Using reactivity series & displacement series
In this topic you will:
- use displacement reactions to identify an unknown metal
- learn about some useful displacement reactions.
Key words
- molten
- ores
Using displacement reactions
Using the fact that more reactive metals can displace less reactive ones from their salts can be very useful in a number of ways. It is possible to use displacement reactions to help identify an unknown metal.
Think Like a Scientist
Your task is to identify a mystery metal from the reactivity series. You can test how it reacts with water, oxygen, dilute acid, and in displacement reactions. This helps narrow down which metal it could be.
Part 1: Planning the investigation
Plan an investigation using chemical tests and observations.
Safety
You do not know what this metal is, so treat it with care and pick it up using forceps.
Method:
Step 1: Identify the chemicals and equipment you will use.
Step 2: Write a step-by-step plan of what you will do.
Step 3: For each test, describe what information it gives and how it helps identify the metal.
Part 2: Carrying out the investigation
Once approved by your teacher, carry out the tests and record what you observe.
Safety
Make sure you have carried out a risk assessment.
Method:
Step 1: Record your results from each test.
Step 2: If new clues make you want to change your plan, ask your teacher first.
Step 3: Keep track of any changes and why you made them.
Show Answer
It depends on your observations. If it reacts strongly with acid and displaces many metals, it may be magnesium or zinc. If it barely reacts, it could be copper.
Show Answer
If it reacted with dilute acid and gave off bubbles, that shows it's reactive. If it displaced copper but not zinc in solutions, it lies between those two in the reactivity series.
Show Answer
Yes, as some tests gave unexpected results, the plan was updated to include more displacement reactions to narrow down which metal it could be.
Example: Displacing Copper
When you place a strip of zinc metal into a solution of copper sulfate, a displacement reaction occurs:
zinc + copper sulfate → zinc sulfate + copper
This happens because zinc is more reactive than copper. The copper is displaced from the solution, and you can see solid copper forming on the zinc strip.
Using displacement reactions in industry (extension material)
Aluminium is a metal that is above zinc and below magnesium in the reactivity series shown in Topic 5.1. Aluminium will displace iron from solid iron oxide if it is heated.
aluminium + iron oxide → aluminium oxide + iron
This reaction releases a lot of energy. It is an exothermic reaction. The temperature gets so high that the iron that is produced is molten (in a liquid state). The melting point of iron is 1535 °C.
In the photograph you can see the reaction being used to weld railway rails together. Often, the rails need to be welded in situ (in other words, on the railway lines and not in a workshop where you have all the welding equipment you will need). The iron oxide and aluminium powder react in a container placed on the rails. The molten iron produced in the reaction is shaped and used to join the rails together. This reaction is called the thermite reaction.
In order for the thermite reaction to take place, the iron oxide and aluminium mixture has to be ignited. This is done using another exothermic reaction — this time between magnesium powder and barium nitrate. This reaction provides the energy to start the displacement reaction between the aluminium and iron oxide.
Example: Thermite Reaction in Railway Welding
To weld two railway rails together in the field, a container is placed over the joint and filled with a mixture of iron oxide and aluminium powder. When ignited, the thermite reaction produces molten iron at a high temperature. This molten iron flows into the gap and solidifies, forming a strong weld between the rails.
Displacement using carbon (extension material)
Carbon is not a metal, but it can be used to displace some metals from their compounds. Carbon will displace zinc, iron, tin and lead from their ores. An ore is a rock that contains a metal compound.
People discovered that carbon could displace iron around 3500 years ago. They discovered that iron ore heated with charcoal (a form of carbon) at very high temperatures produced molten iron. Today this displacement reaction is still carried out, but on a large scale, in a blast furnace.
The iron ore is mainly iron oxide. This reacts with carbon to form iron and carbon dioxide.
The word equation for this reaction is:
iron oxide + carbon → iron + carbon dioxide
This industrial process helps extract iron from iron ore using carbon as a reducing agent.
Common Mistake
It’s easy to think that carbon is a metal because it takes part in displacement reactions. However, carbon is a non-metal that works by reducing metal oxides — not through typical metallic reactivity.
Questions
Show Answer
No, iron cannot displace aluminium from aluminium oxide because iron is less reactive than aluminium. Only a more reactive metal can displace a less reactive one from its compound.
Show Answer
The thermite reaction produces molten iron at very high temperatures, which can fill the gap between rails and solidify to create a strong, durable weld — even on-site without workshop equipment.
Show Answer
Because iron ore and coal (used to make coke) are both needed in large quantities for the blast furnace process. Locating furnaces near these resources reduced transport costs and increased efficiency.
Think Like a Scientist
In this task, you will use carbon to try to extract a metal from its oxide. Carbon will only displace a metal that is less reactive than itself.
You will need:
- safety glasses
- copper oxide powder
- charcoal powder
- spatula
- test tube or boiling tube
- heatproof mat
- Bunsen burner
Safety
Remember your safety! Wear safety glasses.
Method:
1. Place a spatula of copper oxide in a test tube or boiling tube.
2. Add a spatula of charcoal powder on top. Do not mix the powders.
3. Heat the tube strongly in a Bunsen flame for five minutes.
4. Allow the tube to cool and examine the boundary between the layers.
5. Record your observations.
Show Answer
Yes, a reaction occurred if copper metal is visible and the black copper oxide has reduced. This shows that carbon has displaced the copper.
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Copper oxide + carbon → copper + carbon dioxide
Show Answer
It shows carbon is more reactive than copper because it was able to remove oxygen from copper oxide and form copper metal.
Show Answer
Carbon is placed above copper in the reactivity series because it can displace copper from its oxide. However, it cannot displace more reactive metals like zinc or magnesium.