Download 14.1 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
1
2
3
4
Work out formulae of the species before and after the change; balance if required
Work out oxidation state of the element before and after the change
Add electrons to one side of the equation so that the oxidation states balance
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
BALANCING REDOX HALF EQUATIONS
1
2
3
4
Work out formulae of the species before and after the change; balance if required
Work out oxidation state of the element before and after the change
Add electrons to one side of the equation so that the oxidation states balance
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
Step 1
Step 2
Step 3
Iron(II) being oxidised to iron(III)
Fe2+ ——> Fe3+
+2
+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
BALANCING REDOX HALF EQUATIONS
1
2
3
4
Work out formulae of the species before and after the change; balance if required
Work out oxidation state of the element before and after the change
Add electrons to one side of the equation so that the oxidation states balance
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
BALANCING REDOX HALF EQUATIONS
1
2
3
4
Work out formulae of the species before and after the change; balance if required
Work out oxidation state of the element before and after the change
Add electrons to one side of the equation so that the oxidation states balance
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
Step 1
MnO4¯ being reduced to Mn2+ in acidic solution
MnO4¯ ———>
Mn2+
No need to balance Mn;
equal numbers
BALANCING REDOX HALF EQUATIONS
1
2
3
4
Work out formulae of the species before and after the change; balance if required
Work out oxidation state of the element before and after the change
Add electrons to one side of the equation so that the oxidation states balance
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
Step 1
Step 2
MnO4¯ being reduced to Mn2+ in acidic solution
MnO4¯ ———>
+7
Mn2+
+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]
BALANCING REDOX HALF EQUATIONS
1
2
3
4
Work out formulae of the species before and after the change; balance if required
Work out oxidation state of the element before and after the change
Add electrons to one side of the equation so that the oxidation states balance
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
Step 1
Step 2
Step 3
MnO4¯ being reduced to Mn2+ in acidic solution
MnO4¯ ———> Mn2+
+7
+2
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
BALANCING REDOX HALF EQUATIONS
1
2
3
4
Work out formulae of the species before and after the change; balance if required
Work out oxidation state of the element before and after the change
Add electrons to one side of the equation so that the oxidation states balance
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
Step 1
Step 2
Step 3
Step 4
MnO4¯ being reduced to Mn2+ in acidic solution
MnO4¯ ———> Mn2+
+7
+2
MnO4¯ + 5e¯ ———> Mn2+
MnO4¯ + 5e¯ + 8H+ ———> Mn2+
Total charges on either side are not equal;
LHS = 1- and 5- = 6RHS = 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
BALANCING REDOX HALF EQUATIONS
1
2
3
4
Work out formulae of the species before and after the change; balance if required
Work out oxidation state of the element before and after the change
Add electrons to one side of the equation so that the oxidation states balance
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
Step 1
Step 2
Step 3
Step 4
Step 5
MnO4¯ being reduced to Mn2+ in acidic solution
MnO4¯ ———> Mn2+
+7
+2
MnO4¯ + 5e¯ ———> Mn2+
MnO4¯ + 5e¯ + 8H+ ———> Mn2+
MnO4¯ + 5e¯ + 8H+ ———> Mn2+ + 4H2O
Everything balances apart from oxygen and hydrogen O LHS = 4
H
now balanced
RHS = 0
LHS = 8
You must ADD 4 WATER MOLECULES to the RHS; the equation is now balanced
RHS = 0
5.3 Exercise 1
BALANCING REDOX HALF EQUATIONS
Q.
Balance the following half equations...
Na
—>
Na+
Fe2+
—>
Fe3+
I2
—>
I¯
C2O42-
—> CO2
H2O2
1
2
3
4
5
—>
O2
H2O2
—>
H2O
NO3-
—>
NO
NO3-
—>
NO2
SO42-
—>
SO2
REMINDER
Work out the formula of the species before and after the change; balance if required
Work out the oxidation state of the element before and after the change
Add electrons to one side of the equation so that the oxidation states balance
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
If the equation still doesn’t balance, add sufficient water molecules to one side
BALANCING REDOX HALF EQUATIONS
Q.
Balance the following half equations...
Na
—>
Na+
+
e-
Fe2+
—>
Fe3+
+
e-
+
2e-
I2
+
2e-
C2O42H2O2
—>
2I¯
—> 2CO2
—>
O2
+ 2H+ + 2e-
H2O2 + 2H+ + 2e- —>
2H2O
NO3- + 4H+ + 3e- —>
NO
+
2H2O
NO3- + 2H+ + e-
—>
NO2
+
H 2O
SO42- + 4H+ + 2e-
—>
SO2
+
2H2O