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 TheoryAcidBase ArrheniusH + producerOH - producer Bronsted- Lowry H + donorH + acceptor LewisElectron-pair 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 = sulfuric acid Ex: H 2 SO 3 = 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 + ) are joined to water molecules to form hydronium ions (H 3 O + )  H + and H 3 O + 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 1 x M  Basic: pH > 7;[H + ] < 1 x M  Neutral: pH = 7;[H + ] = 1 x 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 A moles of A reacts with n B moles of B

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