Oxidation and Reduction

Oxidation and ReductionOxidation of Food: What a Waste!Fruits and Vegetables oxidised when left in open airSolution: Seal in plastic wrapMore radical: Add lemon juice to the cut fruit

Oxidation ofOxidation of nutrients causes increased activity of cells, leading to aging skinSolution: Beauty products?

People!What is a redox reaction?Redox reduction + oxidationBoth processes occur simultaneouslyHence, one species is oxidised, another is reducedSo, what is oxidation, and what is reduction?3 different versions of the definition:Redoxgain of electronsloss of electronsgain in hydrogenloss of hydrogenloss of oxygengain in oxygenReductionOxidationOxidation Gain of oxygen in a speciesE.g. Mg is oxidized to MgO 2Mg(s) + O2(g) 2MgO(s)

Loss of hydrogen in a speciesE.g. HCl is reduced to Cl2MnO2(s) + 4HCl(aq) MnCl2(aq) + 2H2O(g) + Cl2(g)

ReductionLoss of oxygen in a speciesE.g. CuO is reduced to CuCuO(s) + H2(g) Cu(s) + H2O(l)Gain of hydrogen in a speciesE.g. C2 H4 is reduced to C2 H6 in the hydrogenation processC2 H4(g) + H2(g) C2 H6(g)Oxidation and ReductionIn terms of Electrons (OIL RIG: Oxidation Is Loss, Reduction Is Gain): 2Mg(s) + O2(g) 2MgO(s)

Oxidation : Mg Mg2+ + 2e- (Loss of electrons by Mg) Reduction : O2 + 4e- 2O2- (Gain of electrons by O2)

The two equations above are known as half-equations.When electrolysis was discovered and modern theories of atomic structure and chemical bonding were developed, the terms oxidation & reduction were redefined in terms of electrons.8Oxidation numbersFor the redox reaction such as S(s) + O2(g) SO2(g),would the definition of electron transfer be applicable?A new definition is developed to overcome the problemIn terms of Oxidation numbers:

Oxidation: Increase in oxidation number in a speciesReduction: Decrease in oxidation number in a species

The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred.There is limitation/drawback in using the definition of redox reactions in terms of eletron transfer because Reaction like S(s) + O2(g) SO2(g) , neither reactants nor products are ionic, and there is no obvious transfer of electrons. Another definition is developed on oxidation numbers wich can be applied to reactions involving ionic cpds and covalent cpds. 9Free elements (uncombined state) have an oxidation number of zero.Na, Be, K, Pb, H2, O2, P4 = 0In monatomic ions, the oxidation number is equal to the charge on the ion.Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2The oxidation number of oxygen is usually 2. In H2O2 and O22- it is 1. Simple rules on oxidation numbersThe oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds. In these cases, its oxidation number is 1.5. The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion.Deduce the oxidation number of (i) sulfur in sulfur dioxide(ii) nitrogen in NH4+(iii)carbon in HCO3- (iii) chromium in Cr2O72-

S = +4N = -3C = +4Cr = +6What is the oxidation number of P in H3PO4?What is the oxidation number of C in (NH4)2CO3?What is the oxidation number of C in C3H8(propane)?

11Naming inorganic compoundsThe oxidation number is inserted immediately after the name of ion.ExampleIron(II)chloride, FeCl2 and iron(III)chloride, FeCl3Potassium chromate(VI), K2CrO7 and potassium manganate(VII), KMnO4

Identifying redox reactionsConsider the following reactions2FeCl2(s) + Cl2(g) 2FeCl3(s)Mn(NO3)2(s) MnO2(s) + 2NO2(g) (NH4) 2Cr2O7(s) Cr2O3(s) + 4H2O(g) + N2(g)

What if theres no change in the oxidation numbers during the chemical reaction?Fe from +2 to +3 [O] , Cl from 0 to -1 [R]Mn from +2 to +4 [O], N from +5 to +4 [R]Cr from +6 to +3 [R], N from -3 to 0 [O]13Non-redox reactionsThe oxidation states of the elements remained unchanged in the following reactions:Neutralisation reactions:

Precipitation reactions:

Complex formation:

Another reaction:

Tetraammine copper(II) complex(deep blue solution)ligand

DisproportionationA disproportionation reaction is a redox reaction in which one species is simultaneously oxidised and reduced.Example2H2O2(aq) 2H2O(l) + O2(g)

Reactant : Oxidation number of O in H2O2 is -1Product : Oxidation number of O in H2O is -2Oxidation number of O in O2 is 0

ReductionOxidationConsider another example:Cl2(g) + H2O(l) HOCl(aq) + HCl(aq)Cl from Cl2 is 0 to Cl from HOCL is +1Cl from Cl2 is 0 to Cl from HCl is -117Oxidising and Reducing agentAn oxidising agent is a substance that brings about oxidation by accepting electrons from the substance it oxidises. It is always reduced in the process. A reducing agent is a substance that brings about reduction by donating electrons to the substance it reduces. It is always oxidised in the process. 2Mg + O2 2MgO Mg is oxidised, and thus is the reducing agentO2 is reduced, and thus is the oxidising agent

List of common Oxidising and Reducing AgentsRealise something?H2O2 is both an oxidising and a reducing agent!If a stronger oxidising agent is present, H2O2 is reducing

Identifying Redox reactions(a) Consider the reaction S8 + 12O2 8SO3 (i) Does it involve ionic or covalent compounds? (ii) Is this a redox reaction? Explain (iii) What is the oxidising agent and reducing agent?

Consider each of the 2 reactions below2AgNO3 + Na2S Ag2S + 2NaNO3 5As4O6 + 8MnO4- + 18H2O 20 AsO4 3- + 8 Mn2+ + 36H+ (i) Is this a redox reaction? Explain (ii) What is the oxidising agent and reducing agent?

Balancing redox equationsThe oxidation of Fe2+ to Fe3+ by Cr2O72- in acid solution.(1) Write the unbalanced equation for the reaction in ionic form. Fe2+ + Cr2O72- Fe3+ + Cr3+ (2) Separate the equation into two half equationsOxidation : Fe2+ Fe3+ Reduction: Cr2O72- - Cr3+ (3) Balance the atoms other than O and H in each half eqn Cr2O72- 2Cr3+(4) Add H2O to balance O atoms and H+ to balance H atoms. Cr2O72- 2Cr3+ + 7H2O14H+ + Cr2O72- 2Cr3+ + 7H2O

(5) Add electrons to one side of each half-reaction to balance the charges on the half-reaction. Fe2+ Fe3+ + e-14H+ + Cr2O72- + 6e- 2Cr3+ + 7H2O(6) If necessary, equalize the number of electrons in the two half-reactions by multiplying the half-reactions by appropriate coefficients.6Fe2+ 6Fe3+ + 6e-14H+ + Cr2O72- + 6e- 2Cr3+ + 7H2O

(7) Add the two half-reactions together and balance the final equation by inspection. The number of electrons on both sides must cancel. You should also cancel like species.Oxidation : 6Fe2+ 6Fe3+ + 6e-Reduction : 14H+ + Cr2O72- + 6e- 2Cr3+ + 7H2O

Overall eqn: 14H+ + Cr2O72- + 6Fe2+ 6Fe3+ + 2Cr3+ + 7H2O

(8) Verify that the number of atoms and the charges are balanced.14x1 2 + 6x2 = 24 = 6x3 + 2x3(9) For reactions in basic solutions, add OH- to both sides of the equation for every H+ that appears in the final equation. You should combine H+ and OH- to make H2O.

ExampleBalance the following reaction:

2MnO4- + 5H2O2 + 6H+ 2Mn2+ + 5O2 + 8H2O25ExampleWrite a redox equation for the reduction of acidified manganate(VII) ions and the oxidation of methanol using the balanced half-equations below:2 H2O(l) + CH3OH(l) CO2(g) + H2O(l) + 6H+(aq) + 6e- MnO4-(aq)+ 8H+(aq) + 5e- Mn2+ (aq)+ + 4H2O(l)

5CH3OH(l) + 6MnO4-(aq) + 18H+(aq) 5CO2(g) + 6Mn2+(aq) + 19H2O(l)26Reactivity Series and cell voltagePotassiumSodiumCalciumMagnesiumAluminiumCarbonZincIronTinLeadCopperMercurySilverGoldMagnesiumZincIronTinLeadCopperSilverReactivity decreasePositive electrodeNegative electrodeHigher VoltageLower Voltage2Al + Fe2O3 2Fe + Al2O3 THERMITE REACTION

.Uncontrolled spontaneous reactionHow can this be harnessed to do work?

.Controlled redox reactionZn Zn2+ + 2e Cu2+ + 2e CuZn + Cu 2+ Cu + Zn2++ overall reaction but electrons are forced outside of the cellsTypes of electric charge : + and Opp charges attract but same charge repel.Electric charge is measured in coulombs, C.An electron has a charge of 1.6 x 10^-19 cuolombs. The charge flows through wires carried by electrons from the negative terminal to the positive terminal.The rate at which the electric charge flows through a circuit is called the curren measured in amperes,A.A current of 1 amp(A) = flow of 1 coulomb per second (c/s)Electric current flows through a circuit if there is a pd between 1 points in a circuit.The pd gives energy to the charge. Pd is measured in volt, V. 1 volt gives a charge of 1 coulomb 1 joule of energy. The charge then transfer the energy to other forms heat, light, chemical energy (in a chargeable battery)

Voltaic Cell G = negGalvanicBattery

Voltaic Cells19.2spontaneousredox reactionanodeoxidationcathodereduction-+

A simple voltaic cell can be constructed by placing zinc electrode in a solution of zinc sulfate and a copper electrode in a solution of copper(II)sulfate.The 2 electrodes are connected by an external wire with a high resistance voltage. This external circuit allowes electrons to flow. The circuit is complete with a salt that allows ions to flow in order tomaintain electrical neutrality.A simple salt bridge consists of a fliter paper soak in saturated potassium chloride (nitrate). K and Cl ions are chosen because they do not react with the outer ions in solution or with the electrodes.

Overall equation is Zn(s) + Cu2+(aq) Zn2+ (aq) + Cu(s) same if Zn is placed in CuSO4 solution. Heat is released in this situation.In the voltaic cell, they are physically separated allowing the flow/release of electrical energy.An electric current flows between the anode and cathode because there is a difference in electrical potential between the 2 electrodes, measured by voltmeter (in volts) CELL POTENTIAL.32

Explain what happens in this system.

Follow the circulation of one negative charge in the cell.An electric current flows between the anode and cathode because there is a difference in electrical potential between the 2 electrodes, measured by a voltmeter (in volts) : CELL POTENTIAL.The voltaic cell depends on the nature of the electrodes and the ions the concentrations of ions and the temperature at which the celll is operated.

CELL POTENTIAL, EElectrons are driven from anode to cathode by an electromotive force or emf.For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25 C and when [Zn2+] and [Cu2+] = 1.0 M.Zn and Zn2+,anodeCu and Cu2+,cathode

1.10 V1.0 M1.0 MThe voltaic cell depends not only on the nature of the electrodes and the ions, but alsoon the concentrations of ions and the temperature at which the celll is operated.35CELL POTENTIAL, EFor Zn/Cu cell, potential is +1.10 V at 25 C and when [Zn2+] and [Cu2+] = 1.0 M. is the STANDARD CELL POTENTIAL, Eo a quantitative measure of the tendency of reactants to proceed to products when all are in their standard states at 25 C.

Cell potentials of selected v cellsMetal electrodesCell potenials / VCopper and magnesium2.70Copper and iron0.78Lead and zinc0.64Lead and iron0.32The further apart the two metals are in the reactivity series, the higher the cell potential.Copper and magnesium are furthest apart in the reactivity series. It gives the highest cell potential .Lead and iron are the closest in the reactivity series and give the lowest cell potential.

37Reactivity Series and cell voltagePotassiumSodiumCalciumMagnesiumAluminiumCarbonZincIronTinLeadCopperMercurySilverGoldMagnesiumZincIronTinLeadCopperSilverReactivity decreasePositive electrodeNegative electrodeHigher VoltageLower VoltageElectrolysisElectricity is passed from a battery through a liquid which may be a solution/molten solid.The plates which carry the electricity into the liquid are called electrodesMolten ionic compounds or aqueous solution of ionic compounds that allows electricity to pass through are called electrolytesElectrodesMetal plates or graphite rods that conduct electricity into the electrolyte Eg. Platinum, copperCathode:Electrode that is connected to the negative terminal of the battery. Postively charged ions, cations, moved towards the cathodeAnode:Electrode that is connected to the positive terminal of the battery. Negatively charged ions, anions, moved towards the anode Electrolytic cell

Conduction of electricityConductor is a substance which conducts electricity but is not chemically changed during the conductionPresence of freely moving valence electronsEg. All metals and graphiteInsulator does not allow the passage of electricity.Valence electrons are held in fixed positionsEg. Sulphur, phosphorus, diamond, solid state crystalline salts, wood and glassElectrolytes and non-ElectrolytesElectrolytes:Molten ionic compounds or aqueous solution of ionic compounds that allows electricity to pass through and are decomposed in the processEg. Acids, Alkali, Salts dissolved in water, molten saltsNon-electrolytes:Does not allow passage of electricityEg. Distilled water, alcohol, turpentine, oil, paraffin, organic solvents

When electricity is passed through an electrolyte, chemical decomposition occursThis involves the splitting up of the electrolyteSince all electrolytes are ionic, composed of positively and negatively charged ionsThe process: When an electric current pass through the electrolyte, ions in the solution migrate towards the oppositely charged electrodeThis discharge of ions at the electrodes results in the chemical decomposition of the electrolyte to form its elements.

At the anode, negatively charged ions lose their electron(s) to the anode (connected to positive terminal of battery) to form neutral atoms. The negatively charged ions are said to be oxidised and discharged at the anode. Oxidation occured at the anode.At the cathode, positively charged ions gain electron(s) from the cathode (connected to negative terminal of battery) to form neutral atoms. The positively charged ions are said to be reduced and discharged at the cathode. Reduction occured at the cathode. Rules for Predicting Selective Discharge of Cations

Rules for Predicting Selective Discharge of Anions

Electrolysis of Concentrated Hydrochloric Acid

Carbon rods as electrodesElectrolysis of Molten CompoundsMany ionic compounds are binary compounds. A binary compound is a compound containing only 2 elements. It contains a metal cation and a non-metal anion.The electrolysis of a molten binary compound will yield a metal and a non-metal as products.Electrolysis of Molten Lead(II) BromideCarbon rods as electrodes

Electrolysis of Molten Sodium Chloride

Electrolysis of Water19.8

Chemistry In Action: Dental Filling DiscomfortHg2 /Ag2Hg3 0.85 V2+Sn /Ag3Sn -0.05 V2+Sn /Ag3Sn -0.05 V2+Corrosion

Cathodic Protection of an Iron Storage Tank

Simple Voltaic CellsElectrolytic cellChemical energy changed to electrical energyElectrical energy changed to chemical energy2 electrodes, 2 different metals2 electrodes, same or different metals