Group 2 and Group 7

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  • Created on: 15-05-14 18:59
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The periodic table- Group 2
First ionisation energy
- Decreases down group
- Outermost electron becomes further from nucleus
- More shielding by extra electron shells cancels out the increased nuclear charge
- Radiums 1st ionisation energy is a bit higher than the one before it as the shielding effect cannot
cancel out its nuclear charge
Reactions
Oxygen Water Chlorine
Be Be3N2 + BeO(s) Be(s) + HO(g) BeO(s) + H2(g) Be(s) + Cl2(g) BeCl2(s)
Mg Burns with white flame Mg(s) + HO(g) MgO(s) + H2(g)
Mg3N2 (nitride) + white MgO(s) Can react slowly with cold water too, but Mg(OH)2
barrier soon stops reaction
Ca Burns with brick red flame Ca(s) + 2HO(l) Ca(OH)2(s) +
Ca3N2 + white CaO(s) H2(g)
Sr Burns with red flame Sr3N2 + Sr(s) + 2HO(l) Sr(OH)2(aq) +
All react in same way
white SrO(s) H2(g)
Ba Burns with green flame BaO2 Ba(s) + 2HO(l) Ba(OH)2(aq)
(barium peroxide) + Ba3N2 + H2(g)
- Oxides react with water to form hydroxides: O2- + HO 2OH- (e.g. MgO + HO Mg(OH)2)
- Oxides (but not BeO) react with dilute acids to form salts: CaO(s) + 2HNO3(aq) Ca(NO3)2(aq) + HO(l)
- Hydroxides react with acids to form salt + water
Thermal stability of nitrates and carbonates
- Carbonates and nitrates decompose upon heating and become more stable down group
- All carbonates are insoluble and react with acids- less stable than Group 1 carbonates because
they have a higher charge so more energy is released when charges are brought together
- All nitrates are colourless crystalline solids and are very soluble
- CaCO3(s) CaO(s) + CO(g)
- 2Mg(NO3)2 2MgO(s) + 4NO2(g) + O(g)
- Become more stable because:
- Down group, less energy needed to break XCO3 lattice (=LE(XCO3))
- But less energy received from forming new oxide (= -LE(XO))
- Same energy needed to break down CO3 ion (Hconstant)
- Inter-ionic distance of oxides becomes
smaller down group than those of
carbonates (because ion is smaller)
- So formation of oxide requires more
energy than is needed to break lattice
- From the perspective of polarisation: less energy
is needed to break apart the CO32- ion if it has
been more polarised, and the smaller the cation,
the more polarising it is due to its charge density
- To investigate decomposition, heat a sample in a
boiling tube and test for gases
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Hydroxides and sulfates
- Solubility of hydroxides increases down group (think- its the opposite of the sulfates and BaSO4 is
insoluble)
- Solubility of sulfates decreases down group (solubility can be calculated by titrating a saturated
solution to find the number of moles in the water)
- To investigate: dissolve a spatula of each nitrate in 10cm3 of water each, then add Group 1
hydroxide/carbonate to each and record observations about precipitates
Flame tests
- Dip nichrome wire in HCl, then hold in bunsen flame…read more

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The periodic table- Group 7
Normal In water In organic solvent
Chlorine Green gas Pale green
Bromine Orange-brown liquid Yellow-orange
Iodine Purple solid Pale brown Pink-purple
Reactions
Oxidation reactions with metals
- Iodine anion is so polarisable that compounds with small or highly charged cations are very covalent
- Reactions with hot iron:
- Cl: burns brightly to form iron (III) chloride
- Br: less exothermic
- I: with iodine vapour, forms iron (II) iodide
- This is done by passing gas through combustion tube…read more

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With water
- Hydrogen halides (colourless gas steamy fumes in moist air): HCl + HO Cl-(aq) + H3O+(aq)
(oxonium ion)- ionise completely in water to form strong acids
- Halogens: HO + Cl2 HClO (chloric acid) + HCl (bromine reacts to less extent, iodine is insoluble
in water so hardly reacts)- in NaOH solution it makes HOCl (chlorate (1)) and H+ and C-
- Process to become an acid: evaporate, make both gases ions and hydrate them both- HF is a
weaker acid than HCl…read more

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