Chemistry module 6: Acids and Bases

Inquiry Question 1: What is an acid and what is a base?

How can acids and bases be identified, classified and explained using different scientific models?

Acid

A substance that donates protons (H⁺) according to Brønsted-Lowry theory or produces H⁺ ions in aqueous solution according to Arrhenius theory.

Base

A substance that accepts protons (H⁺) according to Brønsted-Lowry theory or produces OH⁻ ions in aqueous solution according to Arrhenius theory.

Arrhenius acid

A substance that dissociates in water to produce hydrogen ions (H⁺).

Arrhenius base

A substance that dissociates in water to produce hydroxide ions (OH⁻).

Brønsted-Lowry acid

A proton (H⁺) donor.

Brønsted-Lowry base

A proton (H⁺) acceptor.

Lewis acid

An electron pair acceptor.

Lewis base

An electron pair donor.

Proton

A hydrogen ion (H⁺).

Conjugate acid

The species formed when a base gains a proton.

Conjugate base

The species formed when an acid loses a proton.

Conjugate acid-base pair

Two species that differ by one proton (H⁺).

Amphiprotic substance

A substance that can act as either a Brønsted-Lowry acid or a Brønsted-Lowry base.

Indicator

A substance that changes colour depending on the pH of a solution.

Universal indicator

A mixture of indicators that displays a range of colours across the pH scale.

Litmus indicator

An indicator that turns red in acids and blue in bases.

Natural indicator

An indicator extracted from natural sources, such as red cabbage.

Neutralisation reaction

A reaction between an acid and a base producing a salt and water.

Salt

An ionic compound formed when the hydrogen ion of an acid is replaced by a metal or ammonium ion.

Acid-base reaction

A reaction involving the transfer of a proton from an acid to a base.

Carbonate

A compound containing the carbonate ion (CO₃²⁻).

Acid-carbonate reaction

A reaction producing a salt, water and carbon dioxide.

Acid-metal reaction

A reaction producing a salt and hydrogen gas.

Enthalpy of neutralisation

The energy released when one mole of water is formed from the reaction of an acid with a base.

Exothermic reaction

A reaction that releases heat to the surroundings.

Endothermic reaction

A reaction that absorbs heat from the surroundings.

Corrosive

A substance capable of chemically damaging materials or living tissue.

Caustic

A term used to describe strongly alkaline substances that are corrosive.

pH

A logarithmic measure of hydrogen ion concentration in solution.

Strong acid

An acid that completely ionises in water.

Weak acid

An acid that only partially ionises in water.

Strong base

A base that completely dissociates in water.

Weak base

A base that only partially reacts with water.

Lavoisier's acid theory

The early theory proposing that all acids contain oxygen.

Davy's acid theory

The theory proposing that hydrogen is the essential element in acids.

Arrhenius theory

A theory defining acids as H⁺ producers and bases as OH⁻ producers in aqueous solution.

Brønsted-Lowry theory

A theory defining acids as proton donors and bases as proton acceptors.

Lewis theory

A theory defining acids as electron pair acceptors and bases as electron pair donors.

Coordinate covalent bond

A covalent bond where both shared electrons are donated by one atom.

Inquiry Question 2: What is the role of water in solutions of acids and bases?

How does water influence acid-base behaviour?

Hydronium ion (H₃O⁺)

A hydrated hydrogen ion formed when a proton combines with a water molecule.

Hydroxide ion (OH⁻)

A negatively charged ion responsible for the basic properties of solutions.

Ionisation

The formation of ions when a molecular substance dissolves in water.

Dissociation

The separation of ions already present in an ionic compound when it dissolves in water.

pOH

A logarithmic measure of hydroxide ion concentration.

Hydrogen ion concentration [H⁺]

The concentration of hydrogen ions in solution, measured in mol L⁻¹.

Hydroxide ion concentration [OH⁻]

The concentration of hydroxide ions in solution, measured in mol L⁻¹.

Autoionisation of water

The reaction in which water molecules react to produce H₃O⁺ and OH⁻ ions.

Ionic equation

A chemical equation showing only the species involved in the reaction.

Net ionic equation

An ionic equation with spectator ions removed.

Spectator ion

An ion that remains unchanged during a chemical reaction.

Strong electrolyte

A substance that completely ionises or dissociates in solution, producing many ions.

Weak electrolyte

A substance that partially ionises, producing relatively few ions.

Electrical conductivity

The ability of a solution to conduct electricity due to the presence of mobile ions.

Concentrated solution

A solution containing a large amount of solute per unit volume.

Dilute solution

A solution containing a small amount of solute per unit volume.

Dilution

The process of decreasing solution concentration by adding solvent.

Ionisation constant

A value describing the extent to which a weak acid or base ionises in water.

Inquiry Question 3: How are solutions of acids and bases analysed?

How can the concentration and strength of acids and bases be determined experimentally?

Titration

A quantitative analytical technique used to determine the concentration of an unknown solution.

Standard solution

A solution of accurately known concentration.

Primary standard

A highly pure, stable substance used to prepare a standard solution.

Standardisation

The process of determining the exact concentration of a solution using a primary standard.

Analyte

The solution of unknown concentration being analysed.

Titrant

The solution of known concentration added during a titration.

Burette

A piece of laboratory equipment used to accurately deliver variable volumes of solution.

Pipette

A piece of laboratory equipment used to accurately transfer a fixed volume of solution.

Volumetric flask

A flask designed to prepare solutions of precise volume.

Conical flask

A flask used to contain the analyte during titration.

Meniscus

The curved surface of a liquid used when reading volume measurements.

Endpoint

The point during a titration when the indicator permanently changes colour.

Equivalence point

The point where chemically equivalent amounts of acid and base have reacted.

Titre

The volume of titrant required to reach the endpoint.

Concordant titres

Titre values that agree within 0.10 mL.

Phenolphthalein

A colourless indicator in acidic solution that turns pink in basic solution.

Methyl orange

An indicator that is red in acidic solution and yellow in alkaline solution.

Titration curve

A graph showing the change in pH during a titration.

Buffer solution

A solution that resists changes in pH when small amounts of acid or base are added.

Buffer

A mixture of a weak acid and its conjugate base or a weak base and its conjugate acid.

Acid dissociation constant (Ka)

The equilibrium constant describing the ionisation of a weak acid.

pKa

The negative logarithm of the acid dissociation constant.

Strong acid

Acid with a large Ka and small pKa.

Weak acid

Acid with a small Ka and large pKa.

Conductometry

The measurement of electrical conductivity during a chemical reaction.

Conductivity

The ability of a solution to conduct electricity through dissolved ions.

Potentiometric titration

A titration in which the endpoint is determined electronically using electrode measurements.

Digital probe

An electronic sensor used to measure quantities such as pH or conductivity.

Hydrometer

An instrument used to determine the density of a liquid.

Buffer capacity

The ability of a buffer solution to resist changes in pH.

Common ion

A dissolved ion already present in an equilibrium system that affects equilibrium position.