Chemistry (9202) | The reactivity series

The Reactivity Series: A Chemist's League Table

Hello future chemists! Welcome to one of the most useful and predictable topics in chemistry: The Reactivity Series.
Don't worry if chemical reactions sometimes feel confusing—this chapter gives you a clear set of rules, almost like a "league table," that tells us exactly which metals are strong and which are weak.

Understanding the reactivity series is essential for predicting if a chemical reaction will happen, especially when metals are involved. Let's dive in!

1. What Does "Reactivity" Mean?

In simple terms, reactivity is a measure of how easily a substance reacts chemically. For metals, this usually means how easily they lose their outer electrons to form positive ions.

Key Concept: Losing Electrons

• Metals want to lose electrons to achieve a stable electronic configuration.
• A highly reactive metal loses electrons very easily. It is desperate to react! (Think of sodium—it reacts violently with water.)
• A low reactivity metal holds onto its electrons tightly. It reacts slowly, if at all. (Think of gold—it stays shiny and doesn't rust.)

Analogy: Imagine a foot race. The highly reactive metals are the sprinters—they start the reaction immediately and finish fast. The low reactivity metals are the slow walkers—they barely move.

2. The Standard Reactivity Series Order

The reactivity series is simply a list of metals arranged in order of decreasing reactivity (most reactive at the top, least reactive at the bottom).

We also include Carbon (C) and Hydrogen (H) in the series. While they are not metals, their positions are crucial because they help us understand how metals are extracted from their ores.

The Full Series (From Most to Least Reactive)

1. Potassium (K) - Most Reactive
2. Sodium (Na)
3. Calcium (Ca)
4. Magnesium (Mg)
5. Aluminium (Al)
6. Carbon (C) (Non-metal benchmark)
7. Zinc (Zn)
8. Iron (Fe)
9. Lead (Pb)
10. Hydrogen (H) (Non-metal benchmark)
11. Copper (Cu)
12. Silver (Ag)
13. Gold (Au) - Least Reactive

Memory Aid: Mnemonics are Your Best Friend!

To remember this tricky order, try using a mnemonic (a phrase where the first letter of each word matches the first letter of the element):

"Please Stop Calling Me A Careless Zebra Instead Try Learning How Copper Saves Gold."

K=Please, Na=Stop, Ca=Calling, Mg=Me, Al=A, C=Careless, Zn=Zebra, Fe=Instead, Pb=Try, H=Learning, Cu=How Copper, Ag=Saves, Au=Gold.

3. Testing Reactivity: Reactions with Water and Acids

We can prove the order of the reactivity series by observing how vigorously metals react with common substances like water (H₂O) and dilute acids (e.g., HCl).

A. Reactions with Water (H₂O)

The position of the metal determines whether it reacts with cold water, steam, or not at all.

Reactivity Group	Metals	Reaction Type	Products Formed
Highest (K, Na, Ca)	Potassium, Sodium, Calcium	Reacts vigorously with cold water.	Metal hydroxide + Hydrogen gas (\(H_2\))
Middle (Mg, Zn, Fe)	Magnesium, Zinc, Iron	Reacts only with steam (hot water vapour).	Metal oxide + Hydrogen gas (\(H_2\))
Lowest (Cu, Ag, Au)	Copper, Silver, Gold	No reaction with cold water or steam.	No reaction

General Equation (Cold Water):
Metal + Water → Metal Hydroxide + Hydrogen

General Equation (Steam):
Metal + Steam → Metal Oxide + Hydrogen

B. Reactions with Dilute Acids (e.g., Hydrochloric Acid, HCl)

Metals that are above Hydrogen (H) in the series will react with dilute acid, producing a salt and hydrogen gas. Metals below Hydrogen (Cu, Ag, Au) will not react.

General Equation:
Metal + Acid → Salt + Hydrogen gas (\(H_2\))

• K, Na, Ca: React violently and dangerously.
• Mg, Zn, Fe: React vigorously, with the speed decreasing as we go down (Mg is fastest, Fe is slowest).
• Cu, Ag, Au: No visible reaction. They are less reactive than hydrogen and cannot displace it from the acid.

Did you know? The speed at which the hydrogen bubbles are produced is a direct indicator of the metal's position in the series!

4. The Power of Prediction: Displacement Reactions

The most important application of the reactivity series is predicting whether one metal can 'kick out' or displace another metal from its compound (usually a solution of a salt).

The Golden Rule of Displacement

A more reactive metal will displace a less reactive metal from a solution of its salt.

Analogy: Think of two students sitting on a chair. If a stronger (more reactive) student walks in, they can push the weaker (less reactive) student off the chair and take their spot. The weaker student cannot do the same to the stronger one.

Step-by-Step Prediction Example

Will zinc metal (Zn) react with copper sulfate solution (\(CuSO_4\))?

1. Identify the Reacting Metals: Zinc (Zn) and Copper (Cu, which is in the solution).
2. Check the Series: Look at the reactivity list. Zinc is above Copper.
3. Apply the Rule: Since Zinc is more reactive, it can displace Copper.
4. Write the Result: Zinc takes Copper’s place, forming zinc sulfate, and solid copper is produced (often seen as a brown/pink coating).

Equation (Reaction Occurs):
\(Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)\)

What about the reverse? Will copper metal (Cu) react with zinc sulfate solution (\(ZnSO_4\))?

1. Identify the Reacting Metals: Copper (Cu) and Zinc (Zn).
2. Check the Series: Copper is below Zinc.
3. Apply the Rule: Copper is less reactive; it cannot displace Zinc.
4. Write the Result: No reaction occurs.

Equation (No Reaction):
\(Cu(s) + ZnSO_4(aq) \rightarrow \text{No Reaction}\)

Common Pitfall to Avoid

Do not confuse the metal that starts the reaction (the pure element) with the metal that is already in the compound (the ion). Always compare the two metals on the series before deciding!