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chemistry > OZ abcdk(i)n: Electronegativity; intermolecular bonding > Flashcards

OZ abcdk(i)n: Electronegativity; intermolecular bonding Flashcards

1
Q

Define electronegativity.

A

The tendency of an atom to attract electrons within a covalent bond.

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2
Q

What is the trend in electronegativity across the periodic table?

A

Increases up groups and across periods (diagonal trend).

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3
Q

Why does electronegativity decrease down a group?

A
  • More electron shells
  • Distance between nucleus + outer shell increases
  • Weaker electrostatic attraction between nucleus + bonding pairs

Same explanation as trend in ionisation enthalpy

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4
Q

Why does electronegativity increase across a period?

A
  • All outer electrons in a period are in same shell
  • Proton number increases so greater electrostatic attraction between bonding pairs + nucleus

​Same explanation as trend in ionisation enthalpy

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5
Q

What determines an atom’s electronegativity?

A
  • Nuclear charge (related to periods)
  • Atomic radius (related to groups)
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6
Q

Define miscibility.

A

The property of two liquids to mix evenly.

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7
Q
  1. What is a dipole?
  2. What is meant by “polar molecule”?
A
  1. A molecule or part of a molecule with a positive + negative end due to polar bonds
  2. A molecule with a dipole
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8
Q
  1. Explain why you might expect tetrachloromethane to be polar.
  2. Explain why it is non-polar.
A
  1. It contains 4 C-Cl bonds which are polar because chlorine is more electronegative than carbon.
  2. Cl atoms are positioned tetrahedrally + therefore symmetrically around the C, so dipoles cancel out. No overall dipole.
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9
Q

List the types of intermolecular bonding, in order of increasing strength.

A
  • Id-id (instantaneous dipole-induced dipole) interactions
  • Pd-id (permanent dipole-induced dipole) interactions (including hydrogen bonding)
  • Pd-pd (permanent dipole-permanent dipole) interactions
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10
Q

Explain how the following types of dipole arise:

  1. Permanent
  2. Instantaneous
  3. Induced
A

Permanent: significant electronegativity difference between the 2 bonding atoms

Instantaneous: bonding electrons are in constant motion, and sometimes happen to not be evenly distributed

Induced: dipole (either type) attracts or repels electrons in an unpolarised molecule, inducing polarity

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11
Q

What are the requirements for hydrogen bonding?

A
  • Polar bond between hydrogen and an electronegative atom: O/N/F
  • Lone pair on an O/N/F to which H is attracted
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12
Q

Why can water form more hydrogen bonds than:

  1. Hydrogen fluoride?
  2. Ammonia?
A
  • In a HF molecule, fluorine has 3 lone pairs but only 1 hydrogen, so only 1 bond can form
  • A NH3 molecule has 3 hydrogens but nitrogen has only 1 lone pair, so only 1 bond can form
  • A H2O molecule has 2 lone pairs and 2 hydrogens, so 2 bonds can form
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13
Q

Why do straight-chain alkanes have higher boiling points than their branched isomers?

A
  • Closer contact between unbranched molecules
  • Stronger id-id interactions requiring more energy to overcome
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14
Q

Which has the highest boiling point and why?

A

Pentane

  • They are structural isomers with the same number of electrons
  • Less branching means closer contact between molecules, giving stronger id-id interactions
  • Pentane molecules are unbranched so have the strongest id-id interactions, requiring the most energy to overcome
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15
Q

State and explain the intermolecular forces present in a mixture of hydrogen chloride and chlorine.

A
  • Id-id, since instantaneous dipoles arise in every molecule
  • Pd-id, since HCl molecules are permanent dipoles which induce dipoles in chlorine molecules
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16
Q

Explain why butane has a higher boiling point than propane.

A
  • Butane has more atoms / higher Mr than propane, so has more electrons
  • Stronger id-id interactions between butane molecules, requiring more energy to overcome

Same principle explains the increase in boiling points of the halogens down the group

17
Q

Why does ethanol (CH3CH2OH) have a higher boiling point than ethanal (CH3COH)?

A
  • Id-id interactions in both
  • Pd-pd interactions in both due to polar C-O, O-H + C=O bonds
  • Also hydrogen bonding in ethanol due to OH group
  • Ethanol has stronger intermolecular forces requiring more energy to overcome
18
Q

Why does 2-iodobutane have a higher boiling point than 2-fluorobutane?

A

Both have id-id interactions:

  • Iodine has more electrons than fluorine
  • So 2-iodobutane has stronger id-id interactions

Both have pd-pd interactions due to polar carbon-halogen bonds:

  • C-F bond is more polar than C-I since fluorine is more electronegative than iodine
  • So 2-fluorobutane has stronger pd-pd interactions

Difference in id-id bond strength has dominant effect, so 2-iodobutane has higher boiling point

19
Q

Explain why the solubility of alcohols in water decreases as chain length increases.

A

As chain length increases:

Enthalpy change of breaking water-water hydrogen bonds >

enthalpy change of forming instantaneous dipole-induced dipole interactions between alkyl group + water

and hydrogen bonds between water + lone pair on oxygen

20
Q

Explain why butan-1-ol has a higher boiling point than ethanol.

A
  • Both molecules contain one -OH group, so can form 1 hydrogen bond; no comparative effect
  • Butan-1-ol has higher Mr than ethanol, so has more electrons
  • Stronger id-id interactions require more energy to overcome
21
Q

State and explain which of CO2 and SiO2 has a greater boiling point.

A
  • CO2 has id-id interactions, but not pd-pd interactions since symmetricaly arranged polar C=O bonds cancel each other out
  • SiO2 is a giant covalent lattice containing strong covalent bonds
  • Covalent bonds are stronger than id-id bonds, requiring more energy to overcome, so SiO2 has higher boiling point
22
Q
  1. What two factors determine the reactivity of haloalkanes / carbon-halogen bonds?
  2. State the trend in reactivity down the group, and hence which factor has the dominant effect.
A
  1. Bond enthalpy + bond polarity
  2. Reactivity increases down group:
  • Decrease in bond enthalpy down group, promoting reactivity
  • Decrease in bond polarity down group, inhibiting reactivity
  • Bond enthalpy therefore has dominant effect

Both trends are due to increasing distance, and therefore decreasing electrostatic attraction, between outer shell + nucleus

chemistry (50 decks)

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