8 - Reactivity trends

Question 1

are group 2 metals reducing or oxidising agents

Answer 1

Reducing
This is because they have a tendency to lose 2 electrons (from their outer shell) to form positive ions. In chemical reactions, they donate electrons to other species, causing reduction by themselves undergoing oxidation.

Ca-> Ca2+ + 2e Ca is oxidised

another species will gain the 2e-

Question 2

group 2 and oxygen

Answer 2

undergo a redox reaction with O2 to form a metal oxide - MO
2Mg + O2 –> 2MgO
0 —> +2
0—–> -2

Metal is oxidised
Oxygen is reduced

Question 3

group 2 metals with water

Answer 3

undergo a redox reaction and form a alkaline hydroxide - M(OH)2 and hydrogen gas

-> Mg and water react slowly but reaction becomes more vigorous down the group as reactivity increases

Sr + 2H2O -> Sr(OH)2 + H2
0 -> +2
+1 -> 0

Question 4

group 2 and dilute acids

Answer 4

a redox reaction

metal + acid -> salt + hydrogen

• reactivity increases down the go[

Mg + HCl -> MgCl2 + H2
0 -> +2
+1 -> 0

Question 5

trend in reactivity and ionisation energy

Answer 5

As you move down the group, the atomic radius increases due to additional electron shells.

• the shielding effect from inner electron shells increases, reducing the effective nuclear charge felt by outer electrons.
• the outer electrons experience less nuclear attraction
• leading to a decrease in ionization energy as less energy is needed to remove the e-
• enhanced reactivity as the outer electrons are more easily lost in chemical reactions.

Question 6

Group 2 oxides and water

Answer 6

CaO + H2O –> Ca2+(aq) + OH-(aq)

• releases oh- and forms an alkaline solution of the metal hydroxide
• group 2 hydroxides are only slightly soluble so any when the solution become saturated, any additional metal and OH ions form a solid precipitate

Question 7

trend in solubility of Group 2 hydroxides

Answer 7

• solubility of hydroxides in water increases down the group, so the solution contains more OH- and is more alkaline
• Mg(OH)2 is slightly soluble so pH=10
• Ba(OH)2 is more soluble so pH=13

Question 8

experiment to show trend in solubility of group 2 hydroxides

Answer 8

1- add MO to water
2- shake mixture and a saturated solution should form so a solid precipitate form
3- measure the pH

Question 9

group 2 compounds in agriculture

Answer 9

Ca(OH)s is added to fields to increase the pH of acidic soils
- in the form of white lime
Ca(OH)2 + 2H+ -> Ca2+ 2H2O

Question 10

group 2 compounds in medicine

Answer 10

• often used as antacids to treat indigestion
• contain magnesium and calcium carbonates
• and milk of magnesia a suspension of white magnesium hydroxide

Question 11

show the reactions that take place of group 2 compounds in indigestion tablets

Answer 11

Mg(OH)2 + 2HCl -> MgCl2 + 2H2O
CaCO3 + 2HCl -> CaCl + H2O + CO2

Question 12

trend in Tb of halogens

Answer 12

• more electrons
• stronger induced dipole-dipole interactions
• more energy needed to break IF
• boiling point increases down the group

Question 13

are halogens reducing or oxidising agents

Answer 13

oxidising agents
-> 7 3- in outer shells so in reactions they gain one electron to form 1- ions
Cl2 + 2e- -> 2Cl-

Question 14

halogen halide displacement reactions

Answer 14

• displacement reactions sow that reactivity of halogens decreases down the group
• • a solution of each halide is added to aqueos solutions of the other halides
• if a more reactive halogen is added to a less reactive halide then a reaction takes place- the halogen displaces the halide from solution and it changes colour

Question 15

colours of Cl2, Br2 and I2 in water and cyclohexane

Answer 15

water
Cl2 = pale green
Br2 = orange
I2 = brown

Cyclohexane (top layer)
Cl2 = pale green
Br2 = orange
I2 - violet

Question 16

what solutions of group 7 compounds are hard to tell apart in displacement reactions
-> how do you get over this

Answer 16

solutions of iodine and bromine in water are hard to tell apart as they can appear a similar orange-brown colour

• add a non-polar organic solvent like cyclohexane and the mixture is shaken
• the non-polar halogens dissolve more readily in cyclohexane than water - so colours are more easier to tell apart

Question 17

Cl2 added Br-(aq)

Answer 17

pale green -> orange

Question 18

Cl2 added to I- (aq)

Answer 18

pale green -> violet

Question 19

Br2 added to I-(aq)

Answer 19

orange -> violet

Question 20

trend in reactivity of the halogens

Answer 20

• atomic radius increases
• more inner shells so shielding increases
• less nuclear charge so to capture an e-
• so reactivity decreases down a group

Question 21

what is disproportionation

Answer 21

a redox reaction where the same element is oxidised and reduced

e.e reation of water and cold dilute sodium hydroxide

Question 22

reaction of chlorine and water

Answer 22

disproportion reaction takes place

Cl2(aq) + H2O -> HClO(aq) + HCl(aq)
0 -> +1 -1

• chlorine is used as a disinfectant as a water treatment
• ClO- kills the bacteria

Question 23

reaction of chlorine and cold, dilute aqueous sodium hydroxide

Answer 23

disproportiojnation reaction

Cl2 + 2NaOH -> NaClO + NaCl + H2O
0 -> +1 -1

• reaction of water and chlorine is limited by its low solubility so if the water contains dissolved NaOH much more chlorine is dissolved
• so there is a large number of ClO- formed which is useful for household bleach

Question 24

:) and :( of using chlorine

Answer 24

:)
- water is fit to drink as bacteria is killed
-
:(
- chlorine is a toxic gas#- a respiratory irritant where large [] can be fatal
- Cl in drinking water can react with organic hydrocarbons (from decaying vegetation). it forms chlorinated hydrocarbons which are carcinogenic

Question 25

what tests do i need to know

Answer 25

carbonate
sulfate
halide
ammonium

Question 26

carbonate test

Answer 26

add dilute nitric acid to the solution
- carbonate will react with the acid to form CO2 gas
iff bubbles are formed, it could be CO2

Na2CO3 + 2HNO3 -> 2NaNO3 + CO2 + H2O

• to test if the gas is CO2
  -> bubble through lime water - Ca(OH)2(aq)
  -> CO2 react to form a white precipitate of CaCO3

CO2 + Ca(OH)2 -> CaCO3 + H2O

Question 27

sulfate test

Answer 27

• most sulfates are soluble but barium sulfate isn’t BaSO4
• use aqueous barium chloride or barium nitrate (if u are carrying out a halide test use barium nitrate)

Ba2+(aq) + SO4 2- (aq) -> BaSO4 (s)

Question 28

halide test

Answer 28

most halides are soluble but silver halides arent

1) add aqueous silver nitrate to aqueous solution of the halide

Ag+ + X- -> AgX(s)

2) the silver halide precipitates are different colours. Cl= white Br=cream I= yellow
3) to distinguish between them add aqueous ammonia to test the solubility. AgI is insoluble in NH3

Question 29

sequence of tests

Answer 29

1) carbonate
2) sulfates
3) halides
CaseOh

1) neither sulfate of halide ions produce bubbles with dilute acid so it can be carried out without the possibility of an incorrect conclusion
2) barium carbonate is also white and insoluble in water so you carry it out after carbonate to ensure there are no carbonate present and don’t get a false positive
3) Ag2CO3 and AG2SO4 are both insoluble in water and form a precipitate. so carry out this test last to ensure there is no carbonate or sulphate to make a false positive

Question 30

test for ammonium ions

Answer 30

NH4 +
1) added aqueous
sodium hydroxide to a solution of an ammonium ion
2) ammonia gas is produced- no bubbles form as ammonia is very soluble in water
3) warm the mixture and ammonia gas is released
4) test for the ammonia gas by using moist pH indicator paper - it turns paper blue as ammonia is alkaline