Balance in acidic
cr2o7²- + so2cr³+ + so4 2-

The balanced equation is

Cr

2

O

2-

7

+

3SO

2-

3

+

8H

+

→

2Cr

3+

+

3SO

2-

4

+

4H

2

O

Explanation:

Step 1: Separate the skeleton equation into two half-reactions.

Cr

2

O

2-

7

→

Cr

3+

SO

2-

3

→

SO

2-

4

Step 2: Balance all atoms other than

H

and

O

.

Cr

2

O

2-

7

→

2Cr

3+

SO

2-

3

→

SO

2-

4

Step 3: Balance

O

.

Add enough

H

2

O

molecules to balance

O

.

Cr

2

O

2-

7

→

2Cr

3+

+

7

H

2

O

SO

2-

3

+

H

2

O

→

SO

2-

4

Step 4: Balance

H

.

Add enough

H

+

ions to balance

H

.

Cr

2

O

2-

7

+

14H

+

→

2Cr

3+

+

7

H

2

O

SO

2-

3

+

H

2

O

→

SO

2-

4

+

2H

+

Step 5: Balance charge.

Add electrons to the side that needs more negative charge.

Cr

2

O

2-

7

+

14H

+

+

6

e

-

→

2Cr

3+

+

7

H

2

O

SO

2-

3

+

H

2

O

→

SO

2-

4

+

2H

+

+

2

e

-

Step 6: Equalize electrons transferred.

Multiply each half-reaction by numbers to get the lowest common multiple of electrons transferred.

1

×

[

Cr

2

O

2-

7

+

14H

+

+

6

e

-

→

2Cr

3+

+

7

H

2

O

]

3

×

[

SO

2-

3

+

H

2

O

→

SO

2-

4

+

2H

+

+

2

e

-

]

Step 7: Add the two half-reactions.

Cr

2

O

2-

7

+

8

14

H

+

+

6

e

-

→

2Cr

3+

+

4

7

H

2

O

3

SO

2-

3

+

3

H

2

O

→

3SO

2-

4

+

6H

+

+

6

e

-

m

m

m

m

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−

Cr

2

O

2-

7

+

3SO

2-

3

+

8H

+

→

2Cr

3+

+

3SO

2-

4

+

4H

2

O

Explanation:

a. a primary source combines several different accounts of an event. b. a primary source generally benefits from having been written after an event.

explanation: