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F2.2 · Investigate a redox reaction qualitatively

Learn to investigate a redox reaction qualitatively through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Electrochemistry

Connecting visible evidence to electron transfer

A clean iron nail placed in blue copper(II) sulfate solution may develop a reddish-brown coating. The blue colour may also change. These are possible observations to look for when investigating whether a reaction occurs. The particle-level explanation is that iron atoms can transfer electrons to copper(II) ions. This lesson develops that explanation from visible evidence to chemical equations. The observations described here are predictions, not results from an experiment that has been carried out.

What you will learn

  • Describe oxidation and reduction in terms of electron transfer.
  • Distinguish predicted observations from results actually recorded.
  • Use a particle model to explain evidence that may support a redox reaction.
  • Identify the substances oxidized and reduced in a metal-displacement reaction.
  • Interpret balanced molecular and net ionic equations for the reaction.

1. Prerequisite bridge: electrons, ions, and redox

Atoms contain positively charged protons and negatively charged electrons. A neutral atom has equal numbers of protons and electrons. An ion is an atom or group of atoms with a net charge because electrons have been lost or gained.
Oxidation is the loss of electrons. Reduction is the gain of electrons. These changes happen together in a redox reaction: electrons lost by one substance are gained by another. The name redox joins the words oxidation and reduction.
The memory aid OIL RIG means Oxidation Is Loss and Reduction Is Gain. It refers to electrons. Oxygen does not have to be present. A particle's change in charge can help you track electron transfer, but the electron model explains what the change means.
oxidation: loss of electrons
  • Oxidation means loss of electrons.
  • Reduction means gain of electrons.
  • In a redox reaction, oxidation and reduction occur together.

2. Begin with evidence, then build a particle model

A qualitative investigation describes the kind of change or evidence rather than measuring its amount. Begin by recording what you can see, such as a new solid, a change in a metal surface, a solution colour change, or bubbles. These descriptions are observations. They are not yet explanations of what caused the change.
For the proposed iron and copper(II) sulfate investigation, a reddish-brown coating on the nail and a change in the blue solution are possible observations. If they occur, they could fit a particle model in which iron atoms lose electrons and become iron ions in solution. Copper(II) ions gain those electrons and become copper atoms. Copper atoms may collect on the nail as a solid.
A visible change by itself does not establish which particles changed. Compare the evidence with the substances present and with a particle model that accounts for electron transfer. A balanced equation can then check whether the proposed explanation conserves atoms and net charge.
A sound investigation separates prediction from result. State what you expect before beginning, record the starting appearance, and later record what actually happens. Do not report an expected coating or colour change as an experimental result unless it was observed.
  • Record observations separately from explanations.
  • A coating or colour change may support a redox explanation but does not identify the reacting particles by itself.
  • Predictions are not results.

3. Representing the electron transfer

A chemical equation represents substances before and after a reaction. State symbols show whether a substance is a solid, liquid, gas, or dissolved in water. The label (aq) means dissolved in water.
In the iron and copper(II) sulfate reaction, iron becomes iron(II) ions, while copper(II) ions become solid copper. The molecular equation includes sulfate as part of the dissolved compounds. The net ionic equation leaves out sulfate because it remains dissolved and unchanged. An unchanged ion is sometimes called a spectator ion.
Electron-transfer statements show why the reaction is redox. Iron loses two electrons to form iron(II) ions. Each copper(II) ion gains two electrons to form a copper atom. The electrons lost and gained match. Iron is oxidized, and copper(II) ions are reduced.
When checking equations, conserve each type of atom and the total charge. In the net ionic equation, the total charge is the same on both sides. Electrons appear in the separate electron-transfer statements, but cancel when those statements are combined into the overall reaction.
Fe(s)+Cu2+(aq)→Fe2+(aq)+Cu(s)\mathrm{Fe(s) + Cu^{2+}(aq) \rightarrow Fe^{2+}(aq) + Cu(s)}
  • Use state symbols where they clarify the substances present.
  • Omit unchanged ions from a net ionic equation, not from the molecular equation.
  • Check both atoms and net charge.

4. Plan and interpret a qualitative investigation

A useful investigation question is: Does iron react with copper(II) ions in solution? Before beginning, predict what evidence might appear. For this example, the prediction could be a reddish-brown coating on the iron and a change in the blue solution. These are expected possibilities, not guaranteed results.
Use a small amount of copper(II) sulfate solution in a labelled container and place a clean iron nail in it, following teacher instructions. Record the initial appearance of the solution and nail. After a planned observation period, record any changes. If comparing conditions, change one chosen factor and keep the other conditions as similar as possible.
Interpret the record in stages. First identify what was observed. Next consider whether the observations fit the particle model. Then use the balanced equation to check whether the proposed changes conserve atoms and charge. If the observations differ from the prediction, report them accurately and reconsider the explanation rather than altering the record.
Follow classroom safety directions, wear appropriate eye protection, avoid skin contact, and wash your hands after the investigation. Use the teacher's directions for handling and disposal. Never taste chemicals.
  • Make a prediction and record initial and later appearances.
  • Keep conditions similar when making a comparison.
  • Interpret actual observations; do not claim that a proposed investigation has been performed.

Worked example

Interpreting a proposed iron and copper(II) sulfate investigation

A learner proposes placing a clean iron nail in copper(II) sulfate solution. The predicted evidence is a reddish-brown coating and a change in the solution's blue colour. Explain how these predictions could fit a redox reaction, and write the balanced molecular and net ionic equations. Treat the evidence as predictions, not as experimental results.
  1. Separate predictions from results
    The coating and colour change are predicted observations. They may provide evidence of a reaction if they occur, but they cannot be reported as results before the investigation is carried out.
  2. Explain the particle changes
    If the reaction occurs, iron atoms lose electrons and become iron(II) ions in solution. Copper(II) ions gain those electrons and become copper atoms. Copper atoms may form a reddish-brown solid on the nail. A change in the dissolved copper(II) ions could be consistent with a change in the solution's blue colour.
    Fe(s)→Fe2+(aq)+2e−\mathrm{Fe(s) \rightarrow Fe^{2+}(aq) + 2e^-}
  3. Track reduction
    The two electrons lost by an iron atom are gained by a copper(II) ion. Iron is oxidized because it loses electrons. Copper(II) ions are reduced because they gain electrons.
    Cu2+(aq)+2e−→Cu(s)\mathrm{Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)}
  4. Write the molecular equation
    The molecular equation includes sulfate in the dissolved compounds. One iron atom, one copper atom, one sulfur atom, and four oxygen atoms appear on each side, so the equation is balanced.
    Fe(s)+CuSO4(aq)→FeSO4(aq)+Cu(s)\mathrm{Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)}
  5. Write the net ionic equation
    Sulfate ions remain unchanged in solution, so they are omitted from the net ionic equation. One iron atom and one copper atom are present on each side. The total charge is zero on each side.
    Fe(s)+Cu2+(aq)→Fe2+(aq)+Cu(s)\mathrm{Fe(s) + Cu^{2+}(aq) \rightarrow Fe^{2+}(aq) + Cu(s)}
Answer: If the predicted changes occur, they fit the proposed redox model: iron is oxidized to iron(II) ions, and copper(II) ions are reduced to copper metal. The equations are balanced as written. The coating and colour change remain predictions until they are actually observed.
Check: Iron loses two electrons and copper(II) ions gain two electrons. The net ionic equation conserves both atoms and net charge.

Common mistakes and how to avoid them

Assuming oxidation always means gaining oxygen.
Correction: For this redox investigation, identify oxidation as electron loss and reduction as electron gain. Oxygen does not have to be present.
Reporting a predicted coating or colour change as a result.
Correction: Label it as a prediction until it has been observed and recorded.
Leaving sulfate out of the molecular equation because it does not change.
Correction: Include sulfate in the molecular equation. Omit unchanged sulfate ions only from the net ionic equation.
Calling iron reduced because it becomes an ion.
Correction: Iron loses electrons to form iron(II) ions, so it is oxidized. Copper(II) ions gain electrons, so they are reduced.

Lesson summary

  • A redox reaction includes oxidation and reduction together.
  • Oxidation is electron loss; reduction is electron gain.
  • Visible changes can provide evidence, while a particle model explains how electron transfer could produce them.
  • A qualitative investigation describes the kind of change rather than measuring its amount.
  • Balanced equations conserve atoms and net charge.

Check your understanding

Question 1

In the proposed iron and copper(II) reaction, which statement correctly identifies the electron changes?
  1. Iron gains electrons and is reduced; copper(II) ions lose electrons and are oxidized.
  2. Iron loses electrons and is oxidized; copper(II) ions gain electrons and are reduced.
  3. Both iron and copper(II) ions gain electrons.
  4. Neither substance changes its electron count.
Show answer and explanation
Iron loses electrons and is oxidized; copper(II) ions gain electrons and are reduced.
Iron forms iron(II) ions by losing electrons. Copper(II) ions gain those electrons to form copper atoms.

Question 2

Before carrying out the proposed investigation, what status should be given to the expected reddish-brown coating?
  1. It is a confirmed result.
  2. It is a prediction to compare with actual observations.
  3. It proves that the solution has reached equilibrium.
  4. It is evidence that no reaction can occur.
Show answer and explanation
It is a prediction to compare with actual observations.
An expected coating is a prediction. Only an observation made during the investigation can be reported as a result.

Question 3

Which equation shows the reacting particles and leaves out unchanged sulfate ions?
  1. Fe(s)+Cu2+(aq)→Fe2+(aq)+Cu(s)\mathrm{Fe(s) + Cu^{2+}(aq) \rightarrow Fe^{2+}(aq) + Cu(s)}
  2. Fe(s)+Cu2+(aq)→Fe3+(aq)+Cu(s)\mathrm{Fe(s) + Cu^{2+}(aq) \rightarrow Fe^{3+}(aq) + Cu(s)}
  3. Fe(s)+Cu2+(aq)→Fe2+(aq)+Cu2+(s)\mathrm{Fe(s) + Cu^{2+}(aq) \rightarrow Fe^{2+}(aq) + Cu^{2+}(s)}
  4. Fe(s)+Cu2+(aq)→Fe2+(aq)+Cu(s)+2e−\mathrm{Fe(s) + Cu^{2+}(aq) \rightarrow Fe^{2+}(aq) + Cu(s) + 2e^-}
Show answer and explanation
Fe(s)+Cu2+(aq)→Fe2+(aq)+Cu(s)\mathrm{Fe(s) + Cu^{2+}(aq) \rightarrow Fe^{2+}(aq) + Cu(s)}
The first equation balances atoms and charge. Iron and copper(II) ions change, while sulfate ions remain unchanged and are omitted from the net ionic equation.

Key terms

Redox reaction
A reaction in which oxidation and reduction occur together through electron transfer.
Oxidation
Loss of electrons.
Reduction
Gain of electrons.
Net ionic equation
An equation showing the particles that change, with unchanged dissolved ions left out.
Qualitative investigation
An investigation that describes the kind of change or evidence rather than measuring its amount.
Spectator ion
An ion that remains unchanged in solution during the reaction.

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