10th ICSE Electrolysis Notes PDF

# Electrolysis 1. **Electrolysis:** The process due to which a chemical compound in fused state or in aqueous state conducts direct electric current, resulting in the discharge of ions of an electrolyte so as to form neutral atoms at their respective electrodes is called electrolysis. 2. **Elect...

# Electrolysis 1. **Electrolysis:** The process due to which a chemical compound in fused state or in aqueous state conducts direct electric current, resulting in the discharge of ions of an electrolyte so as to form neutral atoms at their respective electrodes is called electrolysis. 2. **Electrolyte:** A chemical compound in fused state or in aqueous state which conducts direct electric current and at the same time decomposes to form neutral atoms at the electrodes is called an electrolyte. 3. **Electrodes:** The metal rods or plates through which current enters or leaves an electrolyte are called electrodes. 4. **Cathode:** The electrode connected to the negative terminal of a battery is called cathode. 5. **Anode:** The electrode connected to the positive terminal of a battery is called anode. 6. **Cations:** The positively charged ions which discharge at the cathode are called cations. 7. **Anions:** The negatively charged ions which discharge at the anode are called anions. 8. **Electrolytic Cell:** A glass vessel (or any other insulating material), containing two electrodes and an electrolyte is collectively called an electrolytic cell. 9. **Ionisation:** The process of formation of positively charged and negatively charged ions from the molecules of polar covalent compounds is called ionisation. 10. **Electrolytic Dissociation:** The process in which separation of ions of an electrovalent compound takes place on heating or in aqueous solution is called electrolytic dissociation. 11. **Electropositive Series:** Depending upon the ease with which metals lose electrons from their valence shell to form ions, they are arranged in a vertical column called electropositive series. 12. **Electro-refining:** The electrolytic process by which impurities are removed from an impure metal is called electro-refining. 13. **Electroplating:** The process of depositing a thin layer of a superior metal on the clean surface of a base metal, by the process of electrolysis is called electroplating. 14. **Electrometallurgy:** The extraction of metals from their fused ores by the process of electrolysis is called electrometallurgy. ## Electrolysis of Lead Bromide using Graphite Electrodes (i) Dissociation of lead bromide: $PbBr_2 \rightarrow Pb^{2+} + 2Br^-$ (ii) Reaction at cathode: $Pb^{2+} + 2e^- \rightarrow Pb.$ (iii) Reaction at anode: $Br^- \rightarrow Br$ and $Br + Br \rightarrow Br_2$ ## Electrolysis of Acidulated Water using Platinum Electrodes (i) Ionisation of sulphuric acid: $H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}$ (ii) Ionisation of water: $H_2O \rightleftharpoons H^+ + OH^-$ (iii) Reaction at cathode: $H^+ + e^- \rightarrow H$ and $ H + H \rightarrow H_2(g)$ (iv) Reaction at anode: $OH^- \rightarrow OH$ and $4OH \rightarrow 2H_2O + O_2(g)$ ## Electrolysis of Acidified Aqueous Copper Sulphate using Copper Electrodes (i) Dissociation of copper sulphate: $CuSO_4 \rightarrow Cu^{2+} + SO_4^{2-}$ (ii) Ionisation of sulphuric acid: $H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}$ (iii) Ionisation of water: $H_2O \rightleftharpoons H^+ + OH^-$ (iv) Reaction at cathode: $Cu^{2+} + 2e^- \rightarrow Cu$ (v) Reaction at anode: $Cu \rightarrow Cu^{2+} + 2e^-$ ## Electrolysis of Acidified Aqueous Copper Sulphate using Platinum Anode and Copper Cathode (i) Dissociation of copper sulphate: $CuSO_4 \rightarrow Cu^{2+} + SO_4^{2-}$ (ii) Ionisation of sulphuric acid: $H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}$ (iii) Ionisation of water: $H_2O \rightleftharpoons H^+ + OH^-$ (iv) Reaction at cathode: $Cu^{2+} + 2e^- \rightarrow Cu$ (v) Reaction at anode: $OH^- \rightarrow OH$ and $4OH \rightarrow 2H_2O + O_2(g)$ ## Silver Plating using Silver Anode (i) Dissociation of sodium argento cyanide: $NaAg(CN)_2 \rightarrow Na^+ + Ag^+ + 2CN^-$ (ii) Ionisation of HCN: $HCN \rightleftharpoons H^+ + CN^-$ (iii) Ionisation of water: $H_2O \rightleftharpoons H^+ + OH^-$ (iv) Reaction at cathode: $Ag^{2+} + e^- \rightarrow Ag$ (v) Reaction at anode: $Ag \rightarrow Ag^+ + e^-$ ## Nickel Plating using Nickel Anode (i) Dissociation of nickel sulphate: $NiSO_4 \rightarrow Ni^{2+} + SO_4^{2-}$ (ii) Ionisation of sulphuric acid: $H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}$ (iii) Ionisation of water: $H_2O \rightleftharpoons H^+ + OH^-$ (iv) Reaction at cathode: $Ni^{2+} + 2e^- \rightarrow Ni$ (v) Reaction at anode: $Ni \rightarrow Ni^{2+} + 2e^-$ ## Electro-refining of Copper using Pure Copper as Cathode and Impure Copper as Anode (i) Dissociation of copper sulphate: $CuSO_4 \rightarrow Cu^{2+} + SO_4^{2-}$ (ii) Ionisation of sulphuric acid: $H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}$ (iii) Ionisation of water: $H_2O \rightleftharpoons H^+ + OH^-$ (iv) Reaction at cathode: $Cu^{2+} + 2e^- \rightarrow Cu$ (v) Reaction at anode: $Cu \rightarrow Cu^{2+} + 2e^-$ ## State one relevant observation for each of the following statements: (a) At the cathode when acidified aqueous copper sulphate solution is electrolyzed with copper electrodes. (b) At the anode when aqueous copper sulphate solution is electrolysed using copper electrodes. (c) At the anode and at the cathode during the electrolysis of: 1. Fused lead bromide using graphite electrodes. 2. Copper sulphate solution using copper electrodes. (d) At the anode, when molten lead bromide is electrolysed using graphite electrodes. (e) Copper sulphate is electrolysed using copper electrodes. **Answer:** (a) A shining coppery colour (red) is formed on the cathode due to the deposition of copper metal. (b) The anode slowly dissolves, but the colour of copper sulphate does not change. Shining reddish colour of copper appears at anode because of the removal of copper atoms. (c) 1. At cathode silvery droplets of molten lead appear. At anode reddish vapours of bromine are given out. 2. At cathode a fresh layer of reddish copper is deposited At anode fresh layer of copper is exposed on account of dissolving in copper sulphate solution. (d) At the anode, reddish vapours of bromine escape in air from molten lead bromide. (e) When copper sulphate solution is electrolysed using copper electrodes, fresh reddish copper metal is deposited at cathode and fresh surface of copper is exposed at anode. The colour of copper sulphate solution does not change. ## Explain the following: (a) Carbon tetrachloride does not conduct electricity. (b) During electrolysis of lead bromide, graphite anode is preferred to other electrodes. (c) The electrical conductivity of acetic acid is less in comparison to electrical conductivity of dilute sulphuric acid at a given concentration. (d) Sodium chloride will conduct electricity only in fused or aqueous solution state. (e) In the electroplating of an article with silver, the electrolyte sodium argento-cyanide solution is preferred over silver nitrate solution. (f) Although copper is a good conductor of electricity, it is a non-electrolyte. **Answer:** (a) Carbon tetrachloride is a covalent compound and hence does not have free ions which are responsible for the conduction of current through a liquid. (b) Bromide ions discharge at anode to form bromine gas. This gas is highly corrosive and reacts with metallic anode. To avoid corrosion of anode, graphite is preferred as it does not react with bromine and is a good conductor of electricity. (c) The acetic acid molecules dissociate very little (about 4%) compared to sulphuric acid molecules (about 90%). Thus, electrical conductivity of dilute acetic acid is less than dilute sulphuric acid. (d) It is because the ions of NaCl can migrate towards opposite electrodes only, they are free to move, i.e., they are either in fused state or solution form. (e) sodium argentocyanide is preferred over silver nitrate as an electrolyte for electroplating an article with silver because it results in a smoother, more uniform coating. Silver nitrate is a solid-state electrolyte that dissociates very fast, which can result in an uneven deposit. (f) In an electrolyte the conduction of electricity takes place due to migration of cations and anions. As no cations or no anions are present in copper therefore it is not an electrolyte. Moreover, a chemical change takes place in an electrolyte during passage of current, but no such chemical change takes place in copper. ## State the reactions occurring at the anode during the electrolysis of: (i) copper sulphate solution using copper anode. (ii) copper sulphate solution using platinum anode. (iii) dilute sulphuric acid using platinum anode. **Answer:** (i) $Cu \rightarrow Cu^{2+} + 2e^-$. Copper atoms ionise to form $Cu^{2+}$ ion. $OH^-$ or $SO_4^{2-}$ do not discharge. (ii) $ 4OH^- - 4e^- \rightarrow 2H_2O + O_2$. $OH^-$ ions discharge in preference to $SO_4^{2-}$ to liberate oxygen gas. (iii) $ 4OH^- - 4e^- \rightarrow 2H_2O + O_2$. $OH^-$ ions discharge in preference to $SO_4^{2-}$ to liberate oxygen gas. ## Complete the table given below: | Electrolyte | Cathode | Anode | Product at Cathode | Product at Anode | |---|---|---|---|---| | Dil. sulphuric acid | Platinum | Platinum | Hydrogen gas | Oxygen gas | | Cu(II) $SO_4$ solution | Copper | Copper | Copper deposits | Copper dissolves | | Cu(II) $SO_4$ solution | Platinum | Platinum | Copper deposits | Oxygen gas | ## State the reactions occurring at anode during the electrolysis of: (i) copper sulphate solution using copper anode. (ii) copper sulphate solution using platinum anode. (iii) dilute sulphuric acid using platinum anode. **Answer:** (i) $Cu \rightarrow Cu^{2+}$ . Copper atoms ionise to form $Cu^{2+}$ ion. $OH^-$ or $SO_4^{2-}$ do not discharge. (ii) $ 4OH^- - 4e^- \rightarrow 2H_2O + O_2$. $OH^-$ ions discharge in preference to $SO_4^{2-}$ to liberate oxygen gas. (iii) $ 4OH^- - 4e^- \rightarrow 2H_2O + O_2$. $OH^-$ ions discharge in preference to $SO_4^{2-}$ to liberate oxygen gas. ## What products are formed when the following chemicals are electrolysed? | Chemical | Product at Cathode | Product at Anode | |---|---|---| | Dilute sulphuric acid | Hydrogen gas | Oxygen gas | | Molten lead bromide | Lead metal | Bromine | | Aqueous $CuSO_4$ solution with copper electrodes | Copper deposits | Copper dissolves | ## Complete the table, giving: (i) ions present (ii) products at electrodes, during electrolysis. | Substance | Ions present | Product at Cathode | Product at Anode | |---|---|---|---| | Molten lead bromide | $Pb^{2+}$ and $Br^-$ ions| Lead | Bromine | | Copper sulphate solution using copper electrodes | $Cu^{2+}$, $H^+$, $OH^-$, $SO_4^{2-}$ ions| Copper deposits | Copper dissolves | ## Electrolysis is used in the purification of metals. Name: (i) anode (ii) cathode (iii) electrolyte you would use to get pure copper from a piece of impure copper. **Answer:** (i) The anode is made from impure copper. (ii) The cathode is made from pure copper. (iii) Electrolyte is saturated copper sulphate solution, acidified with dil. $H_2SO_4$. ## Following questions are related to electroplating of an article with silver: (i) Name the electrode formed by the article to be electroplated. (ii) What ions must be present in the electrolyte? (iii) What should be nature of anode? **Answer:** (i) Article to be electroplated forms cathode. (ii) Silver ions must be present in the electrolyte. (iii) The anode should be of the same metal which is to be electroplated. ## Answer the following questions about electroplating a copper wire with silver. (i) What ions must be present in the electrolyte? (ii) Of what substance must the anode be made? (iii) What should be made cathode? **Answer:** (i) Silver ions must be present in electrolyte. (ii) The anode should be made from silver. (iii) The article to be electroplated should be made the cathode. ## Three different electrolytic cells A, B and C are connected in separate circuits. Electrolytic cell A contains sodium chloride solution. When the circuit is completed a bulb in the circuit glows brightly. Electrolytic cell B contains acetic acid solution and in this case the bulb in the circuit glows dimly. The electrolytic cell C contains sugar solution and the bulb does not glow. Give a reason for each of these observations. **Answer:** Electrolytic cell A has completely ionised sodium chloride solution. Thus, the ions can easily migrate to oppositely charged poles and hence the bulb glows brightly. To conclude sodium chloride solution is a strong electrolyte. Electrolytic cell B has a weak electrolyte as only 5% of the acetic acid molecules ionise. Thus, a weak current flows through it and hence the bulb glows dimly. Electrolytic cell C has a non-electrolytic. Sugar molecules do not ionise and hence no current flows through it. Thus, the bulb does not glow. ## Here is an electrode reaction: $Cu^{2+} + 2e^- \rightarrow Cu$ (i) At which electrode (anode or cathode) would such a reaction take place? Is this an example of oxidation or reduction? (ii) A solution contains magnesium ions ($Mg^{2+}$), iron (II) ions ($Fe^{2+}$) and copper ions ($Cu^{2+}$). On passing an electric current through this solution which ions will be the first to be discharged at the cathode? Write the equation for the cathode reaction. (iii) Why is carbon tetrachloride, which is a liquid, a non-electrolyte? **Answer:** (i) This reaction will take place at the anode of the electrolytic cell. This reaction is an example of oxidation of copper atom to copper ion. (ii) The copper ions will discharge first: $Cu^{2+} + 2e^- \rightarrow Cu$ (iii) Carbon tetrachloride is a covalent compound and has no free cations and anions which are responsible for the conduction of electric current.