Grade by Grade
F. Electrochemistry · Electrochemistry

Predicting Redox Reactions and Investigating Them in the Lab

F2.2 F3.1 F2.1 A1.1 A1.4 A1.5 A1.8 A1.10About 75 minutes
Learning goals
  • I can use a table of oxidizing and reducing agents to predict whether a redox reaction is spontaneous.
  • I can identify the strongest oxidizing agent and the strongest reducing agent in a mixture, including water.
  • I can build a redox table from experimental evidence.
  • I can plan and carry out a safe inquiry that analyses a redox reaction qualitatively.

Words to know

spontaneous reaction
A reaction that happens on its own once the reactants are in contact, without a continuous outside supply of energy (such as electricity).
table of relative strengths
A list of reduction half-reactions ordered from the strongest oxidizing agent (top left) to the strongest reducing agent (bottom right). Often called a redox table.
strongest oxidizing agent (SOA)
Of all the species present, the one highest on the left side of the redox table.
strongest reducing agent (SRA)
Of all the species present, the one lowest on the right side of the redox table.
qualitative analysis
Describing what happens (colour changes, coatings, bubbles) without measuring amounts.

Will it react?

If you drop a copper wire into hydrochloric acid, nothing happens. Drop in a piece of zinc and it fizzes. Leave a steel nail in copper(II) sulfate solution and it turns copper-coloured. Chemists want to predict these results before mixing anything. In Grade 11 you did this for metals with the activity series. In this lesson you will extend that idea into a tool that works for any oxidizing agent and any reducing agent.

From activity series to redox table

The activity series ranks metals by how easily they lose electrons. A metal that loses electrons easily is a strong reducing agent. Its ion holds electrons weakly, so the ion is a weak oxidizing agent.

A redox table writes every pair as a reduction half-reaction:

  • Left column: oxidizing agents. The strongest is at the top (F₂).
  • Right column: reducing agents. The strongest is at the bottom (Li).

The two lists are mirror images. A strong oxidizing agent (like F₂) pairs with a very weak reducing agent (F⁻), and a strong reducing agent (Li) pairs with a very weak oxidizing agent (Li⁺).

oxidizing agentreducing agentF₂(g)+ ne⁻ ⇌2F⁻(aq)Cl₂(g)+ ne⁻ ⇌2Cl⁻(aq)Br₂(l)+ ne⁻ ⇌2Br⁻(aq)Ag⁺(aq)+ ne⁻ ⇌Ag(s)I₂(s)+ ne⁻ ⇌2I⁻(aq)Cu²⁺(aq)+ ne⁻ ⇌Cu(s)2H⁺(aq)+ ne⁻ ⇌H₂(g)Pb²⁺(aq)+ ne⁻ ⇌Pb(s)Fe²⁺(aq)+ ne⁻ ⇌Fe(s)Zn²⁺(aq)+ ne⁻ ⇌Zn(s)Mg²⁺(aq)+ ne⁻ ⇌Mg(s)Li⁺(aq)+ ne⁻ ⇌Li(s)stronger oxidizing agentsstronger reducing agentsCu²⁺ is ABOVEZn on the right:spontaneousZn + Cu²⁺ →Zn²⁺ + Cu
A short redox table. Oxidizing agents get stronger going up the left side; reducing agents get stronger going down the right side. A reaction is spontaneous when the oxidizing agent is above the reducing agent, as for Cu²⁺ with Zn (highlighted).

The spontaneity rule

A redox reaction is spontaneous if the oxidizing agent is above the reducing agent in the table. If the oxidizing agent is below the reducing agent, there is no noticeable reaction.

Why? An oxidizing agent high in the table pulls on electrons strongly. A reducing agent low in the table holds its electrons weakly. When the strong puller meets the weak holder, electrons flow. In the lesson Standard Reduction Potentials and the Standard Hydrogen Electrode you will attach a number (in volts) to each row; for now, the order is all you need.

Oxidizing agent (+ electrons)→ Reducing agent
F₂(g) + 2e⁻2F⁻(aq)
MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻Mn²⁺(aq) + 4H₂O(l)
Cl₂(g) + 2e⁻2Cl⁻(aq)
O₂(g) + 4H⁺(aq) + 4e⁻2H₂O(l)
Br₂(l) + 2e⁻2Br⁻(aq)
Ag⁺(aq) + e⁻Ag(s)
Fe³⁺(aq) + e⁻Fe²⁺(aq)
I₂(s) + 2e⁻2I⁻(aq)
Cu²⁺(aq) + 2e⁻Cu(s)
2H⁺(aq) + 2e⁻H₂(g)
Pb²⁺(aq) + 2e⁻Pb(s)
Ni²⁺(aq) + 2e⁻Ni(s)
Fe²⁺(aq) + 2e⁻Fe(s)
Zn²⁺(aq) + 2e⁻Zn(s)
2H₂O(l) + 2e⁻H₂(g) + 2OH⁻(aq)
Al³⁺(aq) + 3e⁻Al(s)
Mg²⁺(aq) + 2e⁻Mg(s)
Na⁺(aq) + e⁻Na(s)
Li⁺(aq) + e⁻Li(s)
A redox table for this unit, strongest oxidizing agent at the top. Water appears twice: as an oxidizing agent (2H₂O + 2e⁻ → H₂ + 2OH⁻, near the bottom) and as a reducing agent (it is the product side of the O₂ row, near the top).

Predicting step by step

  1. List every species present, as it really exists. Ionic compounds in solution are separate ions. Always include H₂O(l) for aqueous solutions. Acids give H⁺(aq).
  2. Label each species as a possible oxidizing agent (OA), reducing agent (RA), or both. (Water, Fe²⁺ and some others can be both.)
  3. Find the strongest OA (highest on the left) and the strongest RA (lowest on the right).
  4. If the SOA is above the SRA, the reaction is spontaneous. Write the reduction half-reaction (as in the table) and the oxidation half-reaction (the table row reversed), balance electrons and add. If the SOA is below the SRA, write "no reaction".
Worked example

Example 1: Copper in hydrochloric acid

Predict whether copper reacts with hydrochloric acid, HCl(aq).

  1. Species: Cu(s), H⁺(aq), Cl⁻(aq), H₂O(l).
  2. OAs: H⁺, H₂O. RAs: Cu, Cl⁻, H₂O.
  3. SOA: H⁺ (the 2H⁺/H₂ row is above the 2H₂O/H₂ row). SRA: Cu (the Cu row is lower than the Cl⁻ and H₂O rows).
  4. Is H⁺ above Cu? No: the 2H⁺/H₂ row is below the Cu²⁺/Cu row.

No reaction. This matches the activity series: copper is below hydrogen.

Worked example

Example 2: Acidified permanganate and iron(II) sulfate

Predict the reaction when acidified KMnO₄(aq) is added to FeSO₄(aq).

  1. Species: K⁺, MnO₄⁻, H⁺, Fe²⁺, SO₄²⁻, H₂O.
  2. SOA: MnO₄⁻ with H⁺ (near the top). SRA: Fe²⁺ (as a reducing agent, in the Fe³⁺/Fe²⁺ row). K⁺ and SO₄²⁻ are very weak and act as spectators.
  3. MnO₄⁻ is above Fe²⁺, so the reaction is spontaneous.
  4. Reduction: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. Oxidation (reversed row, ×5): 5Fe²⁺ → 5Fe³⁺ + 5e⁻.

Spontaneous: MnO₄⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 5Fe³⁺(aq) + 4H₂O(l). The purple colour fades, which is the titration from the lesson Balancing Redox Equations: The Half-Reaction Method in Acidic Solution.

Worked example

Example 3: Sodium metal in water

Predict what happens when a small piece of sodium is placed in water (a teacher demonstration only).

  1. Species: Na(s), H₂O(l).
  2. SOA: H₂O (as an oxidizing agent, 2H₂O + 2e⁻ → H₂ + 2OH⁻). SRA: Na(s).
  3. The water row is above the Na row, so the reaction is spontaneous.
  4. Reduction: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Oxidation: 2Na → 2Na⁺ + 2e⁻.

Spontaneous: 2Na(s) + 2H₂O(l) → 2Na⁺(aq) + 2OH⁻(aq) + H₂(g). The solution becomes basic and hydrogen gas forms, often with enough heat to ignite it.

Building a table from evidence

A redox table is not memorized from nowhere: it comes from experiments. A classic SCH4U investigation places small pieces of metal into drops of different ion solutions in a well plate and records which combinations react. The more combinations a metal reacts with, the stronger it is as a reducing agent. The more metals an ion reacts with, the stronger it is as an oxidizing agent.

Typical lab plan (A1.1, A1.4, A1.5):

  • Testable question: How do Mg, Zn and Cu compare as reducing agents, and how do Mg²⁺, Zn²⁺, Cu²⁺ and Ag⁺ compare as oxidizing agents?
  • Hypothesis: based on the activity series, Mg > Zn > Cu as reducing agents, so Ag⁺ > Cu²⁺ > Zn²⁺ > Mg²⁺ as oxidizing agents.
  • Variables: independent: the metal–ion combination; dependent: evidence of reaction; controlled: solution concentration (0.10 mol/L), drop volume, size and cleanliness of the metal pieces (sand them), time observed (5 min), temperature.
  • Safety (WHMIS): wear goggles and gloves. Silver nitrate stains skin and is corrosive. Copper(II) and zinc salts are harmful to aquatic life, so collect all solutions for proper disposal instead of pouring them down the drain. Wash hands afterwards.
solution (contains the ion)Mg²⁺(aq)Zn²⁺(aq)Cu²⁺(aq)Ag⁺(aq)Mg(s)−✓✓✓Zn(s)✗−✓✓Cu(s)✗✗−✓metal✓ coating or colour change (reaction)✗ no change− same metal and ion (not tested)Number of reactions: Mg 3, Zn 2, Cu 1 → Mg is the strongest reducing agent
Sample results from the well-plate investigation. Magnesium reacts with three ion solutions, zinc with two and copper with one. Silver ions react with all three metals, so Ag⁺ is the strongest oxidizing agent tested.

Analysing the evidence (A1.8, A1.10)

From the grid: reducing-agent strength is Mg > Zn > Cu. Oxidizing-agent strength is the reverse order of their ions, with silver at the top: Ag⁺ > Cu²⁺ > Zn²⁺ > Mg²⁺. This supports the hypothesis and agrees with the published table.

Sources of error: an oxide coating on unsanded magnesium can hide a slow reaction; very slow reactions may look like "no reaction" in 5 minutes; a cloudy precipitate (not a metal coating) can be mistaken for a reaction. A good report names the error and how to reduce it (sand all metals, observe longer, use a white background).

Just for fun

Chemists sometimes call fluorine the "bully" of the redox table: it is the strongest oxidizing agent on it and will rip electrons from almost anything, including water. Lithium, at the other end, is so eager to give its electrons away that it is kept stored under oil.

Hands-on activity

Vitamin C detective: an iodine redox inquiry

You need (with a grown-up): tincture of iodine or povidone-iodine antiseptic from a pharmacy (it stains and is poisonous if swallowed: a grown-up handles it), 1 tablespoon of cornstarch, water, a kettle or microwave to heat water (grown-up), 6 clear glasses or jars, a medicine dropper or oral syringe, one vitamin C tablet, and 3 or 4 juices to test (for example orange, apple, a juice drink and lemon juice). Wear old clothes.

  1. Starch indicator: a grown-up stirs 1 tablespoon of cornstarch into 1 cup of hot water. Let it cool.
  2. Blue test solution: in a jar, mix 1 cup of water, 1 teaspoon of the starch mixture and 5 drops of iodine tincture. It turns dark blue-black (iodine + starch).
  3. Pour equal amounts (about 50 mL) of the blue solution into 5 glasses.
  4. Dissolve the vitamin C tablet in 50 mL of water. Add it drop by drop to the first glass, swirling, and count the drops until the blue disappears.
  5. Repeat with each juice in a fresh glass of blue solution.
  6. Analyse: fewer drops means more vitamin C (a stronger supply of reducing agent).

The chemistry: vitamin C (ascorbic acid, C₆H₈O₆) is the reducing agent and iodine is the oxidizing agent: C₆H₈O₆ + I₂ → C₆H₆O₆ + 2I⁻ + 2H⁺. Iodide ions do not colour starch, so the blue fades.

For your report: write a testable question, name your independent, dependent and controlled variables, and list one source of error (for example, the colour of orange juice makes the endpoint harder to see). Do not drink any of the test mixtures, and pour them down the drain with plenty of water when finished.

Your turn

1

Which is the stronger oxidizing agent, Cl₂ or Br₂?

2

Which is the stronger reducing agent, Fe(s) or Pb(s)?

3

Predict whether zinc reacts with hydrochloric acid. If it does, write the net ionic equation.

4

Predict whether silver reacts with copper(II) nitrate solution.

5

Predict the reaction when chlorine water is added to sodium iodide solution.

6

Can a copper(II) sulfate solution be stored in an iron container? Explain.

7

In the well-plate lab, why must the metal pieces be sanded first?

More practice: warm-up

1

Where is the strongest oxidizing agent in the redox table?

2

Where is the strongest reducing agent?

3

A reaction is spontaneous if the OA is ___ the RA in the table.

4

Name the species present in NaCl(aq).

5

Which reacts with dilute acid: magnesium or silver?

More practice: on your own

1

A nickel strip is placed in a solution of lead(II) nitrate. Predict the reaction and write the net ionic equation.

2

Predict whether iron(III) chloride solution will dissolve copper. (This reaction is used to etch copper circuit boards.)

3

A student mixes potassium bromide solution and iodine solution and sees no change. Explain using the table.

4

Four metals W, X, Y, Z are tested in each other's ion solutions. W reacts with all three other ions; Y reacts with none; X reacts only with Y's ions. Rank the metals as reducing agents.

5

Aluminium should react with water according to the table, yet an aluminium pot does not dissolve when you boil water. Suggest why.

Challenge

1

Predict what happens when acidified potassium permanganate is added to a solution of sodium chloride. Name the SOA and SRA and write the balanced equation.

2

Iron(II) ions can act as an oxidizing agent or a reducing agent. Predict what happens if Fe²⁺ is mixed with zinc, and separately with acidified hydrogen peroxide. (H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O is above the MnO₄⁻ row.)

3

Design a qualitative inquiry to place the unknown metal "M" (which forms M²⁺ ions) into the redox table between Zn and Fe, or confirm it is not there. Give the tests and the results that would confirm the position.

Check your understanding

Quiz

1.

In a redox table, a reaction is spontaneous when:

2.

Which is the strongest reducing agent: Cu(s), Zn(s), Ag(s) or Fe(s)?

3.

A strip of lead is placed in each solution. In which one will a reaction occur?

4.

In aqueous solutions, which species must always be included when you list possible oxidizing and reducing agents?

5.

In a well-plate lab, metal A reacts with B²⁺ and C²⁺; metal B reacts with C²⁺ only; metal C reacts with none. Which is the strongest oxidizing agent?

6.

Which statement about a substance at the very top of the left side of the redox table is correct?

For parents and teachers

Big idea (F2.2, F3.1): you can predict a redox reaction before you mix anything. List every species (including water), find the strongest oxidizing agent and the strongest reducing agent, and check whether the oxidizing agent is above the reducing agent in the redox table. Lab evidence is how the table was built in the first place.

How to help: Do the vitamin C activity together. Ask your teen to explain which substance is the oxidizing agent and which is the reducing agent, and why fewer drops means more vitamin C. Ask them to name one variable they kept the same.

Common difficulties:

  • Forgetting water, or forgetting that acids supply H⁺(aq).
  • Reading the wrong column: oxidizing agents are on the left (stronger going up), reducing agents on the right (stronger going down).
  • Thinking "spontaneous" means "fast". The table says a reaction is possible; it says nothing about speed (aluminium and its oxide layer is a good example).
  • Writing the oxidation half-reaction without reversing the table row.

Safety: tincture of iodine stains and is poisonous if swallowed; a grown-up should handle it and the hot water. Nothing in the activity should be tasted. In school labs, silver nitrate and copper(II) salts need goggles, gloves and proper disposal.