Electrochemical Cells & The Electrochemical Series

1. A simple cell from an exothermic redox reaction Zn (s) + Cu2+

(aq)

2. Redox Equilibrium – zinc in water

3. Standard Hydrogen Reference Half Cell

4. Standard electrode potential – Zn/Zn2+

5. Standard electrode potential – Cu/Cu2+

6. Standard electrode potential – Fe2+/Fe3+

7. The Electrochemical Series

8. Calculation of Cell e.m.f.

9. Questions

10. Useful Internet Link

Cu2+(aq)H2O (l)

Electrochemical Cell

Zn

V

SALT BRIDGE

Zn(s) Zn2+(aq) + 2e- Cu2+(aq) + 2e- Cu(s)HALF EQUATION HALF EQUATION

Salt Bridge• Maintains electrical neutrality.

• Solution of ionic compound, usually potassium bromide KBr.

• Positive (K+) ions move into negative half cell

• Negative (Br-)ions move into positive half cell

Br- K+

Very High Resistance Voltmeter

• No current drawn.

• Measures the electromotive force [e.m.f.] of the cell.

• The potential for the cell to provide energy

• OXIDATION• ANODE

OXIDATION

An anode is a place of oxidation

Zn(s) Zn2+(aq) + 2e-

• REDUCTION• CATHODE

REDUCTION

A cathode is a place of reduction

Cu2+(aq) + 2e- Cu(s)

Redox Equilibrium[PRESS ZINC]

Zinc Zn

Zn2+

e- e-Zne- e-

Zn2+

Zn(s) Ý Zn2+(aq) + 2e-

e- e-

e- e-e- e-

e- e-

Zn2+

Zn2+ Zn2+

Add stronger oxidising agent Add stronger reducing agentADDING REDOX AGENTS

Redox Equilibrium 2MAGNESIUM

e- e-Mg

Mg2+

e- e-Mg

Mg2+

Mg(s) Ý Mg2+(aq) + 2e-

e- e-

e- e-e- e-

e- e-

Mg2+

Mg2+ Mg2+

Equilibrium for magnesium liesfurther to right than for zinc

Magnesium

• magnesium sheds electrons and forms ions more readily than zinc does.

• can't measure the absolute voltage between the metal and the solution

• don't need to be able to measure the absolute voltage between the metal and the solution.

• enough to compare the voltage with a standardised system [standard hydrogen reference electrode].

Oxidising agent added, e.g. Cu2+:

Zinc

Zn2+Zn2+

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

e- e-

e- e-e- e-

e- e-

Cu2+

Zn2+ Zn2+Cu

EQUATION

Reducing agent added, e.g. Mg:

Zinc

Zn2+

Zn2+

Mg(s) + Zn2+(aq) Mg2+(aq) + Zn(s)

e- e-

e- e-e- e-

e- e-

Mg2+

Zn2+ Zn2+

e- e-Mg

EQUATION

Zn

1 mol dm-3 H+(aq)

Standard Hydrogen ReferenceElectrode

2H+(aq) + 2e- H2(g)HALF EQUATION

HYDROGEN GAS1 atmosphere

298K

Pt

Platinum Electrode

1 mol dm-3 Zn2+(aq)

Eθ Zn2+ / Zn

Zn

V

SALT BRIDGE

Zn2+(aq) + 2e- Zn(s)HALF EQUATION

1 mol dm-3 H+(aq)

Pt

H2 (g)

Pt (s) H2(g), 2H+(aq)Zn2+(aq)Zn(s)CELL NOTATION

Eθ= -0.76VEθ

1 mol dm-3 Cu2+(aq)

Eθ Cu2+ / Cu

Cu

V

SALT BRIDGE

Cu2+(aq) + 2e- Cu(s)HALF EQUATION

1 mol dm-3 H+(aq)

Pt

H2 (g)

Pt (s) H2(g), 2H+(aq)Cu2+(aq)Cu(s)CELL NOTATION

Eθ= +0.34VEθ

1 mol dm-3 Fe2+(aq)

Eθ Fe3+ / Fe2+

Pt

V

SALT BRIDGE

Fe3+(aq) + e- Fe2+(aq)HALF EQUATION

1 mol dm-3 H+(aq)

Pt

H2 (g)

Pt (s) H2(g), 2H+(aq)Fe3+(aq), Fe2+(aq)Pt(s)CELL NOTATION

Eθ= +0.77VEθ 1 mol dm-3 Fe3+(aq)

Electrochemical Series

Li+(aq) + e− Ý Li(s) −3.05VK+(aq) + e− Ý K(s) −2.93VCa2+(aq) + 2e− Ý Ca(s) −2.76VNa+(aq) + e− Ý Na(s) −2.71VMg2+(aq) + 2e− Ý Mg(s) −2.38VAl3+(aq) + 3e− Ý Al(s) −1.68VZn2+(aq) + 2e− Ý Zn(s) −0.76VFe2+(aq) + 2e− Ý Fe(s) −0.44V2H+(aq) + 2e− Ý H2(g) 0.00VCu2+(aq) + e− Ý Cu+(aq) +0.16VCu2+(aq) + 2e− Ý Cu(s) +0.34VO2(g) + 2H2O(l) + 4e- Ý 4OH-(aq) +0.40VCu+(aq) + e− Ý Cu(s) +0.52VI2(s) + 2e− Ý 2I−(aq) +0.54VFe3+(aq) + e− Ý Fe2+(aq) +0.77VAg+(aq) + e− Ý Ag(s) +0.80VBr2(aq) + 2e− Ý 2Br−(aq) +1.09VCl2(g) + 2e− Ý 2Cl−(aq) +1.36VCr2O7

2−(aq) + 14H+ + 6e− Ý 2Cr3+(aq) + 7H2O(l) +1.38VMnO4

−(aq) + 8H+ + 5e− Ý Mn2+(aq) + 4H2O(l) +1.51VF2(g) + 2e− Ý 2F−(aq) +2.87V

1 mol dm-3Cu2+(aq)1 mol dm-3 Zn2+(aq)

Cell e.m.f.

Zn

V

SALT BRIDGE

Cu

Zn (s) Zn2+(aq)Cu2+(aq)Cu(s)CELL NOTATION

EθCELL= +0.34 – (-0.76) = +1.10VEθ

CELL

opposite

Questions

1 mol dm-3 Zn2+(aq)1 mol dm-3 Cu2+(aq)

Cell e.m.f.

Cu

V

SALT BRIDGE

Zn

Cu (s) Cu2+(aq)Zn2+(aq)Zn(s)CELL NOTATION

EθCELL= -0.76 – (+0.34) = -1.10VEθ

CELLQuestions

QUESTIONS

Feasibility Of ReactionsElectrochemical Cells

QUESTIONSFeasibility of Reactions

• For each of the following mixtures predict whether a reaction is or isn’t feasible at s.t.p. If a reaction is feasible then write the balanced ionic equation.

1. Li(s) and Cu2+(aq)2. Zn(s) and Al3+(aq)3. O2(g), H2O(l) and Mg(s)

4. Br2(aq) and Fe(s)

5. Acidified Cr2O72- and I-(aq)

QUESTIONSelectrochemical cells

• For each of the following redox reactions write the cell notation and calculate the e.m.f. of the cell at s.t.p.

1. Li(s) + Ag+(aq) Li+(aq) + Ag(s)2. 3Na(s) + Al3+(aq) 3Na+(aq) + Al(s)3. Cr2+(aq) + Fe3+(aq) Fe2+(aq) + Cr3+(aq)4. O2(g) + 2H2O(l) + 2Cu(s) 4OH-(aq) +

2Cu2+(aq)5. Ca(s) + 2K+(aq) Ca2+(aq) + 2K(s)