OZ abcdk(i)n: Electronegativity; intermolecular bonding Flashcards
Define electronegativity.
The tendency of an atom to attract electrons within a covalent bond.
What is the trend in electronegativity across the periodic table?
Increases up groups and across periods (diagonal trend).
Why does electronegativity decrease down a group?
- More electron shells
- Distance between nucleus + outer shell increases
- Weaker electrostatic attraction between nucleus + bonding pairs
Same explanation as trend in ionisation enthalpy
Why does electronegativity increase across a period?
- 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
What determines an atom’s electronegativity?
- Nuclear charge (related to periods)
- Atomic radius (related to groups)
Define miscibility.
The property of two liquids to mix evenly.
- What is a dipole?
- What is meant by “polar molecule”?
- A molecule or part of a molecule with a positive + negative end due to polar bonds
- A molecule with a dipole
- Explain why you might expect tetrachloromethane to be polar.
- Explain why it is non-polar.
- It contains 4 C-Cl bonds which are polar because chlorine is more electronegative than carbon.
- Cl atoms are positioned tetrahedrally + therefore symmetrically around the C, so dipoles cancel out. No overall dipole.
List the types of intermolecular bonding, in order of increasing strength.
- Id-id (instantaneous dipole-induced dipole) interactions
- Pd-id (permanent dipole-induced dipole) interactions (including hydrogen bonding)
- Pd-pd (permanent dipole-permanent dipole) interactions
Explain how the following types of dipole arise:
- Permanent
- Instantaneous
- Induced
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
What are the requirements for hydrogen bonding?
- Polar bond between hydrogen and an electronegative atom: O/N/F
- Lone pair on an O/N/F to which H is attracted
Why can water form more hydrogen bonds than:
- Hydrogen fluoride?
- Ammonia?
- 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
Why do straight-chain alkanes have higher boiling points than their branched isomers?
- Closer contact between unbranched molecules
- Stronger id-id interactions requiring more energy to overcome
Which has the highest boiling point and why?
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
State and explain the intermolecular forces present in a mixture of hydrogen chloride and chlorine.
- Id-id, since instantaneous dipoles arise in every molecule
- Pd-id, since HCl molecules are permanent dipoles which induce dipoles in chlorine molecules
Explain why butane has a higher boiling point than propane.
- 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
Why does ethanol (CH3CH2OH) have a higher boiling point than ethanal (CH3COH)?
- 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
Why does 2-iodobutane have a higher boiling point than 2-fluorobutane?
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
Explain why the solubility of alcohols in water decreases as chain length increases.
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
Explain why butan-1-ol has a higher boiling point than ethanol.
- 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
State and explain which of CO2 and SiO2 has a greater boiling point.
- 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
- What two factors determine the reactivity of haloalkanes / carbon-halogen bonds?
- State the trend in reactivity down the group, and hence which factor has the dominant effect.
- Bond enthalpy + bond polarity
- 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
