Oxides, Chlorides and Hydrides
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Oxides, Chlorides and Hydrides
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` W e know already that the physical and chemicalproperties of the elements show trends dependenton where they are in the Periodic Table
` W e¶ll now look at how the physical properties andchemical reactions with water for two-elementcompounds change as we go across the PeriodicTable (Periods 2 and 3)
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` Oxides of elements can usually be prepared fromdirect combination with oxygen
S pontaneous reaction of sodium in air, or whitephosphorus in air S low tarnishing of magnesium in air, but rapid burning ina Bunsen flameNitrogen forming nitrogen oxides at very hightemperatures, e.g. during a lightening storm, or in a car
engine
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` BeO, MgO and Al 2O 3 all exist as ionic lattices of positive metal ions and negative non-metal ions
` MgO is used to produce inert linings for furnaces
due to its high melting point
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` Oxides of the non-metals, e.g. CO 2 and P 4O 10have low melting and boiling points as they consistof discrete covalent molecules
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` S ilicon dioxide (silica)` Covalent network
High melting point
` Can disrupt the network with metal ions toproduce glass
S odium ions give soft low melting point glassLead give bright glass with high refractive index
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` Ionic oxides have high conductivity when molten` Non-metal oxides zero conductivity` Metalloid oxides, e.g. BeO and B 2O 3 have poorer
conductivity than ionic oxides, but better thancovalent oxides (when molten)
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` Aluminium is never found in its elemental form` It is extracted from an ore called bauxite` Aluminium oxide is extracted from the bauxite` Electrolysis is then used to get aluminium from
aluminium oxide
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` F rom S tandard Grade we know that metal oxidesthat dissolve give alkalis, and non-metal oxidesthat dissolve give acids
` W e can see that in general, the pH drops as wego across a Period (it would be a Periodicfunction)
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` As usual, there are exceptions to any rule!` Let¶s have a wee look at Al 2O 3` Like any metal oxide, it behaves as a base when it
reacts with acid, to give a salt + water Al2O 3 (s) + 6HCl (aq) 2AlCl3 (aq) + 3H 2O (l)
` However, it can also react with a base to form asalt (i.e. act as an acid)
Al2O 3 (s) +2NaOH (aq) + 3H 2O (l) 2NaAl(OH) 4 (aq)` Oxides which can act as an acid or a base are
called amphoteric oxidesExamples are Al 2O 3 and BeO
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` Oxides which react with acids to form salts arecalled basic oxides . S ome basic oxides aresoluble in water and form alkaline solutions
` Oxides which react with bases to form salts arecalled acidic oxides . S ome acidic oxides aresoluble in water and react to form acids
` S ome oxides can behave as an acidic or basicoxide and form one type of salt with an acid, and a
different one with a base. These are calledamphoteric oxides .
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` All elements in Periods 2 and 3 form chlorides,except the noble gases
` S odium reacts vigorously with chlorine to form
sodium chloride, other elements react moreslowly.` The bonding of the Period 2 and 3 elements with
Cl varies similarly to that with oxygen
Physical properties can be described as periodic
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` Groups 1 and 2 chlorides are ionic lattices` Others are covalent molecules
Melting points and boiling points confirm this
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` Group 1 chlorides are ionic lattices and do notreact with water
` The ions dissociate fullyNaCl (s) NaCl (aq)
S cholar animation` The original material can be obtained by
evaporation of the water ` Moving across a Period, chlorides become more
covalent and some can be hydrolysed by water and give off hydrogen chloride fumes
PCl 3 (s) + 3H 2O H3PO 4 (aq) + 3HCl (g)
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` Ionic chlorides will conduct when molten or insolution
` Electrolysis of molten NaCl is used industrially toproduce the elements sodium and chlorine
` Electrolysis of NaCl solution (commonly calledbrine ) is used to produce chlorine and sodiumhydroxide
This is the basis of the chlor-alkali industryProducts include:x Chlorine, hydrochloric acid, caustic soda, sodium hypochlorite,
dry cleaning solvents, anaesthetics and polymers
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` Hydrogen can form compounds with all of theelements in the 2 nd and 3 rd periods (except thenoble gases)
` LiHMelting point 680°C (decomposes)` NaH
Melting point 800°C (decomposes)
NaCl structure (face-centred cubic)
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` Electrolysis of molten hydrides produceshydrogen at the positive electrode
i.e. Ionic hydrides must contain the H - ion`
Electropositive metal atoms give an electron to thehydrogen atom` 2Na + H 2 2(Na +H-)` Electrolysis of molten NaH:
Na+
+ e-
Na2H - H2 + 2e -
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` Very strong reducing agent (electron donor)` Reduces some metal oxides to the produce the
metal
CuO + NaH Cu + NaOH` Complex hydrides like LiAlH 4 and NaBH 4 are used
in organic chemistry when strong reducingconditions are required
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` Ionic hydrides react with water to producehydrogen gas and the metal hydroxide
NaH (s) + H 2O (l) NaOH (aq) + H 2 (g)
Again, NaH is a reducing agent (it is oxidised to NaOH)` CaH 2 behaves in a similar way
Treated with water to provide small amounts of H 2 gas for weather balloons
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` Research is looking at the possibility of usingmetal hydrides to store hydrogen for poweringtransport
Addition of water will release H 2 gas as it is required` Other group 2 and 3 metal hydrides (BeH 2, MgH 2
and AlH 3) as well as B 2H6 have complexmacromolecular structures
React with water to give a range of products
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` The trends in boiling points are affected by thepresence of hydrogen bonding
` As we get further along a period, the atomsbecome more electronegative than hydrogen sopolar molecules (e.g. HCl are obtained)
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` Reminder ± metal hydrides react with water toproduce hydroxide ions and hydrogen gas
` N on-metal hydrides can be weak bases (e.g.
ammonia), weak acids (e.g. hydrogen sulphide) or strong acids (e.g. hydrogen chloride)
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` Physical and chemical properties show Periodictrends and are related to the bonding present
` Oxides can be acidic, basic or amphoteric
` Chlorides of 2nd
and 3rd
Period elements showtrends in their reaction with water dependent onthe position of the element in the Periodic Table
` Ionic hydrides are very good reducing agents and
can be used as a source of hydrogen (reactionwith H2O gives H 2 (g) and OH - (aq)