Displacement and Neutralisation Reactions
At Olympic Dam in South Australia, OZ Minerals extracted 143,000 tonnes of copper in 2022 using the same displacement reaction you can do with an iron nail.
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If you place an iron nail in a blue copper sulfate solution, the solution slowly turns green and a reddish-brown coating forms on the nail. What do you think is happening? Where is the copper coming from, and where is the iron going?
Metals are ranked by how reactive they are, some react vigorously with water or acid, others barely react at all. Why do you think gold jewellery lasts thousands of years without corroding, while an iron car body rusts within years? What does "reactivity" mean to you?
● Know
- The definition of a displacement reaction
- The metal reactivity series at this level level
- That neutralisation is a reaction type: acid + base → salt + water
● Understand
- Why a more reactive metal displaces a less reactive metal from its compound
- How to use the reactivity series to predict displacement outcomes
- The observations that indicate a displacement reaction has occurred
● Can do
- Predict whether a displacement reaction will occur using the reactivity series
- Describe the practical observations of displacement reactions
- Classify neutralisation as a distinct reaction type
Drop an iron nail into blue copper sulfate solution and within minutes a reddish-brown coating of solid copper appears on the nail as the blue colour fades, one metal literally shoving another out of its compound. A displacement reaction occurs when a more reactive element displaces a less reactive element from its compound. In metal displacement reactions, a more reactive metal displaces a less reactive metal from a solution of its salt.
The general equation is:
Metal A + Salt of Metal B -> Salt of Metal A + Metal B
(where Metal A is more reactive than Metal B)
The reactivity series is a ranking of metals by their reactivity. Common metals from most to least reactive: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, tin, lead, hydrogen, copper, silver, gold. A metal can displace any metal below it in the series from its compounds.
Displacement reactions are redox reactions: the more reactive metal is oxidised (loses electrons) while the less reactive metal ion is reduced (gains electrons). The driving force is the difference in reactivity - more reactive metals have a stronger tendency to lose electrons.
When iron is placed in copper sulfate solution, iron (more reactive) displaces copper (less reactive): Fe(s) + CuSO4(aq) -> FeSO4(aq) + Cu(s). The iron nail becomes coated with reddish-brown copper metal. The blue colour of the solution fades because Cu2+ ions (blue) are replaced by Fe2+ ions (pale green). If you reverse the experiment and place copper in iron sulfate solution, nothing happens because copper is below iron in the reactivity series and cannot displace it.
Australian mining chemistry: Displacement reactions are used in solution mining and metal recovery. At the Olympic Dam mine in South Australia, uranium is extracted from ore using acid leaching, then recovered through precipitation and ion exchange processes that exploit reactivity differences. In gold mining, the carbon-in-pulp process uses activated carbon to displace gold cyanide complexes from solution, concentrating the gold for final recovery by electrolysis.
Displacement happens because the more reactive metal is stronger physically. This is false. Reactivity is a chemical property related to electron configuration and ionisation energy, not physical strength. Sodium is extremely reactive but soft enough to cut with a knife. Gold is unreactive but strong enough for jewellery and electronics. A metal position in the reactivity series depends on thermodynamics and kinetics of electron transfer, not mechanical properties.
You place a clean iron nail into a blue copper sulfate solution. Predict what will happen to the nail and the solution over 30 minutes.
The iron nail becomes coated with reddish-brown copper metal. The blue solution fades as copper ions are displaced and iron ions enter solution.
Use these terms in your explanation: displacement · reactivity series · iron · copper
The reactivity series of metals is one of the most useful predictive tools in chemistry. It ranks metals by how readily they lose electrons to form positive ions. Metals at the top lose electrons easily and are strongly reducing. Metals at the bottom hold onto their electrons and are weakly reducing.
Key positions:
- Potassium, sodium, calcium: Extremely reactive. React with cold water to produce hydrogen gas and metal hydroxides. Must be stored under oil.
- Magnesium, aluminium, zinc: Quite reactive. React with steam and dilute acids. Form protective oxide layers.
- Iron, tin, lead: Moderately reactive. React with dilute acids but not water. Iron rusts in moist air.
- Hydrogen: Included as a reference. Metals above hydrogen react with dilute acids to displace hydrogen gas.
- Copper, silver, gold: Unreactive. Do not react with dilute acids. Found naturally as native metals or easily extracted from ores.
The reactivity series explains why some metals corrode rapidly while others remain pristine for millennia.
The Statue of Liberty in New York is made of copper sheets over an iron framework. When first assembled, engineers did not consider galvanic corrosion. Copper is below iron in the reactivity series, so in the presence of salt water (an electrolyte), iron corrodes preferentially to protect the copper. Over decades, the iron framework severely corroded, requiring major restoration. This case demonstrates how ignoring the reactivity series can cause engineering failures.
Australian corrosion science: Australia harsh coastal environments accelerate metal corrosion. The CSIRO Corrosion Management team studies how reactivity series principles apply to Australian infrastructure. Galvanising (coating iron with zinc) protects steel because zinc is above iron in the reactivity series and corrodes preferentially. Cathodic protection systems for pipelines and offshore platforms exploit the same principle on a massive scale, using sacrificial anodes of zinc or magnesium.
The reactivity series is just a list to memorise. This is false. The series is derived from measurable quantities: standard electrode potentials, reaction rates with acids and water, and reduction potentials. It has predictive power. If you know the reactivity series, you can predict displacement reactions, corrosion behaviour, and extraction methods without memorising every specific reaction. It is a model, not a list.
- Potassium
- Iron
- Copper
- Gold
- Found naturally as the pure metal
- Reacts violently with cold water
- Reacts with steam but not cold water
- Does not react with dilute acids
Displacement reactions have numerous practical applications in industry, construction, and environmental protection.
Metal extraction: Less reactive metals like copper and silver can be extracted by displacement using more reactive metals or carbon. More reactive metals require electrolysis because no common reducing agent is strong enough to displace them from their ores.
Thermite welding: Aluminium powder reacts with iron oxide in a highly exothermic displacement reaction: 2Al + Fe2O3 -> 2Fe + Al2O3. The reaction produces molten iron at about 2,500C, which is poured into gaps between railway tracks to weld them together. This technique is used for repairing railway lines in remote areas where conventional welding equipment is unavailable.
Sacrificial protection: Blocks of zinc or magnesium are attached to steel ships, pipelines, and offshore platforms. These more reactive metals corrode preferentially, protecting the steel. The sacrificial anodes must be replaced periodically as they are consumed.
The Sydney Harbour Bridge is protected from corrosion by multiple systems. The paint provides a physical barrier. Galvanising protects bolts and smaller components. For submerged parts, cathodic protection using sacrificial anodes prevents rusting in the salty harbour water. Engineers monitor the anode consumption and replace them before they are fully depleted. Without this protection, the bridge steel would corrode rapidly in the aggressive marine environment, compromising structural integrity.
Australian infrastructure protection: The oil and gas industry operating off Western Australia and in the Timor Sea uses extensive cathodic protection systems for offshore platforms and subsea pipelines. These systems prevent the billions of dollars of steel infrastructure from corroding in warm, salty water. The design and monitoring of these systems employ electrochemists who apply reactivity series principles at enormous scale.
Once a sacrificial anode is consumed, the protected metal immediately rusts. This is not quite true. While the protected metal loses its active protection when the anode is gone, it may still have passive protection from paint, coatings, or oxide layers. However, if all protection systems fail, corrosion will begin. This is why regular inspection and maintenance of protection systems are essential for long-lived infrastructure.
Wrong: "Any metal can displace any other metal." No � only a more reactive metal can displace a less reactive one. Gold cannot displace zinc.
Right: Only a more reactive metal can displace a less reactive one from its compound. The reactivity series determines the outcome, a metal higher in the series will displace one lower down, never the reverse.
Wrong: "A displacement reaction is the same as a decomposition reaction." No � displacement involves one element replacing another in a compound. Decomposition is one compound breaking apart.
Right: Displacement and decomposition are different reaction types. In displacement, one element replaces another inside a compound; in decomposition, a single compound breaks apart into two or more simpler substances.
Wrong: "Neutralisation is not a real chemical reaction." Yes it is, new substances (salt and water) are formed, and the reaction is usually exothermic with observable temperature change.
Right: Neutralisation is absolutely a chemical reaction, new substances (a salt and water) are produced, and the temperature usually rises, indicating that the reaction is exothermic.
Galvanising and the Mining Industry
Australia is one of the world's largest producers of iron ore and zinc. The reactivity series has direct industrial applications here. Galvanising is the process of coating iron or steel with a layer of zinc. Even if the zinc coating is scratched, the zinc (more reactive) will corrode preferentially, protecting the iron underneath. This is why galvanised steel is used for Australian fencing, roofing and outdoor structures.
In Australian mining, understanding the reactivity series helps engineers choose the best methods for extracting metals from their ores. More reactive metals like aluminium require electrolysis for extraction, while less reactive metals like copper can be extracted using simpler chemical methods.
✍ Copy Into Your Books
▾Displacement Reactions
- General: A + BC → AC + B
- More reactive displaces less reactive
- Look for colour changes and metal deposition
Reactivity Series (key metals)
- Mg > Al > Zn > Fe > Sn > Pb > Cu > Ag > Au
- Any metal displaces metals below it
- Gold is least reactive; magnesium is very reactive
Neutralisation
- acid + base → salt + water
- Usually exothermic (releases heat)
- pH moves toward 7 (neutral)
Predict the Outcome
Describe the Evidence
At the start of this lesson, the hook described an iron nail dropped into blue copper sulfate solution turning reddish-brown within minutes, iron literally pushing copper out of its own compound. Did you have an idea of why that happens before you started?
Now that you understand the reactivity series and how displacement reactions work, can you explain in your own words why iron can displace copper but not the other way around? How has your understanding of what "more reactive" means shifted from your initial thinking?
Q1. 1. Explain what a displacement reaction is and how the reactivity series helps predict whether a displacement will occur. 4 MARKS
Q2. 2. A student places a zinc strip into copper sulfate solution. Describe the expected observations and explain the reaction using the reactivity series. 4 MARKS
Q3. 3. Explain why galvanising iron with zinc protects the iron from rusting, even if the zinc coating is scratched. Use the reactivity series in your answer. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Were your predictions about the iron nail and copper sulfate correct?
- Can you now explain what happens using the reactivity series?
Model answers (click to reveal)
Answers
▾MCQ 1
CIn a displacement reaction, a more reactive element replaces a less reactive element in a compound.
MCQ 2
DMagnesium is the most reactive metal in the list provided. Gold is the least reactive.
MCQ 3
CMagnesium is more reactive than copper, so it displaces copper from copper sulfate. The blue solution fades as colourless magnesium sulfate forms, and reddish-brown copper metal deposits on the magnesium strip.
MCQ 4
DCopper is less reactive than magnesium, so copper cannot displace magnesium from magnesium sulfate solution. No reaction occurs.
MCQ 5
BZinc is more reactive than iron in the reactivity series. This means zinc will corrode (react with oxygen and water) preferentially, protecting the iron underneath. Even if scratched, the zinc continues to sacrifice itself.
Short Answer 1
Model answer: A displacement reaction is a reaction where a more reactive element replaces a less reactive element in a compound. The reactivity series helps predict displacement because it ranks metals from most reactive to least reactive. Any metal can displace any metal below it in the series. For example, zinc (above copper) can displace copper from copper sulfate, but copper (below zinc) cannot displace zinc from zinc sulfate.
Short Answer 2
Model answer: The expected observations are: the blue colour of the copper sulfate solution gradually fades as colourless zinc sulfate forms, and a reddish-brown solid (copper metal) deposits on the surface of the zinc strip. Using the reactivity series, zinc is more reactive than copper. This means zinc displaces copper from the copper sulfate solution. The word equation is: zinc + copper sulfate → zinc sulfate + copper.
Short Answer 3
Model answer: Galvanising protects iron because zinc is more reactive than iron in the reactivity series. When the iron is coated with zinc, the zinc reacts with oxygen and water in preference to the iron, preventing rust. Even if the zinc coating is scratched, the exposed zinc still protects the iron because zinc corrodes preferentially, it 'sacrifices' itself. This is called sacrificial protection and is widely used in Australian fencing, roofing and marine structures.