Predicting Displacement Reactions

Introduction

Suppose a strip of iron is added to zinc sulfate solution, and a strip of zinc is added to iron(II) sulfate solution. The same two metals appear in both trials, but only one direction is expected to give ordinary displacement. The reactivity series provides the deciding comparison before anyone balances an equation or interprets an observed coating.

Core explanation

Use the common order as a guide: Mg, Al, Zn, Fe, Pb, H, Cu, Ag and Au appear in descending reactivity in a useful portion of the school series. Some complete versions also include potassium, sodium, calcium and tin. Find the solid metal first. Next find the metal represented by cations in the aqueous salt. A solid metal higher in the series can usually displace the lower metal from a suitable salt solution. A solid lower in the series is not predicted to displace the higher one under ordinary conditions.

Zinc is above iron. Thus Zn(s) + FeSO₄(aq) → ZnSO₄(aq) + Fe(s) is the expected direction in the simplified comparison. For an iron strip in ZnSO₄, the corresponding Fe + ZnSO₄ → FeSO₄ + Zn is not expected. The latter string is balanced for atoms, yet balance is only a conservation requirement. It does not decide whether reactants have a favourable chemical pathway. Treat “no reaction” as no significant displacement under the stated standard school conditions; complicated conditions might call for another analysis.

Pay attention to the oxidation state represented by the salt. Iron(II) sulfate contains Fe²⁺, so replacing it with Zn²⁺ gives a neat one-to-one net equation: Zn + Fe²⁺ → Zn²⁺ + Fe. Copper(II) nitrate contains Cu²⁺; magnesium can replace copper in Mg + Cu(NO₃)₂ → Mg(NO₃)₂ + Cu. By contrast, silver nitrate contains Ag⁺, so one Mg atom that forms Mg²⁺ can reduce two silver ions: Mg + 2AgNO₃ → Mg(NO₃)₂ + 2Ag. Predicting the direction comes first, then correct formulas and balanced coefficients follow.

A table of test pairs can expose a partial order. If metal A displaces B²⁺ and B displaces C²⁺, the school interpretation is A above B above C. If C does not displace A²⁺, that agrees with the same ordering. One observation may be misleading when a strip has an oxide layer, solution is very dilute or the metal deposit is hard to see. Repeated observations under comparable conditions strengthen a conclusion. A quick reaction and a slow reaction can have the same predicted direction; rate and direction are distinct questions.

The series is particularly helpful with salts in solution because mobile hydrated ions can reach the solid surface. It should not be applied mechanically to any pair of names in a question. A chemical formula might describe an insoluble solid rather than an aqueous salt, or the metal might react with water instead. An oxide-coated aluminium strip, for example, can fail to show an immediate visible displacement even though aluminium ranks highly. The apparent absence of a coating is not always a clean contradiction of the underlying order.

Hydrogen is a useful comparison marker for metal–acid reactions, but do not insert it as though it were a metal ion in every salt-displacement table. The specific reactants and conditions set the task. If asked about Cu in aqueous AgNO₃, compare Cu with Ag, not Cu with H. If asked about Cu in dilute HCl producing H₂, compare Cu with H and note the acid limitation.

Step-by-step reasoning

1. Underline the solid metal and the metal named in the aqueous salt. 2. Locate their positions in the given or known reactivity series. 3. Predict displacement only if the solid is higher, subject to the stated solution and surface conditions. 4. Write chemically valid formulas, balance the equation and describe a possible observation.

Visual explanation

Draw a vertical ladder Zn above Fe above Cu above Ag. Put a metal strip on one rung and an ion target below it. Draw a downward arrow for an expected displacement, but no upward arrow for the reverse pairing.

Real-world analogy

A ranking used for one competition answers only the relevant matchup. The series helps compare a solid metal with the metal ion it meets. It does not automatically tell how quickly a coated strip reacts or what happens in a different type of reaction.

Real-world example

In a classroom comparison, a clean copper wire can develop silver crystals in aqueous silver nitrate. Copper is above silver, so Cu can become Cu²⁺ while Ag⁺ becomes Ag. A silver wire placed in a suitable copper(II) solution would not show the reverse simple displacement.

Why?

Why use the series before balancing? Balanced symbols constrain atom counts, but both a forward and a reversed proposed displacement may balance on paper. Relative reactivity supplies the chemical direction. Formula writing then expresses that direction accurately.

Common misconception

“If an equation balances, it must happen.” Fe + ZnSO₄ → FeSO₄ + Zn balances, but ordinary iron does not displace zinc from zinc sulfate. Fe is below Zn, so the predicted displacement direction is the reverse.

Worked example

Decide whether Mg(s) reacts with AgNO₃(aq). Magnesium is above silver, so displacement is expected. Magnesium forms Mg²⁺, and silver ions are Ag⁺. Two Ag⁺ ions are needed for the two electrons from one Mg atom. The balanced equation is Mg(s) + 2AgNO₃(aq) → Mg(NO₃)₂(aq) + 2Ag(s). Nitrate remains unchanged; the net change is Mg + 2Ag⁺ → Mg²⁺ + 2Ag.

Quick check

1. Will an iron strip normally displace zinc from aqueous zinc sulfate? Answer: No. Iron lies below zinc in the common reactivity series.

Exam focus

Explicitly state which metal is higher. When no displacement is expected, write “no reaction under the stated conditions” rather than inventing products. When it is expected, check ion charges before balancing coefficients.

Advanced insight

The standard series is a compact summary of electrochemical tendencies under reference conditions. Actual solution concentrations can change electrode potentials, and films can change kinetics. This explains why a predicted thermodynamic direction and a dramatic visible result are not identical claims.

Summary

Compare the solid metal with the metal cation in the salt. A higher solid metal can usually displace a lower one from suitable aqueous solution, while the reverse is not ordinarily predicted. Only after choosing the direction should you write formulas and balance the equation.

Practice questions

1. Does Zn(s) displace Fe from FeSO₄(aq)? Answer: Yes. Zn is above Fe, giving ZnSO₄(aq) and Fe(s) in the simple model. 2. Predict Cu(s) placed in aqueous MgSO₄. Answer: No ordinary displacement; Cu is below Mg in the series. 3. Write the balanced reaction of Cu with aqueous AgNO₃. Answer: Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag. 4. Why can aluminium show a delayed visible response despite its high series position? Answer: Its protective oxide coating can limit contact between the metal and solution.