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 (NH 3 )

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 Lewis H + donor Electron-pair acceptor H + acceptor Electron-pair donor

Strong vs. Weak Acids and Bases

 Strong acids – completely ionised in aqueous solution  Ex: HCl; HNO 3 ; H 2 SO 4  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: CH 3 NH 2 , NH 3

Naming Acids

 Single Element:  Hydro_____ic acid  Ex: HCl = Hydrochloric acid  Polyatomic Ion:  ATEic ITEous   Ex: H 2 SO 4 Ex: H 2 SO 3 = sulfuric acid = 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 (H 3 O + )

Dissociation of Water

  Self-ionisation of water: reaction in which TWO water molecules produce ions Ex: H 2 O + H 2 O  H 3 O + + OH   Can also be written as a DISSOCIATION: Ex: H 2 O (l)  H + (aq) + OH (aq)

Dissociation of Water

 In water or aqueous solution, hydrogen ions (H ions (H 3 O + ) + ) are joined to water molecules to form hydronium  H + and H solution 3 O + are both used to represent hydrogen ions in aqueous

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

n A A + n B B products

This means that n reacts with n B A moles of A moles of B

 To work out concentration C A solution A of volume V A titration: (dm -3 (moldm -3 ) of unknown ) at the end of the C A C B x V x V A B = nA = nB E.g. C A = nA X C B x V B nB x V A