bonding

Question 1

explain the shape of an ammonia molecule

Answer 1

3 bond pairs + 1 lone pair arranged for minimum repulsion; bond angles < 109.5; because bond pair-lone pair repulsion stronger than bond pair-bond pair

Question 2

explain why boiling point increases down group 7

Answer 2

number of electrons increases down the group; so more energy to break stronger london forces between molecules

Question 3

explain why lattice energy of CaCO3 is less exothermic than MgCO3

Answer 3

calcium has larger ionic radius and greater shielding; so less attraction between ions

Question 4

describe the effect of impurities on a substance’s melting temperature

Answer 4

melting temp becomes lower; melts over a range of temperatures

Question 5

explain why sigma bonds are stronger than pi bonds

Answer 5

sigma: orbitals overlap directly between nuclei; pi: orbitals overlap sideways so further away from nuclei

Question 6

when can an alkene do E-Z isomerism

Answer 6

when both carbons in a double bond have two different groups attached

Question 7

what is the difference between E-Z and cis-trans isomerism

Answer 7

cis-trans is same as E-Z but judged using a group present on both carbons

Question 8

pure silicon forms a giant covalent lattice; explain why it doesn’t conduct electricity even though it has electrons

Answer 8

its outer electrons are used in bonding so they are not free to move

Question 9

explain why giant covalent lattices have high melting points

Answer 9

high energy needed to break its strong covalent bonds

Question 10

explain why ionic compounds have high melting points

Answer 10

high energy needed to break its strong ionic bonds

Question 11

explain why solutions of ions are able to conduct electricity

Answer 11

ions are free to move

Question 12

suggest why solutions of CaCl2 and BaCl2 of equal concentration have different electrical conductivity

Answer 12

they have different sized cations

Question 13

what causes london forces to be stronger

Answer 13

more electrons

Question 14

what is the strongest intermolecular force

Answer 14

hydrogen bonding

Question 15

what is needed for a hydrogen bond

Answer 15

N-H, O-H or F-H bond on one molecule; lone pair on the other molecule

Question 16

what is ionic bonding

Answer 16

strong electrostatic attraction between opposite charged ions

Question 17

how does electrolysis of sodium chloride solution show the presence of ions

Answer 17

chlorine is evolved at the anode, showing there were negative chloride ions present

Question 18

both Na and NaCl can conduct electricity; compare the conditions needed and the methods of each for conductivity

Answer 18

Na: conducts when solid, delocalised electrons as charge carriers; NaCl: conducts when molten or aqueous, ions as charge carriers

Question 19

describe the structure of a metal

Answer 19

lattice of positive ions in a sea of delocalised electrons

Question 20

what is metallic bonding

Answer 20

strong electrostatic attraction between positive ions and delocalised electrons

Question 21

explain why the melting point of Mg is higher than Na (give 3 reasons)

Answer 21

Mg 2+ has higher charge than Na +; and smaller ionic radius; and contributes more to the sea of delocalised electrons; so more energy to break stronger metallic bonding

Question 22

explain how metals conduct electricity

Answer 22

delocalised electrons flow when a voltage is applied

Question 23

explain why a lattice of Mg 2+ and O 2- is stronger than one of Mg + and O -

Answer 23

both ions have higher charge; Mg 2+ is smaller and has less shielding than Mg +

Question 24

suggest why the melting point of MgO is higher than MgCl2

Answer 24

O 2- ions have higher charge than Cl - ions (and same shielding); so higher energy to break stronger ionic bonding in MgO

Question 25

explain the shape of a water molecule

Answer 25

2 lone pairs + 2 bond pairs arranged for minimum repulsion; 104.5 degree bond angles; because lone pairs repel more than bond pairs

Question 26

explain why hydrogen bonds have a ‘bond angle’ of 180 degrees

Answer 26

two electron pairs around H atom; arranged for min repulsion so 180 degrees

Question 27

describe the formation of london forces

Answer 27

temporary asymmetry in electron density; this creates a dipole that induces more dipoles on other molecules

Question 28

what is a covalent bond

Answer 28

strong electrostatic attraction between two nuclei and a shared bonding pair of electrons

Question 29

what is electronegativity

Answer 29

ability of an atom to attract a bonding pair of electrons in a covalent bond