redox – part 2

Redox – Part 2Redox – Part 2a. demonstrate an understanding of:

iv. how oxidation number is a useful concept in terms of the classification of reactions as redox and as disproportionation

b. write ionic half-equations and use them to construct full ionic equations.

Task: Match up the formulas with theircorrect names

Naming CompoundsNaming Compounds

• Some elements can have more than one oxidation number.

• So to avoid confusion when naming the compounds they form, its helpful to use these numbers in their names.

Naming CompoundsNaming Compounds

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What are the missing

oxidation numbers here for Manganese and Chromium, respectively?

Task Answer – sometimes, instead of using the systematic name, scientists still use the traditional

names for some substances.

Redox and Oxidation Redox and Oxidation NumbersNumbers

• An increase in oxidation number means oxidation has occurred.

• While a decrease in oxidation number means reduction has occurred.

• E.g. What would bethe oxidationnumbers foreach of the

species in thisreaction?0 -1-2 0+1 +1

Therefore,

chlorine has been reduced, and

sulphur has been oxidised.

Redox and Oxidation Redox and Oxidation NumbersNumbers

• Another example is the Thermite Reaction (click here to see it in action!!)

• But what is oxidised and what is reduced?

0 +3-2 -2+3 0

Therefore,

iron has been reduced, and

aluminium has been oxidised.

DisproportionationDisproportionation

• Is a type of reaction in which a substances both oxidised and reduced in the same reaction. E.g. the breakdown of hydrogen peroxide:

Disproportionation – another Disproportionation – another exampleexample

• The reaction of copper(I) oxide with dilute sulphuric acid:

• Study the equation carefully – what disproportionates?

• The answer is copper (Cu) which gets oxidised from +1 to +2 and also reduced from +1 to zero, in the same reaction.

• Other examples of such reactions include reactions of chlorine with water, and chlorine with hot sodium hydroxide solution.

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Oxidation StatesOxidation States

• Some elements show a range of oxidation states.

• One colourful example is the reduction of vanadium (V) to vanadium (II) through sucessive oxidation number.

Vanadium – its different oxidation statesVanadium – its different oxidation states

Ammonium Vanadate (V) (white solid) can be added to dilute HCl to give the orange Dioxovanadium (V) ion:

VO3- + 2H+ VO2

+ + H2O

If granulated zinc is now added it reduces the vanadium over a period of several minutes and gives several colour changes:

Blue [VO(H2O)5]2+ Vanadium (IV)

Green [VCl2(H2O)4] + Vanadium (III)

Violet [V(H2O)6]2+ Vanadium (II)

+5 +5

Common Oxidising AgentsCommon Oxidising Agents

• What is an oxidising agent?– A species that reacts by oxidising something else and getting

reduced itself.

• So, as we will see, electrons in the half equations are always on the left-hand side of the equation, as oxidising agents gain electrons.

More Oxidising AgentsMore Oxidising Agents

More Oxidising AgentsMore Oxidising Agents

Common Reducing AgentsCommon Reducing Agents

• What is an reducing agent?– A species that reacts by reducing something else and getting

oxidised itself.

• So, this time, electrons in the half equations are always on the right-hand side of the equation, as reducing agents loose electrons.

More Reducing AgentsMore Reducing Agents

More Reducing AgentsMore Reducing Agents

Writing balanced equations from ionic Writing balanced equations from ionic half equations - Worked Example…half equations - Worked Example…

Firstly, write down the

half equations you

need.

Worked Example…..cont….Worked Example…..cont….

X5

Worked Example…..cont….Worked Example…..cont….

Notice, that in adding the two equations togetherThe electrons are cancelled out and not

Included in the final equation

Worked Example 2…Worked Example 2…

Again, start with the half

equations

Have a gobefore you turn tothe next slide for

the answer

Worked Example 3…Worked Example 3…

Again, start with the half

equations

Or

?

X

molecular equation

Ionic to Molecular EquationIonic to Molecular Equation• To turn an ionic equation to a molecular equation, you would

simply add the spectator ions.

• Eg. Reaction of potassium manganate (VII) solution with iron (II) sulphate solution in the presence of dilute sulphuric acid:

Ionic equation – which you work out

Add in spectator ions by using cluesIn question

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