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CHAPTER 12 : OXIDATION AND REDUCTION

REDOX REACTION

Definition of oxidation and reduction according to 4 aspects

a) Oxidation occurs when there is b)Reduction occurs when there is

(i) a gain in oxygen (i) a loss of oxygen

(ii)a loss of hydrogen (ii)a gain of hydrogen

(iii)a loss of electron (iii)a gain of electron

(iv)an increase in the oxidation state (iv)a decrease in the oxidation state

RUSTING AS A REDOX REACTION

y Corrosion of a metal is a redox reaction in which a metal is oxidizedby losingelectrons to formpositive ions

y Metals which are more electropositive corrodefastery When iron corrodes , the process is also known as rusting.y Two conditions for the rusting of iron to occur are

-The presence ofwater-oxygen

y Arrangement of metals in the descending order in terms of the ease of corrosion. K Na Al Zn Fe Sn Pb Cu Ag Au

y The diagram below shows the mechanism of rusting

y The following table shows the redox process in rusting by stating the electrodeand write the half equation for each terminal

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Terminal Electrode Half-equation

A Anode FeFe2+

+ 2e-

B Cathode O2 + 2H2O + 4e- 4OH

-

y Overall redox equation in rusting : 2Fe(aq) + O2(g) + 2H2O(l) Fe(OH) 2(s)y Name for the rust : Iron(III) oxidey Chemical Formula for the rust : Fe2O3.xH2O, where x variesy Three main ways to prevent rusting-by using protective coating-byformingalloys

-by using sacrificial metal

y Three metals that can be used in sacrificial protection for an underground ironpipe

-magnesium-aluminium

-zinc

y A reagent that is usually used to detect the presence of iron (II) ion in a laboratoryexperiment.

-Potassium hexacyanoferrate(III) solution

THE REACTIVITY SERIES OF METALS AND ITS APPLICATION

y In the reactivity series, metals are arranged in the order oftheir reactivity withoxygen

y The metal at the top of the series is the most reactive. They burn more vigorouslyand most quickly in oxygen.

y The more reactive the metal is the more powerful it is as a reducingagent.Reactivity Decreases

K Na Mg Al Zn Fe Sn Pb Cu Hg Ag AuVery Reactive Very Unreactive

y When metal X is heated with an oxide of metal Y , metal X will displace metal Yfrom its oxide if metal X is more reactive than metal Y

X + metal Y oxide of X + Y

Reactivity Decreases

K Na Mg AlC Zn H Fe Sn Pb Cu Hg Ag AuVery Reactive Very Unreactive

y If metal remove oxygen from carbon dioxide , the metal is more reactive thancarbon

metal + carbon dioxide metal oxide + carbon

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y If hydrogen is more reactive than the metal oxide , the metal oxide, the metaloxide will be reduced to metal

hydrogen + metal oxide metal + carbon dioxide

y Two application of the reactivity series-to predict the possibility ofreactions involving metals

-to predict the method ofmetal extractionfrom its ore.