ElectrochemistryLesson 3

Spontaneous Reactions

Predicting Spontaneous ReactionsUsing the

Standard Reduction Chart

Does Au3+ react with Cl-?

Predicting Spontaneous ReactionsUsing the

Standard Reduction Chart

If two half-cells are joined, the higher half-reaction on the table undergoes reduction, while the lower half-reaction undergoes oxidation

Possible situations…If both reactants are found on the left, or both are found on the right, NO reaction will occur ( non spontaneous)Same side NO reaction

nonspontaneous- both on same side

Does Au3+ react with Fe3+?

Possible situations…If one reactant is found on the left, one is found on the right, a spontaneous reaction will occur if and only if…

The reactant on the left side(to be reduced) is ABOVE the reactant on the right side( to be oxidized)

Predicting Spontaneous ReactionsUsing the

Standard Reduction Chart

Does Au3+ react with Cl-? If it does, write the spontaneous reaction.

The top reaction is written forward- reduction

The bottom reaction is written in reverse- oxidation

(Au3+ + 3e- Au(s))

(2Cl- Cl2(g) + 2e-)

2

3

2Au3+ + 6Cl- 3Cl2(g) + 2Au(s)

spontaneous- downhill

Does Fe3+ react with Pb?

Does Ag+ react with Br-?

nonspontaneous

Some ions are on both sides of the table and are oxidizing or reducing agents. For example: Fe Fe2+ Fe3+

 Sn Sn2+ Sn4+

  

Cu Cu+ Cu2+

Does Sn2+ react with Cr?

spontaneous

Check both

(Sn2+ + 2e- Sn(s))

(Cr(s) Cr3+ + 3e-)

3

2

2Cr (s) + 3Sn2+ 2Cr3+ + 3Sn(s)

Can you keep HCl in a Cu container? Explain!

nonspontaneous

Can you keep HCl in a Cu container? Explain!

Yes, nonspontaneous.

H+ is a weaker oxidizing agent than Cu2+ and Cu+.

Can you keep HCl in a Zn container? Explain!

Spontaneous- no

H+ is a stronger oxidizing agent than Zn2+

Write the spontaneous reaction.

2H+ + 2e- H2(g)

Zn(s) Zn2+ + 2e-

2H+ + Zn(s) H2(g) + Zn2+

Can you keep HNO3 in a Au container? Explain!

Can you keep HNO3 in a Au container? Explain!

Yes, nonspontaneous. HNO3

is a weaker oxidizing agent than Au3+.

TRY: Can you keep HNO3 in a Cu container? Explain!

Take the strongest ox agent- higher

Take the strongest red agent- lower

Can you keep HNO3 in a Cu container?

No, spontaneous. HNO3 is a stronger oxidizing agent than Cu2+.

Write the spontaneous reaction.

NO3- + 4H+ + 3e- NO(g) + 2H2O x 2

Cu(s) Cu2+ + 2e- x 3

2NO3- + 8H+ + 3Cu(s) 2NO(g) + 4H2O + 3Cu2+

Homework

Hebden: pg. 199 #7-12 (odd); pg. 200 17-18