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Secondary (3)

Chapter (4)

Electrochemistry

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Define Electrochemistry:

Science to study the exchange of chemical energy and electrical energy through Oxidation and Reduction Reactions.

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Section (1) Basic Skills

Oxidation Reduction

  1. Oxidation Process:-
  2. Losing electrons during the reaction.

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  • The Oxidation number will increase.

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  • Electrons written at right side. (get out from the piece)

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  • Half reaction must be balanced.

H2<===> 2H1+ + 2e1-

Pb <===> Pb2+ + 2e1–

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احفظ كلمة

L E O

Losing

Electron

Oxidation

  • Oxidizing Agent: Substance gain electrons.

الـ agent عكس الـ Process

الـ Agent هو المتسبب في ال process

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احفظ كلمة

G E R

Gaining

Electron

Reduction

Reducing agent: Substance lose electrons

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The best reducing agent:

  • Lose electrons easiest.
  • Oxidized easiest.

The best oxidizing agent:

  • Gain electrons easiest.
  • Reduced easiest.

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Practice

1) Given the reaction: Mg + ZnCl2 <===> MgCl2 + Zn.

Which is true of magnesium?

(A) It is oxidized by losing electrons.; (B) It is oxidized by gaining electrons;

(C) It is reduced by losing electrons.; (D) It is reduced by gaining electrons.

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2) In the reaction Zn + Cu2+ <=====> Zn2+ + Cu

the oxidizing agent is

​

(A) Zn (B) Cu (C) Zn2+ (D) Cu2+

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3) Given the reaction: Ca + NiCl2 <===> CaCl2 + Ni.

The oxidation number of the chlorine

​

(A) decreases (B) increases (C) remains the same.

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4) Given the reaction: Ca + NiCl2 <===> CaCl2 + Ni.

The oxidation number of the chlorine

​

(A) decreases (B) increases (C) remains the same.

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5) The Pb in the half-reaction: Pb <===> Pb2+ + 2e1–.

(A) gains protons (B) loses protons

(C) is oxidized (D) is reduced

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6) In the half-reaction Mg <===> Mg2+ + 2e1–

the magnesium

​

(A) gains electrons and is oxidized (B) gains electrons and is reduced (C) loses electrons and is oxidized (D) loses electrons and is reduced.

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A)

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B)

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C)

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D)

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9) On Putting a metal (x) in the salt solution (YCl2), the concentration of the cation Y3+ is changed from 0.1M to 0.01M,

Which of the following exist in the solution?

​

A) Ions of X2+, Y2+ and Cl- B) Ions of X2+, Cl- only

C) Ions of Y2+ and Cl-, X is precipitated at the bottom of the vessel

D) Ions of Cl-, X and Y are precipitated at the bottom of the vessel

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11) When a piece of zinc is added to diluted sulphuric acid then the evolved gas is passed in four solutions at the suitable conditions.

Which one of the following processes occurs?

A)

​

B)

​

C)

​

D)

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12) On adding potassium dichromate solution acidified by sulphuric acid to iron (II) sulphate solution, so the correct equation that expresses the oxidation reduction reaction is

​

​

​

​

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13) the graph that expresses the relationship between the concentration of [Zn2+] ions and the mass of Cu

when a zinc plate is dipped in a copper sulfate solution?

A

B

C

D

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14) The graph that expresses the relationship between the concentration of zinc cations and the concentration of sulfate anions when dipping a zinc rod in a copper II sulfate solution CuSO4

A

B

C

D

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Section (2) Galvanic Cells

 

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Galvanic Cells (Daniel Cell)

Electric Cells produce electric current from spontaneous Oxidation – Reduction Reactions.

This cell consists of two separated half cells in two containers

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This cell consists of two separated half cells in two containers

Cathode

Anode

Zinc Half-Cell

Copper Half-Cell

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Anode

  • Negative pole which includes oxidation process.
  • Consists of zinc metal dipped in ZnSO4 electrolyte solution

At anode Zn0(s) 🡸 🡺 Zn2+(aq) + 2e- oxidation

  • Anode corrodes and dissolves and its mass decreased.

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  • Anode ions concentration increases.

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  • Zinc ions dissolve and 2 electrons transfer in the wire to copper electrode

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This cell consists of two separated half cells in two containers

Anode

Cathode

Zinc Half-Cell

Copper Half-Cell

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Cathode

  • Positive pole which includes reduction process.
  • Consists of copper plate dipped in CuSO4 electrolyte solution.
  • Copper ions receive 2 electrons come from zinc half cell
  • Cathode deposit and its mass increases.
  • Cathode ions concentration decreases.

 

 

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A glass U- shaped tube filled with strong electrolytic solution like (Na2SO4) its ions do not react with the ions present in the two half cells or the electrode material.

Salt Bridge

The importance of the Salt Bridge:

A) Connects between the solution of the two half cells indirectly.

B) Neutralize the excess of positive and negative ions

which forms in the solutions of the two half cells as a result

of oxidation and reduction reactions.

C) Supplies anions (-) to the anode and cations (+) to the cathode to

maintain electrical neutrality.

D) The absence of the salt bridge cause stopping the oxidation and reduction reaction and stopping the flow of electric current in the external wire.

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In order for the salt bridge to work, it must follow some guideline:

  1. Must be highly soluble in water and form a solution that conducts electricity well. In another words it must be a strong electrolyte.

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  • An electrolyte is a solution that contains ions which are free to move around and so conduct electricity.

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  • It must not undergo any reactions with the solution or electrode.

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  • It must not undergo a precipitation reaction with either electrolyte solution.

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  • The flow of positive ions and flow of negative ions are equal in speed.

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A) All of zinc metal dissolves in the zinc half-cell.

​

B) All the copper ions disappear because they are precipitated as a copper atoms (Cu0) in the copper half-cell.

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C) Salt bridge is removed.

The flow of current stopped between two half cells when:-

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Notes

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Notes

A) Oxidation is at Anode, Reduction is at Cathode.

B) Electrons move in a wire and never in a solution.

C) Ions move in a solution and never in a wire.

D) Anode electrons out, cathode open mouth ya5od electrons.

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Notes

E) Electrons enter the cell from the wire through cathode.

F) The negative pole of the cell is the source of electron.

G) Cations move to cathode; anions move to anode.

H) Electric current flows from anode to cathode in the wire. (AWC)

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Notes

J) Electric current flows from cathode to anode in the solution.

H) Electron movement in the wire is in opposite direction of anion in the solution, and the same direction of the cation in the solution.

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  • Anode mass decreases during working.

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  • Cathode mass increases during working.

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  • Concentration of anode ions increases during working.

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  • Concentration of cathode ions decreases during working.

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Ions share in chemical reaction at cathode cannot move in the salt bridge.

In Galvanic Cell

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  • Reactions in all galvanic cells are spontaneous and exothermic.

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  • In any cell:
    • Negative pole is a fan of electrons to push them in the wire.
    • Positive pole is a suction of electrons to receive them from wire.

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  • In a cell the electric current (electrons) flows in the outer circuit from negative electrode to positive electrode.

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  • The rods of the cell are connected by a wire, solutions are connected by the salt bridge.

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  • Every galvanic cell has an External Circuit and an Internal Circuit.

The External Circuit

The Internal Circuit

Is the wires attached to each electrode which allows the passage of electrons from one electrode to another.

Is made by the salt bridge which is ionic in nature.

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Exercise

  1. In the cell in front of you:

Which of the following is correct

​

A)

The cell is a galvanic cell, the concentration of solution (A) increases.

B)

The cell is a galvanic cell, the concentration of solution (B) increases.

C)

The cell is an electrolytic cell, the concentration of solution (A) decreases.

D)

The cell is an electrolytic cell, the concentration of solution (B) decreases

A

Solution (A)

Solution (B)

B

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A)

Ag+ ions increase in the solution.

B)

Copper electrode loses electrons

C)

The electrons travel in the electrolyte.

D)

The mass of copper electrode increases

(2the galvanic cell illustrated by the opposite diagram:

What happens upon the operation of this cell?

Ag

Ag+

Cu

Cu2+

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3) The electrolyte used in the salt bridge of Daniel cell can be……

A) Pb(NO3)2

B)

BaCl2

C)

KNO3

D)

CaCL2

 

A)

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B)

​

C)

​

D)

​

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5) Daniel cell, sodium sulfate solution in the salt bridge can be replaced by all of the following solutions except……

A) Potassium chloride

B)

Sodium nitrate

B) Barium chloride

D)

Potassium sulfate

6) KCI is used as an electrolyte in the salt bridge used in the galvanic cell which is represented by the cell diagram……..

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B)

​

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D)

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Cell Diagram Rules

  1. The anode half-cell is described first; the cathode half-cell follows. Within a given half-cell, the reactants are specified first and the products last. The description of the oxidation reaction is first, and the reduction reaction is last; when you read it, your eyes move in the direction of electron flow. Spectator ions are not included ions do not share in reactions.

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  • A single vertical line (|) is drawn between two chemical species that are in different phases but in physical contact with each other (e.g., solid electrode | liquid with electrolyte).

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  • A double vertical line (||) represents a salt bridge or porous membrane separating the individual half-cells.

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  • The phase of each chemical (s, l, g, aq) is shown.

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Exercise

  1. Write the cell diagram for the next cell equations:-
  2. Fe2+ + Co ⇄ Co2+ + Fe
  3. 3Ag+ + Ni ⇄ Ni3+ + 3 Ag
  4. Cu2+ + Pb ⇄ Pb2+ + Cu
  5. 2Au3+ + 3 Zn

2 Au3+ + Zn2+

 

A)

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B)

​

B)

​

D)

​

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2) If you know that:

​

​

The diagram of the cell composed of these two electrodes is…….

A)

Al0/Al3+ // Cu2+/ Cu0

B)

3Cu0 / 3Cu2+ // 2Al 3+ / 2Al0

C)

2Al0 / 2Al3+ // 3Cu2+/3Cu0

D)

Cu2+ / Cu0 // Al3+ / Al0

3) If you know that the element (X) is a divalent element, and it more active than (Y) which is mono valent, so the cell diagram which represents the formed cell is…………

A) X2+ / X // 2Y+ / 2Y

B)

X / X 2+ // 2Y+ / 2Y

B) 2Y / 2Y+// X2+ / X

D)

2Y / 2Y+ // X / X2+

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Updated Bank El-Ma3reffa Galvanic Cells

  1. Consider the following figure:

Determine which pole is the anode and which one is the cathode.

Y

X

A) X is the anode, because it is on the left, while Y is the cathode, because it is on the right

B) X is the anode, because it accepts electrons, while Y is the cathode, because it loses electrons

C) Y is the anode, because it accepts electrons, while X is the cathode, because it loses electrons

D) This figure is not enough to determine the anode and the cathode

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2)Which of the following describes what happens in the galvanic cell below?

A)

The Zn electrode is oxidized, and its mass decreases with time

B)

Zn2+ ions are reduced, and they deposit on the Zn electrode

C)

The Co electrode is oxidized, and its mass decreases with time

D)

The Co electrode is reduced and its mass increases with time

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(3Which of the following happens upon closing the galvanic cell circuit?

A)

Anions move toward the anode through the porous septum

B)

Cations move toward the cathode through the porous septum

C)

Electrons move through the external wire from the anode toward the cathode

D)

All the answers are correct

 

A)

Electrons moves from the anode towards the cathode via an external wire, while Cu+2 moves from the cathode toward the anode via salt bridge

B)

Electrons moves from the anode towards the cathode via an external wire, while Cu+2 moves from the anode toward the cathode via salt bridge

C)

Electrons moves from the anode towards the cathode via an external wire, while Cd+2 moves from the anode toward the cathode via salt bridge

D)

Electrons moves from the anode towards the cathode via an external wire, while Cd+2 moves from the cathode toward the anode via salt bridge

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A)

Iodide ions are oxidized and lose electrons

B)

chlorine ions are reduced and lose electrons

C)

Potassium ions are oxidized and lose electrons

D)

Iodide ions are oxidized and gain electrons

F)

Potassium ions are oxidized and gain electrons

 

A)

​

B)

Zn(s)

C)

​

D)

Cu(s)

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5) Consider the figure below

What may cause the electric current to stop flowing?

A) The full consumption of the Cu+2 ions

B) Removing the salt bridge

C) The full consumption of the Zn electrode

D) All of the answers are correct