Electrochemistry Chemistry - 5070 Electrochemistry in This

8/14/2019 Electrochemistry Chemistry - 5070 Electrochemistry in This

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Electrical Devices and Circuit Symbols Cell is source of electricity. Battery is a collectionof cells. Switch is used to stop the flow of current. Bulb is used to register

whether current is flowing. Voltmeter is used to measure current. Ammeter is used to measure current. Resistor ensures that a suitable amount of current is flowing. Variable resistor is resistor those rating can be changed. Electrodes are plates which carry electricity into the liquid. Cathode is the electrode connecte

d to the negative terminal of the battery. Anode is the electrode connected to the positive terminal of the battery. Cation are positively charged ion that travels to the cathode during the electrolysis. Anion are negatively charged ion that travels to the anode during the electrolysis.

Electrical Device Cell

Circuit Symbol

Battery Bulb Switch Ammeter Voltmeter

Resistor

Variable resistor (rheostat)


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Conductors and Non-ConductorsConductors A conductor is a substance which conducts electricity but is not chemically changed during the conduction. Conductors have free moving valence electrons which conducts electricity. All metals and graphite are conductors

Non-conductors

A non-conductor is a substance which does not allow the passage of electricity. Non-conductors do not have free valance electrons and thus do not conductelectricity. All non-metals except graphite are non-conductors.

Electrolytes, Weak-electrolyte and Non-electrolyteElectrolytes Electrolytes are compounds which when molten or dissolved in water conductelectricity. All acids, alkalis and salts are electrolytes. They conduct electr

icity by the movement of ions between electrodes.

Weak-electrolyte Weak acids and weak alkalis are classified as weak-electrolytes. They contai

n only few ions.

Non-Electrolyte

Non-electrolyte is a liquid which does not allow the passage of electricity.Distilled water, alcohol, turpentine, oil, paraffin and other organic solvents

are examples of nonelectrolytes. Weak electrolytes Limewater Ammonia solution Aqueous ethanoic acid Aqueous sulphurous acid Aqueous carbonic acid Non-electrolytes Ethanol Tetra chloromethane Tri chloromethane Pure water Sugar solution Molten sulphur

Strong electrolytes Aqueous sulphuric acid Aqueous nitric acid Aqueous hydrochloric acid Aqueous potassium hydroxide Aqueous sodium hydroxide Copper(II) sulphate solution

Electrolysis

The process of decomposing a compound by passage of an electric current is called electrolysis.


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When electricity is passed through electrolyte, anions start moving towards anode and cations towards cathode. Anions are usually non-metal ion for example Cl-,Br- and O2-. Cations are usually metal ion for example Ag+, Cu+2 and Pb+2. Whenanion reach anode they lose their electron to anode, which is ready to gain ele

ctron due to positive charge. When cation reach cathode they gain electron from

cathode, which is ready to donate electron due to negative charge.

Factors Affecting ElectrolysisPosition of ion in reactivity series More reactive ion undergoes discharge.

Cations K+ Na+ Ca2+ Mg2+ Zn2+ Fe2+ Pb2+ H+ Cu2+ Ag+

Anions

SO42NO3ClBrIOH-

Difficulty of discharge decreases

Concentration

If the concentration of particular ion is high, then this can alter the preferential discharge. Concentrated ion will be discharged.

Type of electrode

Type of electrode used can affect the electrolysis. Carbon electrodes are inert electrodes and do not effect electrolysis.


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Electrolysis of concentrated Sodium Chloride solutionIons present in the solution Anions present in solution are OH- (from water) and Cl- (from salt). Cationspresent in solution are H+ (from water) and Na+ (from salt).

ReactionAnode Chloride and hydroxide ions migrate towards anode. Chloride ions are d

ischarged as the concentration of chloride ion is higher. 2 chloride ions lose2 electrons to form chlorine gas. 2 2 + 2 Cathode Sodium and

hydrogen ion travel towards cathode. Hydrogen ion is discharged as sodium is too high up in the reactivity series. 2 hydrogen ions gains 2 electrons to formhydrogen gas. 2 + + 2 2

Electrolysis of dilute sulphuric acidIons present in the solution Anions present in solution are OH (from water) and SO4 2 (from acid). Cation present in solution is H+ from both water and acid.

Reaction

Anode

Hydroxide and sulphate ion migrate towards anode.

Hydroxide ion is preferentially discharged as it is at the bottom of the reactivity series. Hydroxide ion loses 1 electron to form oxygengas which bubbles out and water which stays in the solution. 4 22 + 2 + 4 Cathode Hydrogen ion is discharged as it is only cation present.


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Electrical Cell A simple cell is a device that converts chemical energy into electrical energy. Each simple cell consists of: 2 electrodes of 2 different metals An electrolyte solution containing an acid or an aqueous salt As more reactive metal tends toundergo oxidation more easily, and naturally becomes the negative terminal by l

osing electrons, which then travel via the external circuit to other metal electrode. The less reactive metal becomes the positive terminal, as it has a weaker

tendency to lose electrons. The voltage of the cell depends on the position of the metal in the reactivity series. The far two metals are in the reactivity series more is the voltage.

ReactivityMagnesium Aluminum Zinc Iron Tin Lead Copper Silver

Reactivity decreases


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Dry cell In dry cell electrolyte is a paste. Carbon electrode is positive terminal. Zinc electrode is negative terminal. Ammonium chloride is used as electrolyte.

Credits: Muhammad Hassan Nadeem [email protected]


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Electrochemistry Chemistry - 5070 Electrochemistry in This