👍 Welcome to the Reactivity Series!
Hello future Chemists! This chapter is all about metals, but specifically, it helps us answer a really important question: How easily does a metal react?
The Reactivity Series is simply a list of metals arranged in order of how vigorously they react. Understanding this series allows you to predict the products of reactions, figure out how to extract metals from their ores, and explain why we use certain metals for certain jobs (like wiring or building airplanes!).
Don't worry if the list looks long—we have some easy ways to remember the order!
1. Defining and Ordering the Reactivity Series
What is Reactivity?
In chemistry, a metal's reactivity is determined by how easily it loses its outer electrons to form a positive ion.
- Highly Reactive Metals lose electrons very easily.
- Less Reactive Metals hold onto their electrons tightly.
The Full Reactivity Series (Core Content 9.4.1)
The standard list includes common metals and two non-metals (Carbon and Hydrogen) for comparison, as they are crucial in extraction and acid reactions.
Order (Most Reactive to Least Reactive):
- Potassium (K)
- Sodium (Na)
- Calcium (Ca)
- Magnesium (Mg)
- Aluminium (Al)
- Carbon (C) (A non-metal used as a reference)
- Zinc (Zn)
- Iron (Fe)
- Hydrogen (H) (A non-metal used as a reference)
- Copper (Cu)
- Silver (Ag)
- Gold (Au)
💯 Memory Aid (Mnemonic)
To remember this exact order (K, Na, Ca, Mg, Al, C, Zn, Fe, H, Cu, Ag, Au), try this phrase:
Please Stop Calling Me A Cute Zebra Instead Try Learning How Copper Saves Gold.
(Note: 'I' for Iron, 'T'/'L' are just filler words, but the main element initials are there: K, Na, Ca, Mg, Al, C, Zn, Fe, H, Cu, Ag, Au)
Reactivity in Terms of Ion Formation (Supplement Content 9.4.4)
The list is ordered by the metal's tendency to form positive ions (\(M^+\), \(M^{2+}\), etc.).
- Metals high in the series (K, Na) have a high tendency to lose electrons and form ions. They are powerful reducing agents.
- Metals low in the series (Ag, Au) have a low tendency to lose electrons. They prefer to stay as solid metal atoms.
✔ Key Takeaway: Reactivity Order
The higher a metal is in the series, the more reactive it is. This means it is easier to change from an atom (M) to a positive ion (\(M^+\)).
2. Reactions with Water and Acids (Core Content 9.4.2)
The position of a metal tells us exactly what substances it will react with. We test metals using cold water, steam, and dilute acids.
Reactions with Water
1. Very Reactive Metals (K, Na, Ca)
These react vigorously with cold water to produce a metal hydroxide and hydrogen gas.
Example: Sodium with cold water:
\(2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)\)
Observation: Sodium moves quickly, melts into a ball, and produces hydrogen gas (which may catch fire if the reaction is very vigorous, like with Potassium).
2. Moderately Reactive Metals (Mg, Zn, Fe)
Magnesium reacts very slowly with cold water, but most of these metals only react well with steam (hot water vapour). They produce a metal oxide and hydrogen gas.
Example: Magnesium with steam:
\(Mg(s) + H_2O(g) \rightarrow MgO(s) + H_2(g)\)
Observation: The metal glows brightly (especially Mg) and a gas (hydrogen) is produced.
3. Least Reactive Metals (Cu, Ag, Au)
These metals do not react with water or steam at all.
Reactions with Dilute Acids (e.g., dilute HCl or \(H_2SO_4\))
If a metal is placed above hydrogen (H) in the series, it is reactive enough to push hydrogen out of the acid (a displacement reaction). They form a salt and hydrogen gas.
General Equation: Metal + Acid \(\rightarrow\) Salt + Hydrogen
- Mg, Zn, Fe: React readily, producing hydrogen gas (effervescence).
- K, Na, Ca: React violently, often dangerously (we rarely test these with acids in school).
- Cu, Ag, Au: Do not react with dilute acids because they are less reactive than hydrogen.
Example: Zinc with dilute hydrochloric acid:
\(Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)\)
✔ Quick Review: Reactions
Reactivity decreases as you go down the list. The most reactive metals react with cold water; moderate ones need steam; metals below hydrogen don't react with dilute acids.
3. Displacement Reactions
The reactivity series is most famously used to predict displacement reactions. This is where a more reactive element "kicks out" a less reactive element from its compound (usually an aqueous solution of a salt).
The Golden Rule of Displacement
A more reactive metal can displace a less reactive metal from a solution of its salt.
Why? Because the more reactive metal has a greater tendency to form positive ions (it wants to lose its electrons more).
Analogy: Imagine the reactivity series is a ranking of how much metals want a job (to become an ion). A high-ranking metal (Zinc) can easily steal the job from a low-ranking metal (Copper).
Displacement Example (Core & Supplement 9.4.4)
Consider Zinc metal placed into Copper(II) sulfate solution (\(CuSO_4\)).
Since Zinc (Zn) is higher than Copper (Cu), Zinc will displace Copper.
Reaction:
\(Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)\)
Observations:
- The blue colour of the Copper(II) sulfate solution fades (as \(Cu^{2+}\) ions are removed).
- A reddish-brown solid (Copper metal) forms on the surface of the Zinc.
✅ Understanding Displacement using Electrons (Extended only)
Displacement is a redox reaction (reduction and oxidation).
- Oxidation (Loss of electrons): Zinc metal loses electrons to become Zinc ions:
\(Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-\) (Zinc is the reducing agent) - Reduction (Gain of electrons): Copper ions gain electrons to become Copper metal:
\(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)\) (Copper ion is the oxidising agent)
Metal Displacement Summary
If you mix Iron (Fe) with Magnesium chloride (\(MgCl_2\)), no reaction occurs because Iron is less reactive than Magnesium.
✔ Key Takeaway: Displacement
The metal atom that is more reactive will lose electrons and form ions; the ion of the less reactive metal will gain electrons and become a neutral atom.
4. Deducing the Order of Reactivity (Core Content 9.4.3)
In the lab, you may be given a set of experimental results (like observations of reaction speeds with water or acids) and asked to determine the order of reactivity.
Step-by-Step Deduction:
- Look for the most vigorous reactions: The metal that reacts most violently with cold water (or steam/acid) is the most reactive.
- Use Displacement Tests: If metal X displaces metal Y from its salt solution, then X is more reactive than Y.
- Look for 'No Reaction': If metal Z does not react with steam or dilute acid, it is likely placed below Hydrogen.
- Rank the data: Arrange the metals from the fastest, most vigorous reaction to the slowest, or no reaction at all.
Example: If metal A reacts violently with acid, metal B reacts slowly, and metal C doesn't react at all, the order of reactivity is A > B > C.
5. The Special Case of Aluminium (Al) (Supplement Content 9.4.5)
If you look at the reactivity series, Aluminium (Al) is quite high, just below Magnesium (Mg) and above Carbon (C) and Zinc (Zn).
Based on its position, you would expect Aluminium to be highly reactive and react vigorously with air, water, and acids. However, in reality, aluminium containers and foils are very stable!
Why is Aluminium apparently unreactive?
This is due to a thin, tough layer of aluminium oxide (\(Al_2O_3\)) that forms instantly on the surface of the metal when it is exposed to air.
- When pure aluminium is first made, it reacts immediately with oxygen in the air:
\(4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)\) - This oxide layer is extremely strong, unreactive, and non-porous (it doesn't have holes).
- This oxide layer acts like a natural suit of armour, preventing further contact between the metal underneath and the surrounding air or water, thus making the aluminium seem unreactive.
Did you know? This protective layer is why aluminium is used for aircraft manufacturing, despite being highly reactive chemically. Its stability means it resists corrosion beautifully!
✎ Summary Checklist: Reactivity Series
What you must know (Core & Extended)
- State the order of the reactivity series (K to Au, including C and H).
- Describe and explain the reactions of metals with cold water (K, Na, Ca), steam (Mg, Zn, Fe), and dilute acids (all metals compared to H).
- Deduce the order of reactivity from experimental observations.
- Understand and predict displacement reactions: a more reactive metal displaces a less reactive metal from its salt solution.
For those aiming for top grades (Extended only)
- Explain reactivity in terms of the tendency to form positive ions (losing electrons).
- Explain the apparent unreactivity of aluminium due to the presence of its protective oxide layer (\(Al_2O_3\)).
- Use ionic half-equations to show the oxidation and reduction occurring during displacement (e.g., \(Zn \rightarrow Zn^{2+} + 2e^-\)).
Keep practising the order and those reaction patterns—you've got this!