ACID- BASE BEHAVIOUR OF THE PERIOD 3 OXIDESThese pages look at the reactions of the oxides of Period 3 elements (sodium tochlorine) with water. Argon is obviously omitted because it doesnt form an oxide.A quick summary of the trendThe oxidesThe oxides well be looking at are: Na2O MgO Al2O3 SiO2 P4O10 SO3 Cl2O7 P4O6 SO2 Cl2OThe trend in acid-base behaviorThe trend in acid-base behavior is shown in various reactions, but as a simplesummary: • The trend is from strongly basic oxides on the left-hand side to strongly acidic ones on the right, via an amphoteric oxide (aluminum oxide) in the middle. An amphoteric oxide is one which shows both acidic and basic properties.For this simple trend, you have to be looking only at the highest oxides of theindividual elements. Those are the ones on the top row above, and are where theelement is in its highest possible oxidation state. The pattern isnt so simple if youinclude the other oxides as well.These reactions are all explored in detail on the rest of these pages.Chemistry of the individual oxidesSodium oxideSodium oxide is a simple strongly basic oxide. It is basic because it contains theoxide ion, O2-, which is a very strong base with a high tendency to combine withhydrogen ions. • Reaction with waterSodium oxide reacts exothermically with cold water to produce sodium hydroxidesolution. Depending on its concentration, this will have a pH around 14 (pH >7). Na2O + H2O → 2 NaOH
Magnesium oxideMagnesium oxide is again a simple basic oxide, because it also contains oxide ions.However, it isnt as strongly basic as sodium oxide because the oxide ions arent asfree.In the sodium oxide case, the solid is held together by attractions between 1+ and2- ions. In the magnesium oxide case, the attractions are between 2+ and 2-. Ittakes more energy to break these. Even allowing for other factors (like the energy released when the positive ionsform attractions with water in the solution formed), the net effect of this is thatreactions involving magnesium oxide will always be less exothermic than those ofsodium oxide. • Reaction with waterIf you shake some white magnesium oxide powder with water, nothing seems tohappen - it doesnt look as if it reacts. However, if you test the pH of the liquid,you find that it is somewhere around pH 9 (pH > 7) - showing that it is slightlyalkaline or basic. There must have been some slight reaction with the water to produce hydroxideions in solution. Some magnesium hydroxide is formed in the reaction, but this isalmost insoluble - and so not many hydroxide ions actually get into solution. MgO + H2O → Mg(OH)2Aluminum oxideAluminium oxide is amphoteric. It has reactions as both a base and an acid. • Reaction with waterAluminium oxide doesnt react in a simple way with water in the sense that sodiumoxide and magnesium oxide do, and doesnt dissolve in it. Although it still containsoxide ions, they are held too strongly in the solid lattice to react with the water.Silicon dioxide (silicon(IV) oxide)By the time you get to silicon as you go across the period, electronegativity hasincreased so much that there is no longer enough electronegativity differencebetween silicon and oxygen to form ionic bonds.Silicon dioxide has no basic properties - it doesnt contain oxide ions. Instead, it isvery weakly acidic. • Reaction with waterSilicon dioxide doesnt react with water, because of the difficulty of breaking upthe giant covalent structure.
The phosphorus oxidesWe are going to be looking at two phosphorus oxides, phosphorus (III) oxide, P4O6,and phosphorus (V) oxide, P4O10.Phosphorus (III) oxidePhosphorus(III) oxide reacts with cold water to give a solution of the weak acid,H3PO3 - known variously as phosphorous acid. Its reaction with hot water is muchmore complicated.Phosphorus (V) oxidePhosphorus (V) oxide reacts violently with water to give a solution containing amixture of acids, the nature of which depends on the conditions. We usually justconsider one of these, phosphoric (V) acid, H3PO4 - also known just as phosphoricacid. P4O10 + 6 H2O → 4 H3PO4The sulphur oxidesWe are going to be looking at sulphur dioxide, SO2, and sulphur trioxide, SO3.Sulphur dioxideSulphur dioxide is fairly soluble in water, reacting with it to give a solution ofsulphurous acid (sulphuric(IV) acid), H2SO3. This only exists in solution, and anyattempt to isolate it just causes sulphur dioxide to be given off again.Sulphur trioxideSulphur trioxide reacts violently with water to produce a fog of concentratedsulphuric acid droplets. SO3 + H2O → H2SO4The chlorine oxidesChlorine forms several oxides, but the only two mentioned by IB are chlorine (VII)oxide, Cl2O7, and chlorine (I) oxide, Cl2O. Chlorine (VII) oxide is also known asdichlorine heptoxide, and chlorine(I) oxide as dichlorine monoxide.
Chlorine(VII) oxideChlorine(VII) oxide is the highest oxide of chlorine - the chlorine is in its maximumoxidation state of +7. It continues the trend of the highest oxides of the Period 3elements towards being stronger acids.Chlorine(VII) oxide reacts with water to give the very strong acid, chloric(VII)acid - also known as perchloric acid. The pH of typical solutions will, like sulphuricacid, be around 0.Chlorine(I) oxideChlorine(I) oxide is far less acidic than chlorine(VII) oxide. It reacts with water tosome extent to give chloric(I) acid, HOCl - also known as hypochlorous acid.