In the previous blog post, we have discussed on the importance of the Reactivity Series of Metals as well as the mnemonic (super memory technique) to remember the sequence of metals in terms of their relative reactivities. In case you have missed that blog post which comes with a learning Chemistry Youtube Video, do check it out.
In the Reactivity Series, metals are arranged from the most reactive to the least reactive.
So how is the order of reactivity of metals being determined?
The order of reactivity of metals are determined by the scientists based on the:
- Reaction of metals with cold water or steam
- Reaction of metals with dilute acids such as hydrochloric acid
Determining the Order of Reactivity of Metals
A) Reaction of Metals with Cold Water or Steam
The more reactive metals tend to react with cold water to form metal hydroxide (alkaline solution) and hydrogen gas.
This reaction can be easily represented by the following word equation:
Metal + Water → Metal Hydroxide + Hydrogen Gas
Note that a more reactive metal will react more violently with cold water.
Some metals such as zinc and iron, do not react with cold water but they do react with steam. Such metals will react with steam to form metal oxide and hydrogen gas.
This reaction can be easily represented by the following word equation:
Metal + Steam → Metal Oxide + Hydrogen Gas
Note that a more reactive metal reacts violently with steam.
Check out the table below on the observations and chemical equation for the reaction of metals with cold water and/or steam.
| Metal | Reaction with Cold Water / Steam |
| Potassium | 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g) violent reaction |
| Sodium | 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) violent reaction |
| Calcium | Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g) reacts readily |
| Magnesium | Mg(s) + 2H2O(l) → Mg(OH)2(aq) + H2(g) very slow reaction Mg(s) + H2O(g) → MgO(s) + H2(g) violent reaction |
| Aluminium | Al(s) + 2H2O(l) → No Reaction 2Al(s) + 3H2O(g) → Al2O3(s) + 3H2(g) reacts readily |
| Zinc | Zn(s) + H2O(l) → No Reaction Zn(s) + H2O(g) → ZnO(s) + H2(g) reacts readily |
| Iron | Fe(s) + H2O(l) → No Reaction 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g) reacts slowly |
| Tin | Sn(s) + H2O(l) → No Reaction Sn(s) + H2O(g) → SnO(s) + H2(g) reacts readily |
| Lead | Pb(s) + H2O(l) → No Reaction Pb(s) + H2O(g) → No Reaction |
| Copper | Cu(s) + H2O(l) → No Reaction Cu(s) + H2O(g) → No Reaction |
| Silver | Ag(s) + H2O(l) → No Reaction Ag(s) + H2O(g) → No Reaction |
| Gold | Au(s) + H2O(l) → No Reaction Au(s) + H2O(g) → No Reaction |
B) Reaction of Metals with Dilute Hydrochloric Acid
Most metals react with dilute acids to form a salt solution and hydrogen gas.
This reaction can be easily represented by the following word equation:
Metal + Dilute Acid → Salt Solution + Hydrogen Gas
The reactions of the metals with the dilute acids will also indicate how reactive the metals are and this is used to place them in the Reactivity Series.
A more reactive metal will react more violently with the dilute acid.
Check out the table below on the observations and chemical equation for the reaction of metals with dilute acid.
| Metal | Reaction with HCl(aq) |
| Potassium | 2K(s) + 2HCl(aq) → 2KCl(aq) + H2(g) reacts explosively |
| Sodium | 2Na(s) + 2HCl(aq) → 2NaCl(aq) + H2(g) reacts explosively |
| Calcium | Ca(s) + 2HCl(aq) → CaCl2(aq) + H2(g) reacts violently |
| Magnesium | Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) reacts rapidly |
| Aluminium | 2Al(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2(g) reacts rapidly |
| Zinc | Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) reacts moderately |
| Iron | Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g) reacts slowly |
| Tin | Sn(s) + 2HCl(aq) → SnCl2(aq) + H2(g) reacts slowly |
| Lead | “No apparent reaction” |
| Copper | No Reaction |
| Silver | No Reaction |
| Gold | No Reaction |
Note that lead should react easily with dilute acids since it is higher than hydrogen in the Reactivity Series.
However, do note that when hydrochloric acid is being used, the initial reaction between lead and hydrochloric acid will form an insoluble layer of lead (II) chloride. This becomes a protective layer and prevents further reaction of the hydrochloric acid with the underlying lead metal. As such, reaction slows down and eventually stops. Hence, lead does not appear to react with hydrochloric acid. This is being heavily covered in my Sec 3 GCE O-Level and IP Pure Chemistry Tuition Classes.
The chemical equation with state symbols for the reaction:
Pb(s) + 2HCl(aq) → PbCl2(s) + H2(g)
Similarly, the above applies when sulfuric acid is being used. Lead does not appear to react with sulfuric acid because of the insoluble layer of lead (II) sulfate coated onto the underlying lead metal.
The chemical equation with state symbols for the reaction:
Pb(s) + H2SO4(aq) → PbSO4(s) + H2(g)
Thus, based on the reactions of metals with cold water, steam and dilute hydrochloric acid, we can place metals in order of their reactivity i.e. Reactivity Series of Metals.
YouTube Video Tutorial on Reaction of Metals with Water, Steam and Dilute Acids
You can watch the YouTube Video below to have a greater understanding of the reactions between metals and water, steam as well as dilute acids.
Click on the following link for the video on O-Level Chemistry . IP Chemistry: Reaction of Metals with Water, Steam and Dilute Acids.
Length of Video: 11.27 minutes
Before we end the post, let me give you a quick check multiple-choice question to test your understanding and applications skills. I have given this question previously to my Sec 3 O-Level Pure Chemistry and IP Chemistry Tuition Classes for discussion.
Question:
Which of the following metal will displace hydrogen from aqueous solutions of acids but not from cold water?
A) Calcium
B) Copper
C) Sodium
D) Zinc
Do write your answers in the Comment section below. Have fun!
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