Chapter 6 Shapes of Molecules and Intermolecular Forces Flashcards
What is Electron-Pair repulsion theory
Pairs of electrons around a central atom repel each other, so they move as far apart as possible to minimise this repulsion. This means molecules containing 3 or more atoms have shapes
What is the molecular shape and bonding angles for:
2 pairs of electrons (or 2 regions of electron density) repelling around a central atom
Linear
180°
When working out shapes, how many regions of electron density do double and triple bonds count as
1
What is the molecular shape and bonding angles for:
3 pairs of electrons around a central atom
Trigonal Planar
120°
What is the molecular shape and bonding angles for:
4 bonding pairs around a central atom with no lone pairs
Tetrahedral
109.5°
What is the molecular shape and bonding angles for:
4 pairs of electrons around a central atom with 1 lone pair
Pyramidal
107°
What is the molecular shape and bonding angles for:
4 pairs of electrons around a central atom with 2 lone pairs
Non Linear
180°
What is the molecular shape and bonding angles for:
5 pairs of electrons around a central atom
Trional Bipyramid
90°
120°
What is the molecular shape and bonding angles for:
6 pairs of electrons around a central atom
Octahedral
90°
By how many degrees is the bond angle reduced for each lone pair
2.5°
Define Electronegativity
Electronegativity is a measure of the ability of an atom in a molecule to attract a pair of electrons in a covalent bond to itself
Finish this sentence:
The higher the electronegativity value…
The more strongly the bonding electrons are attracted to that atom
Going up a group does electronegativity increase or decrease
Electronegativity increases up a group
Across a period, does electronegativity increase or decrease
Electronegativity increases across a period (up to group 7)
What is the most electronegative atom
Fluorine
What group on the periodic table has the least electronegative atoms
1
Which elements have the most electronegative atoms
- Nitrogen
- Oxygen
- Fluorine
- Chlorine
What happens to the pull on bonding electrons as nuclear charge increases
It increases
Why does the pull on bonding electrons decrese further from the nucleus
Pull falls rapidly as electrons become further from the nucleus and the number of shielding inner shells increases
What is a non-polar bond
In a non-polar bond, the bonded electron pair is shared equally between the bonded atoms
When is a bond non polar
- The bonded atoms are the same
- The bonded atoms have the same, or similar, electronegativity values.
What is a polar bond
In a polar bond, the bonded electron pair is shared unequally between the bonded atoms
When is a bond polar
A bond will be polar when the bonded atoms are different and have different electronegativity values, resulting in a polar covalent bond
Why is H–F a polar bond
In H-F, F has a higher electronegativity than H so the bonding electrons are more attracted to the F end of the bond. The bond is polarised
What is the separation of opposite charges called
A dipole
Finish this sentence:
The bigger the difference in electronegativity between the bonding atoms…
The more polar the bond and the greater the ionic character
What difference in electronegativity will result in the bond being ionic instead of Polar Covalent
(Give a value)
1.8+
How can a molecule with polar bonds be non-polar
The molecule is symmetrical and the dipoles cancel out
What has to be true for a molecule with polar bonds to be polar
The molecule has to be unsymmetrical so the dipoles do not cancel out
Can polar molecules (eg water) dissolve ions
Yes, polar molecules (particularly water) can form strong enough attractions to ions to permit dissolving in some cases
When is dissolving likely
If the energy needed to break the lattice is similar to that released, it will dissolve
What is hydration
When attractions form between water moleculesand ions releasing energy
Are Nitrates soluble
Yes, all nitrates are soluble
Are Chlorides soluble
Yes, all chlorides are soluble, except for AgCl and PbCl₂
Are Sulfates soluble
Yes, most sulfates are soluble, except for BaSO₄ PbSO₄ and SrSO₄
Are Carbonates soluble
No, all carbonates are insoluble, except (NH₄)₂CO₃ and those of the Group 1 elements
Are sodium, potassium, and ammonium salts soluble
Yes all sodium, potassium and ammonium salts are soluble
What do the solubility rules depend on
Amount of solute and/or solvent and temperature
Define intermolecular forces
Weak interactions between dipoles of different molecules
Which atoms or molecules London forces (induced dipole dipole forces)
All atoms and molecules
Explain London Forces
- Electrons in an atom or molecule are constantly moving
- A temporary uneven distribution of electrons can occur for an instant
- This creates an instantaneous dipole
- The instantaneous dipole induces a dipole in its neighbours
- leading to an attraction
- Each dipole can induce many others
How long do london forces last
Induced dipoles are only temporary. In the next instant of time, the induced dipoles may disappear, only for the whole process to take place amongst other molecules
Which intermolecular forces do non-polar molecules have
London Forces Only
What happens to the london force as the molecule gets larger
Number of electrons increases so size of the induced dipoles also get larger, giving stronger forces
Why do the melting and boiling points increase as the strength of the London force increases
As the strength of the London forces increases, more energy is needed to overcome them, so the melting and boiling point increases
Define Permanent Dipole-Dipole interactions
Dipole-Dipole interactionshese are formed between molecules which are polar (molecules with both polar bonds and a shape that does not allow the dipoles to cancel each other out)
How long do permanent dipole-dipole interactions last
They are permanent and exert stronger forces. Opposites attract
What are simple molecular substances
Simple molecular substances are made up of discrete molecules containing a specific number of atoms and having a consistent molecular formula e.g CH₄, N₂
What is the regular structure of solid simple molecular substances called
Simple Molecular Lattice
Are the intermolecular forces holding simple molecular substances together strong or weak
Weak
What are the properties of simple molecular substances
Summarised
- Low melting and boiling points
- Non-polar molecules tend to dissolve in non-polar solvents, whereas polar molecules tend to dissolve in polar solvents
- Do not conduct electricity
Why do simple molecular substances have low melting and boiling points
Intermolecular forces usually do not need a lot of energy to overcome so most simple molecular substances have low melting and boiling temperatures with many being gases or liquids at room temperature
Describe the solubility of simple molecular substances
Like dissolves like
Non-polar molecules tend to dissolve in non-polar solvents.
Polar molecules tend to dissolve in polar solvents
Why can’t simple molecular substances conduct electricity
There are no mobile charged particles
What is Hydrogen bonding
A special type of permanent dipole-dipole interaction containing:
* An electronegative atom with a lone pair of electrons, e.g. oxygen, nitrogen, or fluorine
* A hydrogen atom attached to an electronegative atom, e.g. H-O, H-N, or H-F
What is the strongest type of intermolecular force
Hydrogen Bonding
What are the anomalous properties of water
- Solid (ice) is less dense than liquid (water)
- Water has a relatively high melting point and boiling point
- Water has a relatively high surface tension
Why is ice less dense than water
- Hydrogen bonds hold water molecules apart in an open lattice structure
- The water molecules in ice are further apart than in liquid water
- Solid ice is less dense than liquid water and floats
How many hydrogen bonds can each water molecule form
4
The hydrogen bonds extend outwards, holding water molecules slightly apart in an open tetrahedral arrangement
The hydrogen bonds extend outwards, holding water molecules slightly apart in an open tetrahedral arrangement
What is roughly the bond angle for each Hydrogen bond
180⁰
DO General trend – boiling point increases. As molecule size (and number of electrons) increases, the strength of the London forces between molecules increases and more energy is needed to overcome them.
FINISH H2O has the highest boiling point as it can form more extensive hydrogen bonding between its molecules
FINISH H2O, NH3, and HF do not follow the trend. They have stronger hydrogen bonds between their molecules which require more energy to break
FINISH urface tension is caused by molecules on the surface experiencing unbalanced hydrogen bonding forces pulling them in.
Molecules in the bulk experience balanced forces in every direction.
FINISH The double helix structure of DNA is held together by hydrogen bonds between base pairs.
