CORE Chemistry (9222) | The reactivity series

Welcome to the Reactivity Series!

Hello future chemist! This chapter is all about predicting how metals behave. We’re diving into the Reactivity Series, which is essentially a ranking system for metals based on how eager they are to react.

Why is this important? Understanding the reactivity series helps us predict:
• Which metals will react with water or acid.
• How we can get pure metals out of their naturally occurring ores.
• Which metal will "win" in a chemical fight (called a displacement reaction!).

Don’t worry if chemical equations seem daunting right now; we’ll break down the patterns using simple analogies. Let’s get started!

Understanding the Reactivity Series: The Metal Rankings

What is Reactivity?

In chemistry, reactivity is all about how easily an element loses electrons to form positive ions. Metals are generally keen to lose electrons.

Key Definition: The reactivity series is a list of metals arranged in order of decreasing reactivity (the most reactive metal is at the top, and the least reactive is at the bottom).

• Metals high up the list are very reactive: they lose electrons easily and react vigorously.
• Metals low down the list are unreactive: they lose electrons poorly and react slowly, if at all.

The Full Line-Up (Including Carbon and Hydrogen)

Although Carbon (C) and Hydrogen (H) are non-metals, we include them in the series because they are essential reference points for predicting reactions and understanding metal extraction.

Here is the standard order you need to know, from Most Reactive (Top) to Least Reactive (Bottom):

K - Potassium
Na - Sodium
Ca - Calcium
Mg - Magnesium
Al - Aluminium
(---------- Gets less reactive ----------)
C - Carbon (Reference Point)
Zn - Zinc
Fe - Iron
Pb - Lead
H - Hydrogen (Reference Point)
Cu - Copper
Ag - Silver
Au - Gold

Memory Aid: A Simple Mnemonic

Using a mnemonic (a phrase to help you remember the order) is essential! Try this simple one:

Please Stop Calling Me A Cute Zebra In Learning How Copper Saves Gold.

(K, Na, Ca, Mg, Al, C, Zn, Fe, Pb, H, Cu, Ag, Au)

Predicting Chemical Changes: Reactions with Water and Acids

We use the reactivity series to predict whether a metal will react with water (H₂O) or dilute acids (like HCl or H₂SO₄), and how fierce that reaction will be.

1. Reaction with Water (\(H_2O\))

Metals react with water (either cold water or steam) to produce two main things: a hydroxide or oxide, and hydrogen gas (\(H_2\)).

Group A: Highly Reactive (K, Na, Ca)

• These react vigorously with cold water.
• They form the metal hydroxide and hydrogen gas.
Example: Sodium floats, melts into a ball, and moves rapidly across the surface of the water, releasing heat and hydrogen gas.

\(2Na\,(s) + 2H_2O\,(l) \rightarrow 2NaOH\,(aq) + H_2\,(g)\)

Group B: Moderately Reactive (Mg, Al, Zn, Fe)

• These metals generally react slowly or not at all with cold water.
• They require steam (very hot water vapour) to react effectively.
• They produce the metal oxide and hydrogen gas.
Did you know? Magnesium reacts very slowly with cold water, but burns brightly and quickly in steam!

Group C: Unreactive (Cu, Ag, Au)

• These metals do not react with water or steam, even when hot.

2. Reaction with Dilute Acids

Most metals above hydrogen in the series will react with dilute acids (like hydrochloric acid, HCl) to produce a salt and hydrogen gas (\(H_2\)).

The Pattern:

1. K, Na, Ca: Too dangerously reactive! We don't usually test these with acid in a lab setting.
2. Mg, Zn, Fe: These react readily. Magnesium is the fastest, producing a rapid fizzing of hydrogen gas.
3. Pb: Reacts slowly, often stopping quickly because an insoluble layer of lead chloride/sulphate forms on the metal surface, protecting the rest of the metal (this is a common exception to the rule!).
4. Cu, Ag, Au: These metals are below Hydrogen in the series. They are not strong enough to displace (push out) hydrogen from the acid. Therefore, no reaction occurs.

General Equation:
Metal + Acid \(\rightarrow\) Salt + Hydrogen Gas

Example (Zinc and Hydrochloric Acid):
\(Zn\,(s) + 2HCl\,(aq) \rightarrow ZnCl_2\,(aq) + H_2\,(g)\)

Displacement Reactions: The Strongest Metal Wins!

This is one of the most important concepts based on the reactivity series. A displacement reaction happens when a more reactive element takes the place of a less reactive element in a compound.

The Analogy of the Bully

Imagine the reactivity series is a list of strength. A metal higher up the list is chemically "stronger" than a metal lower down.

If you put a stronger metal (Metal A) into a solution containing a weaker metal's salt (Metal B's salt), Metal A will bully Metal B out of the salt solution and take its place.

How it Works: Step-by-Step

We add solid Zinc (Zn) to a solution of Copper Sulfate (\(CuSO_4\)).

1. Check the Series: Where are Zn and Cu?
Zn is much higher than Cu. Zn is the stronger metal.
2. Prediction: Since Zn is more reactive, it will displace (push out) the Copper (Cu) from the Copper Sulfate solution.
3. The Result: Zinc takes the sulfate partner, forming Zinc Sulfate (\(ZnSO_4\)). The Copper metal is left naked (it appears as a brown/pink solid coating the zinc).

\(Zn\,(s) + CuSO_4\,(aq) \rightarrow ZnSO_4\,(aq) + Cu\,(s)\)

What if we tried the opposite?
If we put solid Copper (Cu) into Zinc Sulfate (\(ZnSO_4\)), Copper is weaker than Zinc. It cannot displace Zinc. No reaction occurs.

Applications of Displacement Reactions

Displacement reactions are crucial, especially in preventing corrosion. For instance, galvanising involves coating iron (Fe) with a layer of more reactive zinc (Zn). If the layer is scratched, the zinc reacts first, protecting the iron underneath (this is called sacrificial protection).

Common Mistake Alert!
Always check the position of the two metals being compared. If the metal you are starting with is below the metal in the salt solution, nothing will happen!

Extraction of Metals: Reactivity and Industry

Metals rarely exist as pure elements in the ground; they are usually trapped in compounds called ores. We need to use chemical processes to extract the pure metal, and the method depends entirely on the metal’s position in the reactivity series.

The key dividing line for extraction is Carbon (C).

1. Metals Below Carbon (Zn, Fe, Pb, Cu, Ag, Au)

These metals are less reactive than Carbon. This means Carbon is chemically "strong enough" to displace them from their compounds (oxides) when heated.

• Method: Heating the metal oxide with Carbon (a process called reduction).
• Why it works: Carbon is more reactive than these metals and can steal the oxygen from the metal oxide.

Example (Iron Extraction):
Iron Oxide + Carbon \(\rightarrow\) Iron + Carbon Dioxide
\(2Fe_2O_3 + 3C \rightarrow 4Fe + 3CO_2\)

Did you know? Gold and Silver are so unreactive that they are often found naturally in their pure elemental form, meaning they need no chemical extraction at all!

2. Metals Above Carbon (K, Na, Ca, Mg, Al)

These metals are more reactive than Carbon. This means Carbon cannot displace them from their oxides, regardless of how hot you heat the mixture.

• Method: Electrolysis (passing an electric current through the molten metal compound).
• Why it works: This process uses electrical energy to force the unreactive metal ions to gain electrons and turn back into pure metal atoms. This is a very expensive process because it uses a lot of electricity!

Analogy: Think of electrolysis as using a massive, expensive bulldozer (electricity) to separate the metal, while reduction by carbon is like using a strong shovel (cheap, hot coal).

You have successfully mastered the basics of the reactivity series! Keep practicing the order and applying the displacement rules, and you'll be able to predict chemical reactions like a professional!