Oxidation & Reduction Redox reactions e Oxidation and reduction take place together at the same time in the same reaction e These are called redox reactions e Oxidation is areactionin which oxygen is added to anelement ora compound e Reduction is areaction in which oxygen is removed from an element orcompound Example: Identifying the loss and gain of oxygenin an equation zinc oxide + carbon— zinc + carbon monoxide ZnO +C—+Zn+CO © Inthis reaction, the zinc oxide has beenreduced since it has lost oxygen © Thecarbonatom has been oxidised since it has gainedo Thecarbonatom has been oxidised since it has gained fosaVzel—1ay Names using oxidation numbers e Transition elements can bondin different ways by forming ions with different charges e Whennaming, the charge on the ionis shown by using a Roman numeral after aat-e-)(-1ant—anaecmal-l eal) ° e.g.ironcanformions with a 2+ charge, callediron(Il) ions ora 3+ charge, calledironi(IIl) ions ¢ The Roman numeral is the oxidation number of the element e Wheniron reacts with oxygen to form iron oxide, the formula depends on the oxidation state of the ironions o The compound where iron has a 2+ charge has the formula FeO and is called iron(II) oxide o Thecompound where ironhasa 3+ charge has the formula Fe2Oz and is called iron(IIl) oxideRedox & Electron Transfer EXTENDED Oxidation & Reduction e Redoxreactions can also be defined in terms of electron transfer ¢ Oxidationis areactionin which an element, ion or compound loses Cito telat ° The oxidation number of the element is increased © Thiscanbe shownin ahalf equation, e.g. when silver reacts with chlorine, silver is oxidised to silverions: Ag — Ag*t+e7 e Reduction is areaction in which an element, ion or compound gains electrons © The oxidation number of the element is decreased © This canbe shown in ahalf equation, e.g. when oxygeno Thiscanbeshownin a half equation, e.g. when oxygen reacts with magnesium, oxygenis reduced to oxide ions: over ry 10 la Example: Identifying Redox Reactions zinc + copper sulphate — zinc sulphate + copper Zn + CuSO, — ZnSO4+ Cu e Theions present (with state symbols) in the equation are: Zn(s) + Cu2*(aq) + SO42-(aq) >Zn2#(aq) + SevraiCle) EaevlG) ¢ Thespectatorions (those that donot change) are SO,4?-(aq) ¢ These canbe removed and the ionic equation written as: Zn(s) + Cu2*(aq) > Zn2*(aq) + Cu(s) ¢ Byanalysing the ionic equation, we can eniit the reaction into two halfe Byanalysing the ionic equation, we can split the reaction into two half equations by adding in the electrons to show how the changes in charge have occurred: Zn(s) > Zn2*(aq) + 2e7 Cu?*(aq) +2e" > Cu(s) e Itthen becomes clear that zinc has been oxidised asit has lost electrons e Copperions have been reduced as they have gained electrons Exam Tip Use the mnemonic OIL-RIG to remember oxidation and reduction in terms of the movement of electrons: Oxidation Is Loss - Reduction Is Gain.Identifying Redox Reactions EXTENDED Oxidation Number The oxidation number (also called oxidation state) is anumber assigned to an atom orion ina compound which indicates the degree of oxidation (or reduction) It shows the number of electrons that an atom has lost, gained or sharedin forming acompound The oxidation number helps you to keep track of the movement of electrons ina redox process Itis written as a +/- sign followed by a number (not to be confused with charge which is written by anumber followed by a +/- sign) E.g. aluminium in a compound usually has the oxidation state+3e E.g. aluminium ina compound usually has the oxidation state +3 e Afewsimple rules help guide you through the process of determining the oxidation number of any element Table of Rules 4. The oxidation number of any uncombined element is zero for Assigning Oxidation Numbers 2. Many atoms or ions have fixed oxidation number in compounds Group “elements are always +4 Group 2 elements are always +2 Fluorine is always ~4 Hydrogen is +4 (except for inmetal hydrides like NaH, where it is ~1) Oxygen is ~2 (except in peroxides, where it is -“.and FO where it is +2) 3, The oxidation number of an element in mono- ‘atomic ion is always same as the charge 4, The eum of the oxidation number In ‘a. compound is zero 5. The sum of oxidation number in jon is equal to ‘the charge on the ion Zn** oxidation number = +2 Fe®* oxidation number = +3 CL” oxidation number = ~4 Nac oxidation number of NA = +4 oxidation number of 4 sum oxidation number = 0 oxidation number of S = +6 oxidation number of 4 O atoms = 4«(-2 sum oxidation number = 2 e Redoxreactions can be identified by the changes in the oxidation number when a reactant goes to a produWorked example The equation for the reaction between chlorine and potassium iodide is shown below. Clo + 2KI > 2KCl+ lp Identify which species has been: a) Oxidised b) Reduced Val o Thespecies that has been oxidised is iodine = The oxidation number of I” is -1 = The oxidationnumber of= The oxidation number of iodine inlzis O tal in c) iw e oxidation number has creased so the iodine has been oxidised (lost ectrons) “(aq) = lo(s) +2e7 o The species that has been reduc edis chlorideions = The oxidation number of chlorine as Clgis O. = The oxidation number of CI is il le] le) he oxidation number has lecreased so the CI has een reduced (gained c} ectrons) = Clo(g)+2e° — 2CI (aq) Identifying Redox Reactions by Colour Changes e The tests for redox reactions involve the observation of a colour change in the solution being analysedIdentifying Redox Reactions by Colour Cedar Tate [-43 e The tests for redox reactions involve the observation of a colour change in the solution being analysed e Twocommon examples are acidified potassium manganate(VIl), and potassium iodide e Potassium manganate(VIl), KMnOq, is an oxidising agent whichis often used to test for the presence of reducing agents e When acidified potassium manganate(VIl) is added to areducing agent its colour changes from purple to colourlesse Potassium iodide, Kl, is areducing agent which is often used to test for the presence of oxidising agents e When added to an acidified solution of an oxidising agent such as aqueous chlorine or hydrogen peroxide (H20>), the solution turns ared-brown colour due to the formation of iodine, l2: 2KI (aq) + H2SO4 (aq) + H2O2 (aq) > |p (aa) +K2SOq (aq) + 2H20 (1) e The potassium iodide is oxidised as it loses electrons and hydrogen peroxide is reduced, therefore potassium iodideis acting asa reducing agent as it willitself be oxidised: VeOxidising & Reducing Agents EXTENDED Oxidising agent e Asubstance that oxidises another substance, and becomes reducedin the process e Anoxidising agent gains electrons as another substance loses electrons ¢ Commonexamples include hydrogen peroxide, fluorine and chlorine Reducing agent e Asubstance that reduces another substance, and becomes oxidised in the process e Areducing agentloses electrons as another substance gains electrons e Common examples include carbon and hydrogen ¢ The process of reduction is very important in the chemical industry as ameans of extracting metals from thoair nrace Theprocess of reduction is very important in the chemical industry as ameans of extracting metals from their ores Example CuO +H Cu+H2O e Inthe above reaction, hydrogen is reducing the CuO andis itself oxidised as it has lost electrons, so the reducing agent is therefore hydrogen: pee Ae e The CuO is reduced to Cu by gaining electrons and has oxidised the hydrogen, so the oxidising agent is therefore copper oxide (Oa v1 Or]a Worked example Wheniron reacts with bromine to form iron(|l) bromide, aredox reactionreaction occurs: ol) 0 ee 15) 0) What is acting as the reducing agent in this reaction? Pe Step 1 - Write half equations to work out what has gained/lost electrons eS alo 131A a Fe loses electrons; Br2 gains electrons Step 2 - Deduce what has been oxidised/reduced (remember OILStep 2 - Deduce what has been oxidised/reduced (remember OIL RIG) Fe has been oxidised as it has lost electrons Br2 has been reduced as it has gained electrons Step 3 - Identify the reducing agent Fe is the reducing agent as it has been oxidised by losing electrons and caused Brz to be reduced as it gained electrons