Understanding Chemical changes: OxfordAQA IGCSE CORE Chemistry (Short Course) Explained

Chemical changes: reactivity and electrolysis together

Chemical changes in OxfordAQA IGCSE CORE Chemistry (Short Course) brings together two topics that, at first glance, look unrelated: the reactivity series of metals, and electrolysis. In fact they share the same underlying idea, namely how easily an atom loses electrons to form a positive ion. A metal high in the reactivity series loses electrons easily; the same tendency to lose or gain electrons is exactly what happens at the electrodes during electrolysis. Keep that connection in mind and oxfordaqa igcse core chemistry (short course) chemical changes becomes far easier to learn as one coherent story rather than two disconnected topics.

Metals and the reactivity series

Metals can be ranked in a reactivity series based on how they react with water and with dilute acids. For this specification you need to know the order of potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper, and be able to place them in order of reactivity from their reactions.

Very unreactive metals such as gold exist in the Earth as the metal itself, but most metals are found combined as compounds and need a chemical reaction to extract the pure metal.

Extraction methods depend on reactivity

• Metals less reactive than carbon, such as iron, can be extracted from their oxide by reduction with carbon; this is how iron oxide is reduced to iron in a blast furnace. Reduction here means the removal of oxygen from the metal oxide.
• Metals more reactive than carbon, such as aluminium, cannot be extracted this way and instead require electrolysis of a molten compound, which uses large amounts of energy and makes these metals more expensive to produce.

If an exam question asks how a metal is extracted, first decide where it sits relative to carbon in the reactivity series; that single decision determines whether the correct answer is reduction with carbon or electrolysis.

Electrolysis

When an ionic substance is melted or dissolved in water, its ions become free to move. Passing an electric current through this molten or dissolved substance, called the electrolyte, breaks it down into its elements; this process is electrolysis.

• Positive ions move to the negative electrode, the cathode, where they gain electrons (reduction).
• Negative ions move to the positive electrode, the anode, where they lose electrons (oxidation).

Worked example: half equations

During the electrolysis of molten lead bromide, bromide ions lose electrons at the anode. This is written as a half equation:

2Br^- → Br₂ + 2e^-

Notice the charges balance: two negative charges on the left from the bromide ions match the two electrons released on the right. Always check that the number of electrons and the overall charge balance on both sides before treating a half equation as finished.

Electrolysis with a mixture of ions

When the electrolyte contains more than one type of positive or negative ion, the products depend on reactivity and concentration: at the cathode, the less reactive element is usually produced; at the anode, the product formed can depend on the relative concentration of the ions present. The electrolysis of concentrated sodium chloride solution, for example, produces hydrogen at the cathode, chlorine at the anode, and leaves sodium hydroxide solution behind. All three products are important industrial chemicals: hydrogen for various processes, chlorine for bleach and plastics, and sodium hydroxide for soap manufacture.

Aluminium extraction by electrolysis

Aluminium is manufactured by electrolysing a molten mixture of aluminium oxide and cryolite; the cryolite lowers the melting point of the mixture, reducing the energy needed. Aluminium metal forms at the negative electrode, and oxygen forms at the positive electrode. Because the positive electrode is made of carbon, it reacts with the oxygen produced to form carbon dioxide, meaning the carbon electrodes gradually burn away and need periodic replacement.

Electroplating

Electrolysis is also used to electroplate objects with a thin layer of metal, for example copper plating or silver plating, for reasons that include improving appearance, increasing durability, and preventing corrosion.

Why reactivity and electrolysis are taught together

Once chemical changes oxfordaqa igcse explained material clicks into place, most students realise the reactivity series and electrolysis were never really separate ideas. A metal near the top of the reactivity series holds onto its electrons weakly, which is exactly why it forms ions so readily in solution or when molten, and exactly why it is difficult to reduce back to the free metal using carbon. Seeing the reactivity series and electrolysis as two views of the same underlying property, rather than two topics to memorise independently, tends to make revision faster and answers more confident under exam conditions.

It also helps to keep a running list of oxfordaqa igcse core chemistry (short course) practice questions you have got wrong on this topic, since the mistakes tend to repeat: mixing up oxidation and reduction, or picking the wrong extraction method for a metal you have not seen named explicitly before. Revisiting your own error log in the final weeks before the exam is often more productive than working through a fresh set of unfamiliar questions.

Common mistakes in chemical changes

• Mixing up oxidation and reduction at the electrodes; remember loss of electrons at the anode (oxidation) and gain of electrons at the cathode (reduction).
• Assuming every metal is extracted by electrolysis; only metals more reactive than carbon need this method, while less reactive metals are reduced using carbon.
• Forgetting to balance charge, not just atoms, when writing or completing half equations.
• Describing gold as never occurring in nature as a compound; the point is that unreactive metals like gold are found as the free metal, which is different from saying reactive metals cannot form compounds.

A note on required practical work

The specification includes a required practical investigating the products at the anode and cathode during the electrolysis of copper sulfate solution. Even if you are not asked to describe the practical procedure directly, examiners frequently draw on it for questions about observing gas bubbles, colour changes at an electrode, or identifying a product using the standard gas tests you meet in the chemical analysis topic. Reading through a description of this practical, and being able to predict what happens at each electrode using inert graphite versus copper electrodes, is time well spent even on a specification that does not require you to write up a full practical report. Copper electrodes in particular behave differently from inert graphite ones, since the copper anode itself dissolves into solution rather than releasing a gas, which is a detail worth remembering if a question specifies the electrode material.

Self-check questions

1. Place zinc, potassium and copper in order of decreasing reactivity, based on their reaction with dilute acid.
2. Explain why aluminium cannot be extracted from its oxide using carbon.
3. Write and balance the half equation for the reaction occurring at the cathode when copper ions are discharged.
4. Explain why the positive electrode in aluminium extraction needs to be replaced periodically.
5. State two everyday reasons why an object might be electroplated with silver.

Revisiting the reactivity series before your exam

This oxfordaqa igcse core chemistry (short course) explained page is worth bookmarking as a final checklist. In the days before the exam, cover the reactivity table above and try to reconstruct it from memory, along with the reaction each metal has with water and dilute acid. Then check your version of the half equations for electrolysis of both molten lead bromide and sodium chloride solution, since these two examples between them cover most of the reasoning examiners expect you to apply to an unfamiliar electrolyte.

How this topic is examined

Questions on oxfordaqa igcse core chemistry (short course) chemical changes often present an unfamiliar metal or an unfamiliar electrolyte and ask you to apply the general rules above rather than recall a specific memorised fact. Practise completing and balancing half equations, since these appear regularly and are a reliable source of marks once the method is secure. For further igcse 9222 chemical changes practice, work through past-paper questions that combine reactivity series reasoning with electrolysis, since the two are often tested together within the same question.

These oxfordaqa igcse core chemistry (short course) revision notes on chemical changes oxfordaqa igcse connect directly to the structure and bonding topic before it, since ionic bonding explains why these substances can be broken down by electrolysis in the first place, and to the quantitative chemistry topic later, where you will calculate the masses of products formed at each electrode.

Once you are confident distinguishing reduction with carbon from electrolysis, and can write a balanced half equation without hesitation, this oxfordaqa igcse core chemistry (short course) notes topic becomes one of the most predictable parts of the paper, because the underlying rules apply consistently across almost every question style examiners use.