电化学

Electrolysis 电解 is the breaking down (decomposition 分解) of an ionic compound 离子化合物 — when it is molten 熔融 or in aqueous 水溶液 solution — by passing an electric current 电流 through it.

It only works when the substance is molten or dissolved, because then the ions 离子 are free to move and carry the charge. A solid ionic compound cannot be electrolysed because its ions are locked in place.

The electrolytic cell

The set-up is called an electrolytic cell 电解池. Two electrodes 电极 (solid conductors) dip into the electrolyte 电解质 — the molten or aqueous substance being broken down.

• The anode 阳极 is the positive ($+$) electrode.
• The cathode 阴极 is the negative ($-$) electrode.

The electrodes are often inert 惰性 (they do not react), such as platinum 铂 or carbon/graphite 石墨.

What is formed at each electrode

There is a simple rule:

• Metals 金属 or hydrogen 氢气 are formed at the cathode.
• Non-metals 非金属 (other than hydrogen) are formed at the anode.

For aqueous solutions, the product can depend on whether the solution is dilute 稀 or concentrated 浓. Here are the three Core examples:

Worked example. Predict the products of electrolysing molten zinc chloride, $\text{ZnCl}_2$. Because it is molten, only zinc ions and chloride ions are present - there is no water to complicate things. Zinc is a metal, so it forms at the cathode: $\text{Zn}^{2+}$ ions gain electrons and silvery zinc appears. Chlorine is a non-metal, so it forms at the anode: pale green chlorine gas bubbles off. Always check the state first. A molten compound simply gives you its own two elements, while an aqueous one brings water into the competition - which is why concentrated sodium chloride solution gives hydrogen at the cathode rather than sodium.

Aqueous solutions: which ion is discharged? (Supplement)

In the simple aqueous solutions considered here, water supplies hydrogen ions, $\text{H}^+$, and hydroxide ions, $\text{OH}^-$, which compete with the dissolved salt’s ions at the electrodes. Ions are discharged 放电 when they gain or lose electrons to form products. Use the following syllabus rules for the specified solutions with inert electrodes.

• At the cathode: if the metal is more reactive than hydrogen (for example sodium, potassium, calcium), hydrogen forms instead of the metal. If the metal is less reactive than hydrogen (for example copper or silver), the metal forms.
• At the anode: a concentrated solution of a halide 卤化物 (chloride, bromide or iodide) gives the halogen. Otherwise (a dilute solution, or a sulfate) oxygen forms from the hydroxide ions.

The half-equations you need:

Worked example. Predict the products of electrolysing dilute aqueous sodium chloride with inert electrodes. At the cathode, $\text{Na}^+$ and $\text{H}^+$ arrive; sodium is more reactive than hydrogen, so hydrogen forms. At the anode, $\text{Cl}^-$ and $\text{OH}^-$ arrive; the solution is dilute, so oxygen forms: $4\text{OH}^- \rightarrow \text{O}_2 + 2\text{H}_2\text{O} + 4e^-$. Compare the concentrated solution, which gives chlorine at the anode.

Common mistakes.

• Saying sodium forms at the cathode from aqueous sodium chloride. Sodium would react with the water at once; hydrogen forms instead.
• Writing $\text{H}^+ + e^- \rightarrow \text{H}$. Hydrogen gas is $\text{H}_2$, so two ions and two electrons are needed.
• Putting the electrons on the wrong side. At the anode the electrons are products (lost); at the cathode they are reactants (gained).
• Saying the ions "carry electrons through the solution". Electrons move only in the wires; in the electrolyte it is the ions that move.

How the charge moves

During electrolysis:

• Electrons 电子 move through the wires (the external circuit 外电路) from the power supply.
• At the cathode, positive ions gain electrons. Gaining electrons is reduction 还原.
• At the anode, negative ions lose electrons. Losing electrons is oxidation 氧化.
• Inside the electrolyte, the ions move: positive ions go to the cathode and negative ions go to the anode.

You can write a half-equation 半反应式 for each electrode. For molten lead(II) bromide:

Electrolysis of copper(II) sulfate

The product at the anode depends on the electrode:

• With inert carbon electrodes: copper 铜 forms at the cathode and oxygen forms at the anode. The blue colour of the solution slowly fades as copper is removed.
• With copper electrodes: copper forms at the cathode, while the anode itself dissolves into the solution. The blue colour stays the same. This is used to purify copper.

Electroplating

Electroplating 电镀 means covering a metal object with a thin layer of another metal. This improves its appearance and its resistance to corrosion 腐蚀 (rusting and wearing away).

To electroplate an object:

• the object to be coated is made the cathode;
• the plating metal is made the anode;
• the electrolyte is a solution containing ions of the plating metal.

A fuel cell 燃料电池 uses hydrogen and oxygen to make electricity directly. The only chemical product is water.

It is useful to compare a fuel cell with a normal petrol (gasoline) engine in a vehicle.

Advantages of the fuel cell: the only product is water, so it does not pollute the air, and it changes chemical energy into electricity efficiently. Disadvantages: hydrogen is hard to store and transport safely, and producing the hydrogen can still use energy from fossil fuels.

• At the cathode (negative) you get a metal or hydrogen; at the anode (positive) you get a non-metal. Positive ions move to the cathode, negative ions to the anode.
• A molten compound just gives its two elements. An aqueous solution can give something different (concentrated sodium chloride gives hydrogen and chlorine, not sodium).
• Reduction is gain of electrons (at the cathode); oxidation is loss of electrons (at the anode). Remember OIL RIG.
• In a half-equation the atoms and the charges must balance, with the electrons on the correct side: $\text{Pb}^{2+} + 2e^- \rightarrow \text{Pb}$ (cathode).
• To electroplate an object, make it the cathode, make the plating metal the anode, and use a solution containing ions of the plating metal.