group 2

what is the name of group 2 metals?

alkaline earth metals

what is the valence electron structure of group 2 metals?

two electrons in the outermost shell in the s sub-shell

what is the most common reaction of group 2 metals?

redox reactions
each metal atom is oxidised, losing two valence electrons to form a 2+ ion
another species gains the two electrons lost, becoming reduced
therefore, group 2 elements are reducing agents

what are the three redox reactions of group 2?

redox reactions with: oxygen, water, and dilute acids

what happens in a redox reaction of a group 2 element with oxygen?

a metal oxide is formed, with the general formula MO
2M(s) + O2(g) → 2MO(s)
the total increase of oxidation number of M is +4
the total decrease of oxidation number of O2 is -4

what happens in a redox reaction of group 2 elements with water?

an alkaline hydroxide is formed, with the general formula M(OH)2 and hydrogen gas
M(s) + 2H2O(l) → M(OH)2(aq) + H2(g)
the total increase in oxidation number of M is +2
the total decrease in oxidation number of H is -2 (2H -1 to form H2) (2H do not change to form M(OH)2)

what happens in a redox reaction of group 2 with dilute acids?

a salt and hydrogen gas is formed
M(s) + 2HCl(aq) → MCl2(aq) + H2(g)
the total increase in oxidation number of M is +2
the total decrease in oxidation number of H is -2

what is the trend in reactivity of group 2?

reactivity increases down the group
as atomic radii increases, there is more shielding
nuclear attraction decreases
it is easier to lose outer electrons

what is the trend in ionisation energy of group 2?

both first and second ionisation energy decreases down the group
atomic radii increases, more shielding
nuclear attraction decreases, less energy required to remove valence electrons

what do group 2 compounds react with?

water

what happens in the reaction of group 2 oxides with water?

hydroxide ions are released to form alkaline solutions
MO (s) + H2O(l) → Ca2+(aq) + 2OH-(aq)
solutions get more alkaline down the group due to increasing solubility, so solutions formed contain more OH- ions

what happens when the alkaline solution becomes saturated in a reaction of group 2 oxides with water?

any further metal and hydroxide ions will form a precipitate
Ca2+(aq) + 2OH-(aq) → Ca(OH)2(s)

what are two examples of group 2 precipitates in a reaction of group 2 oxides with water?

solid Mg(OH)2 - only slightly soluble in water, solution has low OH- concentration and pH 10
Ba(OH)2(s) - much more soluble in water, solution has a greater OH- concentration and pH 13

what are two uses of group 2 compounds and why?

group 2 oxides, hydroxides and carbonates are used in agriculture and medicine due to their ability to neutralise acids

why are group 2 compounds used in agriculture?

pH of soil can be raised by adding a group 2 base to neutralise excess acid

what are two examples of group 2 compounds used in agriculture?

calcium hydroxide (slaked lime) - reasonably soluble in water and acts relatively quickly
Ca(OH)2(s) + 2H+(aq) → Ca2+(aq) + 2H2O(l)
calcium carbonate (limestone) - cheaper and safer to handle than Ca(OH)2 but largely insoluble in water

what are two examples of group 2 compounds used in medicine?

magnesium hydroxide (milk of magnesia) used as an antacid
barium sulfate used for medical imaging

why is magnesium hydroxide (milk of magnesia) used as an antacid?

neutralises excess HCl in the stomach to relieve indigestion
Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l)
safe to use as Mg(OH)2 has low solubility (conc of OH ions is low and solution is pH 10)

why is barium sulfate used for medical imaging?

patient drinks a suspension of barium sulfate before an X ray of the digestive system
BaSO4 coats tissues and allows soft gut to be seen on image, as barium is a large atom that absorbs X-rays well
safe to ingest as it is insoluble, so not absorbed into bloodstream from gut