Electrolysis| Complete Study Topic

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in water it's ionic bonds break. This allows the ions to move. An ionic substance with freely moving ions is called an electrode and can conduct electricity. Introduction notes to Electrolysis Electrolysis uses energy transferred by electricity to decompose electrolytes. Two electrodes are connected to a direct current (d.c.) electricity supply and placed into the electrolyte. The two types of ions carry opposite charges and so migrate (move) towards the electrode with the opposite charge. https://www.pearsonactivelearn.com/app/library/ebook?id=NTU2MjEyfGJvb2t80Td8MHww (textbook pages 84-85) • Cations are positive ions and are attracted to the negative cathode. • Anions are negative ions and are attracted to the positive anode. This can be shown by placing a purple potassium manganate(VII) crystal on a piece of damp filter paper attached to a microscope slide and connected to a d.c. electricity supply. The purple colour spreads towards the anode. Products from Electrolysis The electrolysis of molten or dissolved ionic salts is carried out by using inert (unreactive) electrodes (usually graphite or platinum). When a molten salt is electrolysed, ions are discharged as atoms or molecules at the electrodes. When molten lead bromide is electrolysed, bromine is produced at the anode and lead is produced i at the cathode. You can predict the electrolysis products of any molten salt. The salt will always decompose into its elements. The metal is produced at the cathode and the non metal is produced at the anode. bromine -graphite electrodes -molten lead bromide molten lead heat A Orange bromine gas is seen around the anode and a pool of grey liquid lead is seen under the cathode. This experiment must be carried out in a fume cupboard as bromine gas is toxic. cathode, the negative electrode H+ OH The positively charged He ions are attracted to H+ anode, the positive electrode So electrolyte Products of Electrolysis OIL RIG Oxidation Is Loss of electrons Reduction Is Gain of electrons The reactivity series shows metals in order of reactivity. The reactivity of a metal is related to its tendency to form positive ions. Electrolysis can be used to split ionic compounds and to extract metals. At the negative electrode- if the metal is more reactive than hydrogen then hydrogen is given off. If hydrogen is more reactive than the metal is given off (eg. Copper). At the positive electrode- if a halogen is present then that is given off (chlorine, bromine and iodine). lons cathode anode Overall Electrolysis of salt solutions Cu² (aq) and Cl(aq) (from salt), H(aq) and OH (aq) (from water) Cu²+ (aq) and H(aq) ions are attracted. Cu²+ (aq) + 2e →→ Cu(s) Copper ions are discharged more (reduction) readily than hydrogen ions, so copper is formed as a brown solid. Cl(aq) and OH (aq) ions are attracted. Chloride ions are discharged more readily than hydroxide ions, so chlorine is formed as a pale green gas. 2Cl(aq) → Cl₂(g) + 2e (oxidation) The copper chloride decomposes but the water does not change. CuCl₂(aq) → Cu(s) + Cl₂(g) Water ionises to every small extent, so in an aqueous solutions of a salt, there some hydrogen ions (H+) and hydroxide ions (OH-), as well as the ions of the dissolved solid. The table on this slide shows the electrolysis of 2 salt solutions, copper chloride solution and sodium chloride solution. In the electrolysis of sodium sulfate solution, sodium ions (Na) and hydrogen ions (H) collect at the cathode, where hydrogen gas forms. At the anode, hydroxide ions (OH) are discharged more readily than sulfate ions (SO2), so oxygen gas is formed (along with water). Overall, water decomposes to form hydrogen and oxygen. The Na and SO² ions stay in solution. Water decomposes to form hydrogen and oxygen during the electrolysis of water acidified with dilute sulfuric acid (which contains H*(aq) and SO² (aq)). lons cathode anode Overall Na (aq) and Cl-(aq) (from salt), H(aq) and OH (aq) (from water) Na (aq) and H'(aq) ions are attracted. H2H*(aq) + 2e → H₂(g) Hydrogen ions are discharged (reduction) more readily than sodium ions, so hydrogen gas is formed. Cl(aq) and OH (aq) ions are attracted. Chloride ions are discharged more readily than hydroxide ions, so chlorine is formed as a pale green gas. 2Cl(aq) → Cl₂(g) + 2e (oxidation) The sodium chloride decomposes to form hydrogen and chlorine. The sodium and hydroxide ions remain in the solution. 2NaCl(aq) + 2H₂O(1)→ H₂(g) + Cl₂(g) + 2NaOH(aq) C sodium chloride solution electrolysis What happens at the electrodes? Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced. Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised. flow of Electrode electrons (-ve) H* NaCl Solution flow of electrons CI CI Electrode |(+ve) +ve long are attracted to the -ve electrode. Here they galo electrons (reduction). Lead is produced at the ve electrods. -ve long are attracted to the +ve electrode. Here they lose electrons (oxidation). Bromine is produced at the ve electrode. Ionic half equations We can show what happens at the electrodes using half equations. For example= At the negative electrode (reduction) = 2H+ +2e--> H2 At the positive electrode (oxidation)= 2C1--> Cl2 +2e- 1. Atoms 2. Change Ionic half equations must balance for charge as well as atoms. Reactivity The reactivity series of metals is a chart showing metals in order of decreasing reactivity. In general, the more reactive a metal is: the more vigorous its reactions are the more easily it loses electrons in reactions to form positive ions (cations) More reactive element will remove a less reactive element from a compound. The reactivity series is a list of metals in order of reactivity, with the most reactive at the top. Metal potassium sodium calcium magnesium aluminium zinc iron Reaction with water react with cold water to form hydrogen and a metal hydroxide react very slowly, if at all, with cold water but react with steam to form hydrogen and a metal oxide Reaction with dilute acid copper silver gold B the reactivity series for some metals react violently react to form hydrogen and a salt solution do not react with cold do not react water or steam Tendency of metal atoms to form cations increasing ability of metal atoms to form positive ions Ores Ore: A rock that contains an economically viable amount of metal Displacement reactions We can use the reactivity series to predict whether reactions will take place. Each metal will react with compounds of the metals below it in the series. When zinc is dipped into copper sulfate solution, a copper coating forms on the surface of the zinc. Some of the zinc takes the place of the copper and forms zinc sulfate solution. zinc copper Zn(s) + CuSO (aq) → Cu(s) + ZnSO (aq) Blue copper sultans changes to colourless This is a displacement reaction. The zinc has displaced the copper. Displacement reactions only work one way. Copper cannot displace zinc from zinc surface solution as its compounds because copper is less reactive. zinc displaces copper. SC11b Ores Specification reference: C4.4; C4.7; H C4.8 Progression questions • Which metals are found uncombined in the Earth's crust? • How is the method of extraction of a metal related to its position in the reactivity series? How are biological methods used to extract some metals? A Gold occurs uncombined in the Earth's crust. 1 Write word equations for the two reactions needed to produce copper. 2 State why copper can increasing reactivity be produced by heating copper oxide with carbon. Metal potassium sodium calcium magnesium aluminium (carbon) zinc iron copper silver gold B reactivity of some metals Very unreactive metals, such as gold and platinum, are found naturally in their native state (as uncombined elements). More reactive metals have reacted with other elements to form compounds in rocks. The process of obtaining a metal from these compounds is extraction. An ore is a rock that contains enough of a compound to extract a metal for profit. Haematite is an ore containing iron oxide. Iron is extracted by heating the iron oxide with carbon. Carbon is more reactive than iron so it displaces it. iron oxide + carbon →→iron + carbon dioxide Ⓡ This method is used for compounds of metals below carbon in the reactivity series shown in table B. Malachite is an ore containing copper carbonate. Malachite is heated to convert it to copper oxide, which is then heated with carbon to produce copper. Metals higher than carbon must be extracted using a more powerful method called electrolysis. This involves passing electricity through a molten ionic compound to decompose it into its elements. Aluminium is produced by electrolysis of aluminium oxide, found in an ore called bauxite: aluminium oxide → aluminium + oxygen A lot of energy is needed to keep metal oxides molten for electrolysis, making it extremely expensive. Electrolysis is only used to extract very reactive metals that cannot be obtained by heating their oxides with carbon. Method of extraction electrolysis of a molten compound t an ore with carbon found as the uncombined element 8.8 3 Name two metals that can only be extracted from their ores by electrolysis. 4 Zinc can be extracted from zinc oxide, ZnO, by heating with carbon. a Write the balanced equation for this reaction. b Explain why electrolysis is not used in the large-scale extraction of zinc. What is an Ore? Gold Ore Iron Ore Copper Ore A rock with gold in it A rock with iron in it A rock with copper in it A rock that contains enough metal or metal compounds so that it can be mined profitably Bioleaching The Earth's supply of metal ores is limited. For example, high-grade copper ores, which contain a high percentage of copper, are becoming harder to find and mine. There are some alternative methods to extract metals from low-grade copper ores that use living organisms. These have advantages and disadvantages compared to the usual extraction methods. Process both bioleaching and phytoextraction bioleaching Advantages no harmful gases (e.g. sulfur dioxide) are produced causes less damage to the landscape than mining conserves supplies of higher grade ores Phytoextraction Phytoextraction involves growing plants that absorb metal compounds The plants are burnt to form ash, from which the metal is extracted. does not require high temperatures phytoextraction can extract metals from contaminated soils Bioleaching uses bacteria grown on a low grade ore. The bacteria produce a solution containing copper ions, called a leachate. Copper is extracted from the leachate by displacement using scrap iron, then purified by electrolysis. This method can also be used for metals such as nickel, cobalt and zinc. LI Disadvantages very slow toxic substances and sulfuric acid can be produced by the process, and damage the environment more expensive than mining some ores growing plants is dependent on weather conditions Steps of Phytoextraction: Plants absorb mineral ions through their roots. Phytoextraction makes use of this: ● ● plants are grown in soil that contains low grade ore the plants absorb metal ions through their roots and concentrate these ions in their cells the plants are harvested and burnt the ash left behind contains metal compounds Phytoextraction is slow but it: reduces the need to obtain new ore by mining conserves limited supplies of high-grade ores reduces the amount of rock waste that must be disposed of after traditional mining Oxidation & Reduction Many metals are extracted from metal oxide ores. In order to obtain the metal from its oxide, the oxygen must be removed. When oxygen is removed from a compound, the compound is reduced. Oxidation is the gain of oxygen by a substance. Reduction is the loss of oxygen from a substance. These examples show how to explain oxidation and reduction. Often you can explain it in terms of change in oxygen content or hydrogen content but sometimes an explanation in terms of electrons is required. Oxidation 1. gain of oxygen 2. loss of hydrogen 3. loss of electrons 4. increase in oxidation number Reduction 1. Loss of oxygen 2. Gain of hydrogen 3. Gain of electrons 4. Decrease in oxidation number https://www.bbc.co.uk/bitesize/guides/z7rswt y/revision/1 Redox Reactions Oxidation is loss of electrons, gain of oxygen or loss of hydrogen. Reduction is gain of electrons, loss of oxygen or gain or hydrogen. Rusting is an example of oxidation. Magnesium is oxidised when it reacts with oxygen to form magnesium oxide. Explain why this is an oxidation reaction. magnesium + oxygen → magnesium oxide - 2Mg + 02 Explanation: Magnesium gains oxygen and gain of oxygen is oxidation. → 2MgO Magnesium reacts with copper(II) sulfate solution according to the equation: Mg(s) + CuSO4(aq) → MgSO4(aq) → Cu(s) Explain, in terms of electrons, why this reaction is a redox reaction. The sulfate ion, SO42-, is the spectator ion in this reaction so the ionic equation is: Mg(s) + Cu2+(aq) → Mg2+(aq) + Cu(s) END OF LIFE RESOURCES LIFE CYCLE ASSESSMENT DISTRIBUTION MANUFACTURING PROCESSING Life Cycle Assessment A life cycle assessment (LCA) can be carried out to work out the environmental impact of a product. An example is shown in diagram C. The LCA also helps people to decide whether it is worthwhile to manufacture and recycle a product. LCAS can be used to compare the effect of using different materials for the same product, for example making a bottle from glass or plastic. Life Cycle Assessment Every UK household is estimated to produce 1 tonne of waste each year. About 45% of this waste is recycled but there is an EU target for recycling to reach 70% by 2030. An estimated 2 million tonnes of waste electrical and electronic (WEEE), such as phones, TV's and toasters, are thrown away in UK each year. These items contain a lot of precious metals worth around £1 billion, including about £36,000,000 of aluminium. A Artists have recycled the waste washed up on the coast of Kenya into sculptures. Recycling metals Many metals can be recycled by melting them down and making them into something new. Some of the main advantages of recycling are as follows: • Natural reserves of metal ores will last longer. • The need to mine ores is reduced. Mining can damage the landscape as well as create noise and dust pollution. • Less pollution may be produced. For example, sulfur dioxide is formed when some metals are extracted from metal sulfide ores. • Many metals need less energy to recycle them than to extract new metal from the ore. • Less waste metal ends up in landfill sites. However, there are some disadvantages of recycling - including the costs and the energy used in collecting, transporting and sorting metals to be recycled. Sometimes it can be more expensive, and require more energy, to recycle than to extract new metal. Dynamic Equilibrium: A reversible reaction where products can, under appropriate conditions, turn back reactions. In some chemical reactions the products react to reform the reactants. These are reversible reactions. In the equations for the reversible reactions (such as the one shown in the picture), a double arrow is show that both forward and backwards reactions occur at the same time. forward reaction Dynamic Equilibrium ammonium chloride NH Cl(s) ammonia + hydrogen NH₂(g) + HCl(g) backward reaction Graph B shows how the percentages of the reactants and products change during the reaction. At a certain point, the forward and backward reactions are still occurring but the percentages of the reactants and products are no longer changing. This is called a dynamic equilibrium because the reactions are still occurring (dynamic) but the substances remain in balance (equilibrium). | I Dynamic Equilibrium In reversible reactions equilibrium means balance. but this balance does not have to be at the halfway point. We may have mostly reactants with just a little product or vice versa. There are two factors that we can change that influences the position of the equilibrium: Temperature, concentration. Finding the conditions that gives the most products is really important in the industrial chemicals reactions. Reaction Rate (mol./s) Forward Reaction Reverse reaction Dynamic Equilibrium Equilibrium reached Concentration (mol.) Reactants Products Constant concentrations at equilibrium Time (s) Time (s) Dynamic equilibrium - this is the equilibrium when the rate of the forward reaction is equal to the rate of the reverse reaction and the amount of reactants and products remain constant. Temperature All reactions are exothermic (give out heat) in one direction and endothermic (take in heat) in the other. Eg. Nitrogen dioxide (NO2) joins to form dintrogen tetroxide (N204) exothermically. Gets cold going backwards (endothermic) Gets hot going forwards (exothermic) The rule is: The hotter a reaction is, the more likely it is to go in the endothermic direction Heating more NO2 in the equilibrium mixture. Cooling would give N204 in the equilibrium mixture. Pressure This applies to the gas reactions. Here the rule depends upon the number gas molecules on each side of the equations. Gets more gas molecules on the side going backwards reaction. Gets less gas molecules in the forward direction. Helpful links https://www.pearsonactivelearn.com/app/library/ebook?id=NTU2MjEyfG Jvb2t80TV8MHww https://www.bbc.co.uk/bitesize/guides/zpxn82p/revision/1 https://www.youtube.com/watch?v=AhTRIL6xjBA https://www.savemyexams.co.uk/gcse/chemistry/aqa/18/revision-notes/ 4-chemical-changes/4-3-electrolysis/4-3-1-the-process-of-electrolysis/ https://www.savemyexams.co.uk/gcse/chemistry/edexcel/18/revision-no tes/3-chemical-changes/3-2-electrolytic-processes/3-2-4-electrolysis--redo x/ https://www.britannica.com/science/electrolysis