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Daniel Png. Chua Zong Han. Rodella Chowdhury. Explain the colour change. The diagram below shows the electrolysis of an aqueous solution containing an electrolyte P using graphite as electrodes. It is known that P contains nitrate ions. Why must we use d. During the electrolysis , a gas Q collected above electrode R. At electrode S, no gas was given off but a pink deposit formed over the electrode S.
Describe two other changes that you would observe after sometime. Explain your answers. Notes On Electrolysis December Electrolysis Notes December The Electrolysis 2 weeks ago 0. Electrolysis Ppt December Discharge of anions is viz. Procedure Anelectric current is passed through the dilute sulphuric acid. Observation At the Anode A colourless gas; collects The gas collected relights a glowing splint; and its volume is half the volume of the gas at cathode.
The gas is oxygen. At the cathode A colourless gas collects The collected gas burns with a pop-sound; and its volume is double the volume of gas at the anode. The gas is Hydrogen gas. Explanations Ions present in the electrolyte Hydrogen ions and Sulphate ions form sulphuric acid. Hydrogen ions and hydroxide ions from water. At the Anode Positive electrode The negatively charged Sulphate ions and hydroxide ions migrate to the anode.
Reason:The 4 electrons lost by the hydroxide ions to form 1 mole 1 volume of oxygen molecules are gained by the four hydrogen ions which form 2 molecules 2 volumes of hydrogen molecules. During the electrolysis, the concentration of the electrolyte H 2 SO 4 aq , increases Reasons:The volumes of hydrogen and oxygen gas liberated are in the same ratio as they are combined in water.
Thus the amount of water in the electrolyte progressively decrease; hence the increased electrolyte concentration. Conclusion Electrolysis of dilute sulphuric acid is thus the electrolysis of water. Note: The Hoffmans voltmeter can be used instead of the circuit above. Electrolysis of dilute sodium chloride Apparatus Procedure An electric current is passed through dilute sodium chloride solution; with carbon rods as the electrodes. Gases evolved of each electrode are collected and tested.
Observations At the anode: A colourless gas is collected The gas relights a glowing splint, and its volume is half the volume of the gas collected at the cathode. The gas is Oxygen, O 2 At the cathode A colourless gas collects The gas burns with a pop sound; and its volume is twice the volume of the gas collected at the anode.
At the Anode Cl - and OH - migrate to the anode OH - are preferentially discharged coz they have greater tendency to lose electrons than the chloride ions; - the OH - aq lose electrons to form water and O 2 g at anode.
Electrolysis f dilute NaCl is thus the electrolyze of water since only water is decomposed. The gas has a pungent irritating smell; and its volume is equal to the volume of the gas evolved at he cathode.
The gas is chlorine Cl 2 g At the Cathode A colourless gas is liberated The gas burns with a pop sound; and its volume is equal to volume of gas evolved at the anode.
Reason ; -OH - aq have higher tendency to lose electrons than Cl - ions. However coz of the higher concentration Cl - aq , relative to OH - aq , the Cl - aq , are preferentially discharged hence the evolution of Chlorine gas.
As soon as the Cl - aq concentration becomes equal to that of OH - aq The Mercury Cathode Cell Is an electrolytic arrangement commonly used for the large scale manufacture of chlorine and sodium hydroxide. Apparatus Electrolyte in the mercury cell is Brine concentrated NaCl Anode is carbon or titanium Cathode is a moving mercury film.
Reactions At the Anode Both Chloride and Hydroxide ions are attracted Due to their high concentrations the chloride ions are preferentially discharged. The Cl - aq lose electrons to form Chlorine gas. The resultant NaOH is of very high party. Its expensive due to the high cost of mercury. Pollution form Mercury; i. Mercury is poisonous and must be removed from the effluent. Electrolysis of Copper II Sulphate solution.
At the cathode A reddish brown coating of Cu solid is deposited. The hydroxide ions have a higher tendency to lose electrons than the SO 4 2- aq They OH - easily loose electrons to form the neutral and unstable hydroxide radical OH - The hydroxide radical OH decomposes to form water and Oxygen.
Cathode equation. Cations: The higher the cation in the electrochemical series; the lower the tendency of discharge at the cathode. Reason:Most electropositive cations require more energy in order to be reduced and therefore are more difficult to reduce. Anions: Discharge is through oxidation and is as follows. Concentration of electrolytes A cation or anion whose concentration is higher is preferentially discharge if the ions are close in the electrochemical series.
Example : dilute and concentrated NaCl Product at the anode. The electrodes used:Products obtained at electrodes depend on the types of electrodes used Examples:in the electrolysis of CuSO 4 aq using carbon and copper rods separately.
Example :Sodium is extracted from molten sodium chloride using carbon electrodes. Purification of metals It can be used in refining impure metals Examples: Refining copper The impure copper is made of the anode.
Their strips of copper are used as the cathode Copper II Sulphate are used as the electrolyte. During the electrolysis the anode dissolves and pure copper is deposited on the cathode. The impurities including valuable amounts of silver and gold from the crude copper collect as a sludge become the anode. Electroplating Is the process of coating one metal with another, using electrolysis so as to reduce corrosion or to improve its appearance.
During electrolysis: the item to be electroplated is made the cathode the metal to be used in electroplating is used as the anode the electrolyte is made from a solution containing the ions of the metal to be sued in electroplating. Electroplated Nickel Silver.
Importance: Prevention arrosion of Aluminum articles Manufacture of sodium hydroxide, chlorine and hydrogen Sodium hydroxide is prepared by the electrolysis of brine, for which 3 methods are available The method depends on the type of electrolytic cell.
These cells are The mercury cell The diaphragm cell The membrane cell The mercury cell Components The electrolyte is concentrated sodium chloride The anodes are made of graphite or titanium, which are placed above the cathode. The cathode consists of mercury, which flows along the bottom of the cell. Chemical reactions Anode Both chloride and hydroxide ions are attracted.
Chloride ions are preferentially discharged due to their high concentration The chloride ions undergo oxidation to form green yellow chlorine gas. They undergo reduction to form sodium solid. The amalgam reacts with water forming hydrogen and sodium hydroxide. Main product: Sodium hydroxide By products Sodium and chlorine.
Advantages of mercury cathode cell The resultant sodium hydroxide is very pure; as it has no contamination from sodium chloride. It is highly concentrated; i. Disadvantages Some of the mercury said its way into the environment leading to mercury pollution; a common case of brain damage in humans. At the operating temperatures 70 o C - 80 o C , mercury vapours escape into the atmosphere and cause irritation and destruction of lungs tissues.
Its operation requires highly skilled man power. The cathode compartment contains a stainless steel cathode. Diagram: The diaphragm cell Chemical reactions The asbestos diaphragm is permeable only to ions, but not to the hydrogen or chlorine molecules. At the anode Chloride ions undergo oxidation to form chlorine gas.
The membrane cell Is divided into 2 compartments by a membrane Most commonly used type of Membrane is the cation exchange membrane. This membrane type allows only cations to pass through it. Components A cation exchange that divides the cell into 2 compartments; an anode and a cathode compartments.
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