chemical bonding 2

Question 1

define electronegativity

Answer 1

the ability of an atom to attract the bonding electrons towards itself in a covalent bond

Question 2

describe non-polar bonds

Answer 2

-both bonding atoms are the same
-both atoms have the same electronegativity
-bonding electrons are evenly distributed between the 2 bonded atoms
-no dipole

Question 3

what is a permanent dipole?

Answer 3

a small difference in charge across a bond that results from a difference in charge because of electronegativities of the bonded atoms

Question 4

what group doesnt make covalent bonds?

Answer 4

group 0

Question 5

why is HCL polar?

Answer 5

-in a H-CL bond, “the cl atom is more electronegative than the H atom”
-the bonding electrons are attracted closer to the cl atom as the electron cloud is more dense around the cl atom there is a permanent dipole across the bond
-the bond is a polar covalent bond because it has a permanent dipole

Question 6

what is more important,
a full charge or a dipole charge?

Answer 6

a full charge

Question 7

what is the most electronegative element in the periodic table?

Answer 7

Fluorine

Question 8

what would make a more polar covalent bond?

Answer 8

-greater difference in electronegativity between the bonded atoms
-greater the permanent dipole

Question 9

why do carbon and hydrogen not form a polar covalent bond?

Answer 9

due to the very small difference in the electronegativity between C and H

Question 10

why cant oil mix with water?

Answer 10

because oil is a hydrogen and carbon chain so is
non-polar

Question 11

why are symmetrical molecules non-polar?

Answer 11

because the dipoles cancel out

Question 12

why is CCl4 non-polar?

Answer 12

-each C-Cl bond is polar because Cl is more electronegative than carbon
-symmetrical so the dipoles cancel out
-therefore the CCl4 molecule is non-polar as there is no net dipole across the molecule

Question 13

why are unsymmetrical molecules containing one or more polar bonds polar?

Answer 13

because the dipoles do not cancel out

Question 14

why is NH3 polar?

Answer 14

-each N-H bond is polar because N is more electronegative than H
-since the NH3 molecule is unsymmetrical, the dipoles do not cancel
-therefore the NH3 molecule is polar

Question 15

what are intermolecular forces?

Answer 15

attractive forces between molecules
found only in covalent structures

Question 16

what are london forces?

Answer 16

very weak intermolecular forces between molecules
exist between all molecules. polar or not

Question 17

explain how london forces arise between molecules?

Answer 17

-at any moment there may be an uneven distribution of electrons in a molecule (due to random movement of electrons/density)
-this causes a temporary dipole to be present
-the temporary dipole in one molecule causes a london force in a neighbouring molecule due to opposite dipole attraction

Question 18

how can london forces become stronger?

Answer 18

if the molecules are bigger, there are more electrons so the distribution of electrons is more uneven

Question 19

what are permanent dipole-dipole attractions?

Answer 19

weak attractive forces between polar molecules

Question 20

why is H-Cl bond permantly polar?

Answer 20

because Cl is more electronegative than H and the molecule is not symmetrical so the dipoles do not cancel

the H delta+ of one HCL molecule attracts the Cl delta- of a neighbouring molecule to produce a permanent dipole-dipole force of attraction between molecules

Question 21

what can permanent dipole-dipole attractions and London forces can both be referred as?

Answer 21

Van der Waals forces

Question 22

what is a hydrogen bond?

Answer 22

a strong dipole-dipole attraction between molecules containing O-H, N-H or F-H bonds

Question 23

why does hydrogen bonding occur between O-H, N-H and F-H bonds?

Answer 23

-all 3 have strong electronegative attraction
-all 3 have lone pairs of electrons

Question 24

what does a hydrogen bond exist between?

Answer 24

-a H delta + atom in one molecule ans
-a lone pair on a highly electronegative atom (0 delta-, N delta - or F delta -)

Question 25

what are the strongest type of intermolecular forces of attraction?

Answer 25

hydrogen bonds

Question 26

describe hydrogen bonding between water molecules

Answer 26

-hydrogen bonds form between the H2O molecules in the direction of the lone pair of electrons on the oxygen atom

Question 27

why is water polar?

Answer 27

-O is more electronegative than H
-makes a permament dipole across the O-H bond
-the molecule is not symmetrical so the dipoles do not cancel

Question 28

explain the trend in boiling points down group 7 in terms of intermolecular forces

Answer 28

-boiling point increases down group 7
-molecules get bigger, more shells so more electrons
-more stronger london forces between molecules
-so more energy is needed to overcome the London forces

Question 29

explain the difference in boiling points of NH3(-33.3), Br2(58.8) and H2O(100)

Answer 29

-NH3 and H2O has hydrogen bonding between molecules
-Br2 has london forces of attraction between molecules
-the hydrogen bonds bewteen water molecules are stronger than the hydrogen bonds between ammonia molecules because O is more electronegative than N and O has 2 lone pairs of electrons wheres nitrogen only has 1 lone pair
-Londn forces of attraction between bromine molecules are stronger than the hydrogen bonds between ammonia molecules. however they are not as strong as hydrogen bonds between water molecules
-more energy is needed to break the hydrogen bonds between water molecules and the least energy needed to break the hydrogen bonds in ammonia molecule

Question 30

why does water have anomalous properties?

Answer 30

due to the hydrogen bonds between water molecules

Question 31

why is ice less dense than liquid water?

Answer 31

-in the solid ice, H2O molecules are held further apart by the hydrogen bonds
-this gives the ice an open lattice structure
-this makes solid ice less dense than liquid water

Question 32

why does water have relatively high melting and boiling points?

Answer 32

-liquid water has a higher boiling point
-ice has a higher melting b
point
-hydrogen bonds are relatively strong and therefore stronger than other intermolecular forces
-so more energy is needed to break the hydrogen molecules

Question 33

suggest why ice has a higher melting point than solid ammonia

Answer 33

-hydrogen bonds between H2O molecules are stronger than the hydrogen bonds between NH3 molecules
-O is more electronegative than N
-dipole across the O-H bond is stronger than across the N-H bonds
-there are also 2 hydrogen bonds between H2O molecules and only one between NH3 molecules

Question 34

order on strength of bonds

Answer 34

strongest- hydrogen bonds
2. permament dipole-dipole attractions
3.London forces

Question 35

describe the hydrogen bonding between water molecules

Answer 35

-well labelled diagram
- H delta+ on one water molecule attracts lone pair of electrons on O delta - atom of another water molecule

Question 36

describe and explain 2 anomolous properties of water due to hydrogen bonding

Answer 36

The boiling point of water is higher than expected (or the melting point of ice), as hydrogen bonds
between molecules are the strongest intermolecular force and more heat energy is needed to break
them
* Ice is less dense than water, as hydrogen bonds are longer than covalent bonds, so they hold the
molecules apart in an open lattice

Question 37

Explain the boiling points of the three substances shown in the table below
Liquid Water, H2O = 100 degrees
Trichloromethane, CHCl3 =62 degrees Chlorine, Cl2 = -34 degrees

Answer 37

• All are simple covalent / molecular substances, with weak intermolecular forces between molecules
• Water is a polar molecule with O-H bonds, so has hydrogen bonding between molecules
• Trichloromethane is a polar molecule, so has permanent dipole-dipole interactions between the
  molecules (as the C-Cl bond is polar and the molecule is not symmetrical)
• Chlorine is a non-polar molecule, so has induced dipole-dipole interactions between molecules
• Hydrogen bonds are the strongest intermolecular forces so more heat energy is needed to overcome
  them; induced dipole-dipole interactions are the weakest intermolecular forces so less heat energy is
  needed to overcome them; permanent dipole-dipole interactions have intermediate strength