1
Q
What is a covalent bond?
A
An electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
2
Q
Why is there a covalent bond?
A
- A covalent bond results from the overlap of atomic orbitals to produce a molecular orbital
- A molecular orbital contains two electrons, shared between the two nuclei
- Both the electron density and the electrostatic attraction are localised
3
Q
What is a lone pair?
A
- It is a pair of electrons that are not used in covalent bonding
- They affect both the shape and reactivity of molecules
4
Q
What is a dative covalent bond?
A
- The lone pair can be offered to an electron deficient compound such as BF3 or H+ to form another covalent bond
- These bonds are called dative covalent bonds
- They are made up of a shared pair of electrons that have both come from the same atom
- These bonds are chemically indistinguishable from covalent bonds
5
Q
Why does the octet rule not always hold?
A
Bonding prediction cannot be based solely on the assumption that compounds are only stable with a noble gas electronic structure!
6
Q
Why can you break the octet rule?
A
- The maximum number is related to the group so Phosphorus can only have 5 e.g. pentachloride could not have hexachloride
- This is because the promotion occurring earlier would be too hard since it is too big jump to the next energy level?
7
Q
Why can you have an expanded octet?
A
- (more than 8 electrons in their outer shell)
- They can promote electrons to higher energy levels
- ONLY from third period downwards can do this as they have access to 3D!
8
Q
How many different fluorides can phosphorus, sulphur and chlorine make?
A
- Phosphorus: PF3, PF5
- Sulphur: SF2, SF4, SF6
- Chlorine: ClF, ClF3, ClF5, ClF7
- Take into account the group of the element
- The charge on the element (i.e. not PF1?)
- Could not be PF4 as then have 1 electron unpaired and you NEED electrons to be paired! (use this as a check!)
- Work backwards as you defo know the group so that would be the maximum and it has to be every other
9
Q
What is ionic bonding?
A
An ionic bond is an electrostatic attraction between positive and negative ions
10
Q
What is a positive ion called?
A
A cation
11
Q
What is a negative ion called?
A
An anion
12
Q
How do you work out the charge of an ion?
A
- Metals make cation and non-metals make anions
- The magnitude of the charge for s and p block elements can be deduced from the group number
- The charge of a d block element is even using roman numerals
- Compound ion charged need to be learnt
13
Q
What is the melting and boiling points of ionic lattices like?
A
- Always very high as there is a strong electrostatic attraction between the oppositely charged ions and so lots of energy is needed to overcome the multiple activation energies between ions
- Higher charge and so stronger attraction: higher Mp/BP
14
Q
What is the average bond enthalpy?
A
- It is a measurement of covalent bond strength
- The larger the value of the average bond enthalpy, the stronger the covalent bond
15
Q
What is the solubility of ionic lattices like?
A
- Polar solvents (water) break down the lattic and surround the ions
- Higher charge, stronger attraction so harder for water to break up
16
Q
What is the conductivity of ionic lattices like?
A
- When aqueous/molten: ions can move and so can carry a charge
- When solid: ions can’t move and so cannot carry a charge
17
Q
Describe the structure of sodium chloride
A
- Giant ionic lattice, resulting from oppositely charged ions strongly attracted in ALL directions
1. Each Na+ ion is surrounded by 6 Cl- ions
2. Each Cl- ion is surrounded by 6 Na+ ions
3. Each ion is surrounded by oppositely charged ions, forming a giant ionic lattice
18
Q
What is valence shell electron pair repulsion theory?
A
The shape adopted by molecules to keep repulsive forces to a minimum
19
Q
What is it called when there are two bonding pairs? What is the angle?
A
- Linear
- 180
20
Q
What is it called when there are three bonding pairs? What is the angle?
A
- Trigonal planar
- 120
21
Q
What is it called when there are four bonding pairs? What is the angle?
A
- Tetrahedral
- 109.5
22
Q
What is it called when there are five bonding pairs? What is the angle?
A
- Trigonal bipyramidal
- 120 and 90
23
Q
What is it called when there are six bonding pairs? What is the angle?
A
- Octahedral
- 90
24
Q
How do double bonds act in the VSEPR theory?
A
- Double bonds repel each other in the same way as single bonds. It does not matter that there are four electrons rather than two
- The shape of a compound with a double bond is calculated in the same way
- A double bond repels other bonds as if it was a single e.g. carbon dioxide
25
How do you know if a compound has a lone pair or double bond?
Draw out a dot and cross diagram
26
How do ions act in the VSEPR theory?
- The shapes of ions can be predicted by considering the areas of electron density around the central atom
- The number on the bottom of the multiple element will tell you the shape so e.g. NH4 with be tetrahedral
27
How do lone pairs act in the VSEPR theory?
- If a molecule has lone pairs on the central tom, the shapes are slightly distorted versions of the regular shapes
- This is because of the extra repulsion cause by the lone pairs
- As a result of the extra repulsion, bond angle tend to be slightly less as the bonds are squeezed together
- BP-BP
28
Why does ammonia have a pyramidal shape?
1. (State how many bonds) Ammonia has three bonding pairs and one lone pair
2. Therefore the shape is based on a tetrahedron
3. The lone pair repulsion is stronger than the bonding pair repulsion
4. Therefore the bond angle (H-N-H) is reduced to 107 degrees, and so it is pyramidal
29
What is it called when there is 1 lone pair and 3 bonding pairs? What is the angle? Example?
- Pyramidal
- 107
- E.G NH3
30
What is it called when there are 2 lone pairs and 2 bonding pairs? What is the angle? Example?
- Non-linear
- 104.5
- H2O
31
What is it called when there are 2 lone pairs and 4 bonding pairs? What is the angle? Example?
- Square planar
- 90
- XeF4
32
What is electronegativity?
- It is the ability of an atom to attract the bonding electrons in a covalent bond
- Electronegativity is quantified using the Pauling scale
33
What affects the ability of an atom to attract electrons?
1. The nuclear charge
| 2. The atomic radius (shielding)
34
What is a non-polar bond?
- In a non-polar bond, the bonded atoms, have the same, or similar electronegativity
- For example, hydrogen has equally balanced electrons
- This bond is a pure covalent bond, and is described as non-polar
35
What is a polar bond?
- If one atom in a covalent bond is more electronegative then it will draw more than its share of electrons and carry a partial negative charge (delta minus)
- The other atom has lost some electrons and as a result has a partial positive charge (delta positive)
- Hydrogen chloride has an imbalanced distribution of electron and is described as a polar covalent bond
- You know the electronegativity of an element by looking at the Paulin scale (F the most)
36
What is a permanent dipole?
The separation of opposite charges
37
How is something determined as to being ionic or covalent?
1. Electronegativity values can be used to predict bonding. If the difference is large, then one of the bonded atoms will gain complete control of the electrons, and bond becomes ionic rather than covalent
(Electronegativity scale?)
38
What is the electronegativity difference in a covalent bond?
0
39
What is the electronegativity difference in a polar covalent bond?
0 to 1.8
40
What is the electronegativity difference in an ionic bond?
Greater than 1.8
41
What is a polar molecule?
1. Molecules containing polar bonds may have an overall imbalance in electron distribution, and this results in a polar molecule
2. However, if the distribution of polar covalent bonds is symmetrical, the dipoles cancel each other out. Therefore, it is possible to have molecules that contain polar bonds but are non-polar overall
42
How do you know if a molecule is polar?
1. Different elements (more than two elements!)
2. Lone pairs (of even XeF4 symmetry)
- Look at sheet for examples!
43
What are intermolecular forces?
They are weak interactions between charged parts (dipoles) of different molecules. The charges that develop within molecules may be temporary or permanent
44
What are London forces?
- (Induced dipole-dipole interactions)
- All molecules experience London forces (non polar, polar, ionic, covalent etc)
1. The RANDOM MOVEMENT of electrons leads to short lived imbalances in electron density
2. This results in an INSTANTANEOUS DIPOLE
3. An instantaneous drizzle INDUCES a dipole in a neighbouring molecule
4. The two molecules experience ELECTROSTATIC ATTRACTION
45
How would you rationalise the boiling points of F2 and Cl2?
- CL2 higher boiling point than F2
1. CL2 has a larger atomic radius
2. Therefore it has more electrons than fluorine
3. Therefore more london forces, so more instantaneous dipoles, so more induced dipoles, creating a stronger electrostatic attraction
4. Therefore more energy is need to overcome them
46
How would you rationalise the boiling points of pentane and 2,2-dimethyl propane?
- Pentane higher boiling point than 2,2-dimethyl propane
1. 2,2-dimethyl propane is branched and pentane is not, and branched alkanes have lower boiling points because
2. Branching results in less surface contact between molecules
3. Therefore there are less induced dipoles and thus fewer London forces
4. Therefore less energy is needed to overcome them
- More info?
47
What are permanent dipole-dipole attractions?
1. Molecules that have a permanent overall dipole will attract each other all the time, as well as having the usual London forces
2. As a result the forces between polar molecules are stronger than non-polar molecules
3. This leads to higher boiling points than expected
48
What is hydrogen bonding?
1. A hydrogen bond is the strongest type of intermolecular force
2. It is an interaction between an electronegative atom with a lone pair (N,O,F) and a hydrogen atom bonded directly to the most electronegative atoms (N,O,F)
49
What is the structure of a metal?
- Metallic bonding is the strong electrostatic attraction between the cations and delocalised electrons
1. Layers of positive ions surrounded by a sea of delocalised electrons
2. Giant metallic lattice
3. Conductivity in a solid state
4. Range of melting points; highest is W lowest is Hg
50
What is the structure of a giant covalent structure?
1. B, Carbon and Si form giant covalent
51
What is the density of ice compared with water as a result of hydrogen bonding?
- Solid ice is less dense than water
1. Hydrogen bonds hold water molecules apart in an open lattice structure
2. The water molecules in ice are further apart than in water
3. Solid ice is less dense than liquid water and floats
- Forms an insulating layer and prevents the water from freezing solid
52
Why does water have a relatively high melting and boilling point as a result of hydrogen bonding?
1. As with all molecules, water has London forces between molecules, and so hydrogen bonds are extra forces, over and above the London forces
2. Therefore a large amount of energy is needed to break the hydrogen bonds in water, so water has a much higher melting and boiling point points than would be expected from just London forces
3. When the ice lattice breaks the rigid arrangement of hydrogen bonds in ice is broken, then water builds the hydrogen bonds break completely
53
What is the structure of a simple molecular lattice? What are some examples?
1. A covalently bonded molecules attracted by intermolecular forces
- The molecules are held in place by weak intermolecular forces
- The atoms within each molecules are bonded together strongly by covalent bonds
2. E.g I2, ice
54
What is the bp and mp of simple molecular?
1. Low mp and bp, as weak intermolecular forces need little energy to be overcomes, but only the weak intermolecular forces are broken, the covalent bonds are strong and do not break
55
What is the solubility of a non polar simple molecular?
1. Soluble in non-polar substances as intermolecular forces form between the molecules and the solvent and the interactions weakens the intermolecular forces in the simple molecular lattice. The intermolecular forces break and the compound dissolves
2. Insoluble in polar substances as there is little interaction between the molecules in the lattice and the solvent molecules. the intermolecular bonding within the polar solvent is too strong to be broken
56
What is the solubility of a polar simple molecular?
1. Polar covalent substances may dissolve in polar solvents as the polar solute molecules and the polar solvent molecules can attract each other (similar to dissolving ionic compound)
2. But the solubility depends on the strength of the dipole and can be hard to predict
57
What is the electrical conductivity of simple molecular like?
1. There are no mobile particles in simple molecular structure
2. With no charged particles that can move, there is nothing to complete an electrical circuit
- Therefore simple molecular structure are non-conductors of electricity
58
Why does the melting point increase with size of ionic lattice?
1. As ionic charge increase, melting point increases
2. The greater the ionic charge, the greater the attraction between ions
3. And the more energy required to break the bonds
59
What is a dative covalent bond?
A dative covalent bond is a shared pair of electrons where both electrons are from the same atom
60
What is an alkali ?
A type of base that dissolves in water forming hydroxide ions, OH- (aq) ions
61
Standard solution?
- A solution of known concentration
- Mass of known substance dissolve it
- Put in volumetric flask
62
What is the shape of and bond angle in an NH4+ ion?
Tetrahedral, 109.5
63
What is a salt?
When the H+ in an acid is replaced by a metal ion or an ammonium ion or a + ion
64
What are some anomalous properties of water?
1. Liquid H2O is denser than solid as in solid state H2O molecules are held apart by hydrogen bonds or ice has an open lattice
2. H2O has a relatively high boiling point or melting point
As relatively strong hydrogen bonds need to be broken or a lot of energy is needed to overcome hydrogen bonds or hydrogen bonds are strong
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