Chapter 10: Metals
Chemical properties of metals
The metal reactivity series below shows how metals are arranged according to its reaction with cold water, steam and dilute hydrochloric acid.
| Reactivity decreases | Elements | Reactions | |||
|---|---|---|---|---|---|
| + Cold water H₂O (l) | + Steam H₂O (g) | + dilute hydrochloric acid HCl (aq) | |||
Most reactive
|
Please | Potassium | Metal + H₂O (l) → Metal hydroxide + H₂ | Metal + H₂O (g) → Metal oxide + H₂ | Metal + HCl → Metal chloride + H₂ |
| Stop | Sodium | ||||
| Calling | Calcium | ||||
| Me | Magnesium | ||||
| A | Aluminium | No reaction with cold water. | |||
| Cool | Carbon | ||||
| Zebra | Zinc | ||||
| I | Iron | ||||
| Like | Lead | No reaction with steam. | Does not seem to react with dil HCl due to insoluble layer of PbCl₂ formed. | ||
| Hot | Hydrogen | No reaction with dilute. HCl. | |||
| Cute | Copper | ||||
| Sexy | Silver | ||||
| Greens | Gold | ||||
| Reactivity decreases | Elements | Reactions | |||
|---|---|---|---|---|---|
| + Cold water H₂O (l) | + Steam H₂O (g) | + dilute hydrochloric acid HCl (aq) | |||
| Most reactive
|
Please | Potassium | Metal + H₂O (l) → Metal hydroxide + H₂ | Metal + H₂O (g) → Metal oxide + H₂ | Metal + HCl → Metal chloride + H₂ |
| Stop | Sodium | ||||
| Calling | Calcium | ||||
| Me | Magnesium | ||||
| A | Aluminium | No reaction with cold water. | |||
| Cool | Carbon | ||||
| Zebra | Zinc | ||||
| I | Iron | ||||
| Like | Lead | No reaction with steam. | Does not seem to react with dil HCl due to insoluble layer of PbCl₂ formed. | ||
| Hot | Hydrogen | No reaction with dil. HCl. | |||
| Cute | Copper | ||||
| Sexy | Silver | ||||
| Greens | Gold | ||||
Metal displacement reaction: This is a reaction where a more reactive metal displaces a less reactive metal from its salt solution. This concept under metals is very closely related to qualitative analysis as students need to be able to determine what they can observe when the reaction is carried out.
| Mg (s) | + | CuSO₄ (aq) | → | MgSO₄ (aq) | + | Cu (s) |
|---|---|---|---|---|---|---|
| Grey solid | Blue solution | Colourless solution | Reddish-brown solid |
In the example above, magnesium, being more reactive than copper, displaces copper from copper(lI) sulfate solution to form magnesium sulfate solution and copper metal.
Now, what will we observe if we placed Zn powder in a test tube containing sodium sulfate solution?
There will be no visible reaction as
- Zinc, being less reactive than sodium, is not able to displace sodium from sodium sulfate. Thus, no displacement reaction occurs.
- Zinc metal is not reactive enough to react with water. Hence, effervescence is not observed.
Question 1
The following observations were made when nickel and iron were placed separately into solutions of metals S, T and U.
The diagram below shows how the cross method is done to find the formula of ionic compounds.
What is the correct order in increasing reactivity of the five metals?
A S < Ni < Fe < T < U
B S < Ni < T < Fe < U
C U < Fe < T < Ni < S
D U < T < Fe < Ni < S
Answer
U cannot be displaced by Nickel or Iron, thus U is the most reactive metal. It has to appear last in the sequence as the question is asking for increasing order of reactivity, from the least reactive metal to the most reactive metal. Options C and D are eliminated.
S was displaced from its salt solution by Nickel and Iron, meaning it is the least reactive metal. It should appear first in the answer.
T was only displaced by Iron but not nickel. This means that Iron is more reactive than T, which in turn is more reactive than Nickel.
Thus the order of reactivity in increasing order (least to most reactive) will be S, Ni, T, Fe, U.
Answer is B.
Thermal decomposition of metal carbonate
The ease of decomposition of a carbonate when heated depends on the position of the metal in the metal reactivity series.
Group I metals form very thermally stable compounds. Hence, Group I carbonates (K₂CO₃ and Na₂CO₃) do not undergo thermal decomposition to form an oxide and carbon dioxide. If a crucible containing Group I carbonates is heated, there will be no change in mass before and after heating.
The other metal carbonates will decompose to form its corresponding metal oxides and carbon dioxide. An exception will be silver carbonate which upon heating will decompose to form silver metal, carbon dioxide and oxygen gas. If we measure the mass of the crucible containing the less thermally stable carbonates before heating and after heating, there will be a decrease in mass as carbon dioxide produced escapes into the air.
| Reactivity decreases | Elements | Carbonate | Observation | ||
|---|---|---|---|---|---|
| Most reactive
|
Please | Potassium | K₂CO₃ | Group l Carbonates are unaffected by heating with Bunsen flame. | |
| Stop | Sodium | Na₂CO₃ | |||
| Calling | Calcium | CaCO₃ | Carbonate decomposes into metal oxides and carbon dioxide. eg.: PbCO₃ → PbO + CO₂ | ||
| Me | Magnesium | MgCO₃ | |||
| A | Aluminium | Al₂(CO₃)₃ | |||
| Cool | Carbon | ||||
| Zebra | Zinc | ZnCO₃ | |||
| I | Iron | FeCO₃ | |||
| Like | Lead | PbCO₃ | |||
| Hot | Hydrogen | ||||
| Cute | Copper | CuCO₃ | |||
| Sexy | Silver | Ag₂CO₃ | Silver carbonate decomposes into silver, carbon dioxide and oxygen on heating. | ||
| Greens | Gold | - | |||
Extraction Method
There are 3 ways to extract a metal – electrolysis of molten salt, reduction using carbon, reduction using hydrogen.
| Reactivity decreases | Elements | Extraction of metal from ores | ||
|---|---|---|---|---|
|
Most reactive
|
Please | Potassium | Electrolysis of its molten salt eg: 2NaCl → 2Na + Cl₂ | |
| Stop | Sodium | |||
| Calling | Calcium | |||
| Me | Magnesium | |||
| A | Aluminium | |||
| Cool | Carbon | |||
| Zebra | Zinc | Reduction using Carbon eg: 2CuO + C → 2Cu + CO₂ | ||
| I | Iron | Reduction using Hydrogen eg: CuO + H₂ → Cu + H₂O | ||
| Like | Lead | |||
| Hot | Hydrogen | |||
| Cute | Copper | |||
| Sexy | Silver | Found chemically uncombined | ||
| Greens | Gold | |||
3.1. Extraction of metals by electrolysis
It is harder to extract very reactive metals from its ore because the ores formed are very stable. Thus, the only way to extract the very reactive metals is via electrolysis of its molten salt. For example, sodium ore is heated to its molten state before an electric current is passed through it to obtain sodium and chlorine.
3.2. Extraction of metals by reduction
The oxides are heated with carbon or hydrogen to give the metal.
Take note that to extract zinc metal from its ore, reduction using hydrogen will not work. Reduction using hydrogen is only possible for metals below zinc.
3.2.1. Blast furnace (Extraction of Iron from Haematite)
The 3 solid raw materials are:
- Haematite, Fe2O3
- Coke, C
- Limestone, CaCO3
The waste gases produced are:
- Nitrogen (from hot air blown in)
- Carbon monoxide
- Carbon dioxide
Take note that molten slag floats on molten iron as it has a lower density than molten iron.
| No. | Explanation | Chemical equation |
|---|---|---|
| 1. | Coke reacts with oxygen in hot air to produce carbon dioxide. | C + O₂ → CO₂ |
| 2. | Carbon dioxide reacts with more coke to form carbon monoxide. | C + CO₂ → 2CO |
| 3. | Carbon monoxide reduces iron(III) oxide to form molten iron. | Fe₂O₃ + 3CO → 2Fe + 3CO₂ |
| 4. | Limestone decomposes to form calcium oxide and carbon dioxide due to the high temperature of blast furnace. | CaCO₃ → CaO + CO₂ |
| 5. | Calcium oxide, which is a base, reacts with silicon dioxide, an acidic impurity found in the haematite ore, to form molten calcium silicate. | SiO₂ + CaO → CaSiO₃ |
Important things to remember:
Question 3
Element X occurs naturally as the uncombined metal; element Y is obtained by chemical reduction of its oxide with carbon; element Z is obtained by electrolysing its molten chloride.
What is the order of reactivity of X, Y and Z?
| Least reactive → Most Reactive | |||
|---|---|---|---|
| A | X | Y | Z |
| B | Y | X | Z |
| C | Z | X | Y |
| D | X | Z | Y |
Answer
Since element X occurs naturally, it is the least reactive metal.
Since element Z is obtained by electrolysis of molten chloride, Z is the most reactive metal.
Thus, the metals arranged in increasing reactivity is X < Y < Z.
Answer is A.