Ox­i­da­tion and re­duc­tion

Jamaica Gleaner - - YL: FEATURE - FRANCINE TAY­LOR-CAMP­BELL Con­trib­u­tor Francine Tay­lor Camp­bell is an in­de­pen­dent con­trib­u­tor. Send ques­tions and com­ments to kerry-ann.hep­burn@glean­erjm.com

TOPIC RE­VIEW

A re­dox re­ac­tion is one in which ox­i­da­tion and re­duc­tion are tak­ing place.

Ox­i­da­tion refers to the ad­di­tion of oxy­gen, the loss of elec­trons from a sub­stance and an in­crease in ox­i­da­tion num­ber.

Re­duc­tion refers to the ad­di­tion of hy­dro­gen, the gain of elec­trons and a de­crease in ox­i­da­tion num­ber of a sub­stance.

Ox­i­da­tion num­ber or ox­i­da­tion state refers to the charge on the ion or species.

Com­mon rules for de­ter­min­ing ox­i­da­tion num­bers.

1. Me­tals – usu­ally based on the charge on the ion or the group num­ber. E.g., Mg +2 (in group 2) and Al +3 (in group 3). The num­ber of elec­trons lost.

2. Non-me­tals – usu­ally based on the elec­trons to be added for ion tobe sta­ble.E.g. Cl -1,O -2, P -3.

3. In com­pounds, the sum of the ox­i­da­tion states must add up to zero.

4. In com­plex ions, the sum of the ox­i­da­tion states is equal to the charge on the ion.

5. Ox­i­da­tion state of an un­com­bined el­e­ment or one in its nat­u­ral state is zero.

6. Some species have the ox­i­da­tion state in the name of the species. E.g., Cop­per I ox­ide (Cu2O) and cop­per II ox­ide (CuO)

Ox­i­da­tion and re­duc­tion usu­ally take place in a re­ac­tion through the ac­tion of ox­i­diz­ing and re­duc­ing agents. In some cases, these re­dox re­ac­tions are ac­com­pa­nied by colour changes.The ox­i­diz­ing agent is usu­ally re­duced in the re­ac­tion. The re­duc­ing agent is usu­ally ox­i­dized in the re­ac­tion

You can de­ter­mine which sub­stance is ox­i­dized or re­duced by track­ing changes in the ox­i­da­tion num­ber.

Com­mon ox­i­diz­ing agents in­clude con­cen­trated sul­phuric acid, oxy­gen, ni­tric acid, chlo­rine, potas­sium man­ganate VII and potas­sium dichro­mate so­lu­tions.

Ex­am­ples of re­duc­ing agents in­clude car­bon monox­ide, hy­dro­gen, hy­dro­gen sul­phide, me­tals, potas­sium io­dide and sul­phur diox­ide.

QUES­TIONS

Man­ganese forms a num­ber of com­pounds in which it ex­hibits dif­fer­ent ox­i­da­tion num­bers.

(a) Potas­sium man­ganate VII (KMnO4) is a com­mon reagent used in lab­o­ra­to­ries.

(i) De­scribe one use of potas­sium man­ganate as a lab­o­ra­tory reagent (in­clude the test it is used for and any colour changes dur­ing the re­ac­tion).

(ii) Cal­cu­late the ox­i­da­tion num­ber of man­ganese in KMnO2 and in MnO2.

(iii) Based on the an­swer given above, de­duce the type of re­ac­tion which oc­curs when KMnO4 is con­verted to MnO2. (b) Ex­plain the main prin­ci­ples in­volved in the breathal­yser test (in­clude colour changes etc).

AN­SWER

(a) Potas­sium man­ganate (VII) is used ex­ten­sively in school lab­o­ra­to­ries.

(i) KMnO4 is used as an ox­i­diz­ing agent. It can be used as a test for alkenes, in which the KMnO4 is acid­i­fied and added to the alkene. If the pur­ple colour of the KMnO4 dis­ap­pears, this con­firms the pres­ence of alkenes or KMnO4 can be used to test for re­duc­ing agents. The acid­i­fied KMnO4 is added to the un­known reagent. If the KMnO4 is de­colour­ized, then the reagent is a re­duc­ing agent. (ii) KMnO4 - if K = +1; O= -2; Mn = x; +1+x+4(-2) =0; x = +7 The ox­i­da­tion num­ber of Mn in KMnO4 = +7 MnO2 - if O= -2; Mn = x; x+2(-2) =0; x = +4 The ox­i­da­tion num­ber of Mn in MnO2 = +4

(iii) In the re­ac­tion, Mn changes ox­i­da­tion num­ber from +7 to +4. The ox­i­da­tion num­ber has de­creased, which in­di­cates a re­duc­tion; the KMnO4 has been re­duced to MnO2.

(b) The breathal­yser test is used to test the al­co­hol lev­els of driv­ers. In this test, the per­son’s breath passes over potas­sium dichro­mate which changes colour from or­ange to green if al­co­hol is present. In this case, the al­co­hol is ox­i­dized while the potas­sium dichro­mate is re­duced. Cr2O7 (aq) + 14H+ ----> 2 Cr3+ (aq) + 7H2O (l) or­ange green

Stu­dents of The Queen’s School give a dra­matic pre­sen­ta­tion on the topic ‘The Rel­e­vance and Im­por­tance of Nu­tri­tion to Main­tain­ing Good Health’ at the National Child Month Com­mit­tee’s Youth Fo­rum, held re­cently at the St Andrew Parish Church Hall.

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