Types Of Reaction 3


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Types Of Reaction 3

  1. 1. REDOX REACTIONS <ul><li>Reaction between a Reducing agent and an Oxidising agent . </li></ul><ul><li>The reducing agent donates electrons to the oxidising agent during the reaction. </li></ul><ul><li>The reducing agent is thus oxidised (increase in oxidation state) while the oxidising agent is reduced (decrease in oxidation state). </li></ul>
  2. 2. Examples of Oxidising Agents <ul><li>Potassium manganate(VII) , KMnO 4 – can be reduced to MnO 2 or Mn 2+ . </li></ul><ul><li>Potassium dichromate(VI) , K 2 Cr 2 O 7 – can be reduced to green Cr 3+ . </li></ul><ul><li>Hydrogen peroxide , H 2 O 2 – can be reduced to H 2 O. </li></ul>
  3. 3. Examples of Reducing Agents <ul><li>Metals – oxidised to cations </li></ul><ul><li>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. </li></ul><ul><li>Hydrogen peroxide , H 2 O 2 – oxidised to O 2 . </li></ul>
  4. 4. Examples of Redox reactions <ul><li>Zn(s) + CuSO 4 (aq)  Cu(s) + Zn(NO 3 ) 2 (aq) </li></ul><ul><li>A pink/orange/brown solid which is Copper is formed and the blue colour of the solution slowly disappeared. </li></ul><ul><li>Reaction is also known as Displacement . </li></ul>Zn coated with Cu which turned black due to air oxidation of Cu to black CuO… Cu + O 2  CuO
  5. 5. Examples of Redox Reaction <ul><li>2KMnO 4 (aq) + 5H 2 O 2 (aq) + 3H 2 SO 4  K 2 SO 4 + 2MnSO 4 + O 2 + 8H 2 O </li></ul><ul><li>Purple colour of KMnO 4 disappeared and bubbles of oxygen is seen. </li></ul>Brown solid formed from incomplete reduction of MnO 4 - to brown MnO 2 .
  6. 6. Examples of Redox Reactions <ul><li>Zn(s) + HNO 3 (aq)  Zn(NO 3 ) 2 (aq) + H 2 (g) </li></ul><ul><li>Zn is oxidised to Zn 2+ while H + from acid is reduced to hydrogen . </li></ul>
  7. 7. QUIZ <ul><li>Which ion cannot be detected using redox reactions? </li></ul><ul><li>A: Cu 2+ B: Mg 2+ C: I - D: Cl - </li></ul><ul><li>Answer : Yes, it is Mg 2+ 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. </li></ul><ul><li>Cu 2+ 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 I 2 using KI. </li></ul><ul><li>I- can be oxidised to I 2 which forms a reddish-brown/yellow solution using acidified KMnO 4 or acidified H 2 O 2 . </li></ul><ul><li>Cl - can be oxidised to Cl 2 , a green gas which bleached litmus paper, using concentrated H 2 SO 4 or acidified KMnO 4 . </li></ul>
  8. 8. End of Lesson 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.
  9. 9. Suggested Answer <ul><li>Place a little of the sample in a test-tube and add dilute HCl. </li></ul><ul><li>Look out for effervescence and test the gas evolved with limewater. CO 3 2- is present if limewater turns cloudy. (Using neutralisation reaction) </li></ul><ul><li>Dissolve a little of the sample in water. </li></ul><ul><li>Place a little of solution in a test-tube and add aqueous NH 3 till excess. </li></ul><ul><li>Both Mg 2+ and Zn 2+ would be present if white precipitate is formed. Zn 2+ is confirmed to be present if the amount of white precipitate decreases on adding excess NH 3 . (Using precipitation reactions) </li></ul>
  10. 10. Last Page <ul><li>6. Place a little of the solution in another test-tube and add aqueous AgNO 3 . </li></ul><ul><li>7. I - is present if a yellow precipitate of AgI is formed. (Using precipitation reaction) </li></ul><ul><li>8. Alternatively, acidified H 2 O 2 can be added to the solution. </li></ul><ul><li>9. I - is present if a reddish-brown/brown solution with black particles of I 2 is obtained. (Using redox reaction) </li></ul>