Lewis Acids and Bases

Active learning helps students visualize the invisible electron pair transfers central to Lewis acids and bases. Hands-on model building, lab observations, and sorting tasks make abstract concepts concrete, reducing misconceptions about what counts as an acid or base.

• Grade 12
• Chemistry
• 4 activities
• 20 min–35 min

Learning Objectives

1. Compare and contrast the Brønsted-Lowry and Lewis definitions of acids and bases, identifying key differences in their scope.
2. Identify the electron pair acceptor (Lewis acid) and electron pair donor (Lewis base) in given chemical reactions, including those without protons.
3. Explain how the Lewis acid-base theory expands the classification of chemical species beyond proton transfer.
4. Predict the formation of coordinate covalent bonds in adducts formed between Lewis acids and bases.

Concept Mapping

⏱ 30 min·Small Groups

Model Building: Lewis Adducts

Distribute molecular model kits or software. Have small groups assemble Lewis acids like BF3 or AlCl3, then pair with bases such as NH3 or OH- to form adducts. Students draw before-and-after diagrams, noting electron pair arrows. Discuss how bonds form without protons.

Prepare & details

Differentiate between Brønsted-Lowry and Lewis definitions of acids and bases.

Facilitation Tip: During Model Building, circulate to ensure students rotate models to view empty orbitals and lone pairs from multiple angles, reinforcing the 3D geometry of adduct formation.

Setup: Tables with large paper, or wall space

Materials: Concept cards or sticky notes, Large paper, Markers, Example concept map

• Understand
• Analyze
• Create
• Self-Awareness
• Self-Management

Concept Mapping

⏱ 35 min·Pairs

Observation Lab: Metal-Ligand Reactions

Prepare solutions of Cu2+ or Fe3+ salts. Pairs add ligands like ammonia or water, observe color changes, and classify species as Lewis acid or base. Record videos for analysis and predict outcomes for new combinations.

Prepare & details

Identify Lewis acids and bases in various chemical reactions.

Facilitation Tip: In the Observation Lab, have students record color changes and precipitate formation firsthand, then connect these visual cues to electron pair donation and acceptance.

Setup: Tables with large paper, or wall space

Materials: Concept cards or sticky notes, Large paper, Markers, Example concept map

• Understand
• Analyze
• Create
• Self-Awareness
• Self-Management

Concept Mapping

⏱ 25 min·Small Groups

Card Sort: Reaction Classification

Create cards with chemical reactions, formulas, and roles. Small groups sort into Brønsted-Lowry, Lewis only, or both categories, justifying choices. Class shares and debates borderline cases.

Prepare & details

Explain how the Lewis theory expands the range of substances considered acids and bases.

Facilitation Tip: For the Card Sort, monitor groups to prevent misclassification by asking them to verbalize their reasoning for each card placement.

Setup: Tables with large paper, or wall space

Materials: Concept cards or sticky notes, Large paper, Markers, Example concept map

• Understand
• Analyze
• Create
• Self-Awareness
• Self-Management

Concept Mapping

⏱ 20 min·Pairs

Virtual Simulation: Electron Flow

Use PhET or ChemCollective simulations. Individuals or pairs manipulate molecules to show electron donation in reactions like H+ with H2O versus BF3 with F-. Export screenshots with annotations for a class gallery walk.

Prepare & details

Differentiate between Brønsted-Lowry and Lewis definitions of acids and bases.

Facilitation Tip: Run the Virtual Simulation in real time so students can pause and discuss electron flow snapshots as a class.

Setup: Tables with large paper, or wall space

Materials: Concept cards or sticky notes, Large paper, Markers, Example concept map

• Understand
• Analyze
• Create
• Self-Awareness
• Self-Management

Teaching This Topic

Start with a quick diagnostic question to surface prior knowledge about acids and bases. Teach Lewis theory by contrasting it with Brønsted-Lowry, emphasizing the broader scope of electron pair behavior. Use analogies like 'electron pair magnets' to build intuition, but transition quickly to formal definitions. Avoid over-reliance on proton transfer examples, and always connect back to the Lewis framework to prevent confusion.

What to Expect

Students will confidently identify Lewis acids and bases in reactions, explain coordinate covalent bond formation, and compare Lewis and Brønsted-Lowry definitions using evidence from their work. Success looks like accurate labeling, clear explanations, and applying the definitions beyond textbook examples.

Watch Out for These Misconceptions

Common Misconception

During the Model Building activity, watch for students who assume all acids must contain hydrogen.

What to Teach Instead

Use the model kits to point out BF3’s empty p-orbital and NH3’s lone pair, then ask students to physically demonstrate how the pair moves into BF3’s orbital, linking the visual to the definition.

Common Misconception

During the Observation Lab, watch for students who label metal cations like Fe3+ as bases because they 'accept' something in reactions.

What to Teach Instead

Have students revisit their lab notes and highlight where Fe3+ accepts electron pairs from ligands, then reclassify it as a Lewis acid using the reaction evidence in front of them.

Common Misconception

During the Card Sort activity, watch for students who group all acids and bases based solely on proton transfer.

What to Teach Instead

Ask groups to justify their placements by describing electron pair movement, and prompt them to re-sort cards like BF3 + NH3 or AlCl3 + Cl- using Lewis definitions.

Assessment Ideas

Quick Check

After the Card Sort activity, present students with five reaction equations, some Brønsted-Lowry and some Lewis. Ask them to label the acid and base in each, then pair-share their answers before revealing the correct classifications.

Discussion Prompt

During the Observation Lab discussion, ask students to compare the color changes in metal-ligand reactions with proton transfer reactions, then discuss how the Lewis definition explains both.

Exit Ticket

After the Virtual Simulation, provide the BF3 + NH3 reaction and ask students to 1. identify the Lewis acid and base, 2. sketch the product with the coordinate bond, and 3. write one sentence explaining why BF3 is a Lewis acid based on the simulation’s electron flow.

Extensions & Scaffolding

• Challenge advanced students to predict the stability of adducts between different Lewis acids and bases using molecular orbital diagrams.
• Scaffolding for struggling students includes a guided worksheet that breaks down the steps of identifying lone pairs, empty orbitals, and bond formation.
• Deeper exploration involves researching industrial applications of Lewis acids in catalysis, such as in Friedel-Crafts reactions, and presenting findings to peers.

Key Vocabulary

Lewis Acid	A chemical species that accepts an electron pair to form a covalent bond. It is an electron pair acceptor.
Lewis Base	A chemical species that donates an electron pair to form a covalent bond. It is an electron pair donor.
Electron Pair Acceptor	A molecule or ion that receives a pair of electrons from another molecule or ion to form a chemical bond.
Electron Pair Donor	A molecule or ion that provides a pair of electrons to another molecule or ion to form a chemical bond.
Coordinate Covalent Bond	A type of covalent bond where one atom contributes both electrons to the shared pair. This often forms when a Lewis acid and base react.