25REDOX REACTIONS AND REDOX EQUATIONSDEFINITIONSMost of the reactions and their equations considered so far have been based upon arearrangement of ions between the components of a mixture.Another very common kind of chemical reaction is the redox reaction.Redoxtands foreduction and oxidation.The original definition of oxidation is "a reaction in which a substance combines withn", while reduction is "a reaction in which oxygen is removed from a substanceThese definitions were extended to cover reactions involving hydrogen gas: if a substancecombined with hydrogen, it was reduced, while if it gave off hydrogen it was combines with oxygen ORSubstance gives off hydrogenSubstance gives off oxygen ORSubstance combines with hydrogenBoth of these old definitions continue to be useful, and should be used.Modern definitions describe oxidation and reduction as the movement of electrons from theouter shell of one kind of atom, ion, or molecule to the outer shell of another kind of atom,ion or molecule.This movement usually involves rearrangement of atoms within the ionsor molecules in the reaction.When an atom, ion, or molecule loses one or more electrons, it is oxidised.When an atom, ion, or molecule gains one or more electrons, it is reduced.IDENTIFYING REDOX REACTIONSHow can a chemical reaction be identified as a redox reaction?If either of the elements oxygen or hydrogen is a reactant or a product in the reaction, thenthe reaction must be redox, according to the old definitions of oxidation and reduction.Thefollowing equations represent examples of redox reactions: oxygen or hydrogen is involved2 in each.
NB Hydrogen is H, not H.+Many reactions do not involve elemental oxygen or hydrogen.To decide if a reaction isredox, involving transfer of electrons from one atom, ion, or molecule to another, oxidationstates may be used.The idea of oxidation state is explained on the following pages.26OXIDATION STATESWhen elements combine to form compounds, they either transfer electrons completely fromatoms of one kind to atoms of another, or they transfer electrons partially by sharing thembetween different atoms.This was explained in pages 11 to 16.Oxidation state is a way to describe the number of electrons that have been transferred orshared between atoms of different kinds.RULE ONEstates that uncombined elements have an oxidation state = zero, sincethere are no electrons being shared with atoms of other elements.22In simple ionic compounds, such as NaS or CaI, each atom has either gained or lost oneor more electrons, forming ions such as Na, Ca, S, I.+2+2--RULE TWOstates that the oxidation state of a simple ion is the same as the chargeon the ion.
It is usually written with the sign before the number. 2For example, in NaS, the oxidation state of sodium is +1, of sulfur is -2; in2CaI, the oxidation state of calcium is +2, of iodide is -1.Oxygen and hydrogen are found in very many different compounds.When oxygen, whichhas electron shells 2,6, forms a compound, it gains two electrons (either by completetransfer or by sharing) to achieve full electron shells 2,8.
Hydrogen has a single electron inits first shell, which it may lose to other atoms, to form an H ion.+RULE THREEstates that in its compounds, oxygen has an oxidation state = -2,while hydrogen has an oxidation state = +1.
(In just a few compounds2this rule is not observed: in peroxides, containing the peroxide group O,2-and in metal hydrides, in which hydrogen is combined directly with a2metal, for example, calcium hydride, CaH.)RULE FOUR:the total oxidation states of an ion equals the charge of the ion.Thetotal oxidation states of an uncharged molecule = zero
.EXAMPLES:Oxidation states of
elements in compounds containing three or moredifferent elements can be determined in the following way:a)If the compound is a molecule (see page 15), give hydrogen a value = +1,oxygen a value = -2.Give the other element a value such that the total of all theoxidation states = 0.For example, it is evident that the oxidation stateof nitrogen in nitric acid = +5.27b)If the compound is ionic, divide it into its ions and deal with each ionseparately.Apply the same rules as above in part a), but let the total of alloxidation states equal the charge of the ion.Forexample, the oxidation state of sulfur in calcium4sulfate, CaSO, is the same as the oxidation state of4sulfur in the sulfate ion, SO, by itself.
The oxidation2-state of sulfur in sulfate = +6.Further examples:What are oxidation states of nitrogen and phosphorus in ammonium dihydrogenphosphate?424424The formula is NHHPO, which can be separated into the ions NH and HPO.Treating+-each kind of ion separately, according to the rules above:The oxidation states are: nitrogen = -3, phosphorus = +5Oxidation states may have fractional values: for example,38the oxidation state of carbon in propane, CH, is
- .The same element may have different oxidation states in different compounds: using sulfuras an example:Name of ionFormulaOxidationstateName of addition, sulfur as the uncombined element has oxidation state = 0.If the same element can exist at different oxidation states, then it must be possible for it tochange from one oxidation state to another.Returning to the original question on page 25: How can a chemical reaction be identifiedas a redox reaction?If two elements in a reaction change in oxidation state, one increasing, the otherdecreasing,
then the reaction is a redox reaction.28WRITING REDOX EQUATIONS : OXIDATION STATE METHODThe rules for writing balanced redox equation by this method are set out with an example.Balance the equation : 1.Check that the equation is a redox equation: write oxidation states under key elements,and show that one element increases in oxidation state while the other decreases.2.Insert coefficients so that the total decrease in oxidation state of one element equals thetotal increase in oxidation state of the other element:
Redox equations worksheet
Balancing Redox Equations WorkSheet
Oxidation Number Method for Balancing Redox Equations Assign oxidation numbers to all elements and identify those that are oxidized and reduced.
If only one element is both oxidized and reduced recombine it after the equation is balanced).Balance electron loss and gain by adding coefficients to the reactants.that each element is balanced.) If ions are present, balance the charge by adding H+ for acid solutions or OH for basic solutions.6.Finish balancing the equation by adding H2O.Check to see that each element is balanced and that the charge is balanced.
C2O42- . following reactions occur in basic solution:
+ OH- ..NO2 NO3- +
+ NO3- Zn(OH)42-
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