Acids and Bases

What are acids and bases?

Arrhenius Acids Hydrogen-containing compounds that

ionize to yield hydrogen ions (H+) in aqueous solutions

Arrhenius Bases Compounds that ionize to yield

hydroxide ions (OH-) in aqueous solutions

What are acids and bases?

Bronsted-Lowry Acid A hydrogen-ion donor

Bronsted-Lowry Base A hydrogen-ion acceptor

What are acids and bases?

All acids and bases in the Arrhenius theory are also acids and bases based on Bronsted-Lowry theory.

Bronsted-Lowry includes some bases not included in the Arrhenius theory. Ex: Ammonia (NH3)

What are acids and bases?

Lewis Acid Substance that can accept a pair of

electrons to form a covalent bond Lewis Base

Substance that can donate a pair of electrons to form a covalent bond

What are acids and bases?

Summary of Acid-Base Definitions

Theory Acid Base

Arrhenius H+ producer OH- producer

Bronsted-Lowry

H+ donor H+ acceptor

Lewis Electron-pair acceptor

Electron-pair donor

Strong vs. Weak Acids and Bases

Strong acids – completely ionised in aqueous solution Ex: HCl; HNO3; H2SO4

Weak acids – ionise only slightly in aqueous solution Ex: Acetic acid – 1% of acetic acid

molecules ionised at any instant

Strong vs. Weak Acids and Bases

Strong bases – dissociate completely into metal ions and hydroxide ions in aqueous solution Ex: Ca(OH)2; NaOH; KOH

Weak bases – react with water to form hydroxide ion and the conjugate acid of the base (No OH in formula) Ex: CH3NH2, NH3

Naming Acids

Single Element: Hydro_____ic acid Ex: HCl = Hydrochloric acid

Polyatomic Ion: ATEic ITEous Ex: H2SO4 = sulfuric acid

Ex: H2SO3 = sulfurous acid

Naming Bases

Bases are named the same way as any other ionic compound

Ex: KOH = potassium hydroxide

Hydrogen Ions from Water

Water that LOSES a hydrogen ion becomes a negatively charged hydroxide ion (OH-)

Water that GAINS a hydrogen ion becomes a positively charged hydronium ion (H3O+)

Dissociation of Water

Self-ionisation of water: reaction in which TWO water molecules produce ions

Ex: H2O + H2O H3O+ + OH-

Can also be written as a DISSOCIATION:

Ex: H2O (l) H+ (aq) + OH- (aq)

Dissociation of Water

In water or aqueous solution, hydrogen ions (H+) are joined to water molecules to form hydronium ions (H3O+)

H+ and H3O+ are both used to represent hydrogen ions in aqueous solution

Concentrations

Acidity or basicity of a solution is discussed in terms of the concentration of hydrogen ions, [H+], or the concentration of hydroxide ions, [OH-]

Acidic: [H+] > [OH-] Basic (Alkaline): [H+] < [OH-] Neutral: [H+] = [OH-]

pH

Acidity is measured in pH

pH = -log[H+]

Acidic: pH < 7; [H+] > 1 x 10-7 M Basic: pH > 7; [H+] < 1 x 10-7 M Neutral: pH = 7; [H+] = 1 x 10-7 M

pOH

Basicity could be measured in a similar manner called pOH

pOH = -log[OH-]

pH + pOH = 14

Measuring pH

Acid-Base Indicators An indicator’s acid and base form have

different colors in solution Limitations: usually work at 25°C

pH Meters Ex: PASCO Probes Make rapid, accurate pH measurements Must be calibrated – put into solution of known

pH

Titrations

Neutralisation reaction: hydronium ions combine with hydroxide ions to form water

An indicator can be used to show when the neutralisation is complete

Titrations

If the mole ratio is known, you can calculate the concentration of a given acid or base

nAA + nBB products

This means that nA moles of A reacts with nB moles of B

To work out concentration CA (moldm-3) of unknown solution A of volume VA (dm-3) at the end of the titration:

CA x VA = nA

CB x VB = nB

E.g. CA = nA X CB x VB

nB x VA