Chemistry · Year 12 · Acid-Base Chemistry · Term 2

Bronsted-Lowry Acids and Bases

Defining acids and bases as proton donors and acceptors and identifying conjugate pairs.

• ACARA Content Descriptions
• ACSCH099

About This Topic

The Bronsted-Lowry model expands the definition of acids and bases beyond simple aqueous solutions, focusing on the transfer of protons (H+ ions). This topic, aligned with ACARA AC9S12U06, introduces students to the concept of conjugate acid-base pairs and amphiprotic substances. It is a vital shift from the Year 10 Arrhenius model, allowing students to explain the basicity of substances like ammonia and the behaviour of acids in non-aqueous environments.

This model is essential for understanding biological systems, such as how amino acids behave in the human body or how Indigenous Australians traditionally used alkaline plant ashes in food preparation. It provides the theoretical basis for all subsequent acid-base calculations and titrations. Students grasp this concept faster through hands-on modeling of proton transfer using physical kits or digital interactives.

Key Questions

1. Differentiate between Arrhenius and Bronsted-Lowry definitions of acids and bases.
2. Identify conjugate acid-base pairs in various chemical reactions.
3. Explain why some substances can act as both an acid and a base (amphiprotic).

Learning Objectives

• Compare and contrast the Arrhenius and Bronsted-Lowry definitions of acids and bases.
• Identify conjugate acid-base pairs in given chemical reactions.
• Explain the amphiprotic nature of substances like water using the Bronsted-Lowry model.
• Predict the direction of proton transfer in acid-base reactions based on conjugate pair strength.

Before You Start

Introduction to Chemical Reactions

Why: Students need a foundational understanding of chemical equations and how reactants transform into products.

Atomic Structure and Ions

Why: Understanding the composition of atoms, particularly hydrogen and its isotopes, is crucial for grasping proton transfer.

Key Vocabulary

Proton donor	A chemical species that donates a hydrogen ion (H+) in a reaction, characteristic of a Bronsted-Lowry acid.
Proton acceptor	A chemical species that accepts a hydrogen ion (H+) in a reaction, characteristic of a Bronsted-Lowry base.
Conjugate acid-base pair	Two species that differ by a single proton (H+), where one is the acid and the other is its conjugate base.
Amphiprotic	A substance that can act as either a Bronsted-Lowry acid (donate a proton) or a Bronsted-Lowry base (accept a proton) in different reactions.

Watch Out for These Misconceptions

Common Misconception

An acid is any substance that contains hydrogen.

What to Teach Instead

Only 'ionisable' hydrogens can be donated as protons. For example, methane (CH4) has hydrogen but is not a Bronsted-Lowry acid. Peer discussion focusing on molecular structure and bond polarity helps students identify which hydrogens are actually 'acidic'.

Common Misconception

Conjugate pairs differ by two or more protons.

What to Teach Instead

A conjugate acid-base pair must differ by exactly one proton (H+). Students often struggle with polyprotic acids; using a step-by-step modeling approach helps them see that each proton loss creates a new, distinct conjugate pair.

Active Learning Ideas

Role Play: The Proton Handover

Students act as different molecules (e.g., HCl, H2O, NH3). One student holds a 'proton' (a ball) and must decide, based on their molecular identity, whether to donate it to another student, identifying the resulting conjugate pairs in the process.

20 min·Whole Class

Think-Pair-Share: Amphiprotic Detectives

Pairs are given a list of ions like HCO3- and HPO42-. They must write two equations for each: one where it acts as an acid and one as a base, then share their reasoning with another pair to verify the conjugate pairs formed.

15 min·Pairs

Gallery Walk: Conjugate Matching

Cards with various acids and bases are posted around the room. Students move in small groups to find and record the matching conjugate partner for each card, explaining the 'one proton difference' rule to their peers.

25 min·Small Groups

Real-World Connections

• Pharmacists use acid-base chemistry principles to formulate medications, ensuring proper absorption and stability within the body's physiological pH range.
• Geologists studying acid mine drainage utilize Bronsted-Lowry concepts to understand how water reacts with sulfide minerals, leading to the release of acidic runoff that impacts aquatic ecosystems.
• Food scientists apply knowledge of acids and bases to processes like cheese making and fermentation, controlling pH to achieve desired textures and flavors.

Assessment Ideas

Quick Check

Present students with the reaction: NH3(aq) + H2O(l) <=> NH4+(aq) + OH-(aq). Ask them to identify the Bronsted-Lowry acid, base, conjugate acid, and conjugate base in this reaction.

Exit Ticket

Provide students with two chemical equations. For each equation, ask them to circle the conjugate acid-base pairs and label each species as either an acid or a base according to the Bronsted-Lowry definition.

Discussion Prompt

Pose the question: 'Why is the Bronsted-Lowry definition more useful than the Arrhenius definition for understanding reactions in non-aqueous solvents like liquid ammonia?' Facilitate a class discussion where students share their reasoning.

Frequently Asked Questions

What makes a substance amphiprotic?

An amphiprotic substance can either donate or accept a proton depending on its environment. Common examples include water (H2O) and the hydrogen carbonate ion (HCO3-). To be amphiprotic, a species must possess both a removable proton and a lone pair of electrons to accept one.

How do you identify a conjugate acid-base pair?

A conjugate pair consists of two species that differ by exactly one hydrogen ion (H+). The acid has the extra H+, and the base has one fewer. For example, in the reaction of NH3 and H2O, NH3/NH4+ is one pair and H2O/OH- is the other.

Why is the Bronsted-Lowry model better than the Arrhenius model?

The Arrhenius model is limited to aqueous solutions and requires the production of OH- ions for a substance to be a base. The Bronsted-Lowry model is more versatile because it focuses on proton transfer, explaining why substances like ammonia (NH3) are basic even though they don't contain OH groups.

How can active learning help students understand the Bronsted-Lowry model?

Active learning, like the 'Proton Handover' role play, physically demonstrates that acidity is a behaviour (donating) rather than just a static property. When students have to physically pass a 'proton' to a partner, the concept of conjugate pairs becomes a tangible relationship between two states of a molecule, making the abstract chemical equations much easier to internalise.