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)
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 = metals Yellow = non-metals Pink= semi-metals • Intro.chem.okstat.eduPeriodic table arrange according to the “octet rule”. All the elements want to have 8electrons 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.
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
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