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OCR A-level Chemistry > Chapter 6 > Flashcards

Chapter 6 Flashcards

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1
Q

What are the shapes and bond angles for all molecules

A
  • 0 lone pairs of electrons leads to a bond angle of 109.5 degrees (with exceptions) each lone pair of electrons decreases the bond angle by 2.5 degrees
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2
Q

What is an example of trigonal planar molecule?

A
  • BF3, 3 atoms surrounding the central atom, with a bond angle of 120 degrees
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3
Q

What is an example of a linear molecule ?

A
  • CO2 since there are no lone pairs of electrons, the bond angle is 180 degrees
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4
Q

What is an example of a Tetrahedral molecule ?

A
  • CH4, no lone pairs and 4 bonded atoms repel each other equally, resulting to a bond angle of 109.5 degrees
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5
Q

What is an example of a Pyramidal molecule ?

A
  • NH3, 1 lone pair of electrons and 3 bonded atoms, therefore bond angle of 107 degrees
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6
Q

What is an example of a non-linear molecule ?

A
  • H20, the central atom has 2 lone pairs of electrons, resulting in a bond angle of 104.5 degrees
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7
Q

What is an example of a Octahedral molecule ?

A
  • SF6, with 6 bonded atoms surrounding the central atom, with no lone pairs of electrons, resulting in a bond angle of 90 degrees
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8
Q

What is an example of a Trigonal Bipyramidal molecule ?

A
  • PF5, 5 bonding pairs of electrons, no lone pairs, however this an exception where 3 atoms have a bond angle of 90 degrees and 2 atoms have a bond angle of 120 degrees
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9
Q

What is electron repulsion theory ?

A
  • The valence shell electron pair repulsion theory (VSEPR) predicts the shape and bond angles of molecules
  • In a molecule, the bonding pairs of electrons will repel other electrons around the central atom forcing the molecule to adopt a shape in which these repulsive forces are minimised
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10
Q

What are the rules when determining the shape and bond angles of a molecule ?

A
  • Electron pairs repel each other as they have the same charge
  • Lone pair electrons repel each other more than bonded pairs
  • Repulsion between multiple and single bonds is treated the same as for repulsion between single bonds
  • Repulsion between pairs of double bonds are greater
  • The most stable shape is adopted to minimize the repulsion forces
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11
Q

What is electronegativity ?

A
  • electronegativity is the power of an atom to attract the pair of electrons in a covalent bond towards itself
  • this means that the electron distribution in a covalent bond between elements with different electronegativities will be unsymmetrical
  • This is because of the ability of the positive nucleus to attract negativity charged electrons, towards it
  • the Pauling scale is used to assign a value of electronegativity
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12
Q

What are the different factors which affect electronegativity ?

A
  • Nuclear charge
  • Atomic radius
  • shielding
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13
Q

How does nuclear charge affect electronegativity ?

A
  • Attraction exists between the positively charged protons in the nucleus and negatively charged electrons found in the energy levels of an atom
  • An increase in the number of protons leads to an increase in nuclear attraction for the electrons in the outer shells
  • Therefore, an increased nuclear charge results in an increased electronegativity
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14
Q

How does atomic radius affect electronegativity ?

A
  • The atomic radius is the distance between the nucleus and electrons in the outermost shell
  • Electrons closer to the nucleus are more strongly attracted towards its positive nucleus
  • Those electrons further away from the nucleus are less strongly attracted towards the nucleus
  • Therefore, an increased atomic radius results in a decreased electronegativity
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15
Q

How does sheilding affect electronegativity ?

A
  • Filled energy levels can shield (mask) the effect of the nuclear charge causing the outer electrons to be less attracted to the nucleus
  • Sodium (period 3, group 1) has higher electronegativity than caesium (period 6, group 1) as it has fewer shells and therefore the outer electrons experience less shielding than in caesium
  • Thus, an increased number of inner shells and subshells will result in a decreased electronegativity
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16
Q

What is the trend in electronegativity down a group ?

A
  • there is a decrease in electronegativity going down the group
  • The nuclear charge increases as more protons are being added to the nucleus
  • However, each element has an extra filled electron shell, which increases shielding
  • The addition of the extra shells increases the distance between the nucleus and the outer electrons resulting in larger atomic radii
  • Overall, there is decrease in attraction between the nucleus and outer bonding electrons
17
Q

What is the trend in electronegativity across a period ?

A
  • Electronegativity increases across a period
  • The nuclear charge increases with the addition of protons to the nucleus
  • Shielding remains relatively constant across the period as no new shells are being added to the atoms
  • The nucleus has an increasingly strong attraction for the bonding pair of electrons of atoms across the period of the periodic table
  • This results in smaller atomic radii
18
Q

What is bond polarity ?

A
  • When 2 atoms in a covalent bond have the same electronegativity the covalent bond is nonpolar
  • When 2 atoms in a covalent bond have different electronegativity the covalent bond is polar and the electrons will be drawn towards the more electronegative atom
19
Q

What happens as a result of polar bonds ?

A
  • the electron distribution is asymmetric
  • the less electronegative atom gets a partial positive charge (delta positive)
  • the more electronegative atom gets a partial negative charge (delta negative)
  • the greater the difference in electronegativity the more polar the bond becomes
20
Q

What is a dipole moment ?

A
  • The dipole moment is a measure of how polar a bond is
  • The direction of the dipole moment is shown by the following sign in which the arrow points to the partially negatively charged end of the dipole
21
Q

What is the difference between polar bonds and polar molecules ?

A
  • some molecules have polar bonds but are overall not polar because the polar bonds in the molecule are arranged in such a way that the individual dipole moments cancel eachother out e.g CCl4
22
Q

What are the 3 types of intermolecular forces ?

A
  • Induced dipole – dipole forces are also called London dispersion forces or van der Waals’ forces
  • Permanent dipole – dipole forces (also called van der Waals’ forces) are the attractive forces between two neighbouring molecules with a permanent dipole
  • Hydrogen Bonding are a special type of permanent dipole - permanent dipole forces
  • Intramolecular forces are stronger than intermolecular forces
    For example, a hydrogen bond is about one tenth the strength of a covalent bond
23
Q

How do induced dipole-dipole forces (London forces) work ?

A
  • The electron charge cloud in non-polar molecules or atoms are constantly moving
  • During this movement, the electron charge cloud can be more on one side of the atom or molecule than the other
  • Because the electron clouds are moving constantly, the dipoles are only temporary
  • Therefore the greater the number of electrons the molecule has, the stronger the induced dipole-dipole forces
24
Q

How do permanent dipole- permanent dipole forces work ?

A
  • Polar molecules have permanent dipoles
  • The molecule will always have a negatively and positively charged end
  • permanent dipole forces are stronger than induced dipole forces
25
Q

How does hydrogen bonding work ?

A

the following is needed for hydrogen bonding:
- species which has an O, N or F (very electronegative) atom bonded to a hydrogen
- When hydrogen is covalently bonded to an O, N or F, the bond becomes highly polarised
- The H becomes so δ+ charged that it can form a bond with the lone pair of an O, N or F atom in another molecule

26
Q

What are the properties of water due to hydrogen bonding ?

A
  • high melting and boiling points: this is due to the strength of the hydrogen bonds requiring lots of energy to break
  • high surface tension
  • lower density of ice compared to water - this is because ice, water molecules are arranged in a rigid open lattice held far apart by hydrogen bonds
27
Q

How does solubility work with polar and non-polar molecules ?

A
  • the general principle is that like dissolves like, so non-polar substances dissolve non-polar solvents, and polar covalent substance generally dissolve in polar solvents as a result of dipole-dipole interactions
28
Q

What are the properties of covalent compounds ?

A

OCR A-level Chemistry (6 decks)

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