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Redox equations

Redox equations. L.O.: Balance redox equations using oxidations numbers Deduce overall equations from half equations. In pairs recap rules for finding oxidation states and for balancing redox equations. BALANCING REDOX HALF EQUATIONS.

carlos-delaney
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Redox equations

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  1. Redox equations L.O.: Balance redox equations using oxidations numbers Deduce overall equations from half equations.

  2. In pairs recap rules for finding oxidation states and for balancing redox equations.

  3. BALANCING REDOX HALF EQUATIONS 1 Work out formulae of the species before and after the change; balance if required 2 Work out oxidation state of the element before and after the change 3 Add electrons to one side of the equation so that the oxidation states balance 4 If the charges on the species (ions and electrons) on either side of the equation do not balance then add sufficient H+ ions to one of the sides to balance the charges 5 If equation still doesn’t balance, add sufficient water molecules to one side

  4. BALANCING REDOX HALF EQUATIONS 1 Work out formulae of the species before and after the change; balance if required 2 Work out oxidation state of the element before and after the change 3 Add electrons to one side of the equation so that the oxidation states balance 4 If the charges on the species (ions and electrons) on either side of the equation do not balance then add sufficient H+ ions to one of the sides to balance the charges 5 If equation still doesn’t balance, add sufficient water molecules to one side Example 1 Iron(II) being oxidised to iron(III) Step 1 Fe2+ ——> Fe3+ Step 2 +2+3 Step 3 Fe2+ ——> Fe3+ + e¯ now balanced An electron (charge -1) is added to the RHS of the equation... this balances the oxidation state change i.e. (+2) ——> (+3) + (-1) As everything balances, there is no need to proceed to Steps 4 and 5

  5. BALANCING REDOX HALF EQUATIONS 1 Work out formulae of the species before and after the change; balance if required 2 Work out oxidation state of the element before and after the change 3 Add electrons to one side of the equation so that the oxidation states balance 4 If the charges on the species (ions and electrons) on either side of the equation do not balance then add sufficient H+ ions to one of the sides to balance the charges 5 If equation still doesn’t balance, add sufficient water molecules to one side Example 2 MnO4¯ being reduced to Mn2+ in acidic solution

  6. BALANCING REDOX HALF EQUATIONS 1 Work out formulae of the species before and after the change; balance if required 2 Work out oxidation state of the element before and after the change 3 Add electrons to one side of the equation so that the oxidation states balance 4 If the charges on the species (ions and electrons) on either side of the equation do not balance then add sufficient H+ ions to one of the sides to balance the charges 5 If equation still doesn’t balance, add sufficient water molecules to one side Example 2 MnO4¯ being reduced to Mn2+ in acidic solution Step 1 MnO4¯ ———> Mn2+ No need to balance Mn; equal numbers

  7. BALANCING REDOX HALF EQUATIONS 1 Work out formulae of the species before and after the change; balance if required 2 Work out oxidation state of the element before and after the change 3 Add electrons to one side of the equation so that the oxidation states balance 4 If the charges on the species (ions and electrons) on either side of the equation do not balance then add sufficient H+ ions to one of the sides to balance the charges 5 If equation still doesn’t balance, add sufficient water molecules to one side Example 2 MnO4¯ being reduced to Mn2+ in acidic solution Step 1 MnO4¯ ———> Mn2+ Step 2 +7 +2 Overall charge on MnO4¯ is -1; sum of the OS’s of all atoms must add up to -1 Oxygen is in its usual oxidation state of -2; four oxygen atoms add up to -8 To make the overall charge -1, Mn must be in oxidation state +7 ... [+7 + (4x -2) = -1]

  8. BALANCING REDOX HALF EQUATIONS 1 Work out formulae of the species before and after the change; balance if required 2 Work out oxidation state of the element before and after the change 3 Add electrons to one side of the equation so that the oxidation states balance 4 If the charges on the species (ions and electrons) on either side of the equation do not balance then add sufficient H+ ions to one of the sides to balance the charges 5 If equation still doesn’t balance, add sufficient water molecules to one side Example 2 MnO4¯ being reduced to Mn2+ in acidic solution Step 1 MnO4¯ ———> Mn2+ Step 2 +7 +2 Step 3 MnO4¯ + 5e¯ ———> Mn2+ The oxidation states on either side are different; +7 —> +2 (REDUCTION) To balance; add 5 negative charges to the LHS [+7 + (5 x -1) = +2] You must ADD 5 ELECTRONS to the LHS of the equation

  9. BALANCING REDOX HALF EQUATIONS 1 Work out formulae of the species before and after the change; balance if required 2 Work out oxidation state of the element before and after the change 3 Add electrons to one side of the equation so that the oxidation states balance 4 If the charges on the species (ions and electrons) on either side of the equation do not balance then add sufficient H+ ions to one of the sides to balance the charges 5 If equation still doesn’t balance, add sufficient water molecules to one side Example 2 MnO4¯ being reduced to Mn2+ in acidic solution Step 1 MnO4¯ ———> Mn2+ Step 2 +7 +2 Step 3 MnO4¯ + 5e¯ ———> Mn2+ Step 4 MnO4¯ + 5e¯ + 8H+ ———> Mn2+ Total charges on either side are not equal; LHS = 1- and 5- = 6- RHS = 2+ Balance them by adding 8 positive charges to the LHS [ 6- + (8 x 1+) = 2+ ] You must ADD 8 PROTONS (H+ ions) to the LHS of the equation

  10. BALANCING REDOX HALF EQUATIONS 1 Work out formulae of the species before and after the change; balance if required 2 Work out oxidation state of the element before and after the change 3 Add electrons to one side of the equation so that the oxidation states balance 4 If the charges on the species (ions and electrons) on either side of the equation do not balance then add sufficient H+ ions to one of the sides to balance the charges 5 If equation still doesn’t balance, add sufficient water molecules to one side Example 2 MnO4¯ being reduced to Mn2+ in acidic solution Step 1 MnO4¯ ———> Mn2+ Step 2 +7 +2 Step 3 MnO4¯ + 5e¯ ———> Mn2+ Step 4 MnO4¯ + 5e¯ + 8H+ ———> Mn2+ Step 5 MnO4¯ + 5e¯ + 8H+ ———> Mn2+ + 4H2O now balanced Everything balances apart from oxygen and hydrogen O LHS = 4 RHS = 0 H LHS = 8 RHS = 0 You must ADD 4 WATER MOLECULES to the RHS; the equation is now balanced

  11. 5.3 Exercise 1

  12. BALANCING REDOX HALF EQUATIONS Q. Balance the following half equations... Na —> Na+ Fe2+ —> Fe3+ I2 —> I¯ C2O42- —> CO2 H2O2 —> O2 H2O2 —> H2O NO3- —> NO NO3- —> NO2 SO42- —> SO2 REMINDER 1 Work out the formula of the species before and after the change; balance if required 2 Work out the oxidation state of the element before and after the change 3 Add electrons to one side of the equation so that the oxidation states balance 4 If the charges on all the species (ions and electrons) on either side of the equation do not balance then add sufficient H+ ions to one of the sides to balance the charges 5 If the equation still doesn’t balance, add sufficient water molecules to one side

  13. BALANCING REDOX HALF EQUATIONS Q. Balance the following half equations... 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 NO3-+ 2H+ + e- —> NO2+ H2O SO42- + 4H+ + 2e- —> SO2 + 2H2O

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