Tasapainota seuraavat hapettumis-pelkistymisreaktiot hapetuslukumuutosten avulla.

a)Al(s)+I2(s)->AlI3(s)
b)PbO(s)+CO(g)->Pb(s)+CO2(g)
c)Sn2+(aq)+Fe3+(aq)->Sn4+(aq)+Fe2+(aq)
d)Cu(s)+NO3-(aq)+H+(aq)->Cu2+(aq)+NO2(g)+H2O(l)
e)Cr2O7.2-(aq)+SO3.2-(aq)+H+(aq)->Cr3+(aq)+SO4.2-(aq)+H2O(l)

. 0     0    +III
Al + I2->AlI3

I=x, x*3+III=0(AlI3 ei ole ioni), x=-I

Al 0 III hapettuu 3 elektroneja siirtyy
I  0 -I pelkistyy 1 elektroni siirtyy

Al:I->1:3
Al(s)+3I2(s)->AlI3(s)
3I2=>I6, Al sitten 2

2Al(s)+3I2(s)->2AlI3(s)(2Al=2Al,6I=6I)

b)
.  -II         -II         0            -II
PbO(s)+CO(g)->Pb(s)+CO2(g)

siis:

II -II     II  -II         0    IV  -II
PbO(s)+CO(g)->Pb(s)+CO2(g)

Pb +II o pelkistyy 2 elektroneja
O  -II  -II-II ei muutosta 0 elektroneja
C  II IV hapettuu 2 elektroneja

Pb:C->2:2(1:1)

PbO(s)+CO(g)->Pb(s)+CO2(g) (1Pb=1Pb, 1C=1C, 2O=2O)

c)
II                        III                IV                      II
Sn.2+(aq)+Fe.3+(aq)->Sn.4+(aq)+Fe.2+(aq)

Sn II   IV hapettuu, 2 elektroneja
Fe III II pelkistyy, 1 elektroni

Sn:Fe -> 1:2

Sn.2+(aq)+2Fe.3+(aq)->Sn.4+(aq)+2Fe.2+(aq)(+2+6=+4+4)

d)
.0              -II             o                O                       -II       I     -II
Cu(s)+NO3-(aq)+H+(aq)->Cu2+(aq)+NO2(g)+H2O(l)

-2*3+x=-1,x=5
-2*2+x=0, x=4

0        V   -II             I                II                   IV  -II     I   -II
Cu(s)+NO3(aq)+H+(aq)->Cu2+(aq)+NO2(g)+H2O(l)

Cu 0 II hapettuu, 2 el.
N V IV pelkistyy. 1el.
O -II -II-II ei muutosta, 0 el.
H +I +I ei muutosta, 0 el.

Cu:N->1:2

Cu(s)+2NO3(aq)+H+(aq)->Cu2+(aq)+2NO2(g)+H2O(l)

O6->O4+O(O2)
Cu(s)+2NO3(aq)+H+(aq)->Cu2+(aq)+2NO2(g)+2H2O(l)
H1(H4)->H4
Cu(s)+2NO3(aq)+4H+(aq)->Cu2+(aq)+2NO2(g)+2H2O(l)
1Cu=1Cu
2N=2N
6O=6O
-2+4=2

Cu(s)+2NO3-(aq)+4H+(aq)->Cu.2+(aq)+2NO2(g)+2H2O(l)

e)
-II                  -II                 I                   III                   -II               I   -II
Cr2O72-(aq)+SO32-(aq)+H+(aq)->Cr3+(aq)+SO42-(aq)+H2O(l)

x*2+(-2)*7=-2, 2x=12, x=6
x+(-2)*3=-2, x-6=-2, x=4
x+(-2)*4=-2, x-8=-2, x=6

VI      -II          IV -II                 I                   III            VI   -II               I   -II
Cr2O72-(aq)+SO32-(aq)+H+(aq)->Cr3+(aq)+SO42-(aq)+H2O(l)

Cr VI III pelkistyy, 3el.
S IV VI hapettuu, 2el.
O -II.-II -II.-II ei muutosta, 0el.
H I  I ei muutosta, 0el.

Cr:S->2:3

Cr2O7.2-(aq)+3SO3.2-(aq)+H+(aq)->2Cr.3+(aq)+3SO4.2-(aq)+H2O(l)

2Cr=2Cr
3S=3S
7O+9O=12O+O(4O)

Cr2O7.2-(aq)+3SO3.2-(aq)+H+(aq)->2Cr.3+(aq)+3SO4.2-(aq)+4H2O(l)

1H(8H)=8H

Cr2O7.2-(aq)+3SO3.2-(aq)+8H+(aq)->2Cr.3+(aq)+3SO4.2-(aq)+4H2O(l)

2Cr=2Cr
3S=3S
7O+9O=12O+4O
8H=8H
-2+3*(-2)+8*1=2*3+3*(-2) (0=0)

Cr2O72-(aq)+3SO32-(aq)+8H+(aq)->2Cr3+(aq)+3SO42-(aq)+4H2O(l)