studysmart chapter 6

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STUDYSMART CHEMISTRY FORM 4

CHAPTER 6 : ELECTROCHEMISTRY

6.1 Understanding properties of electrolytes and non- electrolytes

6.2 Analysing electrolysis of molten compounds 6.3 Analysing the electrolysis of aqueous solutions 6.4 Evaluating electrolysis in industry 6.5 Analysing voltaic Cell 6.6 Synthesising electrochemical series 6.1 UNDERSTANDING PROPERTIES OF ELECTROLYTES AND NON- ELECTROLYTES

Electrolytes are substance that can conduct electricity when they are in molten state or aqueous solution and undergo chemical changes.

Non-electrolytes are substance that cannot conduct electricity either in molten states or

aqueous solution.

6.2 ANALYSING ELECTROLYSIS OF MOLTEN COMPOUNDS

Electrolysis is a process whereby compound is molten or aqueous state is broken down into their constituent elements by passing electricity through them.

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

The electrode which is connected to the positive terminal of an electric source is called the anode while the electrode which is connected to the negative terminal is called the cathode.

In molten electrolyte, the ions move freely.

During electrolysis, the negative ion or anions move to the anode whereas the positive ion or cations move to the cathode.

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Example : Electrolysis of lead(II) bromide, PbBr2

a) Lead(II) Bromide, PbBr2 is an ionic compound PbBr2 Pb2+ + 2Br- b) The positive Lead(II) in Pb2+ are attracted to cathode and undergo discharge whereby each of the ions accepts two electron to form an atom. The half equation below summarized the process. Pb2+ + 2e Pb c) The negative Bromide ions, Br- are attracted to the anode and undergo discharge, whereby each of the ionic accepts an electron to form a neutral bromide atom. Br- Br + e Two bromine atoms combine to form a bromine gas Br + Br Br2 or 2Br- Br2 + 2e d) The overall equation for electrolysis of PbBr2

, Cathode : Pb2+ + 2e Pb Anode : 2Br- Br2 + 2e Overall equation : Pb2+ + 2Br- Pb + Br2

TRY THIS 1 Complete the chart below

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6.3 ANALYSING THE ELECTROLYSIS OF AQUEOUS SOLUTIONS

In an aqueous solution, water will dissociate partially to form hydrogen ions and hydroxide ions. H2O H+ + OH-

Example : Copper(II) Sulphate, CuSO4 consist of copper(II) ions, Cu2+, Sulphate ion SO42-,

hydrogen ion, H+ and hydroxide ion, OH- CuSO4 Cu2+ + SO4

2- H2O H+ + OH-

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

Position of ions in the electrochemical series

The ions that are lower in the electrochemical series will be selectively discharge TRY THIS 4 Select the ion to be discharged from the following pairs of ions. State at which electrode it occurs and write the half equation for the discharge of ion a) Hydroxide ion & Chloride Ion : Equation : ______________________________ at the __________ a) Hydroxide ion & Nitrate ion : Equation : ______________________________ at the __________ a) Hydroxide ion & Copper(II) ion : Equation : ______________________________ at the __________ a) Hydroxide ion & Pottasium Ion : Equation : ______________________________ at the __________ a) Hydroxide ion & Silver ion : Equation : ______________________________ at the __________

Position of ions in the electrochemical series

Concentration of ions in the electrolysis

Types of electrodes used in the electrolysis

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TRY THIS 5 Complete the following table for the electrolysis of 0.1 moldm-3 sodium nitrate solution using carbon electrode.

Set up of apparatus

Write the equation of electrolyte ionization

ANODE CATHODE

Write the formulae of ions that are attracted to the anode and cathode.

Name of the product at anode and cathode

Observation

Confirmatory test (method and observation of the test

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TRY THIS 6 Complete the following table for the electrolysis of 0.001 moldm-3 sulphuric acid using carbon electrode.

Set up of apparatus


ANODE CATHODE



Observation


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TRY THIS 7 Complete the following table for the electrolysis of 0.1 moldm-3 copper(II) Sulphate solution using carbon electrode.

Set up of apparatus


ANODE CATHODE



Observation


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Electrolyte concentration factor When electrolysis is carried out using inert electrodes and concentrated solutions, ions that are more concentrated will be discharged but this only true for halides ions which are Cl-, Br-, and I-

TRY THIS 8 State the selected ions to be discharged at the anode and cathode for the following concentrated solution. a) Concentrated hydrochloric acid solution, using carbon electrode Anode : _____________________________ Cathode : _______________________________ b) Concentrated potassium iodide solution, using carbon electrode Anode : _____________________________ Cathode : _______________________________ c) Concentration of sodium bromide solution, using carbon electrode

Anode : _____________________________ Cathode : _______________________________ TRY THIS 9 Complete the following table for the electrolysis of 0.001 moldm-3 hydrochloric acid and 2.0 moldm-3 hydrochloric acid, using carbon electrode.

Set up of apparatus


0.001 moldm-3 2.0 moldm-3

The ions that move to cathode

Equation of discharged of ion at the cathode

Name of the product at cathode

The ions that move to anode

Equation of discharged of ion at the anode

Name of the product at anode

Observation


The electrolyte concentration after a while

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TRY THIS 10 Complete the following table for the electrolysis of 2.0 moldm-3 sodium iodide solution using carbon electrode.

Set up of apparatus


ANODE CATHODE



Observation


Types of electrode There are 2 types of electrode - Inert electrode An electrode that are acts as conductor only and does not undergo any chemical changes. Normally they are made of carbon or platinum. - Reactive electrode – An electrode that not only acts as conductor but undergoes chemical changes. During the electrolysis, the metal anode will dissolve while the reaction at the cathode is similar to the reaction at the carbon electrode. The consist of metal electrodes such as copper, silver and nickel.

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TRY THIS 11 Complete the following table for the electrolysis of 1.0 moldm-3 copper(II) Sulphate solutin, using carbon electrode and copper electrode.

Set up of apparatus


Carbon Electrode Copper electrode

The ions that move to cathode

Equation of discharged of ion at the cathode

Name of the product at cathode

The ions that move to anode

Equation of discharged of ion at the anode

Name of the product at anode

Observation


The electrolyte concentration after a while

6.4 EVALUATING ELECTROLYSIS IN INDUSTRY

Electrolysis is used in industry for a) Extraction of metals b) Purification of metals c) Electroplating of metals

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A) EXTRACTION OF METALS

Carbon electrode acts as anode and cathode

Name the product formed at Anode:__________________ Cathode: ____________________

Write the half equation for reactions at : ANODE :______________________________________________________________________ CATHODE :____________________________________________________________________

B) PURIFICATION OF METALS BEFORE ELECTROLYSIS AFTER ELECTROLYSIS

Ions Present Cu2+, H+, OH- and SO4

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Reaction at Anode Both SO4

2- and OH- gets attracted here but not discharged. Instead, the copper anode discharged by losing electrons to form Cu2+. So, the electrode size decreases. Cu(s) Cu2 + 2e-

Reaction at Cathode Cu2+ produced from anode gains electrons at cathode to become Cu atoms becoming copper. Hence, the copper is deposited here and the electrode grows. Cu2+ + 2e- Cu

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Overall Change There is no change in solution contents as for every loss of Cu2+ ions at cathode is replaced by Cu2+ ions released by dissolving anode. Only the cathode increases size by gaining copper and anode decreases size by losing copper. We can use this method to create pure copper on cathode by using pure copper on cathode and impure copper on anode. Impurities of anode fall under it.

C) ELECTROPLATING OF METALS

Electroplating is coating an object with thin layer of metal by electrolysis. This makes the object protected and more attractive

Object to be plated is made to be cathode and the plating metal is made as anode. The electrolyte MUST contain plating metal cation.

Half equation that occur at ANODE :______________________________ CATHODE :_______________________________

To ensure that the surface of the iron spoon is thinly and evenly plated with silver metal, a low concentration of silver nitrate solution must be used.

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6.5 ANALYSING VOLTAIC CELL

The changes of energy of a voltaic cells CHEMICAL ENERGY ELECTRICAL ENERGY

A simple voltaic cell can be made by using 2 different metals as electrode immersed to an electrolyte.

Example

* Daniel Cell is an example of voltaic cell that used zinc and copper as electrodes and immersed into a solution of its ions. * The two solution are connected through a salt bridge or porous pot

* The function of a salt bridge or porous pot are: - To prevent the two electrolytes from mixing - To allow the flow of the ions so that the electric circuit is completed.

Mg Mg2+ + 2e (negative terminal) 2H+ + 2e H2 (positive terminal) Overall Equation Mg Mg2+ + 2e 2H+ + 2e H2 . Mg + 2H+ Mg2+ + H2

Zinc Electrode (Negative Terminal) Zn Zn2+ + 2e Copper Electrode (Positive Terminal) Cu2+ + 2e Cu Overall Equation Zn + Cu2+ Zn2+ + Cu

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6.6 SYNTHESISING ELECTROCHEMICAL SERIES

In the electrochemical series, metals are arranged according to the tendency of their atoms to donate electrons

The higher the position of a metal in the electrochemical series, the greater the tendency of the metal atoms to donate electron. Thus, the more electropositive the metal.

K Na Ca Mg Al Zn Fe Sn Pb H Cu Ag Au

The electrochemical series can be constructed based on: - The potential difference between two metals - The ability of a metal to displace other metal

The Importance of the Electrochemical Series

The electrochemical series can be used to determine the following * The terminal of a voltaic cell - more electropositive metal (Negative Terminal) - Less electropositive metal (Positive Terminal)

The standard cell voltage * The further the distance between two metals in the electrochemical series, the higher the voltage of the voltaic cell.

The ability of metal to displace another metal from its salt solution * The more electropositive metal can displace a less electropositive metal from its salt solution. * Example 1 : The reaction between Zinc and Copper(II) Sulphate solution Zn Zn2+ + 2e

Cu2+ + 2e Cu OVERALL Zn + Cu2+ Zn2+ EQUATION

* Example 2 : The reaction between silver and Lead(II) Nitrate solution No reaction occur because silver is less electropositive than lead. Thus it cannot displace Lead

More electropositive (Ease of releasing of electron increase)

Least electropositive

studysmart chapter 6

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