Chemistry Form 4 Chapter 6

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Chapter 6: Electrochemistry

1. Electrolyte  Electrolytes are substances in molten state or aqueous solution that can conduct electricity due to the presence of free moving ions

2. Non-electrolytes  Non-electrolytes are substance that cannot conduct electricity either in molten state or aqueous solution.

3. Electrolysis  Electrolysis is a process whereby compounds in molten or aqueous state are broken down into their constituent elements by passing electricity through them.

4. Electrolytic cell  The electrolytic cell is the set of apparatus needed to conduct electrolysis.  It consists of a battery, an electrolyte and two electrodes.

5. Electrode  Electrodes are electrical conductors.

 Graphite or platinum is usually used as electrodes because they are inert, they do not react with electrolyte or the products of electrolysis.

6. Anode  The electrode which is connected to the positive terminal of an electric source.  Negatively charged ions (anions) in the electrolyte are attracted to the anode. 7. Cathode  The electrode which is connected to the negative terminal of the batteries.

 Positively charged ions (cations) in the electrolyte are attracted to the cathode. 8. Electrolysis of

Molten Compounds

Molten compound:

 A molten compound consists of one type of cations and one type of anion only.  In solid state, ions do not move freely but are held in fixed positions in a lattice.  In molten electrolyte, the ions move freely.

 During electrolysis, the negative ions or anions move to the anode.  The positive ions or cations move to the cathode.

 A new substance is then formed at each electrode.  Example: Electrolysis of molten lead (II) bromide, PbBr2.

 PbBr2 is an ionic compound. It consist Pb 2+

and Br-.

 In solid PbBr2, these ions do not move freely but are held in fixed positions in a lattice.

When it melts, the ions are free to move.  During the electrolysis of molten PbBr2, Br

are attracted to the anode.

 At the anode, Br- undergo discharge whereby each of these ions releases an electron to form a neutral bromine atom.

 Two bromine atoms combine to form a bromine gas, Br2 molecule.

 Thus, Br2 is released at the anode.

 Half equation: 2Br-(l)  Br2(g) + 2e

- At the cathode, Pb2+ undergo discharges whereby each of the ions accepts two electrons to form a lead atom.

 Thus, lead metal is formed at the cathode.  Half equation: Pb2+(l) + 2e-  Pb(s)

 Combining the two half equations, we get the overall equation. Pb2+(l) + 2Br-(l)  Pb(s) + Br2(g)

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Example 1. 2. anion/ -ve ion cation/ +ve ion ammeter - + cathode / -ve electrode anode / +ve electrode - + - + Battery A Molten NaCl Pb2+ + 2e-  Pb 2Br-  Br2 + 2e- anion/ -ve ion cation/ +ve ion ammeter - + cathode / -ve electrode anode / +ve electrode - + - + Battery A Molten PbBr2

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3. 4.

9. Electrolysis of Aqueous Solutions

 An aqueous solution is produced when a solute is dissolved in water.

 An aqueous solution of a salt consists of 2 types of cations (cations of the salt and hydrogen ions, H+), and 2 types of anions (anions of the salt and hydroxide ions, OH-).

 H+ and OH- are always present together with the ions produced from the dissociation of salts in aqueous solutions.

 This is because water dissociates partially to form H+ and OH-. H2O H+ + OH

- There are three factors that may influence the selective discharge of ions during the electrolysis of an aqueous solution.

i. Position of ions in the electrochemical series

- The ions that are lower in the electrochemical series will be selectively discharged

- .

anion/

-ve ion

cation/

+ve ion

ammeter

-

+

cathode /

-ve electrode

anode /

+ve electrode

-

+

-

+

Battery

A

Molten lead (II) oxide

anion/

-ve ion

cation/

+ve ion

ammeter

-

+

cathode /

-ve electrode

anode /

+ve electrode

-

+

-

+

Battery

A

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ii. Concentration of ions in the electrolytes

- If the concentration of a particular ion is high, the ions is selectively discharged. iii. Types of electrodes used in the electrolysis

 The common materials used as electrodes are carbon and platinum because they are inert.  Example: Electrolysis of copper (II) sulphate, CuSO4 solution.

CuSO4  Cu 2+ + SO4 2-H2O H + + OH

- If carbon is used as the electrodes, OH- ions are discharged at the anode because of the position of OH- ion in the electrochemical series.

 If copper is used as the anode, both SO4

ions and OH- ions are not discharge.

 Instead the copper anode dissolves by releasing electrons to form copper (II) ions, Cu2+. Hence, the mass of anode decrease.

 Copper acts as an active electrode here because it takes part in the chemical reactions during electrolysis.

1. 2. 3. 4. anion/ -ve ion cation/ +ve ion ammeter - + cathode / -ve electrode anode / +ve electrode - + - + Battery A

Silver nitrate, AgNO3 solution

Dilute copper (II) chloride CuCl2

anion/ -ve ion cation/ +ve ion ammeter - + cathode / -ve electrode anode / +ve electrode - + - + Battery A anion/ -ve ion cation/ +ve ion ammeter - + cathode / -ve electrode anode / +ve electrode - + - + Battery A

Copper (II) sulphate, CuSO4 solution

anion/ -ve ion cation/ +ve ion ammeter - + cathode / -ve electrode anode / +ve electrode - + - + Battery A

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7. 8. Electrolysis in Industries anion/ -ve ion cation/ +ve ion ammeter - + cathode / -ve electrode anode / +ve electrode - + - + Battery A

Concentrated potassium

chloride, KCl

Concentrated copper (II)

bromide, CuBr

2 anion/ -ve ion cation/ +ve ion ammeter - + cathode / -ve electrode anode / +ve electrode - + - + Battery A Ag C Ag2SO4 Ag  Ag+ + e- Ag+ + e-  Ag anion/ -ve ion cation/ +ve ion ammeter - + cathode / -ve electrode anode / +ve electrode - + - + Battery A Ni Ni NiSO4

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Extraction of metals

- Reactive metals such as aluminium and magnesium can be extracted from their ores be electrolysis. - Aluminium can be extracted from its ore, molten aluminium oxide, Al2O3 using carbon electrodes.

- In this process, a substance known as cryolite, Na3AlF6 is added to aluminium oxide, Al2O3 to lower its

melting point.

Purification of metals

- Pure copper and silver can be obtained through the process of electrolysis.

- In the purification of copper, the impure copper is made to be the anode while the cathode is a thin layer of pure copper.

Electroplating of metals

- In electroplating of metals, electrolysis is used to coat one metal onto another metal.

- In the process of electroplating, a more expensive or attractive metal such as silver or gold is coated onto the object to make it look more attractive and more resistant to corrosion.

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- A simple voltaic cell consists of two different metals immersed into an electrolyte. Chemical energy is converted to electrical energy in the cell.

- The more electropositive metal (metal that is higher position in the ECS) will release electron  Negative terminal

- The less electropositive metal (metal that is lower position in the ECS) will be positive terminal. - The electron flow form negative terminal to positive terminal. Cation which is lower at ECS will be

discharged.

12 Daniell Cell

- In a Daniell cell, zinc and copper are used as electrodes. Each electrode is immersed into a different electrolyte. The electrolytes are connected by a salt bridge or a porous pot.

The porous pot and salt bridges are: i. to allow the flow of ions so

that the circuit is completed

ii. to prevent the two aqueous solution from mixing

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- The electrochemical series is an arrangement of metals based on the tendency of each metal atom to donate electrons.

- The electrochemical series can be constructed based on the potential difference between two metals, and the ability of a metal to displace another metal from its salt solution.

- The electrochemical series is used to determine the terminals and voltage of a cell. It is also used to predict the ability of one metal to displace another metal from its salt solution.

- The further the two metals are in the ECS, the greater the voltage produced by the cell. The Advantages Disadvantages of Various Voltaic Cells

Cell Advantages Disadvantages

Daniell cell  Easily set up in the laboratory  Wet cell – electrolyte easily split  Voltage cannot last

Dry cell  No spillage  Small in size  Easily carried about

 Produces regular current and voltage  Obtained in different sizes

 Does not last

 Cannot be recharged

 Leakage can occur if cell cannot be used anymore

Alkaline cell  Lasts longer than dry cell (10 x)

 Produces a higher and more regular current

 Leakage can occur if cell cannot be used anymore

 Expensive

 Cannot be recharged Mercury cell  Small in size

 Produces regular current for a longer period of time

 Lasts a long time

 Very expensive  Cannot be recharged

 Mercury that is produced is poisonous

Lead-acid accumulator

 Can be recharged

 Produces a high voltage (12V) for a long period time

 Produces a high current (175A) suitable for a heavy duty

 Spillage of acid can occur  Big in size

 Heavy, difficult to be carried about  Expensive

 Loses charge if not used for long

Nickel-cadmium cell

 Can be recharged up to 500 times  Dry cell no spillage

 Smaller than accumulator - portable

 Expensive

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Similarities  Contains an electrolyte

 Consist of an anode and a cathode

 Electron move from the anode to the cathode in the external circuit (connecting wires)  Positive ions and negative ions move in the electrolyte

 Chemical reactions involve the release or acceptance of electrons Differences

Characteristics Electrolytic Cell Voltaic Cell

Energy change  Electrical energy  chemical energy  Chemical energy  electrical energy Electric current

and reaction

 Electric current results in a chemical reaction

 Chemical reaction produces an electric current

Electrode / Terminal

 Cathode: Negative terminal  Anode : Positive terminal

 Cathode: Positive terminal  Anode : Negative terminal Flow of electron  Electron flow from the positive electrode

(anode) to the negative electrode (cathode)

 Electrons flow from the negative electrode (anode) to the positive electrode (cathode)

Negative terminal  Cation receives electrons from the cathode (negative terminal)

 Electron are released at the negative terminal

Positive terminal  Anion release electrons to the anode (positive terminal)

 Electrons are received by the positive terminal

Types of electrodes

 Same or two different types of metal, or graphite electrodes

 Two different types of metal