Chapter 8 - Reactivity Trends

Question 1

what are grp 2 metals known as

Answer 1

alkaline earth metals

Question 2

electron config. of grp 2 metals

Answer 2

• fill 2s-sub-shell
• 2 electrons in outershell
• reducing agents as loses electrons so oxidised

Question 3

grp 2 metals reactions

Answer 3

• oxygen —> makes metal oxide
• water —> makes hydroxide and hydrogen
• dilute acids —> makes salt and hydrogen

Question 4

trend in grp 2 reactivity

Answer 4

increases as more shielding and atomic radius so less nuclear attraction

Question 5

grp 2 metal oxides reactions

Answer 5

water –> releases hydroxide ions and metal ions
- when solution is saturated, any more metal and hydroxide ions will form solid precipitate

Question 6

solubility of hydroxides

Answer 6

• increases down the group, solutions contain more hydroxide ions and are more alkaline
• pH and alkalinity increases

Question 7

grp 2 compounds used in agriculture

Answer 7

• Ca(OH)₂ is added to fields as lime by farmers to increase pH of acidic soils
• neutralises acid in water forming water
  Ca(OH)₂ (s) + 2H⁺ (aq) –> Ca⁺² (aq) + 2H₂O (l)

Question 8

grp 2 compounds used in medicine

Answer 8

• grp 2 bases are used as antacids to treat acid indegestion
• many tables use Calcium Carbonates/ magnesium hydroxide
• ‘milk of magnesia’ is a suspension of white magnesium hydroxide
  Mg(OH)₂ + 2HcCl —> MgCl₂ + 2H₂O

Question 9

what are the halogens

Answer 9

grp 7 elements
exist as diatomic molecules at RTP

Question 10

Flourine

Answer 10

pale yellow gas

Question 11

Chlorine

Answer 11

pale green gas

Question 12

Bromine

Answer 12

red-brown liquid

Question 13

Iodine

Answer 13

shiny grey-black solid

Question 14

b.p down grp 7

Answer 14

• increases down the group, more electrons and London forces so more energy needed, so higher b.p

Question 15

reactivity down the group

Answer 15

• decreases, more shielding and radius so harder to attract electron

Question 16

halogen-halide displacement reactions

Answer 16

• if halogen added is more reactive then it displaces the halide ion

Question 17

Cl^- + Br₂
Cl^- + I₂
Br^- + I₂

Answer 17

no reaction

Question 18

Br^- + Cl₂
I^- + Cl₂
I^- + Br₂

Answer 18

1. turns orange
2. turns purple
3. turns purple

Question 19

what do you add to displacement reaction

Answer 19

cyclohexane to tell colours apart - organic non-polar solvent
chlorine is pale green
bromine is orange
iodine is violet

Question 20

disproportionation

Answer 20

where same species is oxidised and reduced

Question 21

chlorine with water

Answer 21

• used in water purification - kills bacteria
• Cl₂(aq) + H₂ (l) —> HClO (aq) + HCl (aq)
• bacteria killed by chlorate ions in chloric acid, HClO
• also acts as a weak bleach

Question 22

chlorine reacts with cold, dilute aqueous NaOH

Answer 22

• reaction of chlorine with water is limited by low solubility
• if water has dissolved NaOH then more chlorine dissolves
  Cl₂(aq) + 2NaOH (aq) –> NaClO (aq) + NaCl (aq) + H₂O (l)

Question 24

Benefits & Risks of Chlorine

Answer 24

• beneficial in ensuring water is drinkable
• chlorine gas is toxic
• chlorine in drinking water can react with organic hydrocarbons which could cause cancer
• risk is smaller then not added chlorine to water

Question 24

Qualitative analysis

Answer 24

relies on simple observations not measurements

Question 25

Carbonate test

Answer 25

• reacts with acids to form carbon dioxide (dilute nitric acid)
• if bubbles form, bubble through lime water (calcium hydroxide) - turns cloudy
  CO₂ (g) + Ca(OH)₂ (aq) –> CaCO₃ (s) + H₂O (l)

Question 26

sulfate test

Answer 26

add barium nitrate, should form barium sulfate which is insoluble in water

Question 26

halide tests

Answer 26

add silver nitrate
chloride - white precipitate
bromine - cream precipitate
iodide - yellow precipitate

Question 27

how to test halides apart

Answer 27

add ammonia
chloride - redissolves in dilute ammonia
bromine - redissolves in conc. ammonia
iodide - doesn’t redissolve

Question 28

ammonium test

Answer 28

• add aqueous NaOH
• produces ammonia, hard to see bubbles as soluble in water
• warm mixture and releases ammonia gas
• use moist pH indicator paper - it’s alkaline - turns paper blue

Question 29

sequence of tests

Answer 29

1. carbonate
2. sulfate
3. halide

Question 30

why does carbonate test have to be done first?

Answer 30

• sulfates and halides don’t produce bubbles when reacting with dilute acid - carried out without possibility of incorrect conclusion
• if present keep adding acid till bubbles stop to remove presence of ions

Question 31

why does sulfate test have to be done second?

Answer 31

• barium carbonate is also a white precipitate
• so carry out carbonate test before hand to make sure no carbonate ions are present and if precipitate forms it’s due to sulfate ions
• can filter solution after to remove barium sulfate so can test for halides

Question 32

why does halide test have to be done third?

Answer 32

• silver carbonate and silver sulfate are both insoluble and form precipitates
• so carry out test last when possibility of carbonates and sulfates have been ruled out