3.1.2 Group 2 Elements

Question 1

how do alkali metals act as “reducing agents” in redox

Answer 1

• reduce the species they interact with, as in redox:
• metal atom is oxidised
• loses 2 e- in outer s-subshell
• form 2+ ion, with same configuration as noble gas
• e.g. Ca ===> Ca2+ + 2e- ([Ar]4s2 ===> [Ar])

Question 2

how do alkali metals react with oxygen

Answer 2

• undergo redox, forming metal oxides (GF: MO, where M2+ and O2-)

Question 3

give example of magnesium reacting with oxygen

Answer 3

• burns with O2 to give bright light and form white oxide
• 2Mg(s) + O2(g) ===> 2MgO(s)
• Mg oxidised (0 to +2)
• O2 reduced (0 to -2)

Question 4

how do alkali metals react with water

Answer 4

• form an alkaline hydroxide (GF: M(OH)2) and hydrogen gas
• originally a slow reaction, like with Mg, but increases down group as does reactivity

Question 5

give example of strontium reacting with water

Answer 5

Sr(s) + 2H20(l) ===> Sr(OH)2(aq) + H2(g)

-Sr oxidised (from 0 to +2)
-H reduced from +1 to 0)

Question 6

how do alkali metals react with dilute acids

Answer 6

metal + dilute acid ===> salt + hydrogen
- reactivity increases down the group
- can have MONObasic acids, e.g. HCl, which donate 1 proton per molecule, or DIbasic (H2SO4) and TRIbasic (H3PO4)

Question 7

give example of magnesium reacting with dilute hydrochloric acid

Answer 7

Mg(s) + 2HCl(aq) ===> MgCl2(aq) + H2(g)

• Mg oxidised ( 0 to +2 )
• H reduced ( +1 to 0 )

Question 8

what is the pattern of reactivity down group 2 elements

Answer 8

reactivity INCREASES

Question 9

what is the main energy input when alkali metals react and how does it occur

Answer 9

• 2 ionisation energies
• lose 2 e- and form 2+ gaseous ions
• other energy changes do take place, but IEs are the main one

Question 10

why does ionisation energy DECREASE down group 2 elements

Answer 10

• increase in atomic radius
• increase in shielding
• (increase in nuclear charge too, but this is outweighed by other factors)
• decrease in ES attraction between nucleus and outer shell electrons
• so more reactive
• and more stronger reducing agents

Question 11

what are group 2 compounds

Answer 11

oxides, hydroxides, carbonates

Question 12

how do oxides react with water

Answer 12

• release hydroxide ions (OH-) and form alkaline solutions
• go on to form metal hydroxides

Question 13

give example of CaO reacting with water

Answer 13

CaO(s) + H2O(l) ===> Ca2+ + 2OH-

• ions add to make Ca(OH)2 (s)

Question 14

why do most reactions of oxides with water result in solid precipitate formed

Answer 14

• hydroxides aren’t very soluble
• one solution is saturated, form solid precipitate form
• which is typical form - (s) and (aq)

Question 15

what happens to the alkalinity of hydroxides as you go down the group

Answer 15

INCREASES

Question 16

why does alkalinity increase down group 2

Answer 16

• solubility increases down group
• so solution contains more OH- ions
• increasing alkalinity
• pH range from 10 (Mg) to 13 (Ba)

Question 17

explain an experiment to show the varying alkalinity of the group 2 hydroxides

Answer 17

• add spatula of each oxide to water
• shake
• should get a saturated solution and remaining white precipitate which has not dissolved, and insufficient amount of H2O at this scale to dissolve all hydroxide
• measure pH of each hydroxide
• value should increase

Question 18

what are the 2 uses of alkaline compounds

Answer 18

1) AGRICULTURE - neutralise acidic soil
2) MEDICINE - neutralise stomach acid

Question 19

explain use of alkaline compounds in agriculture

Answer 19

• Ca(OH)2, calcium carbonate, is added to fields as lime, which increases the pH of acidic soil, neutralising it, and forming H2O
• Ca(OH)2(aq) + 2H+(aq) ===> Ca2+(aq) + 2H20(l)

Question 20

explain the use of alkaline compounds in medicine

Answer 20

• used as antacids to treat stomach indigestion
• neutralise excess stomach acid, which is mainly HCl
• use MgCO3 (magnesium carbonate), CaCO3 (calcium carbonate)
• or use “milk of magnesia”, Mg(OH)2 (magnesium hydroxide), which is a suspension of white Mg(OH)2 in water, as only slightly soluble in water

CaCO3(s) + 2HCl ===> CaCl2 + H20 + CO2
Mg(OH)2(s) + 2HCl ===> MgCl2 + 2H20