Module 3

Question 1

Define first ionisation energy

Answer 1

The energy required to remove 1 e- from each atom in 1 mole of gaseous atoms of an element to form 1 mole of gaseous 1+ ions

Question 2

Why first ionisation energies show general increase across a period?

Answer 2

• across the period, nuclear charge increases
• e- added to same shell so similar shielding
• greater nuclear attraction on outer e-

Question 3

Why first ionisation energy of B less than that of Be?

Answer 3

• In B, e- removed from 2p orbital
• In Be, e- removed from 2s orbital
• 2p sub shell at higher energy level + its e- easier to remove

Question 4

Explain why first ionisation energy of O is less than that of N

Answer 4

• in O, one of the 2p orbitals contains paired e- whereas in N, all 3 orbitals are singly occupied
• the paired e- in O repel + e- easier to remove

Question 5

Explain trend in ionisation energies down a group

Answer 5

• down a group, atomic radius increases
• more inner shells so shielding increases
• e- added to a new shell further from nucleus thus nuclear attraction on outer e- decreases

Question 6

Why Ice has higher melting point than solid NH3?

Answer 6

• H2O molecule can form more H bonds per molecule because O has 2 lone pairs in the H2O against 1 lone pair for N in NH3
• O is more electronegative than N and will attract the e- more making the hydrogen bond stronger

Question 7

Explain trend in boiling point down group 7

Answer 7

• bp increases down group
• because no. Of electrons increases resulting in stronger Ldn forces
• more energy required to break stronger intermolecular forces

Question 8

I2 and Sr reacted together. Why is it difficult to predict whether this reaction is more or less reactive than Br2 and Ca?

Answer 8

• Sr more reactive than Ca as further down group 2
• I less reactive than Br as further down halogens
• difficult predicting which is more dominant effect

Question 9

Benefit of adding Cl2 to water?

Answer 9

Chlorine kills bacteria

Question 10

Why Cl2 shouldn’t be added to water?

Answer 10

• forms carcinogens

- can form chlorinated hydrocarbons

Question 11

Chlorine more reactive than bromine. Why?

Answer 11

• Cl gains e- more easily than Br
• atom of Cl smaller than atom of Br
• in Cl atom, fewer inner shells between outer e- + nucleus than Br so decreased shielding
• in Cl atom, nuclear attraction on an electron to be gained greater than Br

Question 12

Compounds of magnesium that can be used to treat heartburn?

Answer 12

• magnesium carbonate
• magnesium hydroxide
• magnesium oxide

Question 13

Why is 2nd ionisation enthalpy important when explaining group 2 relativity?

Answer 13

Group 2 elements react by losing 2 electrons to form 2+ions

Question 14

Reactivity increases down group 2. Explain this in terms of first ionisation energy

Answer 14

• down a group, there are more inner shells between the outer e- + the nucleus thus increasing shielding
• e- added to new shell further from the nucleus, thus nuclear attraction on outer e- decreases
• therefore less energy needed to lose an e-

Question 15

What is meant by disproportionation?

Answer 15

Disproportionation is the simultaneous oxidation + reduction of the same element in the same redox reaction