What you'll learn
Chemical changes is the unit of AQA GCSE Combined Science: Trilogy that deals with reactions of metals and acids, and with using electricity to split compounds apart. It brings together three connected ideas: the reactivity series, which ranks metals by how readily they form positive ions; acids and their reactions with metals, bases and carbonates to make salts; and electrolysis, which uses an electric current to break down ionic compounds. By the end you should be able to place metals in order of reactivity from their reactions, explain oxidation and reduction in terms of oxygen and of electrons, explain why some metals are extracted by reduction with carbon and others by electrolysis, write the general equations for acid reactions, describe neutralisation and the pH scale, and predict the products of electrolysis for molten and aqueous compounds. This unit is assessed on Chemistry Paper 1 and includes the required practicals on making a soluble salt and on electrolysis.
Key terms and definitions
Reactivity series — metals arranged in order of how readily they react, which reflects how easily they form positive ions
Oxidation — gain of oxygen, or loss of electrons
Reduction — loss of oxygen, or gain of electrons
Displacement reaction — a reaction in which a more reactive metal takes the place of a less reactive metal in a compound
Acid — a substance that produces hydrogen ions in aqueous solution
Alkali — a soluble base that produces hydroxide ions in aqueous solution
Base — any substance that neutralises an acid, including metal oxides, hydroxides and carbonates
Neutralisation — the reaction of hydrogen ions with hydroxide ions to produce water
Salt — the compound formed when the hydrogen in an acid is replaced by a metal
Electrolysis — the breaking down of an ionic compound using an electric current, when molten or in solution
Electrolyte — the molten or dissolved ionic compound that conducts during electrolysis
Inert electrode — an electrode that does not react, usually made of graphite or platinum
Core concepts
The reactivity series
When metals react, they lose electrons and form positive ions. The more readily a metal does this, the more reactive it is. The order you need, from most to least reactive, is potassium, sodium, lithium, calcium, magnesium, zinc, iron, copper, and then the very unreactive silver and gold. Carbon and hydrogen are included as reference points even though they are not metals, because their positions determine how a metal can be extracted.
Reactivity can be deduced experimentally. A metal's reaction with water and with dilute acid shows how vigorous it is: potassium, sodium and lithium react with cold water, magnesium reacts very slowly with water but readily with acid, zinc and iron react with acid, and copper reacts with neither.
Displacement reactions
A more reactive metal will displace a less reactive metal from a compound. If iron is placed in copper sulfate solution, the iron takes the place of the copper, producing iron sulfate and copper metal. The blue colour of the solution fades and a brown deposit forms.
Displacement reactions are a reliable way of ordering metals when a question gives you a table of which metals react with which solutions. If metal A displaces metal B, then A is more reactive than B.
Oxidation, reduction and extraction
Originally oxidation meant gaining oxygen and reduction meant losing it. When magnesium burns it gains oxygen, so it is oxidised; when copper oxide is heated with carbon, the copper oxide loses oxygen, so it is reduced.
In terms of electrons the definitions are the more useful pair, and the standard memory aid is OIL RIG: oxidation is loss of electrons, reduction is gain. Both definitions describe the same reactions, and a question may ask for either.
Unreactive metals such as gold are found in the Earth as the metal itself. Most metals are found as compounds and must have oxygen removed, or otherwise be separated, to obtain the metal. Metals less reactive than carbon can be extracted by reduction with carbon, because carbon will take the oxygen from them; iron is extracted this way. Metals more reactive than carbon, such as aluminium, cannot be, and must be extracted by electrolysis, which is a much more expensive process because of the large amounts of energy needed.
Acids, alkalis and the pH scale
Acids produce hydrogen ions in aqueous solution; alkalis produce hydroxide ions. The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is. A pH below 7 is acidic, 7 is neutral and above 7 is alkaline. pH can be measured with universal indicator, which gives an approximate value from its colour, or with a pH probe and meter, which gives a numerical value and is more accurate.
Neutralisation is the reaction between hydrogen ions and hydroxide ions to produce water, which can be written as hydrogen ions plus hydroxide ions giving water.
Reactions of acids
Three general reactions are needed, and each produces a salt.
An acid plus a metal gives a salt plus hydrogen. Magnesium with hydrochloric acid produces magnesium chloride and hydrogen. These are redox reactions: the metal is oxidised as it loses electrons, and the hydrogen ions are reduced as they gain them.
An acid plus a base, meaning a metal oxide or metal hydroxide, gives a salt plus water. Copper oxide with sulfuric acid produces copper sulfate and water.
An acid plus a metal carbonate gives a salt plus water plus carbon dioxide. Calcium carbonate with hydrochloric acid produces calcium chloride, water and carbon dioxide, and the effervescence is a useful test.
The salt produced depends on the acid used. Hydrochloric acid produces chlorides, sulfuric acid produces sulfates, and nitric acid produces nitrates.
Making a soluble salt
In the required practical, an insoluble base such as copper oxide is added to warm dilute acid until no more dissolves, which shows the acid has all reacted. The excess solid is removed by filtration, leaving a solution of the salt. The solution is then heated gently to evaporate some of the water, and left to crystallise slowly, producing pure crystals which are dried.
Adding the base in excess is the step that matters most: it guarantees no acid remains, so the crystals are not contaminated.
Electrolysis of molten compounds
When an ionic compound is melted, the ions become free to move and can carry charge. Passing a current decomposes the compound into its elements.
Positive ions move to the negative electrode, the cathode, where they gain electrons and are reduced. Negative ions move to the positive electrode, the anode, where they lose electrons and are oxidised. A helpful reminder is that positive ions go to the negative electrode, because opposite charges attract.
Molten lead bromide therefore produces lead at the cathode and bromine at the anode.
Aluminium is extracted from aluminium oxide by electrolysis after mixing it with cryolite to lower the melting point and so reduce the energy needed. The carbon anodes must be replaced regularly because the oxygen produced reacts with them at the high temperature to form carbon dioxide.
Electrolysis of aqueous solutions
In solution, water also provides hydrogen ions and hydroxide ions, so there is competition at each electrode and the rules change.
At the cathode, hydrogen is produced unless the metal is less reactive than hydrogen, in which case the metal is deposited. Copper sulfate solution therefore gives copper at the cathode, but sodium chloride solution gives hydrogen.
At the anode, oxygen is produced from the hydroxide ions unless the solution contains halide ions, in which case the halogen is produced. Sodium chloride solution therefore gives chlorine at the anode.
Worked examples
Example 1: Ordering metals from displacement data (3 marks)
Metal X displaces copper from copper sulfate solution. Metal Y does not react with copper sulfate solution but does react with dilute acid. Place X, Y and copper in order of reactivity, most reactive first.
X displaces copper, so X is more reactive than copper. Y does not displace copper, so Y is less reactive than X but, because Y reacts with dilute acid while copper does not, Y is more reactive than copper. The order is therefore X, then Y, then copper.
Example 2: Predicting electrolysis products (4 marks)
Predict the products of the electrolysis of aqueous potassium iodide, and explain your reasoning.
At the cathode, the competing positive ions are potassium ions and hydrogen ions. Potassium is more reactive than hydrogen, so hydrogen is produced rather than potassium metal. At the anode, the competing negative ions are iodide ions and hydroxide ions. Because a halide ion is present, iodine is produced rather than oxygen. The products are hydrogen at the cathode and iodine at the anode.
Example 3: Identifying oxidation and reduction (3 marks)
In the reaction between magnesium and dilute hydrochloric acid, state which species is oxidised and which is reduced, in terms of electrons.
Magnesium atoms each lose two electrons to form magnesium ions with a charge of plus 2, so magnesium is oxidised. Hydrogen ions each gain an electron and combine in pairs to form hydrogen molecules, so the hydrogen ions are reduced. Because both oxidation and reduction occur, this is a redox reaction.
Common mistakes and how to avoid them
The most common single error is sending positive ions to the positive electrode. Opposite charges attract, so positive ions go to the negative cathode. Writing this rule at the top of the answer space prevents the mistake.
Students frequently confuse oxidation and reduction when switching between the oxygen and the electron definitions. OIL RIG covers only the electron version; check which definition the question is using.
In salt-naming questions, many answers give the wrong anion. Match the acid to the salt: hydrochloric gives chloride, sulfuric gives sulfate, nitric gives nitrate.
Another regular slip is saying that all metals react with acid to give hydrogen. Metals below hydrogen in the reactivity series, such as copper, do not react with dilute acid at all.
Finally, in the soluble salt practical, students often omit the reason for adding excess base. The reason is to ensure all the acid has reacted, and it is usually worth a mark on its own.
Exam technique for "Chemistry: Chemical Changes"
When a question gives reaction data for unnamed metals, build the order step by step and write it down before answering, rather than trying to hold it in your head.
Word equations should always be checked against the three general patterns. If you have written an acid and a carbonate producing only a salt and water, you have forgotten the carbon dioxide.
For electrolysis, answer in a fixed order: identify the ions present including those from water if aqueous, decide what happens at the cathode, then at the anode, then state the products. Half equations, where asked, must balance for both atoms and charge.
Where a question asks why a particular extraction method is used, the answer is always the metal's position relative to carbon. More reactive than carbon means electrolysis; less reactive means reduction with carbon.
Quick revision summary
Metals form positive ions, and the reactivity series ranks how readily they do so, with carbon and hydrogen included as reference points. A more reactive metal displaces a less reactive one from its compound. Oxidation is gain of oxygen or loss of electrons; reduction is the reverse. Metals below carbon are extracted by reduction with carbon; those above it require electrolysis, which is more expensive. Acids produce hydrogen ions and alkalis produce hydroxide ions, and neutralisation forms water. Acid plus metal gives a salt plus hydrogen; acid plus base gives a salt plus water; acid plus carbonate gives a salt, water and carbon dioxide. Soluble salts are made by adding excess insoluble base, filtering, then crystallising. In electrolysis, positive ions are reduced at the cathode and negative ions oxidised at the anode. In solution, hydrogen forms at the cathode unless the metal is less reactive than hydrogen, and oxygen forms at the anode unless a halide is present.