IGCSE化学 电解 阳极 阴极 电镀 燃料电池
IGCSE 化学 0620 的 Topic 4 叫 Electrochemistry，分成 4.1 电解与 4.2 氢氧燃料电池两块，Cambridge 官方 syllabus 里这一章篇幅不长，考频却很高。Paper 3 与 Paper 4 各 80 分，一道电解题通常占 6 到 10 分，熔融溴化铅、浓氯化钠溶液、稀硫酸、硫酸铜溶液这四个体系反复被考。Core 只要求判断阴阳极与产物，Extended 还要写离子半反应式并解释电荷怎么转移。很多学生以为电解就是通电把物质分开，写出产物就算答完，其实阅卷时扣得最多的是现象描述不完整与阴阳极写反。下面按卷面顺序拆成十二块，每一块都给出考场上真正会被检验的细节。
Topic 4 of Cambridge IGCSE Chemistry 0620 splits into 4.1 Electrolysis and 4.2 Hydrogen-oxygen fuel cells, a short chapter that turns up in almost every session. On Paper 3 and Paper 4 an electrolysis question usually carries 6 to 10 marks, and the four systems examiners keep returning to are molten lead(II) bromide, concentrated sodium chloride solution, dilute sulfuric acid and copper(II) sulfate solution. Core candidates only identify the electrodes and the products; Extended candidates also build ionic half-equations and explain how charge moves.
卷面位置与两条考试路线
0620 的考生走 Core 或 Extended 两条路线，Core 考 Paper 1 与 Paper 3，成绩上限是 C，Extended 考 Paper 2 与 Paper 4，最高可以拿 A*。Paper 1 与 Paper 2 都是 45 分钟 40 道四选一，各占 30%，每道题只考一个点，电极极性或者某一极的产物都能单独成题。Paper 3 与 Paper 4 都是 1 小时 15 分钟 80 分，各占 50%，平均每分不到一分钟，电解大题常与化学计量拼在一起。剩下 20% 是实验技能，Paper 5 是 1 小时 15 分钟的实验考，Paper 6 是 1 小时 40 分的笔试替考，报考时选定其中一卷。Cambridge 官方 syllabus 写明 0620 适用于 2026、2027 与 2028 年考试，电解的编号是 4.1，燃料电池是 4.2。阅卷时这一步最容易失分：学生把选择题的判断习惯带进大题，只写产物不写现象。
Core candidates sit Paper 1 with Paper 3 and stop at grade C, while Extended candidates sit Paper 2 with Paper 4 and can reach A*; both multiple-choice papers last 45 minutes for 40 marks and 30% of the grade. The two theory papers each run 1 hour 15 minutes for 80 marks and 50%, which leaves under a minute per mark, so electrolysis is often stitched onto a mole calculation rather than examined alone. The last 20% comes from one practical paper chosen at entry, either the 1 hour 15 minute laboratory test or the 1 hour written Alternative to Practical.
电解的定义与电解池的三个部件
考纲给电解的定义是 decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current，关键词有三个：离子化合物、熔融或水溶液、通电。要电解的物质必须有能自由移动的离子，固态氯化钠的离子被锁在晶格里，通了电也没有反应。电解池里接电源正极的叫阳极 anode，接负极的叫阴极 cathode，泡在中间的熔融物或溶液叫电解质 electrolyte。很多学生以为电解质一定要是溶液，其实熔融的溴化铅同样是电解质，而且没有水参与，产物判断反而更干净。石墨与铂本身不参加反应，被考纲点名为惰性电极；铁、铜这类金属电极在特定题目里会参加反应，第八块会专门讲。写定义时把 molten 与 aqueous 两个词都写进去，漏掉一个通常只给一半分。
The syllabus defines electrolysis as the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current, and three words carry the marks: ionic compound, molten or aqueous, and electric current. The electrolyte must contain ions that are free to move, which is why solid sodium chloride does nothing when a current is passed through it; the ions are locked in the lattice. The electrode connected to the positive terminal is the anode, the one on the negative terminal is the cathode, and graphite or platinum is called inert because it does not take part in the reaction.
电荷在电路与电解液里怎么走
Extended 层要求把电荷转移写清楚，官方口径有三条：外电路里电子从电源负极流向阴极、再从阳极回到电源正极；电极表面发生电子的得到与失去；电解质内部靠离子移动导电。阴极表面的离子得到电子，这一步叫 reduction，阳极表面的离子失去电子，这一步叫 oxidation，这两句话是后面写半反应式的前提。常见的错误是把电流方向当成电子方向，电流从正极流出，电子从负极流出，两者正好相反。离子在溶液里的迁移方向也要交代：阳离子移向阴极，阴离子移向阳极，所以阴离子聚集的那一极一定是阳极。熔融溴化铅的例子最干净，Pb2+ 移向阴极拿走两个电子，Br- 移向阳极交出电子，中间没有水的干扰。阅卷时错得最多的是把三条里的离子移动漏掉，AO1 的 1 分直接没了。
Extended candidates have to describe the transfer of charge in three parts: electrons travel through the external circuit from the negative terminal to the cathode and back from the anode, electrons are lost or gained at the electrode surfaces, and ions carry the current inside the electrolyte. Reduction happens where electrons are gained, at the cathode, and oxidation happens where electrons are lost, at the anode; mixing those two up collapses every half-equation that follows. A current arrow drawn the wrong way round is the commonest slip, because conventional current flows out of the positive terminal while electrons leave the negative one.
熔融溴化铅的两个产物
熔融 PbBr2 通电以后，阴极得到银灰色液体金属铅，阳极放出红棕色气体溴蒸气，两个现象都要按颜色与状态写全。半反应式写成 Pb2+ + 2e- -> Pb 与 2Br- -> Br2 + 2e-，两式相加要回到 PbBr2 -> Pb + Br2。判断二元熔融化合物的产物有一条简单规则：金属或氢出现在阴极，非金属出现在阳极，所以 PbBr2 给铅与溴，熔融 NaCl 给钠与氯。很多学生把溴的颜色写成黄绿色，那是氯气的颜色，溴是红棕色，颜色写错在现象题里直接丢分。溴蒸气有毒，实验在通风橱里做，考纲不考安全操作，但现象题给了这两个字就要写出来。这一步最容易失分的是忘记写电极名称，只写铅与溴，阅卷时不认。
Molten lead(II) bromide gives a silvery-grey liquid metal at the cathode and a red-brown vapour of bromine at the anode, and both observations need the colour as well as the state. The half-equations read Pb2+ + 2e- giving Pb and 2Br- giving Br2 + 2e-, and adding them returns the overall change from PbBr2 to Pb plus Br2; for any binary molten compound the metal or hydrogen forms at the cathode and the non-metal at the anode. Examiners see the colour of bromine reported as yellow-green every year, which is the colour of chlorine, so a wrong colour costs the observation mark outright.
浓氯化钠溶液：阴极氢气 阳极氯气
浓氯化钠溶液用惰性电极电解，阴极放出氢气，阳极放出氯气，溶液里剩下氢氧化钠。现象要写三条：阴极的气体用点燃的木条检验会发出爆鸣声，阳极气体是淡黄绿色并且让湿润的石蕊试纸褪色，溶液由中性变碱性，滴入酚酞显红色。半反应式是 2H+ + 2e- -> H2 与 2Cl- -> Cl2 + 2e-，水自身电离出的氢离子在阴极得到电子，这是金属离子与氢离子竞争的第一例。学生最常把阳极写成氧气，理由是溶液里有水，其实氯离子浓度足够高时优先放电，氯气才是产物。稀氯化钠溶液的行为相反，阳极得到氧气，考纲 Supplement 第 10 条专门要求比较稀与浓两种卤化物，答题时把浓度这个词写出来。氯气有毒，工业电解食盐水要用隔膜把两极的产物分开。
Concentrated sodium chloride solution with inert electrodes gives hydrogen at the cathode and chlorine at the anode, leaving sodium hydroxide behind in the solution. The syllabus asks for three observations, so the cathode gas should be tested with a lit splint and give a squeaky pop, the anode gas is pale yellow-green and bleaches damp litmus, and the solution turns alkaline so phenolphthalein goes pink; the half-equations are 2H+ + 2e- giving H2 and 2Cl- giving Cl2 + 2e-. Dilute sodium chloride behaves differently and leaves oxygen at the anode, which is why the Supplement asks candidates to compare halide solutions of different concentration.
稀硫酸：氢氧体积比 2 比 1
稀硫酸用铂或石墨电极电解，阴极得到氢气，阳极得到氧气，两个气体的体积比是 2 比 1。这个比例与水的组成一致，因为被电解的其实是水，硫酸只负责增加导电性，浓度会慢慢升高。半反应式写成 4H+ + 4e- -> 2H2 与 4OH- -> O2 + 2H2O + 4e-，两边都配到 4 个电子才好直接相加。题目常给收集管里的液面图，要求判断哪一根管子装的是氢气，装氢气的那根体积更大。常见的错误是把氧气的体积写成两倍，或者忘记说明两种气体都无色。氢气的检验用点燃的木条，听见爆鸣声，氧气的检验用带火星的木条，重新燃烧起来，这两个方法在 Paper 6 里出现频率很高。工业上这套装置没有经济价值，制氢多走天然气重整，这一点在能源题里会用到。
Dilute sulfuric acid with platinum or graphite electrodes gives hydrogen at the cathode and oxygen at the anode in a 2 to 1 volume ratio, because the substance actually being decomposed is water and the acid only makes the solution conduct. The half-equations are 4H+ + 4e- giving 2H2 and 4OH- giving O2 + 2H2O + 4e-, both balanced to four electrons so that they can simply be added. A common mistake is to double the oxygen volume or to leave out the fact that both gases are colourless, and the two gas tests, a lit splint for hydrogen and a glowing splint for oxygen, appear on Paper 6 again and again.
硫酸铜溶液配惰性电极
硫酸铜溶液配碳或铂电极，阴极得到红色铜沉积，阳极冒出氧气气泡，蓝色溶液慢慢变浅。阴极半反应式是 Cu2+ + 2e- -> Cu，阳极是 4OH- -> O2 + 2H2O + 4e-，铜全部析出以后剩下的溶液接近无色。这一块图像题最多：给出电解前后的溶液照片或者阴极质量的读数，要求解释阴极质量增加的原因。很多学生以为阳极的氧气来自硫酸根，其实硫酸根不放电，氧来自水，写理由时要把这一点说清楚。铜的沉积让阴极质量增加，增加的质量与通过的电荷量成正比，Extended 的计算题会把这个关系接到摩尔数上。阅卷时这一步最容易失分的地方是没有提到溶液颜色的变化，只写两极产物与半反应式。
Aqueous copper(II) sulfate with carbon or platinum electrodes deposits red-brown copper at the cathode while oxygen bubbles off at the anode, and the blue colour of the solution fades as the copper ions are used up. The cathode half-equation is Cu2+ + 2e- giving Cu and the anode half-equation is 4OH- giving O2 + 2H2O + 4e-, so the sulfate ion stays in solution throughout and the oxygen comes from water. Data questions give the mass gained by the cathode and expect a link between that mass and the charge passed, which is how the topic connects to mole calculations on the Extended paper.
硫酸铜溶液配铜电极与铜的精炼
换用铜电极以后，阳极的铜原子失去电子溶解进溶液，阴极的铜离子得到电子沉积出来，溶液浓度基本不变，颜色也保持蓝色。阳极半反应式写成 Cu -> Cu2+ + 2e-，阴极仍是 Cu2+ + 2e- -> Cu，两个式子放在一起就是铜在两极之间搬运。工业上利用这一点精炼铜：粗铜做阳极，薄纯铜片做阴极，电解液是硫酸铜溶液，阳极里比铜活泼的杂质溶进溶液，比铜不活泼的杂质沉成阳极泥。学生最常把两极的功能写反，以为还是阳极出氧气、阴极出铜，铜电极在阳极会参加反应，这是它与惰性电极最大的区别。黄金与白银这类贵金属留在阳极泥里，是铜精炼的副产品。考纲 Supplement 第 9 条要求描述两种电极材料下的产物与现象，答题时先交代电极是 inert 还是 copper。
With copper electrodes the anode dissolves as its atoms lose electrons and the cathode gains mass as copper ions are reduced, so the concentration stays constant and the blue colour does not fade, which is the basis of copper refining in industry. The anode half-equation is Cu giving Cu2+ + 2e- and the cathode half-equation is unchanged, so the metal is simply transported from one electrode to the other. Candidates who swap the two electrodes lose the mark, because an active copper anode takes part in the reaction instead of releasing oxygen the way graphite does.
阴极与阳极的放电竞争顺序
溶液里有多种离子的时候，谁先放电要看离子本身的活泼程度与浓度。阴极上，铜、银、金这类不活泼金属的离子比氢离子容易得到电子，先析出金属；钠、钾、镁这类活泼金属的离子在水溶液里永远不析出金属，得到的是氢气。阳极上，卤素离子优先于水放电，所以浓度足够高的氯化钠溶液里得到氯气，稀溶液或者硫酸盐溶液里得到氧气。常见的错误是拿元素周期表的位置直接推产物，忽略浓度这个条件，考纲要求比较稀与浓卤化物正是为了考这一点。石墨阳极在放出氯气或氧气时会慢慢消耗，工业电解槽要定期更换。题目给的信息通常够判断，关键是把浓度、离子种类与电极材料三个条件逐一核对。
When several ions are present, the ion that is discharged first depends on how reactive it is and on how concentrated it is inside the cell. Copper, silver and gold ions take electrons more readily than hydrogen ions and deposit as metals, whereas sodium, potassium and magnesium ions stay in solution and hydrogen is released instead; at the anode a halide ion is discharged in preference to water. Comparing dilute with concentrated halide solutions is the point of the Supplement, because a candidate who predicts products from the periodic table alone will miss the concentration condition.
半反应式的写法与氧化还原命名
写半反应式有四个动作：先写出正确的离子与产物，再配原子数，然后加减电子把电荷配平，最后检查两边总电荷是否相等。阴极得电子，电子写在式子左边，阳极失电子，电子写在式子右边，这一条记反了整题的分都会跟着塌。氧化发生在阳极，还原发生在阴极，两个英文词的字母是配对的，anode oxidation 与 cathode reduction 一起记最省事。常见的错误是电荷不配平，例如把 2Br- -> Br2 + 2e- 写成只给一个电子，或者把水里的氢与氧忘掉。Extended 卷的半反应式通常给 2 分，一步错就只剩一半。把阴阳极的两个式子加起来能回到总反应式，说明配平没错。
Building a half-equation takes four moves: write the correct ion and product, balance the atoms, add or remove electrons to balance the charge, then check that both sides carry the same total charge. Electrons sit on the left at the cathode and on the right at the anode, and the pairing of anode with oxidation and cathode with reduction is the quickest way to keep that straight. Adding the two half-equations should return the overall equation, which is the fastest self-check available in the exam hall.
电镀：物件接阴极 镀层金属接阳极
电镀的原理与铜的精炼一样，被镀的物件接阴极，镀层金属接阳极，电解液里含镀层金属的离子。通电以后阳极的金属溶解，阴极的物件表面覆盖一层薄而均匀的金属，考纲写明电镀的目的是改善外观与提高抗腐蚀能力。镀银用硝酸银溶液，镀镍用硫酸镍溶液，具体盐名不必死记，答对两极接什么就够了。很多学生把物件接成阳极，物件会自己溶解，镀层根本镀不上，这是这一题最典型的错误。镀层要薄而致密，电流太大长出来的镀层粗糙，容易脱落。工业上镀锌的铁皮叫镀锌铁，镀锡的铁皮做食品罐头，考纲不要求这些细节，但选择题里出现过。
Electroplating works the way copper refining works: the object to be plated is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal. The anode dissolves and the object gains a thin, even coating, which the syllabus describes as improving appearance and resistance to corrosion; silver plating uses silver nitrate solution and nickel plating uses nickel sulfate solution. Candidates who connect the object as the anode watch it dissolve instead of being coated, which is the classic error on this question.
氢氧燃料电池与四周复习排期
氢氧燃料电池把氢与氧的化学能直接变成电能，唯一的化学产物是水，考纲 4.2 第一条的措辞就是 with water as the only chemical product。与汽油发动机相比，优点是产物里没有二氧化碳与氮氧化物，效率更高，运转安静；缺点是氢的储存与运输困难，制氢本身要消耗能源，铂催化剂昂贵，加氢站的基础设施远远不如加油站。Extended 层要求把优缺点写成对比，题目通常给 4 分，优点与缺点各两点。常见的错误是写不排放任何物质，水同样是排放物，只是它不造成污染。用天然气重整制氢会附带二氧化碳，电解水制氢要用可再生电力才算低碳，这两句在环境题里常被要求补充。四周复习可以这样排：第一周定义与阴阳极判断，第二周四个体系的产物与现象，第三周半反应式与电荷配平，第四周燃料电池与历年真题的电解大题。
A hydrogen-oxygen fuel cell turns the chemical energy of hydrogen and oxygen straight into electricity, and the syllabus states that water is the only chemical product. Compared with a petrol engine it produces no carbon dioxide or nitrogen oxides, runs more efficiently and far more quietly, but hydrogen is awkward to store and transport, making the fuel itself takes energy, the platinum catalyst is expensive, and there is no refuelling network to match petrol stations. A four-mark comparison question usually wants two advantages and two disadvantages, so water still counts as an emission even though it is not a pollutant.
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