acidic and basic oxides
Post on 21-May-2015
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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
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