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Redox equations

Redox equations. Starter . What is redox ?. Redox – what?. Oxidation and reduction happen at the same time. There is no net gain or loss of electrons. e -. You can’t just create them or destroy them!. Example: the thermit reaction. Fe 2 O 3 + Al  Fe + Al 2 O 3.

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Redox equations

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  1. Redox equations

  2. Starter • What is redox?

  3. Redox – what? Oxidation and reduction happen at the same time There is no net gain or loss of electrons. e- You can’t just create them or destroy them!

  4. Example: the thermitreaction Fe2O3 + Al  Fe + Al2O3 Fe3+ Al0Fe0 Al3+ Fe3+ + 3e- FeReduction Al  Al2O3 + 3e-Oxidation Ionic half-equations

  5. Remember: OIL -Oxidation Is Loss of electrons RIG -Reduction Is Gain of electrons And often... (as a quick and simple way to tell): Oxidation is gain in oxygen or loss of hydrogen Reduction is loss of oxygen or gain of hydrogen

  6. Oxidation Reduction Oxidation and reduction can be seen as movement up or down a scale of oxidation states Oxidation state

  7. Redox in presence of acid Example: a past exam question a) Identify, as oxidation or reduction, the formation of NO2 from NO3- in the presence of H+ and deduce the half-equation for the reaction. NO3- NO2 Reduction +5 +4

  8. Steps to take NO3- NO2 Write a balanced equation for the species Work out “before and after” oxidation states Balance oxidation states with electrons +5 +4 NO3-+ e- NO2 If all the charges don’t balance, add H+ ions to one of the sides to balance them NO3-+ e-+ 2H+  NO2 If the equation still doesn’t balance, add enough water to one side so it balances NO3-+ e-+ 2H+  NO2 + H2O Beautiful!

  9. Balance the half equations Give these a go Na  Na+ Fe2+ Fe3+ I2 I¯ C2O42- CO2 H2O2 O2 H2O2  H2O NO3- NO NO2 NO3- SO42-  SO2

  10. All good? Na  Na++ e- Fe2+  Fe3+ + e- I2+ 2e- 2I¯ C2O42-2CO2+ 2e- H2O2 O2 + 2H+ + 2e- H2O2 + 2H+ + 2e-2H2O NO3-+ 4H+ + 3e- NO+ 2H2O NO2+ H2O  NO3- + 2H+ + e- SO42- + 4H+ + 2e SO2 + 2H2O

  11. Exam Question • 1. Mercury thermometers are not used in some laboratories because of the danger of mercury vapour. This vapour is very easily absorbed through the lungs into the blood. In the blood, mercury reacts with hydrogen peroxide to form mercury(II) oxide. • Hg + H2O2 → HgO + H2O • The mercury(II) oxide formed accumulates within organs in the body. • Use oxidation numbers to show that the reaction between mercury and hydrogen peroxide is an example of both oxidation and reduction. • [Total 2 marks]

  12. Self marking • 1. Oxidation because oxidation state of Hg changes from 0 to +2 so oxidation (1)Reduction because oxidation number of O changes from –1 to –2 (1) • Or • Correct identification of all the oxidation numbers (1)Correct identification of oxidation and reduction (1) 2 • Allowecf for the identification of oxidation and reduction from wrong oxidation numbers • [2]

  13. Combining half equations Just a mashing together of two half-equations! ... followed by some satisfying cancelling-out.

  14. Back to our exam question b) Deduce the overall equation for the reaction of copper with NO3- in acidic conditions to give Cu2+. Reduction NO3-+ e-+ 2H+  NO2 + H2O Oxidation Cu Cu2++ 2e- Now what?

  15. Steps to take to combine equations Multiply the equations so that the number of electrons in each is the same Add the two equations and cancel out the electrons on either side of the equation If necessary, cancel out any other species which appear on both sides of the equation Cu Cu2++ 2e- 2NO3-+ 2e-+ 4H+  2NO2 + 2H2O X 2 Cu + 2NO3-+ 2e-+ 4H+  Cu2++ 2e- + 2NO2 + 2H2O Cu + 2NO3-+ 4H+  Cu2++ 2NO2 + 2H2O

  16. Give these a go • Fe2+ ions are oxidised to Fe3+ ions by ClO3- ions in acidic conditions. The ClO3- ions are reduced to Cl- ions. Write the overall reaction. • Write an overall reaction for MnO4- reducing H2O2to O2and creating Mn2+ . Fe2+  Fe3+ + e- ClO3-+ 6e- + 6H+  Cl- + 3H2O 6Fe2+ + ClO3-+ 6H+  Fe3+ + Cl- + 3H2O 2MnO4¯+ 5H2O2 + 6H+2Mn2++ 5O2+ 8H2O

  17. 2. An unusual compound of iron has been detected on the surface of the planet Mars. This compound contains the ferrate(VI) ion. • When chlorine is bubbled through a suspension of iron(III) oxide in concentrated aqueous sodium hydroxide, a solution of aqueous sodium ferrate(VI) forms. • The two relevant redox systems are shown below. • Cl2(aq) + 2e– → 2Cl–(aq) • Fe2O3(s) + 10OH–(aq) → 2FeO42–(aq) + 5H2O(I) + 6e– • Construct the redox equation for the reaction between chlorine, iron(III) oxide and hydroxide ions. • [Total 2 marks]

  18. Self marking • 2. Fe2O3 + 3Cl2 + 10OH– → 2FeO42– + 5H2O + 6Cl– (2) 2 • Allow one mark if electrons shownAllow one mark if correct reactants and products but not balanced • [2]

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