Printmaking

Review of Chemical Reactions

Acid/Base

Oxidation-Reduction Reactions (RedOx)

Redox Half-reactions:

Previously we have dealt mostly with complete reactions, though we did see some half-reactions to start with

In a half-reaction, electrons appear as:

In a ‘whole’ reaction, the number of electrons lost by the species being oxidized balances the number of electrons gained by the species being reduced, and thus no electrons appear in the balanced reaction

For decades until the 1960s, chemists tended to write half-reactions as oxidations

In the 1960s, the convention changed, and now tables of half-reactions are written as reductions.

Of all possible chemical half-reactions there must be:

You are now going to go to the lab and conduct some experiments to find out what the ordering is for several half reactions.

In each case you combine two, half-reactions to make a ‘whole’ reaction

In each case, one half-reaction will go as written (undergo reduction) and one will be reversed (go in the opposite way as written) (undergo oxidation)

These are the half-reactions you will examine (all written as reductions, as per the convention):

We will use the fact that a very small amount of Fe³⁺ in the presence of SCN turns red-orange. Fe²⁺ is almost colorless in the presence of SCN.

Fill in the following table with brief observations.

In each case, add a drop or two of the species in the first column to the species you are checking in one of the other columns.

What does it mean if a spot appears on a metal strip after a drop of one of the liquids is added?

What does it mean if no spot appears?

If metal ion A+ reacts with metal B, what is happening? What is reduced to what? What is oxidized to what?

Using your experimental results from above, arrange the half reactions in order from most likely to undergo reduction (least likely for the reverse reaction to take place, i.e., least likely for the oxidation to take place) to least likely to undergo reduction (most likely for the reverse reaction to take place, i.e., most likely for the oxidation to take place).

How closely does your ordering compare to the ‘standard values’ (given here with their ‘standard’ reduction potentials:

A large positive value of E means that the reaction tends to go ‘as written’

A not-so-large positive value means the reaction is less likely to go ‘as written’

A negative value means the reaction is likely to go in the opposite direction as written

But really what is meant is that when you combine two half reactions, the one with the more positive value will go as written and the other one will go in the opposite direction as written.

Some practice (using the table of standard values and your observations):

Printmaking

Review of Chemical Reactions

Acid/Base

Oxidation-Reduction Reactions (RedOx)

Redox Half-reactions:

Previously we have dealt mostly with complete reactions, though we did see some half-reactions to start with

In a half-reaction, electrons appear as:

In a ‘whole’ reaction, the number of electrons lost by the species being oxidized balances the number of electrons gained by the species being reduced, and thus no electrons appear in the balanced reaction

For decades until the 1960s, chemists tended to write half-reactions as oxidations

In the 1960s, the convention changed, and now tables of half-reactions are written as reductions.

Of all possible chemical half-reactions there must be:

You are now going to go to the lab and conduct some experiments to find out what the ordering is for several half reactions.

In each case you combine two, half-reactions to make a ‘whole’ reaction

In each case, one half-reaction will go as written (undergo reduction) and one will be reversed (go in the opposite way as written) (undergo oxidation)

These are the half-reactions you will examine (all written as reductions, as per the convention):

We will use the fact that a very small amount of Fe3+ in the presence of SCN­ turns red-orange. Fe2+ is almost colorless in the presence of SCN­.

Fill in the following table with brief observations.

In each case, add a drop or two of the species in the first column to the species you are checking in one of the other columns.

What does it mean if a spot appears on a metal strip after a drop of one of the liquids is added?

What does it mean if no spot appears?

If metal ion A+ reacts with metal B, what is happening? What is reduced to what? What is oxidized to what?

Half Reaction

Most easily reduced/least easily oxidized

Least easily reduced/Most easily oxidized

How closely does your ordering compare to the ‘standard values’ (given here with their ‘standard’ reduction potentials:

A large positive value of E means that the reaction tends to go ‘as written’

A not-so-large positive value means the reaction is less likely to go ‘as written’

A negative value means the reaction is likely to go in the opposite direction as written

But really what is meant is that when you combine two half reactions, the one with the more positive value will go as written and the other one will go in the opposite direction as written.

Half-reaction

e°

Au3+ + 3 e- → Au (s)

+1.50

2 H3O + + NO3- e- → 3 H2O + NO2 (g)

+0.89

Ag+ + e- → Ag (s)

+0.80

Fe+3 + e- → Fe+2

We will use the fact that a very small amount of Fe³⁺ in the presence of SCN turns red-orange. Fe²⁺ is almost colorless in the presence of SCN.

Au³⁺ + 3 e^-→ Au (s)

2 H₃O ⁺ + NO₃^- e^- → 3 H₂O + NO₂ (g)

Ag⁺ + e^-→ Ag (s)

Fe⁺³ + e^-→ Fe⁺²

Cu⁺² + 2 e^-→ Cu (s)

2 H₃O ⁺ + 2e^- → 2 H₂O + H₂ (g)

Sn⁺² + 2 e^-→ Sn (s)

0.16

Zn⁺² + 2 e^-→ Zn (s)

0.76

2 H₂O + 2e^- → OH^- + H₂ (g)

0.83

Na⁺ + e^-→ Na

2.71