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chemistry > Topic 4: Chemical Bonding And Structure > Flashcards

Topic 4: Chemical Bonding And Structure Flashcards

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1
Q

Define an ionic bond.

A

An ionic bond refers to the electrostatic attraction between the electric charges of a cation and an anion.

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2
Q

Define an ion.

A

An ion is formed when one or more electrons are transferred from one atom to another. The driving force for this electron transfer is usually the formation of a noble gas electron configuration.
Usually between metals and non-metals, but remember that the definition involves action and anion, eg. Ammonium cation and chloride anion.

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3
Q

What happens during oxidation?

A

An atom loses one or more electrons.

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4
Q

What happens during reduction?

A

An atom gains one or more electrons.

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5
Q

Describe ionic compounds.

A

Are typically solid.

Have lattice-type structures that contain a 3D repeating units of positive and negative ions.

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6
Q

What is the octet rule?

A

Octet rule states the elements tend to lose, gain or share electrons in order to acquire a noble gas core electron configuration

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7
Q

What are the physical properties of ionic compounds?

A

Ionic compounds have a high melting and boiling points, because of the strong electrostatic forces of attraction between the ions in their lattice structures.

Very low volatility.

In solid state, they do not conduct electricity, because the ions are fixed in the lattice, but in liquid state they do.

Dissolve in polar solvents, such as water, but do not in non-polar. The partial charged of H and O in water are attracted to the ions in the lattice, so individual ions are pulled out of the lattice and become surrounded by water molecules.

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8
Q

Define volatility.

A

Volatility refers to the tendency of a substance to vaporise.

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9
Q

Define a covalent bond.

A

A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei in order to attain a noble gas electron configuration.

It’s a region of relatively high electron density between nuclei.

Usually between non-metals.

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10
Q

What is a Lewis symbol?

A

A Lewis symbol is a simple method of representing the valence electrons of an element, where you draw dots around the element symbol showing all the valence electrons.

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11
Q

Describe the bond strength between single, double and triple bond.

A

Triple bond is the strongest, then double and then single.

Opposite is true for bond length.

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12
Q

Define polar covalent bond.

A

Polar covalent bond, where the electrons shared between the atoms are unequally distributed, which causes the molecule to have a slight electrical dipole with one end being slightly positive and the other slightly negative. Denoted with a delta + and delta -.

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13
Q

Define electronegativity.

A

Electronegativity is the relative attraction that an atom of an element has for the shared pair of electrons in a covalent bond.
Fluorine is the most electronegative.
Trends mirror those of ionization energies across a period and down a group.

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14
Q

What is the main use of electronegativity values?

A

The main use is to estimate whether a bond is ionic, pure-covalent (non-polar) or polar covalent based on the electronegativity value differences.
Where for ionic bonds: >1.8
Pure covalent =0
Polar covalent: 0

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15
Q

What is the bond angle in linear geometry (two electron domains)

A

180

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16
Q

What is the bond angle in trigonal planar (three electron domains)

A

120

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17
Q

What is the bond angle in tetrahedral (four electron domains)

A

109.5

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18
Q

What is an electron domain?

A

Electron domain is the field occupied by a pair of electrons. Can be occupied by either bonding pairs of electrons or lone pair, both affecting the shape of the molecule.

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19
Q

What is an electron domain geometry?

A

Geometry based on the total number of electron domains predicted from VSEPR theory.

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20
Q

What is a molecular geometry?

A

Gives the shape of the molecule. The lone pairs of electrons affect the bond angles in a molecule and they occupy more space than bonding pairs.

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21
Q

What is the name and the bond angle in a ammonia?

A

Trigonal pyramidal.

107

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22
Q

What is the name and the bond angle in water?

A

V-shaped.

104.5

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23
Q

What are the factors that play a role in determining the bond angle?

A
  • no. of bonding and lone pairs of electrons
  • electronegativity differences
  • multiple bonds (occupies more space)
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24
Q

What is the bond angle of AB2E?

A

<120

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25
Q

What is the bond angle of AB3E?

A

<109.5

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26
Q

What is the bond angle of AB2E2?

A

<109.5

27
Q

Define resonance.

A

Resonance involves using two or more Lewis structures to represent a particular molecule or ion. A resonance structure is one of two or more alternative Lewis structures for a molecule or ion that cannot be described fully with one Lewis structure alone.

Represented with a double-headed arrow.

28
Q

Define delocalization.

A

In delocalization electrons are shared by ore than two atoms in a molecule or ion as opposed to being localised between a pair of atoms.

29
Q

Define molecular polarity.

A

Molecular polarity shows whether a molecule itself (rather than the bond) is polar or non-polar. e.g. a non-polar molecule might have polar bonds.

30
Q

How to deduce molecular polarity?

A

1) using VSEPR deduce the molecular geometry
2) for each bond present deduce the bond polarity using the electronegativity differences and draw associated dipole moments
3) using vector addition, sum all the dipole moments to establish the net dipole for the molecule. If there is a net dipole it means that the molecule is polar.

31
Q

Define an allotrope.

A

Allotropes are different structural modifications of the same element. Both physical and chemical properties may vary.

32
Q

List 4 allotropes of carbon

A

graphite
diamond
graphene
C60 fullerene

33
Q

What is a covalent network solid?

A

A covalent network solid is one in which the atoms are held together by covalent bonds in a giant 3D lattice structure. e.g. graphite, diamond and graphene.

34
Q

What is graphite?

A
  • example of covalent network solid
  • layers of hexagonal rings consisting of carbon atoms, connected by weak intermolecular forces of attraction, called London forces
  • each carbon atom adopts a trigonal planar geometry and is covalently bonded to 3 other carbons at a bond abgle of 120 degrees
  • good conductor of electricity
  • has delocalised pi electrons
35
Q

What are the properties of covalent network solids?

A
  • high melting points, >1000 degrees Celsius
  • poor conductors of electricity, except for graphite and graphene
  • insoluble in common solvents
  • hard
36
Q

What is a diamond?

A
  • in the lattice structure of diamond, each carbon atom is covalently bonded to 4 other carbon atoms, in a tetrahedral arrangement with 109.5 degrees bond angle
  • very hard substance due to the covalently bonded interlocking structural arrangement of tetrahedra
  • valence electrons are localised in a single sigma covalent bonds and therefore they can’t move freely
37
Q

What is graphene?

A

Graphene is one of the thinnest and strongest known materials

  • 2D crystal
  • one atom thick
  • single planar sheet of carbon atoms arranged hexagonally
  • each carbon is covalently bonded to three other carbons
  • carbon atoms are densely packed in a honeycomb crystalline lattice
  • excellent thermal and electrical conductor (300x more efficient than copper)
  • rolled up sheet of graphene forms a carbon nanotube
  • folded up into a cube carbon nanotube forms a fullerene
38
Q

List three 2D materials.

A 
Graphene
Boron nitride (BN)
Molybdenum disulphide (MoS2)
39
Q

What is C60 fullerene?

A
  • forms when vaporised carbon condenses in an atmosphere of an inert gas
  • there are 20 hexagonal surfaces and 12 pentagonal surfaces
  • is not a covalent network solid
  • composed of individual molecules with strong covalent bonds, but with weak van der Waals forces between the molecules.
  • black solids
  • do not dissolve in water, but can dissolve in non-polar solvents, such as benzene.
  • in organic solvents, they form coloured solutions, the colour depends on the solvent and varies from red to brown to magenta
  • have the ability to fit inside a hydrophobic cavity that forms the active sire of HIV protease enzymes and thereby inhibit them.
40
Q

What are carbon nanotubes?

A
  • have electrical conductivity about 10x better than copper and are 100x stronger than steel
41
Q

What is quartz?

A

Quartz is silicon dioxide (SiO2)

  • 3D covalent network solid
  • consists of arrays of SiO4 tetrahedra arranged in lattice
  • each silicon is bonded covalently to four oxygen atoms and each oxygen atom is bonded to two silicon atoms
  • Si-O-Si geometrical arrangement is bent due to the presence of two lone pairs for each oxygen
  • has huh melting and boiling point due to strong covalent bonds
  • insoluble in water
  • solid crystalline SiO2 doesn’t conduct electricity, but molten does.
42
Q

What is amorphous?

A

Solid form with no ordered structure

43
Q

What is coordinate covalent bonding?

A

The shared pair of electrons comes from only one of the two atoms as opposed to typical covalent bond where the shared electrons come from both atoms.

Eg. 
NH4^+
H3O^+
CO
Al2Cl6
44
Q

What are intermolecular forces?

A

Are interactions between molecules within a compound

  • responsible for the bulk properties of matter, such as melting and boiling points
  • much weaker than covalent bonds
45
Q

What are the three main intermolecular forces of attraction?

A

_ London forces

  • dipole-dipole forces
  • hydrogen bonding

The first two are collectively called van der Waals forces

46
Q

What are the relative strengths of the three intermolecular forces?

A

London forces < dipole-dipole forces < hydrogen bonds

47
Q

What are London forces?

A

London (dispersion) forces exist in all molecules

  • electrons constantly change position resulting in a temporary dipole, which results in electrostatic attractions between the partial positive charge and the partial negative charge of the neighbouring molecule
  • it’s only temporary!
48
Q

What affects the magnitude of London forces?

A

_ number of electrons —> the greater the number, the larger the distance between the valence electrons and the nucleus, the attraction is weaker and so the electron cloud can be polarised more easily

  • size (volume) of the electron cloud —> in a large electron cloud, the attraction of electrons to the nucleus will not be as great as in a smaller cloud and hence the electrons in a larger cloud can be polarised more easily.
  • shapes of molecules —> straight chain molecule allows for the interaction across the full length of the molecule as opposed to branched molecule; the London forces between are stronger for straight chain molecules
49
Q

What are dipole-dipole forces?

A

_ exist in all polar molecules with a permanent dipole moment.
- the attraction is between the positive end of one permanent dipole and the negative end of another on an adjacent molecule

50
Q

What are dipole-dipole forces?

A

Forces that exist in all polar molecules with a permanent dipole moment. It occurs between the positive end of one permanent dipole and the negative end of another permanent dipole on an adjacent molecule.

51
Q

What is hydrogen bonding?

A

Occurs between molecules when there is a H-F, O-H or N-H bond present, where the bonds are polar covalent.
Often has a large influence on both the properties and structure of materials.

52
Q

What does the strength of the hydrogen bond depend on?

A

Depends on the electrostatic attraction between the lone pair of electrons of the electronegative atom and the nucleus of the proton. ie the hydrogen bonding is stronger in HF than in H2O, because F is more electronegative than oxygen.

53
Q

Describe the hydrogen bonding in water when it’s in liquid and solid state.

A

Because of the hydrogen bonding, the solid state of water (ice) has a lower density than water in the liquid state, which is why ice floats on water. In ice, hydrogen bonds form with adjacent water molecules resulting in a regular and ordered network in the lattice. In liquid phase, the hydrogen bonding is more random resulting in a higher density.

54
Q

What is a metallic bond?

A

It’s the electrostatic attraction between a lattice of positive ions and delocalised electrons.

55
Q

What are delocalised electrons?

A

Delocalised electrons are not associated with a particular nucleus of a metal, but move freely throughout the entire crystalline lattice forming a “sea” of mobile electrons.

56
Q

Name three factors that influence the strength of a metallic bond.

A

1) the number of valence electrons that can become delocalised
2) the charge of the metal ion
3) the ionic radius of the metallic positive ion

57
Q

What is an alloy?

A

An alloy is a mixture that consists of two or more metals or a metal combined with an alloying element composed of one or more non-metals.
Alloys have enhanced properties like strength, hardness and durability.

58
Q

What is the composition and use of brass?

A

copper and zinc

door handles, window fittings, screws

59
Q

What is the composition and use of steel?

A

iron, carbon, and other metals like tungsten

bridges, buildings

60
Q

What is the composition and use of dental amalgam?

A

mercury, silver and tin

teeth fillings

61
Q

Why are metals good conductors?

A

Because of the mobiles delocalised electrons, they can move through the metallic structure and thus carry an electric current.

impurities in a metal increase electrical resistance.

62
Q

Why are metals malleable?

A

Because the positive ions can slide past one another leading to rearrangement of the shape of the solid. The metallic bonds within the lattice do not have a defined direction, thus the cations can slide in any direction.

63
Q

Why does calcium have a higher melting point than potassium?

A

1) calcium has two delocalised electrons per atom, whereas potassium has only one, therefore the electrostatic attraction between the cation and delocalised electrons is greater in calcium
2) calcium forms a 2+ ion, therefore greater electrostatic attraction
3) the size of the ionic radius is smaller for calcium, which implies that the delocalised electrons will be more strongly attracted to the Ca2+ ion.

64
Q

List four conditions of VSEPR theory.

A

1) The electron pairs around the central atom repel each other
2) Bonding pairs and lone pairs of electrons arrange themselves to be as far apart as possible
3) Bond angles are maximised
4) Molecules and ions take up the shape that minimises the repulsion between bonding and lone pairs of electrons.

chemistry (25 decks)

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