6.5 Voltaic Cell

Voltaic cell
■ Voltaic cell

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An electrochemical cell that derives electrical energy from reaction taking place within the cell.

■ Simple voltaic cell

	►	Consists of two different metal electrodes.
	►	Connected to the end of the wire and the other end dipped into an electrolyte solution.
	►	Potential difference generated depends on the difference in the position of the metal used in the electrochemical series.
	►	Electrolyte Negative terminal Positive terminal Metal More electropositive metal Less electropositive metal Flow of electron More electropositive metal will release electron and dissolve to form metal ion Cation in the electrolyte(lower position in the electrochemical series) will accept electrons and be discharged to form copper atom Electrons donated by the metal move from the negative terminal to the positive terminal. Electric current flows in the opposite direction of electrons flow.

■ Simple voltaic cell: zinc plate and copper plate immersed in copper(II) sulphate solution

	►	Zinc located higher in the electrochemical series(more electropositive) will act as the negative terminals.
	►	Copper located lower in the electrochemical(less electropositive) will act as the positive terminal.
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	►	Electrolyte: Copper(II) Sulphate solution Negative terminal Positive terminal Metal Zinc Copper Flow of electron Zinc electrode will release electron and dissolve to form ZN2+ ion Cu2+ ion in the electrolyte will accept electrons and be discharged to form copper atom Half-equation Zn → Zn2+ + 2e- CU2+ + 2e- → Cu Electrons donated by the metal move from the negative terminal to the positive terminal. Electric current flow in the opposite direction of electrons flow. Overall ionic equation : Zn + CU2+ → Zn2+ + Cu

■ This video shows the flow of ion in simple voltaic cell.

Daniel cell
■ Daniel cell

	►	Consists of two different metal electrodes immersed in two separate electrolyte solutions.
	►	Electrolyte solution can be separated either by a porous pot or a salt bridge.
	►	Function of porous pot or salt bridge: to separate two electrolyte solutions but allow ions to pass through to complete a circuit.
	►	Chemical energy → electrical energy. Electrolyte Negative terminal Positive terminal Metal More electropositive metal Less electropositive metal Flow of electron More electropositive metal will release electron and dissolve to form metal ion Cation in the electrolyte(lower position in the electrochemical series) will accept electrons and be discharged to form copper atom Electrons donated by the metal move from the negative terminal to the positive terminal. Electric current flows in the opposite direction of electrons flow.

■ This video shows the flow of ion in simple Daniel cell.

■ Arrangement of a Daniel cell.

Electrolyte solution separated by a salt bridge.

Electrolyte solution separated by a porous pot.

Electrolyte solution separated by a salt bridge(U-tube).

Chemical reactions: Electrolyte: Copper(II) Sulphate solution Negative terminal Positive terminal Metal Zinc Copper Flow of electron Zinc electrode will release electron and dissolve to form Zn2+ ion Cu2+ ion in the electrolyte will accept electrons and be discharged to form copper atom Electrons donated by the metal move from the negative terminal to the positive terminal. Electric current flow in the opposite direction of electrons flow. Overall ionic equation: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

Other voltaic cell
■ Voltaic cell can be divided into two types:

	►	Primary cells : non-rechargeable cells
	►	Secondary cells : rechargeable cells

■ Primary cell:

	►	Positive terminal : carbon rod 2MnO2(s) + 2NH4+(aq) + 2e- → Mn2O3(s) + 2NH3(g) + H2O(l)
	►	Negative terminal : zinc Zn(s) → Zn2+(aq) + 2e-
	►	Electrolyte: Mixture of ammonium chloride and zinc chloride.
	►	Advantages: light and supplies constant current.
	►	Disadvantages: not long-lasting and not rechargeable
	►	Positive terminal : Manganese(IV) oxide 2MnO2(s) + H2O(l) + 2e- → Mn2O3(s) + OH-(aq)
	►	Negative terminal : zinc Zn(s) → Zn2+(aq) + 2e-
	►	Electrolyte: Mixture of ammonium chloride and zinc chloride.
	►	Advantages : long-lasting and supplies high voltage than dry cell for long period.
	►	Disadvantages : expensive than dry cell and not rechargeable.
	►	Positive terminal : Mercury(II) oxide HgO(s) + H2O(l) + 2e- → Hg(l) + 2OH-(aq)
	►	Negative terminal : zinc Zn(s) → Zn2+(aq) + 2e-
	►	Electrolyte: Mixture of zinc hydroxide and potassium hydroxide solution.
	►	Advantages : long-lasting and supplies constant current.
	►	Disadvantages : not rechargeable.

■ Secondary cell:

	►	Positive terminal: Lead(IV) oxide PbO2(s) + 4H+(aq) + 2e- → Pb2+(aq) + 2H2O(l)
	►	Negative terminal : lead Pb(s) → Pb2+(aq) + 2e-
	►	Electrolyte: Dilute sulphuric acid.
	►	Advantages : rechargeable and supplies high voltage for long period.
	►	Disadvantages : heavy, expensive and use electrolyte which corrodes easily.
	►	Positive terminal: Nickel(IV) oxide NiO2(s) + 2H2O(l) + 2e- → Ni(OH)2(s) + 2OH-
	►	Negative terminal : cadmium Cd(s) + 2OH- → Cd(OH)2(s) + 2e-
	►	Electrolyte: Potassium hydroxide.
	►	Advantages : long-lasting, rechargeable and concentration of its electrolyte does not change
	►	Disadvantages : expensive

■ This video shows the functions and how a car battery works in a car.

Similarities and differences of electrolytic cell and voltaic cell
■ Similarities of electrolytic cell and voltaic cell

	►	Consists of an anode and a cathode [a positive terminal and a negative terminal]
	►	Contains an electrolyte
	►	Chemical reaction involves donating or receiving electrons
	►	Positive and negative ions move to the electrodes in the electrolyte
	►	Electrons move from the anode to cathode [negative terminal to positive terminal in the outer circuit]

■ Electrolytic cell differs from voltaic cell in some aspects

Differences	Electrolytic cell	Voltaic cell
Cell structure
Made of	Carbon or different/same metal immersed in the electrolyte	Two different metals immersed in the electrolyte
Electric current	Produces chemical reaction	Produced by chemical reaction
Energy changes	Electrical energy to chemical energy	Chemical energy to electrical energy
Cathode	Cations receive electrons	Electrons are accepted
Anode	Anions release electrons	Electrons are released
Requirement of battery	Uses battery	Does not use battery

← 6.4 Electrolysis in Industry

6.6 Electrochemical Series →

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