Periodicity/Groups 2 and 7

Question 1

What is the definition of 1st ionisation energy?

Answer 1

The energy required to remove 1 electron from each atom in one mole of GASEOUS atoms to form one mole of GASEOUS 1+ ions

Question 2

Write the equation for the 1st ionisation energy of Na

Answer 2

Na(g) –> Na+(g) + e-

Question 3

Write the equation for the 2nd ionisation energy of Na

Answer 3

Na+(g) –> Na2+(g) + e-

Question 4

What is ionisation energy proportional to?

Answer 4

The strength of the attraction between the nucleus and outer e- (nuclear attraction)

Question 5

What does the strength of nuclear attraction depend on?

Answer 5

Radius
Shielding
Charge

Question 6

How does atomic radius affect nuclear attraction?

Answer 6

As atomic radius increases, nuclear attraction weakens (outer e- further away from nucleus)

Question 7

How does electron shielding affect nuclear attraction?

Answer 7

Shielding reduces attractive force from protons on outer e- (more inner shells = larger shielding effect)

Question 8

How does nuclear charge affect nuclear attraction?

Answer 8

More protons in nucleus = greater attraction on outer e-

Question 9

How does ionisation energy change down a group?

Answer 9

Radius increases (more shells)
Shielding increases (more inner shells)
Charge increases BUT is outweighed by R and S

Therefore nuclear attraction on outer e- DECREASES so outer e- is easier to remove - ionisation energy DECREASES

Question 10

How does ionisation energy change across a period?

Answer 10

Radius decreases (increasing nuclear charge pulls in outer e-)
Shielding remains constant (same no. inner shells)
Charge increases (more protons)

Therefore nuclear attraction on outer e- INCREASES so outer e- is harder to remove - ionisation energy INCREASES

Question 11

Why is the 1st ionisation energy of B less than Be?

B: 1s(2)2s(2)2p(1)
Be: 1s(2)2s(2)

Answer 11

The outer e- is in a 2p subshell in B and 2s in Be - 2p is higher in energy than 2s so outer e- in B is easier to remove

Question 12

Why is the 1st ionisation energy of O less than N?

   1s      2s         2p N: ⬆️⬇️ ⬆️⬇️ ⬆️⬆️⬆️

O: ⬆️⬇️ ⬆️⬇️ ⬆️⬇️⬆️⬆️

Answer 12

The outer e- in N is UNPAIRED whilst in O it is PAIRED - paired e- experience more repulsion than unpaired e- so the outer e- in O is easier to remove

Question 13

What is the trend in reactivity down group 2?

Answer 13

Atomic radius decreases ionisation energy
Shielding decreases ionisation energy
Nuclear charge increases ionisation energy but is ignored as it is outweighed by R and S

Therefore easier for outer e- to be removed so reactivity INCREASES down group 2

Question 14

What is the solubility trend in group 2 hydroxides?

Answer 14

Increases down the group

Question 15

What does an increase in pH down group 2 hydroxides show?

Answer 15

More OH- ions released down the group

Question 16

How are hydroxides used in agriculture?

Answer 16

Neutralise acidic soil

Question 17

What are the colours of Cl2, Br2 and I2 (halogens) in water and cyclohexane (non-polar organic solvent)?

Answer 17

Water Cyclohexane

Cl2 Pale green Pale green

Br2 Orange Orange

I2 Brown Violet

Question 18

What is a displacement reaction?

Answer 18

A reaction between a halogen and a halide ion

Question 19

What colour are halides?

Answer 19

Colourless

Question 20

What is the reactivity trend between Cl2, Br2 and I2?

Answer 20

Cl2 Br2 I2
——————————->
reactivity decreases

Question 21

What are the displacement reactions?

Answer 21

Halogen/halide Chloride Bromide Iodide
A O A O A O

 Chlorine          X        X       orange     brown violet

 Bromine         X        X       X         X    brown violet

 Iodine             X         X      X         X        X          X

Question 22

Which halides will chlorine, bromine and iodine displace?

Answer 22

• Chlorine will displace bromide and iodide ions
• Bromine will displace iodide ions
• Iodine won’t displace anything (least reactive)

Question 23

Write the ionic equations for all the displacement reactions that CAN occur

Answer 23

Cl2 + 2Br- –> 2Cl- + Br2

Cl2 + 2I- –> 2Cl- + I2

Br2 + 2I- –> 2Br- + I2

Question 24

What are the halide reactions?

Answer 24

NaCl + silver nitrate –> white precipitate
+ dilute ammonia –> dissolved
+ conc. ammonia –> dissolved

NaBr + silver nitrate –> cream precipitate
+ dilute ammonia –> remained cream ppt.
+ conc. ammonia –> dissolved

KI + silver nitrate –> yellow precipitate
+ dilute ammonia –> remained yellow ppt.
+ conc. ammonia –> ppt. yellow to white colour change

Question 25

Why can chlorine behave as an oxidising agent?

Answer 25

Highly electronegative so can accept electrons to form a Cl- ion

Question 26

Why is cyclohexane added to the test tubes when carrying out displacement reactions?

Answer 26

Colours are more distinct in organic solvents

Question 27

What is required in order for a displacement reaction to happen?

Answer 27

The halogen needs to be more reactive than the halide present

Cl2 Br2 I2
——————————>
reactivity decreases

Question 28

What is the reactivity trend down group 7?

Answer 28

Atomic radius increases (more shells)
Shielding increases (more inner shells) 

Therefore there is less nuclear attraction to GAIN (group 7 gain an e-) an e- so reactivity DECREASES and oxidising strength decreases