipyriamidal = PF5

Question 2

State the 8 main molecular shapes and angles needed in chemistry…

Answer 2

1. Linear = 180^
2. Trigonal Planar = 120^
3. Tetrahedral = 109.5^
4. Trigonal Bipyramidal = 120^ and 90^
5. Octahedral = 90^
6. Non-Linear = 104.5^
7. Pyramidal = 107^
8. Square Planar = 90^

Question 3

What is the difference between triple, double and single covalent bonds?

and REPULSION?!?!?!?

Answer 3

NO DIFFERENCE!
* These bonded regions will all bring about the exact same molecular shapes
* Keep in mind tht BONDED PAIRS STILL DO REPELL, BUT NOT AS MUCH AS LONE PAIRS

Question 4

How can we draw ionically charged molecules as compared to normall molecules??

Answer 4

• IF +VE: make sure to TAKE AWAY ELECTRONS from central atom
• IF -VE: make sure to ADD ELECTRONS to the central atom…

use the common sense with positive and negatives in ions since GCSE..

Question 5

What is Electronegativity?

Answer 5

• This is the ability for an atom to attract electron pairs towards its nucleus in a COVALENT BOND

Question 6

Explain the 3 main factors contributing to Electronegativity

Answer 6

• Nuclear Charge = An increase will cause increased attraction between positive nucleus and negative shared electron pairs = increased electronegeativity
• Atomic Radius = An increase will cause increased distance from positive nucleas, weakening attraction = decreased electronegativity

Question 7

What will occur during differences in electronegativity?

Answer 7

• This will be due to a shared pair of electron being attracted more by one atom than the other

Question 8

What determines what kind of bond will be formed?

ionic, colavlent, polar colavent……….

Answer 8

• If eletronegativity difference IS LESS THAN 0.4, this makes a pure covalent bond
• if electrongeativity IS MORE THAN 0.4 AND LESS THAN 1.7, this makes a polar covalent bond
• IF MORE THAN 1.7, this makes an ionic bond

Question 9

How can we represent electronegativity on a molecule?

Answer 9

• Arrowhead with a line points in direction of partial negative charges

Question 10

What can we say about molecular symmetry?

Answer 10

• If dipoles occur in EQUALL AND OPPOSITE DIRECTIONS, this will cause DIPOLE CANCELLING = nonpolar molecule
• If not, there WILL be an overall PERMANENT DIPOLE, bieng a POLARISED MOLECULE

Therefore, a molecule may have POLAR COVALENT BONDS but overall be a NON POLAR MOLECULE due to SYMMETRY AND DIPOLE DIRECTION

Question 11

What is a Dipole?

Answer 11

• This is a molecule with a separation of partial + and - charges!

Question 12

Name the Enthalpies of the intermolecular forces in order……..

plus a lil sneaker bonding……

Answer 12

1. Induced Dipole-Dipole = 10 kjmol-1
2. Permanent Dipole-Dipole = 20 kjmol-1
3. Hydrogen Bonding = 40 kjmol-1
   4. COVALENT BOND = 150 to 500 kjmol-1

Question 13

Explain the occurence of Induced D-D Interactions….

Answer 13

1. Due to the RANDOM MOVEMENT OF ELECTRONS, this may cause an instantaneous dipole in and atom/molecule
2. This will cause the neighbouring atom to also have an induced dipole (chain effect)
3. The forces of attraction are known as LONDON FORCES

instantaneous = super-quick temporary moment

ALL ATOMS AND MOLECULES EXPERIENCE THIS

Question 14

Explain the factors that affect Induced D-D Interactions..

Answer 14

1. DISTRIBUTION OF ELECTRON AT ANY TIME, as more eletrons a a pole = greater induced london force
2. NUMBER OF ELECTRONS, as more eletrons randomly moving = potential for greater induced dipole

Question 15

Explain the occurence of Permanent D-D Interactions…

look at this area, you’ll be silly not to………

Answer 15

1. Only be can be experienced with PERMANENT dipoles only
2. When polar molecules interact via partial charges of one another, held via this intermolecular force

!!!!!!!!!!!!!!!!!polar molecules can experience BOTH INDUCED AND PERMANENT D-D INTERACTIONS, giving them the HIGHER MP/BP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Question 16

Ok, so so what really are Van der Waal’s Forces

another wired name down here………

Answer 16

• They refer to both INDUCED and PERMANENT D-D Interactions
• This is an outdated term, and should never be used unless otherwise instructed
• HOWEVER, London Forces only refers to INDUCED interactions, not permanent

London Forces = Dispersion Forces

Question 17

Explain the occurence of Hydrogen Bonding

VERY UNIQUE INDEED…….,…

Answer 17

1. Only occurs under FON Elements, with lone pairs available
2. Must be a Hydrogen Atom convieniently around to INTERACT with the FON elements

FON = Flourine, Oxygen, Nitrogen !

Question 18

How can we represent a hydrogen bond in a drawing?

Answer 18

USE DOTTED LINES!

Question 19

Why would Ice happen to float in water?

Answer 19

1. When frozen, hydrogen bonds will slow and eventually stop moving
2. They will freeze, arranged in an open lattice that has a lot of internal space
3. An INCREASE in volume for SAME mass will mean decrease in density (and float…)

Question 20

Why does water have to be odd with its MP/BP?

Answer 20

• Strong hydrogen bonds! = a lot of energy to break this intermolecular force! = high tempratures needed to do so!

* STRONG HYDROGEN BONDS ARE ALWAYS THE REASON FOR ANYTHING TO DO WITH WATER

Question 21

What general structures of compounds do we get?

Answer 21

1. Giant Ionic Lattices (NaCl, MgO)
2. Giant Covalent Structures (Diamond, Graphite)
3. Simple Molecular compounds (Cl2, F2)

Question 22

Why is the hydrogen bonding weaker in ammonia than water?

btw there are 3 points to mention

Answer 22

1. This is due to oxygen having greater electronegativity, more lone pairs, able to form more hydrogen bonds per molecule
2. Hence, all these factors will mean that depsite both havin hydrogen bonds, ammonia remains a gas at ambient temprature

Question 23

What is it meant by “weighted mean mass?”

Answer 23

• This is the average masses taking into account relative abundances of the isotopes!!!!!

Question 24

Why would a simple molecular compound not carry electrical charge at all?

Answer 24

• Due to NO MOBILE CARRIERS able to carry ELECTRICAL CHARGE through the structre!!!!!