Some Chemistry of the Periodic Table

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Oxides, Chlorides and Hydrides



` 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)



` 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



` 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



` Oxides of the non-metals, e.g. CO 2 and P 4O 10have low melting and boiling points as they consistof discrete covalent molecules



` 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



` 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)



` 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



` 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)



` 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



` 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 .



` 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



` Groups 1 and 2 chlorides are ionic lattices` Others are covalent molecules

Melting points and boiling points confirm this



` 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)



` 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



` 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)



` 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 -



` 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



` 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



` 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



` 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)



` 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)



` 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)

Some Chemistry of the Periodic Table

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