Halfequations
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Transcript of Halfequations
AS Chemistry
OXIDATION STATES, HALF EQUATIONS and
REDOX REACTIONS
OXIDATIONOXIDATION REDUCTIONREDUCTION
= OXYGEN= OXYGEN
= HYDROGEN= HYDROGEN
= ELECTRON= ELECTRON
= OXIDATION STATE= OXIDATION STATE
REDOX REACTIONS = reactions involving REDuction and OXidation
Definitions:
GAINGAIN LOSSLOSS
LOSSLOSS GAINGAIN
LOSSLOSS GAINGAIN
INCREASEINCREASE DECREASEDECREASE
Remember “OILRIG” : Oxidation Is Loss ; Reduction Is Gain (of electrons)
Oxidation states (also called oxidation numbers) are numbers assigned to EACH ATOM that takes part in a reaction.
Oxidation states are assigned using a set of International rules.
Rules for deciding Oxidation States (Numbers) :
5. In a BINARY (2 elements) COMPOUND
1. In all UNCOMBINED ELEMENTS, atom’s ox. no. = 0 .
2. In all COMPOUNDS, sum of ox. no.’s equals zero.
3. In all IONS, sum of ox. no.’s equals ion charge.
4. In all COMPOUNDS : Gp 1 elements + 1
+ 2
- 1
+ 3
the more electronegative atom given NEGATIVE ox. no. and the less electronegative atom given POSITIVE ox. no.
In most COMPOUNDS,
6. H = + 1 except when bonded to a metal- metal must have the positive ox. no. (Rule 5)
7. O = - 2 except when bonded to F or in peroxides, e.g. Na2O2- F must have the negative ox. no. (Rule 4)
LEA
RN
and PR
AC
TIS
E
Gp 2 elements
Gp 3 elementsFluorine
ASSIGN AN OXIDATION NUMBER / STATE TO EACH ATOM IN :
Cl2 CO32-
Ca2+ SO32-
Al3+ ClO-
H2O IO4-
CO2 CH4
ClF MnO4-
NO3- Na2S4O6
CuCl CuBr2
N2 C2O4
2-
BrF5 Mn2O3
SF6 CO
S2- BrF
VCl2 Na2S
NO2- BrO3
-
NH4+ H2SO4
SO42- I-
S2O32- NH3
CCl4 Cr2O7
2-
Cl(0)
Ca(+2)
Al(+3)
H(+1) O(-2)
O(-2) C(+4)
F(-1) Cl(+1)
O(-2) N(+5)
Cl(-1) Cu(+1)
N(0)
F(-1) Br(+5)
F(-1) S(+6)
S(-2)
Cl(-1) V(+2)
O(-2) N(+3)
H(+1) N(-3)
O(-2) S(+6)
O(-2) S(+2)
Cl(-1) C(+4)
O(-2) C(+4)
O(-2) S(+4)
O(-2) Cl(+1)
O(-2) I(+7)
H(+1) C(-4)
O(-2) Mn(+7)
Na(+1) & O(-2) S(+2.5)
Br(-1) Cu(+2)
O(-2) C(+3)
O(-2) Mn(+3)
O(-2) C(+2)
F(-1) Br(+1)
Na(+1) S(-2)
O(-2) Br(+5)
O(-2) & H(+1) S(+6)
I(-1)
H(+1) N(-3)
O(-2) Cr(+6)
Rem. OXIDATION REDUCTION
= OXIDATION No. INCREASE DECREASE
Work out the oxidation number change for each of the following process and use it to decide whether it is an OXIDATION or a REDUCTION.
PROCESSPROCESS Ox. No.’sOx. No.’s OxidationOxidation ReductionReduction
ClCl2 2 Cl Cl--
CaCa Ca Ca2+2+
NONO2 2 NO NO
33--
MnOMnO44- - Mn Mn2+2+
SOSO22 SO SO
442-2-
IOIO44- - I I
22
HH22SOSO4 4 S S2-2-
BrBr2 2 BrO BrO--
NHNH44+ + NH NH
33
CrCr22OO77
2- 2- Cr Cr3+3+
Cl(0) (-1)
Ca(0) (+2)
N(+4) (+5)
Mn(+7) (+2)
S(+4) (+6)
I(+7) (0)
S(+6) (-2)
Br(0) (+1)
N(-3) (-3)
Cr(+6) (+3)
NONE
Half Equations= equations showing the SEPARATE oxidation (loss of e-) and
reduction (gain of e-) processes in any redox reaction
e.g. 1 2Ca(s) + O2(g) 2CaO(s)
Ca atoms - Ca oxidised O2 mols - O2 reduced
Oxidation :Reduction :
HALF EQUATIONS :
0 +2
0 -2
Ca Ca2+ + 2e-
O2 + 4e- 2O2-
e.g. 2 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)
Na atoms - Na oxidised H2O mols - H2O reduced
Oxidation :Reduction :
HALF EQUATIONS :
0 +1H(+1) H(0)
Na Na+ + e-
2H2O + 2e- 2OH- + H2
General Method for Writing Half Equations
e.g.1 MnO4- Mn2+ (NOT balanced ; occurs in acid)
Mn(+7) (+2) MnO4- is reduced
Number of electrons in half-equation
3. Complete the balance (for atoms and charges) by inserting H2O and H+ or OH- as appropriate
1. Write formulas of “redox” particles and balance “changed” atoms
Reduction : MnO4- Mn2+
CHANGE in oxidation number of “redox” atoms
2. Insert e- on left for reduction, right for oxidation
Reduction : MnO4- + 5e- Mn2+
=
Reduction : MnO4- + 8H+ + 5e- Mn2+ + 4H2O
i.e. 4 O 4H2O 8H+
e.g.2 Cl2 ClO4- (NOT balanced ; occurs in alkali)
Cl(0) (+7) Cl2 is oxidised
Number of electrons in half-equation
3. Complete the balance (for atoms and charges) by inserting H2O and H+ or OH- as appropriate
1. Write formulas of “redox” particles and balance “changed” atoms
Reduction : Cl2 2 ClO4-
CHANGE in oxidation number of “redox” atoms
2. Insert e- on left for reduction, right for oxidation
Reduction : Cl2 2 ClO4- + 14e-
=
Reduction : Cl2 + 16OH- 2 ClO4- + 14e- + 8H2O
i.e. 8 O 8H2O 16OH-
e.g.3 Cu + HNO3 Cu2+ + NO2 (NOT balanced ; occurs in acid)
Cu(0) (+2)
N(+5) (+4)
Cu oxidised
and HNO3 reduced
Number of electrons in half-equation
3. Complete the balance (for atoms and charges) by inserting H2O and H+ or OH- as appropriate
1. Write formulas of “redox” particles and balance “changed” atoms
Oxidation : Reduction :Cu Cu2+ HNO3 NO2
CHANGE in oxidation number of “redox” atoms
2. Insert e- on left for reduction, right for oxidation
Oxidation : Reduction :Cu Cu2+ + 2e- HNO3 + e- NO2
=
Oxidation :Reduction :
Cu Cu2+ + 2e-
HNO3 + H+ + e- NO2 + H2O
Write a half-equation for each of the following changes.
1. Cl2 to Cl- 6. Br- to Br2
2. Pb2+ to Pb 7. Al to Al3+
3. H2SO4 to H2S 8. At- to At2
4. HNO3 to NO 9. Fe to Fe2+
5. H2SO4 to SO2 10. Br- to Br2
Combining half-equations to produce the full equation
(a) An oxidation half-equation must be combined with a reduction half-equation
(b) Combine in the ratio which balances out the electrons lost during oxidation with those gained during reduction.
i.e. oxidation number changes must balance
Oxidation :
Reduction :
Cu Cu2+ + 2e-
HNO3 + H+ + e- NO2 + H2O
X 1
X 2
Add :
Cu + 2HNO3 + 2H+ + 2e- Cu2+ + 2e- + 2NO2 + 2H2O
Example 1
Remember(a) An oxidation half-equation must be combined with a
reduction half-equation
(b) Combine in the ratio which balances out the electrons lost during oxidation with those gained during reduction.
Oxidation :
Reduction :
Fe2+ Fe3+ + e-
MnO4- + 8H+ + 5e- Mn2+ + 4H2O
X 5
Add :
5Fe2+ + MnO4- + 8H+ + 5e- 5Fe3+ + 5e- + Mn2+ + 4H2O
Example 2
X 1
Remember(a) An oxidation half-equation must be combined with a
reduction half-equation
(b) Combine in the ratio which balances out the electrons lost during oxidation with those gained during reduction.
Oxidation :
Reduction :
Cl2 + 2OH- ClO- + H2O + e-
Cl2 + 2e- 2Cl-
X 2
Add : 2Cl2 + 4OH- + 2e- 2ClO- + 2e- + 2Cl- + 2H2O
Example 3
X 1
Cl2 + 2OH- ClO- + Cl- + H2OCancel by 2
Note : Chlorine is BOTH oxidised and reduced. Such a reaction is called a DISPROPORTIONATION
Use the half-equations written earlier and combine them to form the overall equation for.
1. Cl2 to Cl- with Br- to Br2
2. Pb2+ to Pb with Al to Al3+
3. H2SO4 to H2S with I- to I2
4. HNO3 to NO with Fe to Fe2+
5. H2SO4 to SO2 with Br- to Br2
The End