CHEM1020 Module 3 - Acids & Bases

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University of Queensland
Gwen Lawrie

ACIDS & BASES DEFINITIONS OF ACIDS & BASES Arrhenius definition Acid: a substance which produces hydrogen ions in aqueous solution Base: a substance which produces hydroxide ions in aqueous solution Bronsted-Lowry definition (not limited to aqueous solution) Acid: a proton donor Base: a proton acceptor Acid or base properties of a substance relate to the position of equilibrium  Position of equilibrium defines the strength of an acid ACIDS + - HA (aq) H 2 (l)-> H 3 (aq) A (aq) * in aqueous solution H = H O (strongest acid that can exist in water) 3 Strong acid ionises completely in water to give quantitative formation of H O – acid dissociation concentration large 3 i.e. hydrochloric, nitric, sulfuric Weak acid exists mostly as non-dissociated molecules in water (equilibrium equation) – acid dissociation concentration very small Monoprotic acid: only one proton BASES B (aq) H 2 (l) BH +(aq) OH -(aq) Strong base reacts completely with water to give quantitative formation of OH i.e. lithium hydroxide, sodium hydroxide, potassium hydroxide Weak base reacts incompletely with water to form less than stoichiometric amounts of OH - CONJUGATE ACID-BASE PAIRS Every acid (HA) has a conjugate base related to it by loss of a proton + - HA + H O2 H O 3 A Every base (B) has a conjugate acid related to it by gain of a proton + + B + H HB AUTOPROTOLYSIS OF WATER Reaction between two water molecules H2O (l) H 2 (l)-> H 3 +(aq) OH -(aq) Ion product constant (autodissociation constant) K = [H O ][OH ] =1.0 x 10 -1at 25C w 3 In neutral solution [H O ]=[OH ]= 1.0 x1 10 mol L -1 3 PH SCALE pH = -log 10[H 3 ]+ pOH = -log [10 ] - pK w -log K10 wH + pOH = 14.00 Acidic: pH = 1 – 6 Neutral: pH = 7 Basic: pH = 8 – 14 PH AND K CALCULATIONS DISSOCIATION + - Strong acid: 1 mol HA = 1 mol H + 1 mol A Weak acid: 1 mol HA does not equal 1 mol H + 1 mol A - For weak acid, define K – acid dissociation constant a Large K a reaction lies to the right Small K a reaction lies to left pK a -log 10 a higher K a lower pK = atronger the acid = lower pH For a weak base, define K b base dissociation constant pK b -log 10 b CONJUGATES OF WEAK ACIDS AND BASES K a =bK w pK a pK = bK w If Ka is known for a weak acid, Kb can be calculated for its conjugate base 1. The stronger an acid, the weaker its conjugate base and vice versa 2. The conjugate base of a strong acid is such a weak base that it has no Bronsted base properties in water 3. The conjugate base of a weak acid is always a weak base POLYPROTIC ACIDS Polyprotic acid has two or more acidic protons  Dissociate in a stepwise manner + - 1. H 2 + H O2 H O 3 HA 2. HA + H O 2---> H O +3A + 2- Where K >a1 a2and HA can act as an acid or a base ACID-BASE PROPERTIES OF SALTS Sodium Chloride (NaCl) + - In water, NaCl dissociates completely to Na and Cl  Na has no acid-base properties (always a spectator ion) -  Cl is the conjugate base of a strong acid (HCl) – such a weak base that it has no Bronsted acid-base properties Aqueous solution has pH of 7.0 Sodium fluoride (NaF) +  Na has no acid-base properties  F is conjugate base of a weak acid (HF) and is a weak base Aqueous solution has pH > 7 Ammonium chloride (NH Cl) 4  Cl is the conjugate base of a strong acid (HCl) – no Bronsted properties  NH i4 conjugate acid of a weak base – it is a weak acid Aqueous solution will have a pH < 7 THE COMMON ION EFFECT If you add the salt of a weak acid to the solution of the same acid then the equilibrium will shift towards neutral pH  Same is true for a base B UFFERS A solution containing:  Weak acid and its salt (conjugate base)  Weak base and its salt (conjugate acid) Has the capacity to resist changes in pH when additional amounts of strong acid or base are added + - H 3 (aq)+ A (aq)-> HA (aq) H 2 (l) HA (aq)+ OH
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