RI 2022 Redox Reactions v2.0

Content Raffles Institution Year 5 H2 Chemistry 2022 Lecture Notes 1 b - Redox Reactions • Redox processes: electron transfer and changes in oxidation number (oxidation state) Learning Outcomes Candidates should be able to: (a) describe and explain redox processes in terms of electron transfer and/or of changes in oxidation number (oxidation state) (b) construct redox equations using the relevant half-equations Lecture Outline References 1 Redox Reactions Similar to those in Lecture Notes 1 a 2 Balancing Redox Equations 3 Redox ntrations -1-

_1 _ __._I_R_e_d_o_x_R_e_a_c_ti_o_n_s ___________________________ , • The term "redox" is used as an abbreviation for the processes of reduction and oxidation which occur simultaneously. A redox reaction is an oxidation-reduction reaction. 1.1. Definitions of oxidation and reduction • There are different ways to define oxidation and reduction. The focus will be on (c) and (d). (a) loss/gain • Oxidation involves gain of oxygen . of oxygen • Reduction involves loss of oxygen . oxidation I i • Example: Fe203 + 3CO 2Fe + 3C02 I t reduction (b) loss/gain • Oxidation involves loss of hydrogen . of hydrogen • Reduction involves gain of hydrogen . • Oxidation of ethanol to ethanal: CH3CH20H CH3CHO • Reduction of ethanal to ethanol: CH3CHO CH3CH20H (c) loss/gain • Oxidation involves loss of electron(s) . of electrons • Reduction involves gain of electron(s} . oxidation I i • Example: Zn + Cu2+ Zn2+ + Cu I t reduction OIL RIG oxidation involves loss of e- reduction involves gain of e- (d) increase/decrease • Oxidation involves an increase in oxidation number. in oxidation number • Reduction involves a decrease in oxidation number . oxidation I + 0 0 +4 - 2 • Example: C + 02 CO2 I t reduction • An oxidising agent (electron acceptor) oxidises the other reagent while it undergoes reduction by receiving the electrons lost by the oxidised species, i.e. an Oxidising Agent is itself Reduced. • A reducing agent (electron donor) reduces the other reagent while it undergoes oxidation by giving electrons to cause the reduction, i.e. a Reducing Agent is itself Oxidised. -2-

1.2. Redox processes in tenns of electron transfer • Consider the reaction between zinc metal and copper(II) sulfate solution. Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s) Or Zn(s) + Cu2•(aq) Zn2•(aq) + Cu(s) • The overall reaction can be separated into two simpler processes involving electron transfer. Oxidation half-equation: Zn(s) Zn2•(aq) + 2e- Reduction half-equation: Cu2•(aq) + 2e- Cu(s) Overall equation: Zn(s) + Cu2•(aq) Zn2•(aq) + Cu(s) • The two separate equations can be termed ion-electron equations, but they are more commonly known as half-equations. • One half-equation represents the oxidation process while the other represents the reduction process. Addition of the two half-equations gives the overall redox equation. • In the above redox reaction, • Zn acts as a reducing agent. It loses two ele