Class13 ChemistryG12 Notes and Homework

8/4/2019 Class13 ChemistryG12 Notes and Homework

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Galvanic Cells• Redox reactions involve the transfer of electrons from one reactant to

another. An electric current is a flow of electrons in a circuit.

• These two concepts form the basis of electrochemistry, which is the

study of the processes involved in converting chemical energy to

electrical energy, and in converting electrical energy to chemical

energy.

•

rom

, we

now

a

a

z nc

s r p

reac s

w

a

so u on

containing copper(II) ions, forming zinc ions and metallic copper. The

is exothermic: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

• The reaction occurs on the surface of the zinc stri where electrons

are transferred from zinc atoms to copper(II) ions when these atoms

and ions are in direct contact. A common technological invention,

called a galvanic cell, uses redox reactions, such as the one described

above, to release energy in the form of electricity.



• ga van c ce , a so ca e a vo a c ce , s a

device that

converts

chemical

energy

to

electrical

.

• The key to this invention is to prevent the

direct contact with each other. Instead, electrons flow from one reactant to the other through an

external circuit, which is a circuit outside the

reaction vessel.

• T is ow o e ectrons t roug t e externa circuit

is an electric current.



Example: The Daniell Cell

• One half of the cell consists of a piece of zinc placed in a zinc

.

copper placed in a copper(II) sulfate solution.

• A porous barrier, sometimes called a semi‐permeable membrane, separates t ese two a ‐ce s. It stops t e copper II ons rom

coming into direct contact with the zinc electrode.

•

electrical conductors. The conductors that carry electrons into and out of a cell are named electrodes.

• e z nc su a e an copper su a e ac as e ec ro y es.

Electrolytes are substances that conduct electricity when dissolved

in water. *NOTE

The fact that a solution of an electrolyte conducts electricity does

.

electrolyte solution conducts electricity because of ion movements, and the loss and gain of electrons at the electrodes.



• The redox reaction takes place in a galvanic cell when an external

circuit such as a metal wire connects the electrodes. The

oxidation half ‐reaction occurs in one half ‐cell, and the reduction half ‐reaction occurs in the other half ‐cell. For the Daniell cell:

+ −

Reduction (gain of electrons): Cu2+(aq) + 2e− → Cu(s)

• this example, zinc atoms undergo oxidation at the zinc electrode.

Thus, the zinc electrode is the anode of the Daniell cell.

• e e ec ro e a w c re uc on occurs s name e ca o e. Here, copper(II) ions undergo reduction at the copper electrode.

Thus, the copper electrode is the cathode of the Daniell cell.

• Free electrons cannot travel through the solution. Instead, the external circuit conducts electrons from the anode to the cathode

. Hello,

I am a RED CAT a e

CATHODE)



• At the anode of a alvanic cell electrons are released

by oxidation. For example, at the zinc anode of the

positive zinc ions. Thus, the anode of a galvanic cell is

. ,

of a galvanic cell is positively charged. In galvanic cells,

negative electrode to the positive electrode.



• Each half ‐cell contains a solution of a neutral compound. In a Daniell cell, these

.

electrolyte solutions remain neutral when electrons are leaving the anode of one

half ‐cell and arriving at the cathode of the other half ‐cell?

• ‐ ,

porous barrier. Negative ions (anions) migrate toward the anode, and positive ions

(cations) migrate toward the cathode.

• e separa or e ween e a ‐ce s oes no nee o e a porous arr er. n

alternative device, called a salt bridge, contains an electrolyte solution that does

not interfere in the reaction. The open ends of the salt bridge are plugged with a

orous material such as lass wool to sto the electrol te from leakin out

quickly. The plugs allow ion migration to maintain electrical neutrality.



• ,

NH4Cl(aq). As positive Zn ions are produced at the anode, negative Cl ions migrate

from the salt bridge into the half ‐cell that contains the anode. As positive Cu(II)

ons are remove rom so u on a e ca o e, pos ve ammon um ons m gra e

from the salt bridge into the half ‐cell that contains the cathode. Other electrolytes,

such as sodium sulfate or potassium nitrate, could be chosen for the salt bridge. Neither of these electrolytes interferes in the cell reaction.

• Silver nitrate would be a poor choice for the salt bridge, however. Positive silver

ions would mi rate into the half ‐cell that contains the cathode. Zinc dis laces both

copper and silver from solution, so both copper(II) ions and silver ions would be reduced at the cathode. The copper produced would be contaminated with silver.



Galvanic Cell Notation

•

galvanic cells. The shorthand representation of a Daniell cell is

as follows:

Zn | Zn2+ || Cu2+ | Cu

The phases or states may be included.Zn s Zn2+ aq Cu2+ aq Cu s

• In a diagram, the anode may appear on the left or on the

.

• In the shorthand representation, however, the anode is alwa s shown on the le t and the cathode on the ri ht.

• Each single vertical line, |, represents a phase boundary

between the electrode and the solution in a half ‐cell. For examp e, t e rst s ng e vert ca ne s ows t at t e so z nc

and aqueous zinc ions are in different phases or states. The

double vertical line re resents the orous barrier or salt

bridge between the half ‐cells. Spectator ions are usually omitted.



Inert Electrodes• The zinc anode and copper cathode of a Daniell cell are both

metals, and can act as electrical conductors. However, some

electrodes, such as gases or dissolved electrolytes. Galvanic

cells that involve such redox reactions use inert electrodes.

• An inert electrode is an electrode made from a material that is neither a reactant nor a roduct of the cell reaction. The

figure shows a cell that contains one inert electrode. The chemical equation, net ionic equation, and half ‐reactions for

t is ce are given e ow:

Chemical equation:

Pb(s)

+

2FeCl3(aq)

→2FeCl2(aq)

+

aq

Net ionic equation: Pb(s) + 2Fe3+(aq) → 2Fe2+(aq) + Pb2+(aq)

Oxidation half ‐reaction: Pb(s) → Pb2+(aq) + 2e−

Reduction half ‐reaction: Fe3+(aq) + e− → Fe2+(aq)



‐ ,

so an inert platinum electrode is used in this half ‐cell. The platinum electrode is chemically unchanged, so it does not appear in the chemical

‐ . ,

representation of the cell.

Pb | Pb2+ || Fe3+, Fe2+ | Pt

• comma separa es e ormu as e an e or e ons nvo ve n e

reduction half ‐reaction. The formulas are not separated by a vertical line,

because there is no phase boundary between these ions. The Fe3+ and

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Class13 ChemistryG12 Notes and Homework

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