Olevel Chemistry Notes
TOPIC# :Oxidation and Reduction
Oxidation and Reduction is one the most important and easiest topics in O levels and frequently occuring topic in MCqs.
So, oxidation and reduction are two simultaneuosly occuring processes which combinely called redox reaction.
there are four things to remember about oxidation reaction. In any reaction:
1. Gain of oxygen is oxidation
2. loss of hydrogen is oxidation.
3. loss of electrons os oxidation
4. increase in oxidation state is oxidation.
if in any reaction any one of the thing is seen to be occuring the reaction is called oxidation.similarly, there are four things about reduction reactions and if any one condition seems to be satisfied the reaction is called reduction. four conditions are:
1. Loss of oxygen
2. gain of hydrogen
3. gain of electrons.
4. decrease in oxidation state.
so in essence, reduction seems to be reverse of oxidation or vice versa. now we will see how they occur simultaneiusly and how you will identify a reaction to be redox reaction. In past Papers, the most frequently asked question related to this topic is they ask in a given equation that which of the substance is being oxidised and which is reduced and write the ionic equation for the redox reaction.
so lets see,
a very simple example is of metal with steam or water.
Mg(s) + H2O(g)= MgO(s) + H2(g).
see in above reaction that Mg has gained oxygen which means it oxidised consequently H2O molecule lost O2 which means it reduced. equation that involve simple loss and gain of O2 and H2 are easy to identify but real difficulty comes with equation that involve increase and decrease of oxidation states. for instance,
Zn(s) + Cu(aq) = Zn(aq) + Cu(s)
this reation doesnot invlove oxygen nor hydrogen but it is still a redox reaction. why? because there involved increase and decrease of oxidation states. notice that oxidation of Zn(s) is 0 on left side of eqn while on the right side its oxidation state increased to +2 while reverse happened with copper its oxidation state decreased from +2 to 0.
now are there any substances that can cause other substances to be reduced or oxidisied?? yes! there are. these substances are called oxidising and reducing agents. most of the time students confuse the role these agents play during the reaction with their names. one thing STRICTLY to remember is that OXIDISING AGENTS are substances that causes other substances to be oxidised while they themselves are reduced during the reaction same is with reducing agents they are oxidised during the reaction. for example,
Cl2(g) + H2S(g) = 2HCl(g) + S(s).
during this reaction notice that Hydrogen sulphide is oxidised to sulphur while chlorine is reduced to HCl. it will be a lot easier to solve question related to oxising and reducing agents if you memorise some the common names of the agents for instances, Metals are reducing agents whereas, group 7 gases are oxidising agents other common oxidising agents are KmnO4, K2Cr2O7 H2O2, ozone, MnO2, AGCl2. some of the common reducing agents beside Metals are: KI, H2S, SO2, NH3, Tn2Cl, Fe2Cl.
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