Acids and Bases

● Arrhenius acids give hydronium ions, H3O + (aq) [or hydrogen ions, H+ (aq)] after dissolving in water. Examples: HCl (aq), HNO3 (aq), CH3COOH (aq) and H2SO4 (aq) ● Arrhenius bases give hydroxide ions, OH-(aq) in water. Examples: NaOH (aq), KOH (aq), Ca(OH)2 (aq)

● When an acid dissolves in water, the acid molecule react with H2O molecules by giving out a hydrogen ion, to form hydronium ions, H3O + and a negative ion. ● For example: HCl (aq) + H2O (l) → H3O + (aq) + Cl- (aq)

number of moles of hydrogen ions/ protons produced per mole of acid molecules when dissolved in water.

Monoprotic acids produce one mole of H+ (aq) ions per mole of acid molecules. Examples: HCl (aq), HNO3 (aq) and CH3COOH (aq) ■ CH3COOH (aq) + H2O (l) ⇌ CH3COO- (aq) + H+(aq)

• diprotic acids produce two moles of H+ (aq) ions and triprotic acids produce three moles of H+ (aq) ions per mole of molecules. Examples: H2SO4 (diprotic) and H3PO4 (triprotic)

In 1923, Johannes Nicolaus Brønsted (from Denmark) and Thomas Martin Lowry (from England) published essentially the same theory about how acids and bases behave independently. Both names have been used for the theory. ● The theory classifies a substance as an acid or a base in terms of whether it donates or receives H+ (aq) ions/ protons when dissolved in water or in a reaction. In this theory, acid and base must work together so that the protons donated by the acid can be received by the base.

donates H+ (aq) ions → proton donors ○ Examples: HCl (aq), HNO3 (aq), CH3COOH (aq) and H2SO4 (aq)

receives H+ (aq) ions → proton acceptors Examples: NaOH (aq), KOH (aq), NH3 (aq), Na2CO3 (aq), NaHCO3 (aq)

● Consider the following generic equation for a reaction between an acid, HA (aq) and a base, B (aq): HA (aq) + B (aq) → A- (aq) + BH+ (aq) ● When looking at the backward process, BH+ donates H+ ion to A- to form B and at the same time, A-receives H+ ion from BH+ to form HA. ○ BH+ (aq) is a Brønsted-Lowry acid and A- (aq) is a Brønsted-Lowry base (for the backward reaction).

● In conclusion, two conjugate acid-base pairs can be identified in each acid-base reaction. ● The identities of ‘acid’ or ‘base’ mainly describe the role of the chemical species as proton donors/ acceptors. A Brønsted-Lowry acid and its conjugate base always differ by one proton. A Brønsted-Lowry base and its conjugate acid always differ by one proton.

● Water has a tendency to dissociate (break up) into ions when in solution: H2O (l) + H2O (l) ⇌ H3O + (aq) + OH- (aq) ● Pure water dissociates only slightly - about one water molecule out of every 10 million dissociates and the rest remain undissociated (in molecular form).

● Water molecule ionizes (separates into ions) to form a hydronium/ hydrogen ion and a hydroxide ion: H2O (l) + H2O (l) ⇌ H3O + (aq) + OH- (aq) ● As seen in the equation, water can either → Identify the conjugate acid-base pair in this reaction. donate or receive H+ (aq) ion, which means it can be both a proton donor and a proton acceptor. ○ Water can act as both Brønsted-Lowry acid and base. ○ It is described as being amphoteric

● Apart from water, the conjugate base of many polyprotic acids are also amphoteric.

Example 1: HPO4 2- (aq) ○ It is the conjugate base of H2PO4- (aq)

● Example 2: HCO3 - (aq) ○ It is the conjugate base of H2CO3

HS- Conjugate base of H2S

● Acids have sour taste, while bases have bitter taste and slippery feeling. ● They can be distinguished using universal indicator, pH paper, litmus solution or pH meter. ● They can only show these properties when dissolved in water.

Litmus solution It is just used to identify whether an aqueous solution is acidic or basic. Acidic: Red Neutral: Purple Basic: Blue

Universal indicator It can measure the acidity and the alkalinity of an aqueous solution by providing an approximate pH value.

pH paper, pH meter

● Methyl orange and phenolphthalein are indicators mainly used in titration experiments to identify the end-point and hence determine the concentration of a solution. ● These indicators show a sharp colour change when the acid/ base is neutralised.

● In any type of aqueous solutions (acidic, neutral or basic), they all contain both H+ (aq) ions and OH- (aq) ions. ● This is due to the self-ionization / dissociation of water molecules, although the extent of dissociation is very small. H2O (l) + H2O (l) ⇌ H3O + (aq) + OH- (aq)

● The acidity of a solution depends on the relative concentration of H + (aq) ions and OH- (aq) ions in an aqueous solution.

● Concentration refers to the amount (number of moles) of solute dissolved in a unit volume (dm3 ) of solution. ● In 1 dm3 solution, if solution A contains larger amount of solute than solution B, we will describe them as either: ○ solution A is more concentrated than solution B, or ○ solution B is more dilute than solution A. Note: 1 dm3 = 1000 cm3