Oxides

Oxides

• Iron oxide is the most common example of an oxide = rust (this is an example of a metal oxide)

• Oxides contain oxygen– Eg 2Mg(s) + O2 (g) → 2MgO(s)

– This equation is true for any substance that forms an oxide and could be generalised to • Substance A + Oxygen → substance A oxide

• Non-metals form acidic oxides– They have covalent bonds (ie bond between non-

metal and non-metal)• Metals form basic oxides– They have ionic bonds (ie bond between metal

and non-metal)

Soluble metals

• We can show that metals make basic oxides by examining some equations.

• When a metal is soluble, we can show that there is an increase in OH- ions:– BaO(s) + H2O(l) → Ba2+

(aq) + 2OH-(aq)

– When barium reacts with water it forms an barium ion + the hydroxide group (which makes things basic)

– Barium has given its 2 electrons to the water molecule, which converts the water to hydroxide

– The water can also be tested using a pH meter• Metals tend to give electrons

Insoluble metals

• To determine if a metal oxide is basic when it does not dissolve, add it to an acid

• If the oxide is acting as a base, then a neutralisation reaction will occur.– Acid + base → salt + water– CuO(s) + 2HCl(aq) → CuCl2(aq) + H2O(l)

Soluble non-metals

• The following are equations which demonstrate why non-metals make acidic oxides

• SO2(g) + H2O(l) → H2SO3↔ H+ + HSO3-

– Sulphur grabs electrons to form a strong covalent bond

– When sulfuric acid is mixed with water it dissociates into a H+ ion and a Hydrogen sulfate ion

– This could be tested with a pH meter

Non-soluble non-metal

• We can also use a neutralisation reaction to demonstrate that a non-metal oxide is acidic

• Acid + base → salt + water• SO3(g) + 2NaOH(l) → Ns2SO3+ H2O(l)

Important non-metal oxides

• CO2

• SO3

• SO2

• NO2

Summary

• An acid oxide is one which either:– Reacts with water to form an acid or– Reacts with bases to form salts (or does both)

• A basic oxide is one that:– Reacts with acids to form salts but– Does not react with alkali solutions (such as NaOH or

KOH) – this is something that the amphoteric oxides do.• Amphoteric oxides:– React with acids to form salts AND– React with alkalis

Oxides and the Periodic Table

Blue = metalsYellow = non-metalsPink= semi-metals

Periodic table arrange according to the “octet rule”. All the elements want to have 8 electrons in their outershell. Looking at the PT, we notice that elements on the left tend to “give” electrons, and those on the right tend to “gain” electrons.

• Intro.chem.okstat.edu

Metals

• Likely to give electrons– Have less than 4 electrons in their outershell– Easier to loose a couple than to take a lot.

• Periodic trends– Metallic property decreases across a period

• More basic oxides are on the left• Less basic oxides are on the right of the metals

– Metallic property increases down a group• More basic oxides are at the bottom of a group• This is because outershell electrons are further away from the

positive nucleus – so the electrons are more easily lost

Non-metals

• Likely to take electrons – more electronegative• Electronegative property increases across a

period – Because non-metals are more likely to take

electrons, they form strong covalent bonds with oxygen.

• Electronegative property decreases down the group – this is related to the distance of the outershell from the nucleus

Amphoteric oxides

• These elements can go either way, they sometimes act as an acid and sometimes act as a base, depending on whether they are giving or taking electrons.

• Tend to be the semi-metal, or weaker metals• Examples include:– ZnO– Al2O3

– PbO– SnO

Neutral oxides

• Carbon and Nitrogen can form neutral oxides as well as acidic oxides– eg CO, NO, N2O• The higher the oxidation number, the more acidic

• These do not react with either acids or bases