2.6.1 Activity Series of Metals

The activity series is a practical and extremely useful application of the electrochemical series. It ranks metals according to their ability to displace other metals or hydrogen from their compounds. This section explains how the activity series is derived, what it means, and how it can be used to predict whether a single displacement reaction will occur.

Definition

The activity series (also known as the reactivity series) is a list of metals arranged in order of their decreasing ease of oxidation to their respective ions in aqueous solution. In this ranking, metals and hydrogen are arranged in order of their ability to replace other metals and hydrogen from their compounds.

Simple Explanation: It is a "ladder" of metal reactivity. Metals at the top are the most reactive (they lose electrons easily), while metals at the bottom are the least reactive.

Core Concept

The core concept is that a more reactive metal (one higher in the activity series) can displace a less reactive metal (one lower in the series) from its compound. This is because the more reactive metal has a greater tendency to lose electrons and become an ion, while the less reactive metal ion has a greater tendency to gain those electrons and become a neutral atom.

The activity series is directly derived from the electrochemical series: metals with the most negative standard reduction potentials are the most active (most easily oxidized), and metals with the most positive reduction potentials are the least active.

Detailed Explanation

How the Activity Series Works

A displacement or replacement reaction occurs when an element displaces another element that is a part of a compound. The general equation is: A + XY \longrightarrow X + AY In this reaction, atom A replaces atom X in the compound XY. For this to happen, A must be more reactive (higher in the activity series) than X.

Example 1: Zinc and Copper(II) Sulphate

When a piece of zinc metal is placed in a blue solution of copper(II) sulphate, the blue colour slowly fades away, and grey metal is replaced by red-orange copper metal. Zn_{(s)} + Cu^{2+}_{(aq)} \longrightarrow Zn^{2+}_{(aq)} + Cu_{(s)}

• In this reaction, Cu ions in the solution are reduced to Cu metal, and Zn atoms are oxidized to Zn ions.
• The electrochemical series shows that the standard reduction potential of copper (+0.34 V) is greater than that of zinc (-0.76 V). This means it is easier to oxidize Zn to its ions and reduce Cu^{2+} ions to its atoms.
• Thus, Zn can replace Cu^{2+} ions from its solution.

Example 2: Copper and Zinc Sulphate

However, when copper metal is placed in a zinc sulphate solution, no replacement reaction occurs. Cu_{(s)} + Zn^{2+}_{(aq)} \longrightarrow \text{No Reaction}

• This is because the standard reduction potential of copper is greater than that of zinc. Copper is less easily oxidized than zinc.
• Copper does not have enough "reducing power" to transfer electrons to zinc ions. Thus, Cu cannot replace Zn^{2+} ions from its solution.

Other Examples

• Mg and Al can also displace Cu^{2+} ions because they are higher in the activity series (more negative reduction potentials) than copper.
• Ag cannot displace Cu^{2+} ions because silver is lower than copper in the activity series (more positive reduction potential).
• Na and K can displace H_2 from water because they are above hydrogen in the activity series.
• Cu and Ag cannot displace H_2 from water because they are below hydrogen in the activity series.

Rules for Predicting Reactions

1. Metals higher on the list transfer electrons to metal cations lower on the list. The greater the separation between the species, the more vigorous the reaction will be. 2. Example: When powdered barium is heated with lead(II) oxide, a replacement reaction occurs because Ba is more active than Pb. Ba_{(s)} + PbO_{(s)} \longrightarrow BaO_{(s)} + Pb_{(s)} Barium is oxidized to form barium oxide, and lead is reduced to elemental lead. 3. Example: When iron pellets are added to a solution of MgCl_2, no reaction will occur because Fe is below Mg in the activity series. Iron cannot displace magnesium.

Intermediate Level

The activity series is a ranking of metals based on their standard reduction potentials. A metal with a more negative reduction potential (like zinc, -0.76 V) is more easily oxidized and is therefore more active. A metal with a more positive reduction potential (like copper, +0.34 V) is less easily oxidized and is less active. In a single displacement reaction, a more active metal (higher in the series) will displace a less active metal (lower in the series) from its salt solution. The reaction is spontaneous if the metal added is higher in the series than the metal in the solution.

Important Points

• - Activity Series: A ranking of metals by their ability to displace other metals and hydrogen.
• - Order: Metals and hydrogen are arranged in order of decreasing ease of oxidation.
• - Most Active Metals: Have the most negative reduction potentials (e.g., K, Na, Ca, Mg, Al, Zn).
• - Least Active Metals: Have the most positive reduction potentials (e.g., Cu, Ag, Au).
• - Displacement Reaction: A more active metal (higher in the series) can displace a less active metal (lower in the series) from its compound.
• - Example: Zn displaces Cu, but Cu cannot displace Zn.

Remember

• - Activity Series: Ranking of metals by reactivity.
• - Higher in the series = more reactive = more easily oxidized.
• - Lower in the series = less reactive = less easily oxidized.
• - A metal can displace another metal from solution only if it is higher in the series.
• - Zn displaces Cu.
• - Cu cannot displace Zn.
• - Na and K displace H_2 from water; Cu and Ag do not.

Common Mistakes

• - Confusing the activity series with the electrochemical series. The activity series is a ranking of metals by their ease of oxidation, while the electrochemical series is a list of standard reduction potentials of all half-reactions (including non-metals). The activity series is derived from the electrochemical series.
• - Reversing the trend. Remember that a metal higher in the series (more negative reduction potential) is more reactive. A metal lower in the series (more positive reduction potential) is less reactive.
• - Thinking that any metal can displace any other metal. Displacement only occurs if the metal added is higher in the series than the metal in the solution.
• - Forgetting that hydrogen is included in the series. Metals above hydrogen can displace it from water or acids, while metals below hydrogen cannot.
• - Assuming that a reaction will occur simply because two substances are mixed. You must check their relative positions in the activity series.

Quick Revision

The activity series is a ranking of metals in order of decreasing ease of oxidation. Metals higher in the series are more reactive and are more easily oxidized. This series is used to predict whether a single displacement reaction will occur. A more reactive metal (higher in the series) can displace a less reactive metal (lower in the series) from its compound. For example, zinc (higher) can displace copper (lower) from copper sulphate solution, but copper cannot displace zinc from zinc sulphate solution. The activity series also includes hydrogen, allowing us to predict whether a metal can displace hydrogen from water or acids.

60-Second Revision

The activity series ranks metals by reactivity (ease of oxidation). A more reactive metal (higher in the series, more negative reduction potential) can displace a less reactive metal (lower in the series) from its compound. For example, Zn displaces Cu, but Cu cannot displace Zn. Metals above hydrogen can displace it from water/acids; metals below cannot.

Frequently Asked Questions (FAQs)

Q1. What is the activity series of metals? Answer: The activity series is a ranking of metals in order of their decreasing ease of oxidation (i.e., decreasing reactivity) to their respective ions in aqueous solution. Metals at the top are the most easily oxidized and are the most reactive, while metals at the bottom are the least easily oxidized and are the least reactive.

Q2. How does the activity series relate to the electrochemical series? Answer: The activity series is derived from the electrochemical series. Metals with the most negative standard reduction potentials (like potassium, sodium, and zinc) are the most easily oxidized and are at the top of the activity series. Metals with the most positive reduction potentials (like copper, silver, and gold) are the least easily oxidized and are at the bottom of the activity series.

Q3. Why can zinc displace copper from a copper sulphate solution? Answer: Zinc is higher than copper in the activity series (it has a more negative reduction potential, -0.76 V vs. +0.34 V). This means zinc is more easily oxidized than copper. Therefore, zinc can transfer electrons to copper ions, causing zinc to be oxidized (to Zn^{2+}) and copper ions to be reduced (to Cu). This is a spontaneous displacement reaction.

Q4. Why can't copper displace zinc from a zinc sulphate solution? Answer: Copper has a more positive reduction potential (+0.34 V) than zinc (-0.76 V), so it is less easily oxidized than zinc. Copper does not have enough reducing power to transfer electrons to zinc ions. Therefore, no reaction occurs. Copper is lower than zinc in the activity series.

Q5. What does the activity series tell us about a metal's ability to displace hydrogen? Answer: Metals above hydrogen in the activity series (which have a more negative reduction potential than hydrogen, 0.00 V) can displace hydrogen from water or acids. For example, sodium and potassium react vigorously with water to produce hydrogen gas. Metals below hydrogen, such as copper and silver, cannot displace hydrogen from water.

Q6. Can a metal that is lower in the activity series displace a metal that is higher? Answer: No. For a displacement reaction to occur, the metal that is added must be more reactive (higher in the activity series) than the metal in the compound. A less reactive metal cannot displace a more reactive metal from its compound.

Q7. Give an example of a displacement reaction from the textbook. Answer: When zinc metal is placed in a copper(II) sulphate solution, zinc displaces copper: Zn_{(s)} + Cu^{2+}_{(aq)} \rightarrow Zn^{2+}_{(aq)} + Cu_{(s)} This occurs because zinc is more active (higher in the activity series) than copper. The blue colour of the solution fades, and red-brown copper metal deposits on the zinc.

Q8. What happens when iron pellets are added to a solution of MgCl_2? Answer: No reaction occurs. Iron is below magnesium in the activity series, so it cannot displace magnesium from its compound. Magnesium is more reactive than iron.

Q9. What is the general rule for predicting if a single displacement reaction will occur? Answer: A single displacement reaction will occur if the metal that is added is higher in the activity series (more reactive) than the metal in the compound. If the added metal is lower in the series, no reaction will occur.

Multiple Choice Questions (MCQs)

• Q1. Which of the following metals is most easily oxidized?
• A. Copper
• B. Zinc
• C. Silver
• D. Gold
• Correct Answer: B
• Explanation: Zinc has a more negative reduction potential (-0.76 V) than copper, silver, and gold, making it the most easily oxidized of the choices given.

• Q2. What is the activity series of metals?
• A. A list of metals by atomic number.
• B. A ranking of metals by their ease of oxidation.
• C. A list of standard reduction potentials of all elements.
• D. A ranking of metals by density.
• Correct Answer: B
• Explanation: The activity series ranks metals in order of decreasing ease of oxidation, which corresponds to their reactivity.

• Q3. Which of the following metals can displace copper from a copper sulphate solution?
• A. Silver
• B. Gold
• C. Zinc
• D. Platinum
• Correct Answer: C
• Explanation: Zinc is higher than copper in the activity series (more negative reduction potential), so it can displace copper from its salt solution.

• Q4. Which of the following metals cannot displace hydrogen from water?
• A. Sodium
• B. Potassium
• C. Copper
• D. Calcium
• Correct Answer: C
• Explanation: Copper is below hydrogen in the activity series (more positive reduction potential), so it cannot displace hydrogen from water.

• Q5. Which of the following metals can displace zinc from a zinc sulphate solution?
• A. Copper
• B. Silver
• C. Magnesium
• D. Gold
• Correct Answer: C
• Explanation: Magnesium is higher than zinc in the activity series, so it can displace zinc from its compound. Copper, silver, and gold are lower than zinc.

Q6. The activity series is derived from which of the following? A. Atomic numbers B. Standard reduction potentials C. Boiling points D. Solubility rules Correct Answer: B Explanation: The activity series is directly derived from the standard reduction potentials of the metals. Metals with more negative reduction potentials are higher in the activity series.

• Q7. When zinc is placed in a copper sulphate solution, which substance is the reducing agent?
• A. Zn
• B. Cu^{2+}
• C. Zn^{2+}
• D. Cu
• Correct Answer: A
• Explanation: Zinc is oxidized to Zn^{2+}, so it acts as the reducing agent. It reduces copper ions to copper metal.

• Q8. Which of the following statements about the activity series is correct?
• A. Metals lower in the series are more reactive.
• B. Metals higher in the series are more easily oxidized.
• C. Metals higher in the series are stronger oxidizing agents.
• D. The activity series is the same as the electrochemical series.
• Correct Answer: B
• Explanation: The activity series ranks metals in order of decreasing ease of oxidation. Metals higher in the series are more easily oxidized (more reactive).

• Q9. Which of the following reactions will not occur based on the activity series?
• A. Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu
• B. Cu + Zn^{2+} \rightarrow Cu^{2+} + Zn
• C. Mg + Cu^{2+} \rightarrow Mg^{2+} + Cu
• D. Ba + PbO \rightarrow BaO + Pb
• Correct Answer: B
• Explanation: Copper is lower than zinc in the activity series, so it cannot displace zinc from its compound. This reaction is non-spontaneous.

Q10. What happens when powdered barium is heated with lead(II) oxide? A. No reaction occurs. B. Barium is reduced and lead is oxidized. C. Barium displaces lead, forming barium oxide and lead metal. D. Lead displaces barium. Correct Answer: C Explanation: Barium is higher than lead in the activity series, so it can displace lead from lead(II) oxide. The reaction is Ba + PbO \rightarrow BaO + Pb.

Multiple Choice Questions (MCQs)

• Q1. What does the activity series rank metals by?
• A. Atomic number
• B. Ease of oxidation
• C. Density
• D. Melting point
• Correct Answer: B
• Explanation: The activity series ranks metals in order of decreasing ease of oxidation, which is their reactivity.

Q2. Which of the following metals is lowest in the activity series? A. Sodium B. Zinc C. Copper D. Magnesium Correct Answer: C Explanation: Copper has a more positive reduction potential (+0.34 V) than the other metals listed, so it is the least easily oxidized and is lowest in the activity series among the choices.

• Q3. Why can zinc displace copper from copper sulphate solution?
• A. Zinc is lower than copper in the activity series.
• B. Zinc is higher than copper in the activity series.
• C. Zinc is a non-metal.
• D. Copper is more reactive than zinc.
• Correct Answer: B
• Explanation: Zinc is higher than copper in the activity series (more negative reduction potential), so it is more easily oxidized and can displace copper.

Q4. Which of the following will displace hydrogen from an acid? A. Copper B. Silver C. Gold D. Zinc Correct Answer: D Explanation: Metals above hydrogen in the activity series (more negative reduction potential) can displace hydrogen from acids. Zinc is above hydrogen, while copper, silver, and gold are below.

• Q5. What is the general equation for a displacement reaction?
• A. A + B \rightarrow AB
• B. AB \rightarrow A + B
• C. A + XY \rightarrow X + AY
• D. AB + CD \rightarrow AD + CB
• Correct Answer: C
• Explanation: In a displacement reaction, an element A replaces another element X in a compound XY, giving X + AY.

• Q6. Which metal can displace copper from a copper sulphate solution?
• A. Silver
• B. Gold
• C. Iron
• D. Platinum
• Correct Answer: C
• Explanation: Iron is higher than copper in the activity series, so it can displace copper. Silver, gold, and platinum are lower than copper.

• Q7. What happens when iron pellets are added to a solution of MgCl_2?
• A. Iron displaces magnesium.
• B. Magnesium displaces iron.
• C. No reaction occurs.
• D. Both metals dissolve.
• Correct Answer: C
• Explanation: Iron is below magnesium in the activity series, so iron cannot displace magnesium. Therefore, no reaction occurs.

Q8. The activity series is a ranking of metals in order of: A. Increasing atomic number. B. Decreasing ease of oxidation. C. Increasing reduction potential. D. Decreasing atomic mass. Correct Answer: B Explanation: The activity series ranks metals by their ability to displace other metals and hydrogen, which is based on their ease of oxidation. Metals higher in the series are more easily oxidized.

• Q9. Which of the following is true about the activity series?
• A. It includes only metals.
• B. It includes non-metals only.
• C. It lists elements by their standard reduction potentials relative to the SHE.
• D. It is a ranking of metals by their reactivity.
• Correct Answer: D
• Explanation: The activity series is a ranking of metals by their reactivity (ease of oxidation), and it is derived from the electrochemical series.

• Q10. If a metal is high in the activity series, it is a:
• A. Strong oxidizing agent.
• B. Strong reducing agent.
• C. Weak reducing agent.
• D. Weak oxidizing agent.
• Correct Answer: B
• Explanation: A metal high in the activity series is easily oxidized (loses electrons), so it is a strong reducing agent.

Multiple Choice Questions (MCQs)

• Q1. What is the activity series?
• A. A list of elements by atomic number.
• B. A ranking of metals by their reactivity (ease of oxidation).
• C. A list of standard reduction potentials of non-metals.
• D. A list of compounds by solubility.
• Correct Answer: B
• Explanation: The activity series ranks metals in order of decreasing ease of oxidation, which indicates their reactivity.

• Q2. In the activity series, a more reactive metal is:
• A. Lower in the series.
• B. Higher in the series.
• C. Always a non-metal.
• D. Always a gas.
• Correct Answer: B
• Explanation: A more reactive metal is higher in the activity series and is more easily oxidized.

• Q3. Which of the following metals can displace zinc from zinc sulphate solution?
• A. Copper
• B. Silver
• C. Magnesium
• D. Gold
• Correct Answer: C
• Explanation: Magnesium is higher than zinc in the activity series, so it can displace zinc. Copper, silver, and gold are lower than zinc.

• Q4. Why does copper not displace hydrogen from water?
• A. Copper is above hydrogen in the activity series.
• B. Copper is below hydrogen in the activity series.
• C. Copper is a non-metal.
• D. Copper reacts with water to form a different product.
• Correct Answer: B
• Explanation: Copper is below hydrogen in the activity series (more positive reduction potential), so it cannot displace hydrogen from water.

Q5. Which of the following reactions will occur? A. Cu + ZnSO_4 \rightarrow CuSO_4 + Zn B. Zn + CuSO_4 \rightarrow ZnSO_4 + Cu C. Ag + CuSO_4 \rightarrow Ag_2SO_4 + Cu D. Au + ZnSO_4 \rightarrow Au_2(SO_4)_3 + Zn Correct Answer: B Explanation: Zinc is higher than copper in the activity series, so it can displace copper from copper sulphate. The other reactions involve metals lower in the series trying to displace metals higher in the series, which is non-spontaneous.

• Q6. What happens when barium is heated with lead(II) oxide?
• A. No reaction.
• B. Barium displaces lead.
• C. Lead displaces barium.
• D. Barium oxide decomposes.
• Correct Answer: B
• Explanation: Barium is higher than lead in the activity series, so it displaces lead from lead(II) oxide, forming barium oxide and lead metal.

Q7. The activity series is based on: A. Atomic mass. B. Standard reduction potentials. C. Density. D. Melting point. Correct Answer: B Explanation: The activity series is derived from the standard reduction potentials of the metals. Metals with more negative reduction potentials are higher in the series.

• Q8. Which of the following is a strong reducing agent?
• A. A metal low in the activity series.
• B. A metal high in the activity series.
• C. A non-metal high in the activity series.
• D. A noble gas.
• Correct Answer: B
• Explanation: A metal high in the activity series is easily oxidized and is a strong reducing agent.

• Q9. What happens when iron is added to a solution of magnesium chloride?
• A. Iron displaces magnesium.
• B. Magnesium displaces iron.
• C. No reaction occurs.
• D. Both metals react with water.
• Correct Answer: C
• Explanation: Iron is below magnesium in the activity series, so it cannot displace magnesium from its compound. No reaction occurs.

• Q10. Which metal can displace hydrogen from dilute hydrochloric acid?
• A. Copper
• B. Silver
• C. Gold
• D. Zinc
• Correct Answer: D
• Explanation: Zinc is above hydrogen in the activity series, so it can displace hydrogen from acids. Copper, silver, and gold are below hydrogen.