* KS4 Science GCSE/IGCSE Chemistry ELECTROLYSIS, Industrial Applications, Cells and Batteries Notes
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Doc Brown's Chemistry Extra Electrochemistry Information Notes on Electrolysis, batteries and fuel cells Revision KS4 Science GCSE/IGCSE/O level Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science (revise courses equal to US grades 9-10) Index for this page 1. Introduction to electrolysis * 2. Summary of practical experimental arrangements for laboratory (lab) electrolysis experiments and electrode reactions * 3. Summary of Industrial Electrolysis Process links * 4. Simple cells or batteries * 5. Fuel cells Associated LINKS to other pages on this site: full LIST of GCSE-KS4 Chemistry, Earth Science and Radioactivity REVISION NOTES * Metal Extraction * Industrial chemistry * Metal reactivity (redox introduction) * Types of chemical reaction * Electrolysis calculations * EMAIL query?comment |
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1. Introduction to electrolysis - electrolytes and non-electrolytes Electrolysis is the process of electrically inducing chemical changes in a conducting melt or solution e.g. splitting an ionic compound into the metal and non-metal.
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A simple experiment to show the movement of coloured ions
A rectangle of filter paper is soaked in an ammonia-ammonium chloride solution and mounted on a microscope slide. The paper is connected to a d.c. supply with clips. A 'line' of copper chromate solution is placed in the middle of the paper and the current switched on. The copper chromate is green-brown in solution but gradually it disappears and separates, in different directions, into a yellow and blue bands. The yellow band is due to negative chromate ions, CrO42--, moving towards the positive electrode. The blue band is due to positive copper ions, Cu2+, moving towards the negative electrode. All due to opposite charges attracting in the electric field produced by the potential difference (the voltage!). |
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2a. Summary of arrangements for laboratory electrolysis experiments Gas tests and this section followed by examples of electrode reactions to which the equation numbers refer. |
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Aqueous solutions with inert electrodes (carbon or platinum) Graphite electrodes 'upwardly' dipped into an solution of the electrolyte. The cell can be made from plastic pipe and a big rubber bung with two holes in it. A more elaborate format is to use a Hoffman Voltammeter (see below) using platinum electrodes and accurately calibrated collecting tubes like burettes. |
Molten salts with carbon electrodes Carbon (graphite) electrodes dipped into molten salt which has been strongly heated in a crucible. It is difficult to collect the gases at the electrodes! The salts may be very high melting, so sometimes a small amount of another salt impurity is added to lower the melting point. |
Metal electrodes dipped in aqueous salt solutions M = metal, usually dipped in the metal sulphate solution. For electroplating in general: The (-) is made the metal/conducting surface to be coated, and the (+) is made of the plating metal which dissolves and replaces any deposit formed on the (-) electrode (for more details). |
| Brine - concentrated sodium chloride solution (brine) with carbon (graphite) gives equal volumes of hydrogen gas (-) and green chlorine gas (+) with sodium hydroxide left in solution, via equations 2 and 3. However in very dilute solution, reaction 8 occurs too giving oxygen gas as well as chlorine gas. | Molten lead(II) bromide gives silver beads of lead (-) and brown fumes of bromine (+), illustrated above, see equations 2 and 10. | Copper(II) sulphate with copper electrodes, the copper deposits (-) and the copper dissolves (+), equations 4 and 5. The blue colour of the Cu2+ ions stays constant because Cu deposited = Cu dissolved. Both involve a 2 electron transfer so it means mass of Cu deposited (eq 4) = mass of Cu dissolving (eq 5). Compare with carbon (graphite) electrodes. |
| Copper(II) sulphate with carbon (graphite) electrodes, the copper deposits (-) and oxygen gas (+), equations 4 and 8. | Molten sodium chloride gives silvery sodium (+) and pale green chlorine gas (-), see equations 2 and 11. See also extraction of sodium in introduction. Compare with brine solution. | |
| Copper(II) chloride with carbon (graphite) electrodes, the copper deposits (-) and chlorine gas (+), equations 2 and 4. Compare with copper electrodes. | Molten anhydrous zinc chloride gives zinc (+) and chlorine (-), equations 1 and 2. | |
Method
1b
Dilute sulphuric acid (= acidified water) gives hydrogen and oxygen gases (2 : 1 volume ratio) via equations 3 and 8. This is one way of showing water is a compound i.e. by splitting into two gaseous elements. |
Molten anhydrous calcium chloride gives calcium (+) and chlorine (-), equations 13 and 2. |
Chemical Tests for some of the Gases formed by these electrolysis experiments.
For more details and tests see the qualitative chemical analysis page |
| Concentrated hydrochloric acid gives equal volumes of hydrogen (-) and chlorine (+) via equations 2 and 3. However in very dilute solution, equation 8 occurs too, giving oxygen as well as chlorine. | ||
| For most sulphate salts of reactive metals e.g. sodium/magnesium sulphate the electrolysis products of the aqueous salt solution are hydrogen at the negative (-) cathode electrode (Eq 3) and oxygen at the positive (+) anode (Eq 8) with inert electrodes such as carbon or platinum. | ||
| 2b. Summary of ELECTRODE REACTIONS - half-cell electrode equations | ||
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Eq. no. |
(-) negative cathode electrode where reduction of the attracted positive cations is by electron gain to form metal atoms or hydrogen [from Mn+ or H+, n = numerical positive charge]. The electrons come from the positive anode (see below). (+) positive anode electrode where the oxidation of the atom or anion is by electron loss. Non-metallic negative anions are attracted and may be oxidised to the free element. Metal atoms of a metal electrode can also be oxidised to form positive metal ions which pass into the liquid electrolyte. The released electrons move round in the external part of the circuit to produce the negative charge on the cathode electrode. |
The electrode equations are shown on the left with examples of industrial processes where this electrode reaction happens below on the right. Unless otherwise stated, the electrodes are inert i.e. they do not chemically change e.g. platinum or carbon-graphite. PLEASE NOTE - all electrode equations are a summary-simplification of what happens on an electrode surface in electrolysis. There may be e.g. two equations which are totally equivalent to each other to describe WHAT IS ACTUALLY FORMED e.g. the formation of hydrogen or oxygen and in some cases other products may be formed too. |
| 1 |
(-) Na+(l) + e- ==> Na(l) (sodium metal) |
sodium ion reduced to sodium metal atoms: typical of electrolysis of molten chloride salts to make chlorine and the metal |
| 2 |
(+) 2Cl-(l/aq) - 2e- ==> Cl2(g) |
chloride ion oxidised to chlorine gas molecules: electrolysis of molten chloride salts(l) or their concentrated aqueous solution(aq) or conc. hydrochloric acid(aq) to make chlorine |
| 3 |
(-) 2H+(aq) + 2e- ==> H2(g) (hydrogen gas) or 2H3O+(aq) + 2e- ==> H2(g) + 2H2O(l) or 2H2O(l) + 2e- ==> H2(g) + 2OH-(aq) All three equations amount to the same overall change i.e. the formation of hydrogen gas molecules and as far as I know any is acceptable in an exam? |
hydrogen ion or water reduced to hydrogen gas molecules: electrolysis of many salt or acid solutions to make hydrogen |
| 4 |
(-) Cu2+(aq) + 2e- ==> Cu(s) (copper deposit) |
copper(II) ion reduced to copper atoms: deposition of copper in its electrolytic purification or electroplating using copper(II) sulphate solution, electrode can be copper or other metal to be plated |
| 5 |
(+) Cu(s) - 2e- ==> Cu2+(aq) (copper dissolves) |
copper atoms oxidised to copper(II) ions: dissolving of copper in its electrolytic purification or electroplating (must have positive copper anode) |
| 6 |
(-) Al3+(l) + 3e- ==> Al(l) (aluminium) |
aluminium ions reduced to aluminium atoms: extraction of aluminium in the electrolysis of its molten oxide ore(l) |
| 7 |
(+) 2O2-(l) - 4e- ==> O2(g) (oxygen gas) |
oxide ion oxidised to oxygen gas molecules: electrolysis of molten oxides e.g. anode reaction in the extraction of aluminium from molten bauxite. |
| 8 |
(+) 4OH-(aq) - 4e- ==> 2H2O(l) + O2(g) (oxygen gas) or (+) 2H2O(l) - 4e- ==> 4H+(l) + O2(g) (oxygen gas) Both equations amount to the same overall change i.e. the formation of hydrogen gas molecules and as far as I know either is acceptable in an exam? |
hydroxide ions or water molecules are oxidised to oxygen gas molecules: electrolysis of many salt solutions such as sulphates, sulphuric acid etc. gives oxygen (chlorides ==> chlorine in concentrated solution, but can also give oxygen in diluted solution) |
| 9 |
(-) Pb2+(l) + 2e- ==> Pb(l) (lead deposit) |
lead(II) ions reduced to lead atoms: electrolysis of molten lead(II) bromide(l) |
| 10 |
(+) 2Br-(l/aq) - 2e- ==> Br2(g/l) (bromine) |
bromide ions oxidised to gas/liquid bromine molecules: electrolysis of molten bromide salts(l) or their concentrated aqueous solution(aq) or conc. hydrobromic acid(aq) to make bromine |
| 11 |
(-) Zn2+(aq) + 2e- ==> Zn(s) (zinc deposit) |
zinc ions reduced to zinc atoms: galvanising steel (the electrode) by electroplating from aqueous zinc sulphate solution, (or from molten zinc chloride?) |
| 12 |
(-) Ag+(aq) + e- ==> Ag(s) (silver deposit) |
silver ions reduced to silver atoms: silver electroplating from silver salt solution(aq), electrode can be other metal |
| 13 |
(-) Ca2+(l) + 2e- ==> Ca(s) (calcium metal) |
calcium ions reduced to calcium atoms e.g. in molten calcium chloride or bromide etc. |
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Electrolysis calculations
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3. Summary of Industrial Processes using Electrolysis
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5. Fuel Cells - another sort of battery
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equation | summary
of hydrogen-oxygen fuel cell It uses costly platinum electrodes and an acid electrolyte such as phosphoric acid, H3PO4 |
| 1. oxidation | 2H2(g) ==> 4H+(aq) + 4e- (at negative anode electrode*) | |
| 2. reduction | O2(g) + 4H+(aq) + 4e- ==> 2H2O(l) (at positive cathode electrode*) | |
| 3 = 1 + 2 redox | 2H2(g) + O2(g) ==> 2H2O(l) | |
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* Note the +ve and -ve electrode charges are reversed compared to electrolysis, because the system is operating in the opposite direction. Other notes on ADVANCED chemistry pages: The alkaline hydrogen-oxygen fuel cell is described in Equilibria Part 7 Redox Chemistry and organic fuel cells are described in Redox Chemistry Part 3. |
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| Chimica Ulteriori informazioni Elettrochimica Note sul Elettrolisi, batterie e celle a combustibile O di livello Chimica informazioni note di studio per la revisione di Chimica e Scienza (rivedere i corsi pari a US voti 9-10) Introduzione alla elettrolisi 3. Sintesi delle disposizioni pratiche sperimentali di laboratorio (Lab) esperimenti di elettrolisi e reazioni di elettrodo * 3. Summary of Industrial Electrolysis Process links * 4. Sintesi dei collegamenti processo di elettrolisi industriale * 5. Fuel cells celle o batterie Simple * 5. celle a combustibile (italian) * (afrikaans) Doc Brown se Chemie Ekstra Electro Inligting Aantekeninge oor Elektrolyse, batterye en brandstofcellen Indeks vir hierdie bladsy 1. * 2. Inleiding tot die elektrolise 3. Oorsig van praktiese eksperimentele reëlings vir laboratorium (lab) elektrolise eksperimente en elektrode reaksies 4. Oorsig van produksie Elektrolyse proses skakels* 5. Fuel cells Eenvoudige selle of batterye * (korean) 박사가 브라운 화학 여분의 전기 정보 참고 전기 분해에, 전지 및 연료 전지 색인 페이지에 대한이 * 2. 전기 분해 소개* 3. 요약 생물 무기 협정과 실용의 (연구실) 전기 실험 과 전극 반응 4. 요약 링크의 산업용 전기 분해 공정 단순 전지 또는 배터리 * 5. 연료 전지 연결된 링크 사이트에 해당 페이지의 기타 : 개정 참고 방사능의 전체 목록 중 GCSE - KS4 화학, 지구 과학 * 금속 추출 * 공업 화학 * 금속 반응 (redox 소개) *의 반응 화학 물질의 종류 * 전기 계산 * 이메일 쿼리? 덧글 * Chimie Electrochimie des notes d'information sur l'électrolyse, les piles et les piles à combustible supplémentaire Index pour cette 2. Introduction à l'électrolyse Résumé des modalités pratiques expérimentales en laboratoire (laboratoire) des expériences d'électrolyse et de réactions d'électrode Résumé de l'électrolyse des liens de procédés industriels cellules simples ou de batteries Les piles à combustible Les liens associés à d'autres pages sur ce site: la liste complète des Chimie, Sciences de la Terre et NOTES DE REVISION radioactivité * Extraction de métaux * Chimie industrielle * la réactivité des métaux (introduction d'oxydo-réduction) * Types de réaction chimique * Electrolyse calculs * (spanish) Doc Brown de Química Electroquímica información adicional Notas sobre la electrólisis, baterías y pilas de combustible Índice de la página 2. Introducción a la electrólisis Resumen de las modalidades prácticas experimentales de laboratorio (laboratorio) experimentos de electrólisis y reacciones de electrodo Resumen de la Industria de la electrólisis enlaces Proceso células o las baterías Las pilas de combustible * (portuguese) Doc Brown Química Eletroquímica Informação Notas sobre eletrólise, baterias e células de combustível extra Índice desta página Introdução à eletrólise Resumo das modalidades práticas de laboratório experimental de laboratório () experimentos de eletrólise e reações de eletrodo Resumo do processo de eletrólise links Industrial Simple células ou baterias * 5. células de combustível * (chinese) 督布朗的化学 额外的电化学信息札记电解,电池和燃料电池 此网页索引 介绍电解 实验室实验总结安排的实际(实验室)电解实验和电极反应 摘要工业电解过程的联系 简单的电池或电池 * 5。燃料电池 * (vietnamese) Doc Brown Hóa học Điện thêm thông tin ghi chép về điện phân, pin và các tế bào nhiên liệu Thay đổi KS4 Khoa học O cấp Hóa Thông Tin học cần chú ý cho sửa đổi lại cho Khoa học Hóa học Khoa học Khoa học (sửa đổi các khóa học bằng cấp của Mỹ 9-10) 1. Index cho trang này 2. Giới thiệu về điện phân * 3. Tóm tắt các thử nghiệm thực tiễn cho việc sắp xếp phòng thí nghiệm (trong phòng thí nghiệm) các thí nghiệm điện phân và phản ứng điện cực * 4. Tóm tắt quá trình công nghiệp điện phân các liên kết * 5. Fuel cells Đơn giản các tế bào hoặc pin * 5. tế bào nhiên liệu * (german) Extra Elektrochemie Information Hinweise zur Elektrolyse, Batterien und Brennstoffzellen Index für diese Seite 1. * 2. Einführung in die Elektrolyse 3. Zusammenfassung der praktischen Versuchsanordnungen im Labor (Labor-) Experimente Elektrolyse und Elektrode Reaktionen * 4. Zusammenfassung der Industrial Elektrolyse Links * 5. Fuel cells Einfache Zellen oder Batterien * 5. Brennstoffzellen * |
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