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Chemistry OCR A A-level yr 12 > structure and bonding > Flashcards

structure and bonding Flashcards

1
Q

describe the structure of ionic compounds

A

giant ionic lattice. Each ion is surrounded by 6 of the oppositely charged ion.

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2
Q

properties of ionic bonds

A
  • high melting/boiling point due to strong ionic bonds
    -soluble in polar solvents e.g water as the polar molecules breakdown the lattice and surround each ion in solution.. However in a compound made of ions with large charges, the ionic attraction maybe too strong to breakdown the ionic structure therefore it isn’t very soluble.
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3
Q

what are the two processes solubility requires in an ionic substance?

A

the ionic lattice must be broken down and the water molecules must attract and surround the ions. Therefore solubility depends on the strength of the ionic attraction.

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4
Q

electric conductivity of ionic compounds

A

can only conduct electricity when dissolved or molten as the ions are mobile and can carry the charge.

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5
Q

what is covalent bonding?

A

the strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms

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6
Q

why can some elements have more than 8 electrons in their outer shell?

A

elements with 3 or more shells have d-orbitals which can accommodate for more than 8 electrons, also called the expansion of the octet

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7
Q

what is an octet?

A

8 electrons in the outer shell

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8
Q

what is a dative covalent bond

A

a covalent bond where a lone pair of electrons from one atom is bonded to another. Dative covalent bonds are represented with arrows which must point away from the atom that is providing the lone pair.

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9
Q

what is average bond enthalpy?

A

measurement of covalent bond strength

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10
Q

what does the electron pair repulsion theory state?

A

the shape of a molecule is determined by the electron pairs (outer shell) surrounding the central atom. This is based on the fact that pairs of electrons repel all other electron pairs. The electron pairs now move as far apart as possible to minimise this repulsion.

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11
Q

why do lone pairs repel more strongly than bonding pairs?

A

lone pairs are slightly closer to the central atom. This leads to more repulsion and decreases other bond angles by 2.5 degrees.

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12
Q

what is the name of the shape and bond degrees of methane, ammonia and water?

A

-methane (CH4) has 4 bonded pairs and 0 lone pairs therefore it has a tetrahedral shape and the bond angle is 109.5 degrees
-ammonia (NH3) has 3 bonded pairs and 1 one pair, it has a pyramidal shape and the bond angle is 107 degrees
-water has 2 bonded pairs and two lone pairs, it has a non-linear shape and the bond angle is 104.5 degrees.

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13
Q

what is electronegativity?

A

the ability of an atom to attract the pair of electrons in a covalent bond

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14
Q

what are the three factors electronegativity depends on?

A

1) number of protons in the nucleus
2) atomic radius, the lower the atomic radius the higher the electronegativity
3)shielding, the greater the shells, the lower the electronegativity

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15
Q

how is electronegativity measured?

A

The Pauling scale is used to compare the electronegativity of atoms of different elements

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16
Q

what is a dipole?

A

separation of a charge, with a positive end and a negative end

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17
Q

what is polar?

A

where one end is slightly negative and the other is slightly positive

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18
Q

how can you show bond polarity?

A

using the delta sign. Delta negative goes on the element which is more electronegative.

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19
Q

what is overall polarity also called?

A

dipole moment

20
Q

why is water polar and carbon dioxide isn’t?

A

-in water, the O-H bonds each have a permanent dipole which act in different directions and don’t oppose each other. Therefore the overall oxygen end has a delta negative charge and hydrogen has a delta positive charge.
-In carbon dioxide, the C double bond O bonds each have a permanent dipole which acts in different directions but directly opposes each other. Therefore the overall dipole is 0.

21
Q

How is a London force formed?

A

Due to the uneven distribution of electrons(1), an instantaneous dipole will exist, but its position is constantly changing. The instantaneous dipole induces a dipole on a neighbouring molecule. The induced dipole further dipoles on the neighbouring molecules, which then attract one another . This force is called a London force.

22
Q

what does the strength of the London forces depend on?

A

the number of electrons

23
Q

what are permanent dipole dipole interactions?

A

interaction that act between the permanent dipoles in different polar molecules

24
Q

what are the three types of intermolecular forces?

A

-induced dipole-dipole interactions (London forces)
-permanent dipole-dipole interaction
-hydrogen bonding

25
Q

what is a hydrogen bond are what are the two conditions needed?

A

hydrogen bonding is a type of permanent dipole-dipole interaction containing:
-an electronegative atom with lone pairs of electrons e.g oxygen, nitrogen and fluorine
-a hydrogen atom attached to the electronegative atom
Hydrogen bonds are the strongest forces.

26
Q

where does the hydrogen bonding occur?

A

between the hydrogen which is delta positive and the lone pairs of electrons from the other atom.

27
Q

why does water have a high melting/ boiling point?

A

due to strong hydrogen bonds

28
Q

why is ice less dense than water?

A

in ice, hydrogen bonds are stabilised, moving water molecules further apart than in liquid. This make ice less dense due to air pockets in the open lattice structure.

29
Q

describe the structure of a simple molecular substance?

A

simple molecular lattice. The atoms are held by strong covalent bonds but the molecules are held by weak intermolecular forces.

30
Q

why are non-polar substances soluble in non-polar solvents only?

A

because the solvent molecules form London forces to the substance molecules.

31
Q

why can’t non-polar substances dissolve in polar solvents?

A

Because the polar solvent will be bonded to itself than to the substance

32
Q

what is ionic bonding?

A

the electrostatic attraction between positive and negative ions

33
Q

Describe how the pairs of electrons matches to its shape

A

2 bonded pair: linear (180 degrees)
3 bonded pair: trigonal planar (120 degrees)
4 bonded pair: tetrahedral (109.5 degrees)
3 bonded pair and 1 lone pair: pyramidal (107 degrees)
2 bonded pair and 2 lone pair: non-linear (104.5 degrees)
5 bonded pair: trigonal bipyramid (90 degrees and 120 degrees)
6 bonded pair: octahedral (90 degrees)

34
Q

hydrogen bonds can be formed between hydrogen and which set of atoms?

A

fluorine, oxygen or nitrogen

35
Q

instead of saying weak intermolecular forces you should say?

A

weak instantaneous dipole-induced dipoles

36
Q

what does a giant ionic lattice mean?

A

repeating pattern(1) of oppositely charged ions(1)

37
Q

why do metals have high melting points?

A

because of the strong electrostatic attraction between the cations (positive metal ions) and delocalised electrons

38
Q

Why do giant covalent lattices have high melting point?

A

due to strong covalent bonds

39
Q

name common giant covalent lattices and name their structure?

A

-diamond, silicon, graphite
-tetrahedral structure (diamond)

40
Q

do giant covalent lattices conduct electricity?

A

no except graphite/graphene due to delocalised electrons that can carry the charge

41
Q

what is the bond angle for graphite/graphene?

A

120 degrees by electron pair repulsion

42
Q

why is graphite slippery?

A

weak London forces

43
Q

how does attraction between nuclei and outer electrons in gaseous atoms vary across period 3 (2)?

A

The attraction increases ad the proton number increases(1). All outer electrons experience the same shielding (1).

44
Q

why does H-I have a higher boiling point than H-Br?(1)

A

induced dipole dipole interactions

45
Q

is SF6 non-polar?

A

yes

Chemistry OCR A A-level yr 12 (21 decks)

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