Chemistry 30 Unit C - Electrochemistry

Predicting Redox Reactions

Diploma Alert! ans = 2112

Ans: D

Diploma Alert!

Ans: A

Diploma Alert!

Ans: D

Review: Building Redox Tables

ex) In a school lab, four metals were combined with each of four solutions. The evidence collected is represented by the following chemical equations. Construct a table of half reactions for these metals.

Test 1: Be(s) + Cd2+

(aq) Be2+

(aq) + Cd(s)

Test 2: Cd(s) + 2H+(aq) Cd

2+(aq) + H2(g)

Test 3: Ca2+(aq) + Be(s) no reaction

Test 4: Cu(s) + 2H+(aq) no reaction

Predicting Redox ReactionsRedox reactions all involve the transfer of electrons.

But for the most part, particles don't want to give up their electrons. They will only do so when confronted by a Strong Oxidizing Agent.

Today, we will look again at predicting whether reactions will take place or not by labeling RAs and OAs and using the spontaneity rule.

ex) Copper metal is placed into an acidic potassium permanganate solution. Does a redox reaction occur, and if so, what is the reaction equation?

Step 1: Write out all ions possible. Label them as OAs or RAs using your redox table in the data booklet and these guidelines:

-Aqueous solutions contain HOH(l) molecules.

-Acidic solutions contain the H+(aq) ion.

-Basic solutions contain the OH-(aq) ion.

-Some OAs or RAs only occur in pairs (ex. MnO4-(aq) and H

-(aq)).

- HOH(l), Fe2+

(aq), and Sn2+

(aq) may act as either OAs or RAs.

Step 1:

Cu(s) K+(aq) MnO4

-(aq) H

+(aq) HOH(l)

RA

OA OAOA

RA

Step 2: Identify the strongest OA and the strongest RA. The reaction will occur between these two.

Reactions occur between the SOA and

the SRA.

Cu(s) K+(aq) MnO4

-(aq) H

+(aq) HOH(l)

SRA

OA SOAOA

RA

In this case, copper is the SRA because it is closest to the bottom right hand side of the table.

Permanganate and hydrogen are the SOA because they are located close to the top of the left hand side of the table.

Step 3: Write out the half reactions for the SOA and SRA. Add them together and balance the number of electrons.

MnO4-(aq) + 8H

+(aq) + 5e

- Mn2+(aq) + 4HOH(l)

MnO4-(aq) + 8H

+(aq) + 5e

- Mn2+(aq) + 4HOH(l)

Cu(s) Cu 2+

(aq)+ 2e-

2

5

2MnO4-(aq) + 16H

+(aq) + 5Cu(s) 2Mn

2+(aq) + 8HOH(l) + 5Cu

2+(aq)

Cu(s) Cu 2+

(aq)+ 2e-

2MnO4-(aq) + 8H

+(aq) + 5Cu

2+(aq) Mn

2+(aq) + 4HOH(l) + 5Cu(s)

You now have a balanced redox reaction.

To check to make sure this thing will actually happen spontaneously, use the spontaneity rule.

Since the OA was higher than the RA, the reaction is spontaneous.

OA

RA

MnO4-(aq) H

+(aq)

Cu(s)