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Chemistry > Chapter 6 > Flashcards

Chapter 6 Flashcards

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1
Q

What is the electron repulsion theory?

A

That the electron pairs which surround the central atom determine the shape if the molecule or ion. The electron pairs repel each other so that they are arranged as far apart as possible. The arrangement of electron pairs minimises repulsion and holds the bonded atoms in a definite shape. Different number of electrons forms different shapes.

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2
Q

What does a 4 electron pair atom form?

A

Four bonded pairs of electrons surrounds the central atom, with the pairs repelling each other as far apart as possible in 3D space. This results in a tetrahedral shape with 4 equal bond angles of 109.5 degrees

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3
Q

Give an example of a tetrahedral?

A

Methane, is symmetrical so forms 4 C-H covalent bonds, with the electron pairs surrounding the central carbon atom. It has 4 equal H-C-H bond angles of 109.5.

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4
Q

What does a solid line indicate?

A

A bond in the plane of the paper.

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5
Q

What does a solid wedge represent?

A

Comes out of the plane of paper

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6
Q

What does a dotted wedge represent?

A

It goes into the plane of the paper.

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7
Q

Explain lone pair repulsions?

A

A lone pair of electrons are slightly closer to the central atom and occupies more space than a bonded pair. Resulting in the lone pair repelling more strongly than a bonding pair.

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8
Q

Show the increase in repulsion?

A

Bonded/ Bonded pair < Bonded/lone pair < Lone/ lone pair

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9
Q

Describe the shape and bond of a compound with 2 electron pairs?

A

E.g Carbon - Linear shape and 180 degrees

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10
Q

Describe the shape and bond of a compound with 3 electron pairs?

A

Trigonal Planar, 120 degrees

E.g Boron trifluoride

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11
Q

Describe the shape and bond of a compound with 4 electron pairs?

A

Tetrahedral shape, 109.5 degrees

E.g Methane

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12
Q

Describe the shape and bond of a compound with 6 electron pairs?

A

Octahedral shape, 90 degrees

e.g SF6

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13
Q

Describe the shape and bond of a compound with 5 electron pairs?

A

Bipyrimidal, 90 and 120 degrees

e.g Phosphorus pentafluoride

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14
Q

Describe the molecular shaped of four bonded electron pairs?

A

Methane, ammonia and water all have 4 electron pairs surrounding the central atom. But ammonia and water have a mix of lone pairs. The 4 electron pairs repel each other as far apart as possible into a tetrahedral arrangement. With the lone pairs repelling more strongly, so the lone pairs repel the bonded atoms, closer together decreasing the bond angle between the bonded pairs.

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15
Q

What is the bond angle reduced by for every lone pair?

A

2.5 degrees

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16
Q

Molecular shapes from multiple bonds?

A

Each multiple bond is treated as a bonding region. For carbon dioxide the four bonded pairs around the central atom are arranged into double bonds which count as two bonded regions. The two bonded regions repair one another as far apart as possible. This gives CO2 a linear shape with all three atoms aligned in a straight line.

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17
Q

What happens when there are greater number of electron pairs?

A

The bond angle decreases

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18
Q

Describe the shape of the ammonium ion?

A

NH4+ has four bonded pairs surrounding the central nitrogen. An ammonium ion has the same number of bonded pairs of electrons around the central atom as a methane molecule. It has the same tetrahedral shape and bond angles 109.5° as a methane molecule.

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19
Q

Describe the shape of a carbonate ions?

A

As it has three regions of electron density surrounding the central atom it has a trigonal planar shape with bond angles of 120°.

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20
Q

What is the shape of a nitrate ion?

A

As it has three regions of electron density surrounding the central atom it has a trigonal planar shape with bond angles of 120°.

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21
Q

What is the shape of a sulphate ion?

A

As it has four regions of electron density surrounding the central sulfur atom it has a tetrahedral shape with bond angles of 109.5°.

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22
Q

What is electronegativity?

A

It is the measure of the ability of an atom in a molecule to attract pair of electrons in a covalent bond.

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23
Q

What happens in molecules in covalent bonds which have the same element?

A

The bonded pair is shared evenly

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24
Q

What happens when the bonded atoms are different elements?

A

The nuclear charges are different, the atoms maybe different sizes and the shared pair of electrons may be closer to one nucleus than the other. The shared pair that in the covalent bond can experience more attraction from one bonded atom than the other.

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25
Q

How is the electronegativity measured?

A

It is measured by the pauling scale which compares the electronegativity of atoms between different elements. The values depend on the elements position within the periodic table.
Across the periodic table:
the nuclear charge increases (the pull on the bonding electrons)
the atomic radius decreases (the electrons get further away from the nucleus.

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26
Q

What does a large Pauling scale indicate?

A

The atoms of the element are very electronegative. (noble gases aren’t indicated on this scale)

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27
Q

How does electronegativity increase?

A

As you go up and across the periodic table.

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28
Q

What is the most electronegative atom ?

A

Fluorine with a value of 4.0.

Non metals: N,O,F,Cl are the most electronegative

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29
Q

What are the least electronegative atoms?

A

Group 1 metals.

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30
Q

Using electronegativity how do you know if a compound is ionic or covalent?

A

If there is a large difference in electronegativity (one bonded atom having a greater attraction from for the shared pair than the other) the more electronegative atom would have gained control over the electrons in the bond will then be ionic.

covalent = 0
polar covalent = 0-1.8 difference
ionic = greater than 1.8 difference

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31
Q

How are non-polar bonds formed?

A

When the atoms bonded on the same or have similar electronegativity. In molecules like hydrogen and oxygen the bonded atoms come from the same element and the electron pair is shared equally which is known as a pure covalent bond. Carbon and hydrogen have similar electronegativity is from non polar bonds.

32
Q

What happens to the electron pair in nonpolar bonds?

A

It is shared equally between the bonded atoms.

33
Q

How are polar bonds formed?

A

It is when the bonded atoms are different and have different electronegativity values resulting in a polar covalent bond. The bonded electron pair is shared unequally between the bonded atoms.

34
Q

Given example of the formation of a polar covalent bond?

A

Hydrogen chloride. Hydrogen has a value of 2.1 and chlorine has a value of 3. Chlorine is more electronegative so has a greater attraction for the bonded pair of electrons and hydrogen resulting in the polar covalent bond. The H-CL bond is polarised with a small partial positive charge on the H and a small parcel negative charge on the Cl.

35
Q

What is the dipole?

A

It is the separation of opposite charges. A dipole in a polar covalent bond which doesn’t change is called a permanent dipole.

36
Q

Give an example of a polar molecule?

A

HCl is a polar molecule as the bond has one permanent dipole acting in the direction of HCl bond. Molecules with more than one polar bond can have dipoles which either reinforce one another forming a large type all over the whole molecule or cancel out the dipoles if they act in the opposite direction

37
Q

Why is a water molecule polar?

A

The 2 O– H bonds have a permanent dipole. The 2 dipoles don’t oppose each other. O has a partial negative charge and the H has a partial positive charge.

38
Q

Why is carbon dioxide nonpolar?

A

The double bond C-O bonds have a permanent dipole, which oppose each other so the dipoles cancel each other out.

39
Q

How do you know if a molecule is polar?

A

If the polar bond is:
Symmetrical - dipoles cancel out, non polar
Unsymmetrical - dipoles don’t cancel, polar

40
Q

Are polar solvents soluble?

A

Dissolving is only likely if the forces in the solution stronger or similar to the combined strength of the forces between the ions and the solid lattice and the strength between the forces of the polar molecules.

41
Q

Are nitrate soluble?

A

Yes, they are all soluble

42
Q

Are chloride’s soluble?

A

All apart from AgCl and PbCl2

43
Q

Are sulfate’s soluble?

A

Most apart from BaSO4, PbSO4, SrSO4

44
Q

Are carbonates soluble?

A

All are insoluble apart from (NH4)2CO3 and those in group 1

45
Q

Are sodium, potassium and ammonium salts soluble?

A

Yes

46
Q

What are intermolecular forces?

A

They are weak interactions between dipoles of different molecules. They fall into three main categories.

47
Q

What are the three main categories of intermolecular forces?

A
  1. Induced dipole – dipole interactions (London forces) 2. Permanent dipole – dipole interactions
  2. Hydrogen bonding.
48
Q

What are the intermolecular forces are responsible for?

A

The physical properties such as melting and boiling points whereas covalent bonds determine the identity and chemical reactions of molecules.

49
Q

Give the order from strongest to weakest intermolecular forces?

A

Hydrogen bonding, permanent dipole – dipole interactions, London forces

50
Q

What are induced dipole – dipole interactions (London forces)?

A

They are weak intermolecular forces found between all molecules whether they are polar or nonpolar. They act between induced dipoles in different molecules.

51
Q

Describe the origin of induced dipoles?

A

The movement of electrons produces a changing dipole in a molecule. At an instant an instantaneous dipole will exist but its position is constantly shifting. The instantaneous dipole induces a dipole on a neighbouring molecule. Induced dipole induces further dipoles on neighbouring molecules. An induced dipole is that only temporary. In the next instance in time that induced dipoles may disappear only for the whole process to take place amongst other molecules.

52
Q

What is the strength of London forces?

A

The more electrons in the molecule:
The larger the instantaneous and induced dipole.
The greater the induced dipole – dipole interactions The stronger the attractive forces between molecules so more energy is needed to overcome the intermolecular forces increasing the boiling point.

53
Q

What are permanent dipole – dipole interactions?

A

These acts between permanent dipole is in different polar molecules. These dipoles don’t come on go so exert full time forces which are stronger. Opposite dipoles attract one another.

54
Q

Eventhough HCl and F2 have the same number of electrons and same shape why is their boiling point different?

A

The fluorine molecule is non polar so only contains London forces between the molecules. Hydrogen chloride is polar so contains both London forces and permanent dipole dipole interactions between the molecules. Extra energy is then required to break the additional permanent dipole – dipole interactions between the hydrogen chloride molecules therefore has a higher boiling point.

55
Q

What is a simple molecular substance?

A

It is made up of simple molecules – small units containing a definite number of atoms and a definite formula.

56
Q

What structure is formed in a solid state of a simple molecular substance?

A

A simple molecular lattice. In the lattice the molecules are held in place by weak intermolecular forces with the atoms within each molecule bonded strongly by covalent bonds.

57
Q

Do simple molecular substances have low or high melting points and boiling points?

A

Simple molecular substances are bonded covalently. At room temperature they exist as solids liquids or gases. All can be solidified into simple molecular by reducing the temperature. Within the simple molecular lattice there are weak intermolecular forces which can be broken by low energy at low temperatures so have low melting and boiling points

58
Q

What forces break when the lattice is broken when melting?

A

The intermolecular forces break but the covalent bonds don’t.

59
Q

Describe the solubility of simple molecular substances which are non polar in non polar solvents?

A

When the compound is added to a nonpolar solvents the intermolecular forces formed between the molecules and solvent. Interactions week and the intermolecular forces in the simple molecular lattice. The intermolecular forces break and the compound dissolves. Non polar simple molecular substances tend to be soluble in non polar solvents.

60
Q

Describe the solubility of simple molecular substances which are non polar in polar solvents?

A

When is added to a polar solvent there is little interaction between the molecules in the lattice and solvent molecules. Intermolecular bonding is too strong within the polar solvents to break therefore so tend to be insoluble in polar solvents.

61
Q

Describe the solubility of simple molecular substances which are polar in polar solvents?

A

Polar covalent substances may dissolve in polar solvents as the polar solute molecules and the polar solvent can attract one another.

62
Q

What does solubility depend on?

A

The dipole strength, it can be hard to predict some compounds e.g. ethanol has both polar and nonpolar parts that can dissolve in both polar and nonpolar solvents.

63
Q

Describe the electrical conductivity of simple molecular structures?

A

There are no mobile charge particles in simple molecular structures as no charge particles can move there is nothing to complete an electrical circuit therefore they are non-conductors of electricity.

64
Q

What is a hydrogen bond?

A

It is a special type of permanent dipole – dipole interaction found between molecules containing an electronegative atom with a lone pair of electrons e.g. nitrogen oxygen fluorine. And a hydrogen atom attached to an electronegative atom.

65
Q

In hydrogen bonding where does the bond act?

A

It acts between a lone pair of electrons on an electronegative atom in one molecule and a hydrogen atom in a different molecule. They’re the strongest type of intermolecular forces of attraction.

66
Q

How is a hydrogen bond indicated?

A

By a dashed line

67
Q

Is ice less dense or more dense than water?

A

As hydrogen bonds hold water molecules apart and then open lattice structure the water molecules in ice are held further apart than water meaning solid ice is less dense than liquid water which floats. It is one of the only substances where the solid is less dense than the liquid. It forms an insulating layer which prevents the water from the freezing over protecting aquatic species.

68
Q

How does the arrangement of water allow it to be less dense in ice form than in water form?

A

The oxygen molecule has two lone pairs with two hydrogen is attached to each water molecule forming four hydrogen bonds. The hydrogen bonds extend outwards holding the molecules slightly apart forming an open tetrahedral lattice full of holes. The bond angle of around the hydrogen atom involved in hydrogen bonds is close to 180. The holes in the open lattice decrease the density of water freezing.

69
Q

What happens when ice melts?

A

The ice lattice collapses and the molecules move closer together so liquid water is denser than solid ice.

70
Q

Does water have a high or low melting and boiling point?

A

Water does have London forces but hydrogen bonds are stronger in London forces so greater amount of energy is needed to break the hydrogen bonds in water so it has a much higher melting and boiling point. When ice melts the rigid arrangement in the ice lattice is completely broken. When water boils the hydrogen bonds break completely.

71
Q

Describe the boiling point of hydrides?

A

H20, NH3 and HF don’t follow the general trend. They have strong hydrogen bonds between molecules so require more energy than the other molecules. The general trend is that the boiling point increases as the molecule size increases so the strength of London forces between the molecules increase so more energy is required overcome them

72
Q

Does water have a high surface tension or low surface tension?

A

It has a high surface tension caused by molecules on the surface experiencing unbalanced hydrogen bonding forces pulling them in. Molecules in bulk experience balance forces in every direction.

73
Q

How many hydrogen bonds does adenine and thymine form?

A

2

74
Q

How many hydrogen bonds does guanine and cytosine form?

A

3

75
Q

State the density of water?

A

At 4°C it has a maximum density of 1.029 g/cm³. At 0°C the density drops drastically to 0.917 g/cm³ cubed. Between four and zero the density starts decreasing. As water calls it contracts and the water molecules get closer together.

Chemistry (23 decks)

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