CIE A Level Chemistry: Brønsted–Lowry Theory of Acids and Bases (Chapter 7.2)

Acid

A substance that neutralises a base forming a salt and water.

Base

A compound that neutralises an acid forming a salt and water.

Brønsted-Lowry Acid

A species that gives away a proton (H+).

Brønsted-Lowry Base

A species that accepts a proton (H+) using its lone pair of electrons.

Amphoteric

Species that can act both as acids and bases.

Strong Acid

An acid that dissociates almost completely in aqueous solutions.

Weak Acid

An acid that partially (or incompletely) dissociates in aqueous solutions.

pH

The negative log of the concentration of H+ ions, calculated using pH = -log[H+(aq)].

Hydrochloric Acid

HCl, an example of a strong acid.

Nitric Acid

HNO3, an example of a strong acid.

Sulfuric Acid

H2SO4, an example of a strong acid.

Ethanoic Acid

CH3COOH, an example of a weak organic acid.

Ammonia

NH3, a base that forms ammonium ion and hydroxide ions in water.

Sodium Hydroxide

NaOH, a common alkali that dissociates to form sodium ions and hydroxide ions.

Potassium Hydroxide

KOH, a common alkali that dissociates to form potassium ions and hydroxide ions.

Aqueous Ammonia

NH4OH, a common alkali that dissociates to form ammonium ions and hydroxide ions.

Dissociation

The process by which an acid or base separates into its ions in solution.

Hydrogen Ion

An ion formed when hydrogen loses an electron, also called a proton.

Dot and Cross Diagram

A diagram used to show the behavior of molecules in acid-base reactions.

Hydrogen Sulfide

H2S, an example of a weak acid.

Hydrocyanic Acid

HCN, an example of a weak acid.

Carbonic Acid

H2CO3, an example of a weak acid.

Concentration of H+ Ions

The amount of H+ ions in a solution, which determines the pH.

Equilibrium

A state where the concentrations of reactants and products remain constant over time.

Hydroxide Ion

An ion formed when a base dissolves in water, represented as OH-.

Weak acid

An acid that does not fully dissociate in an aqueous solution.

Acid dissociation constant (K)

A constant used to find the concentration of H+ ions in a weak acid.

Strong acid

An acid that completely dissociates in an aqueous solution.

pH of a strong acid

Use [strong acid] for [H+].

pH of a weak acid

Use K to find [H+].

Strong base

A base that dissociates almost completely in aqueous solutions.

Weak base

A base that partially dissociates in aqueous solutions.

pH Scale

A numerical scale that shows how acidic or alkaline a solution is, ranging from 1-14.

pH of water

Water has a pH of 7.

Equilibrium constant expression for water

K = [H+][OH-] / [H2O].

Ionic product of water (Kw)

Kw = 10^-14 mol^2/dm^6 at 298K.

pH calculation

pH = -log [H+(aq)].

Acidic solutions

Always have more H+ than OH- ions.

Basic solutions

Always have more OH- than H+ ions.

pH of strong acid example

HCl has a pH of 1 in a 0.1 mol/dm^3 solution.

pH of weak acid example

CH3COOH has a pH of 2.9 in a 0.1 mol/dm^3 solution.

pH meter

An accurate device used to measure the pH of a solution.

Universal indicator paper

A less accurate method to measure pH by comparing color changes.

Dissociation of a strong base

A strong base almost completely dissociates in aqueous solution.

Dissociation of a weak base

A weak base does not fully dissociate in aqueous solution.

Hydrogen ions in aqueous solutions

Can be represented as H2O or H+ in chemical equations.

Sulfuric acid (H2SO4)

Acts as both a strong acid and a weak acid depending on the ionization stage.

Degree of dissociation

Refers to the extent to which an acid or base dissociates in solution.

Concentration terms

Dilute and concentrated describe the amount of solute in a solution.

Equilibrium position of strong acid

Position of equilibrium is right, indicating complete dissociation.

Equilibrium position of weak acid

Position of equilibrium is left, indicating partial dissociation.

Electrical conductivity of acids and bases

Can be used to distinguish between strong and weak acids and bases.

Reactivity of acids and bases

Can also help distinguish between strong and weak acids and bases.

Universal Indicator

A pH indicator that changes color with acidity.

Electrical Conductivity

Ability of a solution to conduct electricity.

Conductivity Meter

Device used to measure electrical conductivity.

Hydrogen Ion Concentration

Higher in strong acids, affects reactivity.

Neutralisation Reaction

Reaction between acid and base producing salt and water.

Spectator Ions

Ions not involved in the main reaction.

Salt Formation

Produced from the reaction of an acid and a base.

Titration

Technique to determine unknown solution concentration.

Titrant

Solution of known concentration in titration.

Analyte

Solution of unknown concentration in titration.

Endpoint

Point where titrant and analyte completely react.

Equivalence Point

Moles of acid equal moles of alkali.

pH Titration Curve

Graph showing pH change during titration.

Strong Acid + Strong Alkali

Initial pH around 1-2, final pH around 13-14.

Weak Acid + Strong Alkali

Initial pH around 2-3, final pH varies.

Acid + Metal Carbonate Reaction

Produces salt, water, and carbon dioxide.

Salt from Hydrochloric Acid

Produces chloride salt.

Salt from Sulfuric Acid

Produces sulfate salt.

Salt from Nitric Acid

Produces nitrate salt.

Salt from Ethanoic Acid

Produces ethanoate salt.

pH of Water

Neutral pH of 7.

Bubbles in Reaction

Indicates hydrogen gas production from acid reaction.

0.1 mol/dm³ Concentration

Standard concentration used in acid reactions.

HCl

Strong acid with initial pH around 1-2.

NH₃

Weak alkali that reacts with H⁺ ions.

pH at Equivalence Point (Strong Acid + Weak Alkali)

Approximately 5.5, solution remains acidic.

pH at Equivalence Point (Weak Acid + Strong Alkali)

Approximately 9, solution becomes slightly basic.

CH₃COOH

Weak acid, also known as ethanoic acid.

NaOH

Strong alkali that reacts with weak acids.

pH Surge

Rapid increase in pH near equivalence point.

Weak Acid + Weak Alkali

No vertical region in titration curve.

Point of Inflexion

Indicates equivalence point in weak acid-weak alkali titration.

Titration Curve

Graph showing pH changes during titration.

Methyl Orange

Indicator changing from red to yellow, pH 3.1-4.4.

Phenolphthalein

Indicator changing from colorless to pink, pH 8.3-10.0.

Strong Acid + Weak Alkali

Only methyl orange is suitable indicator.

Titration Example

10.0 cm³ of 0.150 mol/dm³ aminoethanoic acid.

Volume of NaOH to Equivalence Point

15.0 cm³ calculated using concentration formula.

Final pH After Titration

pH of 12.5 after adding 20.0 cm³ NaOH.

Titration Calculation Formula

Vol acid x Conc acid = Vol base x Conc base.

Initial pH of Ethanoic Acid

Approximately 2-3 before NaOH addition.

Weak Acid Characteristics

Dissociates partially in solution.

Strong Alkali Characteristics

Completely dissociates in solution.

pH Range for Methyl Orange

Effective between pH 3.1 and 4.4.

pH Range for Phenolphthalein

Effective between pH 8.3 and 10.0.

Acid-Base Neutralization

Reaction between H⁺ and OH⁻ ions.

Titration Endpoint

Point where reaction is complete.