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C2.5 · Predict single-displacement products using activity series

Learn to predict single-displacement products using activity series through clear examples and targeted practice.

Ontario Grade 11 Chemistry

Chemical Reactions

Using the activity series to decide whether one element can replace another

A piece of metal placed in a solution containing ions of another metal may become coated with a different metal. This visible change can be explained using particles: atoms of the more active metal enter the solution as ions, while ions of the less active metal form neutral atoms. The activity series helps predict whether this kind of change can occur. You will use it to decide whether a displacement is predicted, then write and check the products.

What you will learn

Describe a single-displacement reaction and recognize its general pattern.
Use an activity series to decide whether a proposed displacement is predicted.
Write correct products when a displacement occurs, and state when there is no reaction.

1. Bridge: elements, ions, and reaction patterns

An element is a substance made of one kind of atom. An ion is an atom or group of atoms with an electric charge. A metal atom can form a positive ion by losing electrons. For example, a zinc atom can form a zinc ion, written as

\mathrm{Zn^{2+}}

. The superscript shows the ion’s charge. A subscript shows the number of atoms in a formula.

A chemical equation uses formulas to show starting substances and products. Reactants are written before the arrow, and products after it. A single-displacement reaction is one in which a free element replaces a different element in a compound. Its general pattern is one element plus one compound producing a new compound and a different free element.

An equation must be balanced: each kind of atom must appear in equal numbers on both sides. When writing an ionic equation, the total charge must also match on both sides. Write correct formulas first. Then balance by changing coefficients, not subscripts.

\mathrm{A + BC \rightarrow AC + B}

A free element may replace a different element that is part of a compound.
Write correct formulas first, then balance with coefficients.
A coefficient changes the number of particles represented. A subscript is part of a substance’s formula.

2. Use the activity series to decide

The activity series is an ordered list used to compare elements for single-displacement predictions. In a course reference table, an element higher on the list is more active than an element lower on it. For a metal replacing a metal, the free metal must be higher than the metal it would replace. If it is lower, predict no reaction.

For example, if magnesium is above copper in the activity series, magnesium can replace copper ions in a solution of a copper compound. Copper cannot replace magnesium from a magnesium compound because copper is lower in the series. If a question does not provide the needed order, consult the activity series in your course reference.

Some course activity series include hydrogen. A metal above hydrogen can replace hydrogen from an acid in a single-displacement prediction. A metal below hydrogen cannot. Use the series supplied for your course when making this comparison.

Some course tables also list halogens, which are non-metal elements. For a halogen displacement, compare the free halogen with the halogen in the compound using the halogen entries in the table. A more active halogen can replace a less active halogen. Compare like entries; do not apply the metal comparison to halogens.

\text{higher activity} \;\Rightarrow\; \text{displacement predicted}

For a metal replacing a metal, the free metal must be higher in the activity series.
For a metal replacing hydrogen from an acid, compare the metal with hydrogen if hydrogen is included in the supplied series.
For a halogen replacing a halogen, compare the halogens using their entries in the series.

3. Connect particles to product formulas

When a metal displaces another metal, the free metal forms positive ions in the compound. The displaced metal ions form neutral metal atoms. The charges of the two metals do not have to be the same. For example, magnesium can replace silver ions: magnesium forms

\mathrm{Mg^{2+}}

, while silver forms neutral silver atoms. Write each product’s formula from the ions and the compound’s composition, rather than copying the displaced ion’s charge onto the new metal.

Use a course formula table when needed. A polyatomic ion is a group of atoms that carries an overall charge and is treated as a unit in a formula. Sulfate,

\mathrm{SO_4^{2-}}

, is one example. In zinc sulfate, zinc ions and sulfate ions combine in a one-to-one ratio because their charges balance.

State symbols show the physical state or solution form when it is known. The symbol

(aq)

means dissolved in water,

(s)

means solid, and

(g)

means gas. Add states when they are given or clear from the question. Do not invent a state when the question provides no basis for it.

After deciding that displacement is predicted, write the new compound and the displaced element. Check that each formula is correct, then balance the equation. A correct prediction includes both the activity comparison and the resulting products.

The new metal ion may have a different charge from the displaced metal ion.
Use ion charges and formula tables to write the new compound correctly.
Check formulas and atom counts; also check total charge for ionic equations.

4. A reliable prediction routine

First identify the free element and the element in the compound that might be replaced. Compare those two elements using the appropriate entries in the activity series. If the free element is more active, predict a displacement. If it is less active, write no reaction.

When a reaction is predicted, write the new compound and the displaced element as products. Use correct ion charges and formula rules to determine the compound’s formula. Then balance using coefficients and include relevant states. Finally, check that the activity comparison, products, formulas, and atom counts agree.

This process gives a course-level prediction. It does not require claiming that a reaction has been observed or inventing measurements.

Identify the free element and the element it might replace.
Use the activity series before deciding whether to write products.
Check product formulas, states when known, and balance.

Worked example

Zinc placed in copper(II) sulfate solution

Predict the products when solid zinc is placed in aqueous copper(II) sulfate. Use the activity series, then write a balanced equation with states.

Identify the possible replacement
Zinc is the free element. Copper is the metal in the compound that zinc could replace. In the course activity series, zinc is above copper.
Decide whether displacement occurs
Zinc is more active than copper, so it can replace copper ions. The products are zinc sulfate and copper metal. Zinc forms $\mathrm{Zn^{2+}}$ , and sulfate is $\mathrm{SO_4^{2-}}$ , so the zinc sulfate formula is $\mathrm{ZnSO_4}$ . The charges of the two metal ions need not be the same in other reactions.
$\mathrm{ZnSO_4 + Cu}$
Write and check the equation
The formulas and atom counts match with coefficients of one. Zinc and copper ions each have a charge of $2+$ in their compounds, and sulfate has a charge of $2-$ . The complete equation shows zinc sulfate dissolved in water and copper as a solid.
$\mathrm{Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)}$

Answer: The predicted products are aqueous zinc sulfate and solid copper. The balanced equation is

\mathrm{Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)}

Check: Each side contains one zinc atom, one copper atom, one sulfur atom, and four oxygen atoms. Zinc is above copper in the activity series, so the displacement is predicted.

Common mistakes and how to avoid them

Assuming every combination of an element and a compound will react.

Correction: Use the activity series first. If the free element is below the element it would replace, predict no reaction.

Reversing the activity comparison.

Correction: The free element must be higher in the series than the element being replaced.

Changing subscripts to balance an equation.

Correction: Subscripts define a substance’s formula. Balance by changing coefficients only.

Giving the new metal ion the same charge as the displaced metal ion.

Correction: Determine each metal ion’s charge separately, then use the charges to write the new compound formula.

Writing a displaced metal as an ion among the products.

Correction: In a metal-for-metal displacement, the displaced metal is written as a neutral element.

Lesson summary

A single-displacement reaction occurs when one element replaces another element in a compound.
Use the activity series to decide whether the free element can replace the element in the compound.
If displacement is predicted, write correct product formulas, include relevant states, and balance the equation.
If the free element is lower in activity, the predicted result is no reaction.

Check your understanding

Question 1

A metal is above copper in the activity series. What does the series predict when it is placed in a solution containing copper ions?

It can replace copper, so a single-displacement reaction is predicted.
Copper replaces the metal because copper is in solution.
No reaction is predicted because a metal cannot replace a metal.
The activity series cannot be used for metal ions.

Show answer and explanation

It can replace copper, so a single-displacement reaction is predicted.

A free metal higher in the series can replace a less active metal from a compound.

Question 2

A free metal is below magnesium in the activity series. What is predicted if it is proposed to replace magnesium from a magnesium compound?

The free metal replaces magnesium.
Magnesium replaces the free metal.
No displacement is predicted.
Both metals become gases.

Show answer and explanation

No displacement is predicted.

The free metal is less active than magnesium, so it cannot replace magnesium according to the activity-series rule.

Question 3

Magnesium is placed in a solution containing silver ions. Which statement correctly describes the predicted products?

Magnesium ions form, and silver forms as a neutral metal.
Silver ions form, and magnesium forms as a neutral metal.
Both metals remain ions in the same compound.
The magnesium ion must have the same charge as the silver ion.

Show answer and explanation

Magnesium ions form, and silver forms as a neutral metal.

Magnesium is more active than silver. Magnesium forms its positive ion, while displaced silver ions become neutral silver atoms. Their ion charges are not required to be the same.

Key terms

Activity series: An ordered list used to compare elements when predicting whether one can replace another in a single-displacement reaction.
Single-displacement reaction: A reaction in which one element replaces another element in a compound.
Reactant: A starting substance in a chemical reaction.
Product: A substance formed in a chemical reaction.
Aqueous: Dissolved in water; represented by the state symbol $(aq)$ .
Polyatomic ion: A group of atoms that carries an overall charge and is treated as a unit in a formula.

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation C2.5. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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