Group 2 Flashcards
(26 cards)
1st & 2nd ionisation energies (G2)
Decrease down group: nuclear charge increases, outermost electron energy increases, more quantum shells and shielding.
Reactivity (G2)
Increases down group: energy needed to remove 2 electrons decreases.
G2 reactions with oxygen
Bright flame and formation of white solid (more vigorous down group).
2M(s) + O2(g) -> 2MO(s)
Reactions with chlorine (G2)
Combine with chlorine when heated in the gas (more vigorous down group).
M(s) + Cl2(g) -> MCl2(s)
Reactions with water (G2)
More vigorous down group (Mg doesn’t proceed completely).
M(s) + 2H2O(l) -> M(OH)2(aq) + H2(g)
Calcium hydroxide = only slightly soluble in water, liquid goes cloudy as precipitate of Ca(OH)2 forms.
Magnesium and steam
Magnesium oxide and hydrogen formed v. quickly.
Heated in tube with magnesium cool inside. Steam in one end, hydrogen out other, burned on way out for safety purposes.
Mg(s) + H2O(g) -> MgO(s) + H2(g)
Oxides with water (G2)
Basic oxides = form alkalis. Colourless solutions formed.
MO(s) +H2O(l) -> M(OH)2(aq)
Also simplified to:
O^2- + H2O -> 2OH^-
Solubility and pH of hydroxides (G2)
Increase down group.
Testing for CO2
Bubble through aqueous calcium hydroxide (limewater). Calcium carbonate formed (insoluble in water).
CO2 + Ca(OH)2 -> CaCO3 + H2O
Milk of Magnesia
Aqueous magnesium hydroxide as indigestion remedy. Neutralises enough HCl in stomach to stop indigestion.
Mg(OH)2 + 2HCl -> MgCl2 + 2H2O
Oxides and hydroxides with acids (G2)
Form salt & water. White solid -> colourless solution. Exothermic.
MgO + H2SO4 -> MgSO4 + H2O
CaO + 2HNO3 -> Ca(NO3)2 + H2O
Use in agriculture (G2)
Lime (mostly calcium hydroxide) used. Neutralises excess acidity in soil.
Ca(OH)2 +[2HNO3] -> Ca([NO3])2 + 2H2O
Solubility of G2 sulfates
All G2 nitrates & chlorides are soluble. G2 sulfate solubility decreases down group. Strontium sulfate and barium sulfate are insoluble.
Testing for sulfate ions
Add dilute hydrochloric/nitric acid (removes impurities). Add barium nitrate. Positive = white precipitate forms.
Ba(NO3)2 (aq) + Na2SO4 (aq) -> BaSO4 (s) + 2NaNO3 (aq)
Barium meals
Barium sulfate is insoluble. Makes it possible to see soft tissues in X-rays.
Thermal stability
How stable a compound is when heated, ie. does it not decompose at all (v. thermally stable) or does it decompose as much as possible (not thermally stable)?
Factors affecting thermal stability (G2)
1) Charge on G2 cation = 2x G1 cation.
2) Ionic radius of G2 cation = < G1 cation of same period.
3) Nitrate (NO3^-) and carbonate (CO3^2-) anions are more complex than Cl^- ion.
Cation with greatest influence on anion= Be^2+ (big. charge, small. size).
Thermal stability of nitrates
Nitrates G1 & G2= white solids. Decompose to nitrites/oxides & give off NO2 (brown fumes)/O2 when heated. If contains water of crystallisation, steam observed.
No b. fumes= metal nitrate -> metal nitrite + O2 (less. decomp.).
B. fumes= metal nitrate -> metal oxide + NO2 + O2 (great. decomp.).
Great. decomp. when: 2+ cation/smallest 1+ cation.
Thermal stability of carbonates
All white solids. Do not decompose/decompose to oxides + CO2. No observations possible (colourless gas + white solid).
Li decomposes, other G1s no, except at v. high temps.
G2s decompose with more difficulty down group, eg. CaCO3 -> CaO + CO2.
Decomp. occurs when: 2+ cation/smallest 1+ cation.
Flame test procedure
1) Wear safety goggles & lab coat. Light Bunsen within fume cupboard.
2) Dip nichrome/platinum wire in HCl then put in flame. Repeat until no traces of impurities.
3) Dip wire in HCl, then in compound, then in flame. Record colour.
Problems with flame tests
Colours are subjective, sodium impurities may mask other colours.
Flame colours G1
Li: red, Na: yellow/orange, K: lilac, Rb: red/purple, Cs: blue/violet.
Flame colours G2
Be: no colour, Mg: no colour (bright light), Ca: (brick) red, Sr: (crimson) red, Ba: (apple) green.
Cause of flame colours
Electrons absorb energy and move to higher energy level from ground state (lowest possible energy levels). Electron returns to ground state & releases energy. If energy corresponds to radiation in visibl light spectrum, characteristic colour appears.
