Types Of Reaction 3

REDOX REACTIONSREDOX REACTIONS

• Reaction between a Reducing agent and an Oxidising agent.

• The reducing agent donates electrons to the oxidising agent during the reaction.

• The reducing agent is thus oxidised (increase in oxidation state) while the oxidising agent is reduced (decrease in oxidation state).

Examples of Oxidising AgentsExamples of Oxidising Agents(1) Potassium manganate(VII), KMnO4 –

can be reduced to MnO2 or Mn2+.

(2) Potassium dichromate(VI), K2Cr2O7 – can be reduced to green Cr3+.

(3) Hydrogen peroxide, H2O2 – can be reduced to H2O.

Examples of Reducing AgentsExamples of Reducing Agents(1) Metals – oxidised to cations

(2) Potassium iodide, KI – the iodide is oxidised to iodine which appears as a black solid which sublimes to give a violet vapour. In water, it gives a yellow or reddish brown solution.

(3) Hydrogen peroxide, H2O2 – oxidised to O2.

Examples of Redox reactionsExamples of Redox reactions• Zn(s) + CuSO4(aq) Cu(s) + Zn(NO3)2(aq)

• A pink/orange/brown solid which is Copper is formed and the blue colour of the solution slowly disappeared.

• Reaction is also known as Displacement.Zn coated with Cu which turned black

due to air oxidation of Cu to

black CuO…Cu + O2 CuO

Examples of Redox ReactionExamples of Redox Reaction• 2KMnO4(aq) + 5H2O2(aq) + 3H2SO4 K2SO4 + 2MnSO4 + O2 + 8H2O

• Purple colour of KMnO4 disappeared and bubbles of oxygen is seen.

Brown solid formed from incomplete reduction of

MnO4- to

brown MnO2.

Examples of Redox ReactionsExamples of Redox Reactions

• Zn(s) + HNO3(aq) Zn(NO3)2(aq) + H2(g)

• Zn is oxidised to Zn2+ while H+ from acid is reduced to hydrogen.

QUIZQUIZ• Which ion cannot be detected using redox

reactions?

A: Cu2+ B: Mg2+ C: I- D: Cl-

Answer: Yes, it is Mg2+ as ions are harder to be reduced with increasing reactivity of their corresponding metals. The more reactive the metals, the less reactive would be their ions.

• Cu2+ can be reduced to pink solid Cu using a more reactive metal such as Zn, or reduced to a white precipitate of Copper(I) iodide, CuI in a brown solution of I2 using KI.

• I- can be oxidised to I2 which forms a reddish-brown/yellow solution using acidified KMnO4 or acidified H2O2.

• Cl- can be oxidised to Cl2, a green gas which bleached litmus paper, using concentrated H2SO4 or acidified KMnO4.

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Reaction Challenge:

A sample of sodium nitrate was accidentally contaminated with zinc iodide and magnesium carbonate.

As a budding chemist, describe how you would use any of these 3 types of reaction to confirm the contamination.

Suggested AnswerSuggested Answer1. Place a little of the sample in a test-tube and add dilute

HCl.2. Look out for effervescence and test the gas evolved

with limewater. CO32- is present if limewater turns

cloudy. (Using neutralisation reaction)3. Dissolve a little of the sample in water.4. Place a little of solution in a test-tube and add aqueous

NH3 till excess.5. Both Mg2+ and Zn2+ would be present if white

precipitate is formed. Zn2+ is confirmed to be present if the amount of white precipitate decreases on adding excess NH3. (Using precipitation reactions)

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6. Place a little of the solution in another test-tube and add aqueous AgNO3.

7. I- is present if a yellow precipitate of AgI is formed. (Using precipitation reaction)

8. Alternatively, acidified H2O2 can be added to the solution.

9. I- is present if a reddish-brown/brown solution with black particles of I2 is obtained. (Using redox reaction)