A guide for A level students KNOCKHARDY PUBLISHING ACIDS AND BASES A guide for A level students KNOCKHARDY PUBLISHING

Acid & Bases www.argonet.co.uk/users/hoptonj/sci.htm INTRODUCTION This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards. Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available. Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at... www.argonet.co.uk/users/hoptonj/sci.htm Navigation is achieved by... either clicking on the grey arrows at the foot of each page or using the left and right arrow keys on the keyboard

Acid & Bases CONTENTS Brønsted-Lowry theory of acids and bases Lewis theory of acids and bases Strong acids and bases Weak acids Weak bases Hydrogen ion concentration and pH Ionic product of water Kw Relation between pH and pOH Introduction to buffer solutions Check list

Before you start it would be helpful to… Acid & Bases Before you start it would be helpful to… know the simple properties of acids, bases and alkalis

BRØNSTED-LOWRY THEORY ACIDS AND BASES BRØNSTED-LOWRY THEORY ACID proton donor HCl ——> H+(aq) + Cl¯(aq) BASE proton acceptor NH3 (aq) + H+(aq) ——> NH4+(aq)

BRØNSTED-LOWRY THEORY ACIDS AND BASES BRØNSTED-LOWRY THEORY ACID proton donor HCl ——> H+(aq) + Cl¯(aq) BASE proton acceptor NH3 (aq) + H+(aq) ——> NH4+(aq) Conjugate systems Acids are related to bases ACID PROTON + CONJUGATE BASE Bases are related to acids BASE + PROTON CONJUGATE ACID

BRØNSTED-LOWRY THEORY ACIDS AND BASES BRØNSTED-LOWRY THEORY ACID proton donor HCl ——> H+(aq) + Cl¯(aq) BASE proton acceptor NH3 (aq) + H+(aq) ——> NH4+(aq) Conjugate systems Acids are related to bases ACID PROTON + CONJUGATE BASE Bases are related to acids BASE + PROTON CONJUGATE ACID For an acid to behave as an acid, it must have a base present to accept a proton... HA + B BH+ + A¯ acid base conjugate conjugate acid base example CH3COO¯ + H2O CH3COOH + OH¯ base acid acid base

ACIDS AND BASES LEWIS THEORY ACID lone pair acceptor BF3 H+ AlCl3 BASE lone pair donor NH3 H2O LONE PAIR ACCEPTOR LONE PAIR DONOR LONE PAIR DONOR LONE PAIR ACCEPTOR

STRONG ACIDS AND BASES STRONG ACIDS completely dissociate (split up) into ions in aqueous solution e.g. HCl ——> H+(aq) + Cl¯(aq) MONOPROTIC 1 replaceable H HNO3 ——> H+(aq) + NO3¯(aq) H2SO4 ——> 2H+(aq) + SO42-(aq) DIPROTIC 2 replaceable H’s

STRONG ACIDS AND BASES STRONG ACIDS completely dissociate (split up) into ions in aqueous solution e.g. HCl ——> H+(aq) + Cl¯(aq) MONOPROTIC 1 replaceable H HNO3 ——> H+(aq) + NO3¯(aq) H2SO4 ——> 2H+(aq) + SO42-(aq) DIPROTIC 2 replaceable H’s BASES completely dissociate into ions in aqueous solution e.g. NaOH ——> Na+(aq) + OH¯(aq)

WEAK ACIDS Weak acids partially dissociate into ions in aqueous solution e.g. ethanoic acid CH3COOH(aq) CH3COO¯(aq) + H+(aq) When a weak acid dissolves in water an equilibrium is set up HA(aq) + H2O(l) A¯(aq) + H3O+(aq) The water stabilises the ions To make calculations easier the dissociation can be written... HA(aq) A¯(aq) + H+(aq)

WEAK ACIDS Weak acids partially dissociate into ions in aqueous solution e.g. ethanoic acid CH3COOH(aq) CH3COO¯(aq) + H+(aq) When a weak acid dissolves in water an equilibrium is set up HA(aq) + H2O(l) A¯(aq) + H3O+(aq) The water stabilises the ions To make calculations easier the dissociation can be written... HA(aq) A¯(aq) + H+(aq) The weaker the acid the less it dissociates the more the equilibrium lies to the left. The relative strengths of acids can be expressed as Ka or pKa values The dissociation constant for the weak acid HA is Ka = [H+(aq)] [A¯(aq)] mol dm-3 [HA(aq)]

WEAK BASES Partially react with water to give ions in aqueous solution e.g. ammonia When a weak base dissolves in water an equilibrium is set up NH3 (aq) + H2O (l) NH4+ (aq) + OH¯ (aq) as in the case of acids it is more simply written NH3 (aq) + H+ (aq) NH4+ (aq)

WEAK BASES Partially react with water to give ions in aqueous solution e.g. ammonia When a weak base dissolves in water an equilibrium is set up NH3 (aq) + H2O (l) NH4+ (aq) + OH¯ (aq) as in the case of acids it is more simply written NH3 (aq) + H+ (aq) NH4+ (aq) The weaker the base the less it dissociates the more the equilibrium lies to the left The relative strengths of bases can be expressed as Kb or pKb values.

Hydrogen ion concentration [H+(aq)] Introduction hydrogen ion concentration determines the acidity of a solution hydroxide ion concentration determines the alkalinity for strong acids and bases the concentration of ions is very much larger than their weaker counterparts which only partially dissociate.

Hydrogen ion concentration [H+(aq)] pH hydrogen ion concentration can be converted to pH pH = - log10 [H+(aq)] to convert pH into hydrogen ion concentration [H+(aq)] = antilog (-pH) pOH An equivalent calculation for bases converts the hydroxide ion concentration to pOH pOH = - log10 [OH¯(aq)] in both the above, [ ] represents the concentration in mol dm-3 STRONGLY ACIDIC 100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14 10-0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH OH¯ [H+] WEAKLY ACIDIC NEUTRAL STRONGLY ALKALINE WEAKLY ALKALINE

Ionic product of water - Kw Despite being covalent, water conducts electricity to a very small extent. This is due to the slight ionisation ... H2O(l) + H2O(l) H3O+(aq) + OH¯(aq) or, more simply H2O(l) H+(aq) + OH¯(aq)

Ionic product of water - Kw Despite being covalent, water conducts electricity to a very small extent. This is due to the slight ionisation ... H2O(l) + H2O(l) H3O+(aq) + OH¯(aq) or, more simply H2O(l) H+(aq) + OH¯(aq) Applying the equilibrium law to the second equation gives Kc = [H+(aq)] [OH¯(aq)] [ ] is the equilibrium concentration in mol dm-3 [H2O(l)]

Ionic product of water - Kw Despite being covalent, water conducts electricity to a very small extent. This is due to the slight ionisation ... H2O(l) + H2O(l) H3O+(aq) + OH¯(aq) or, more simply H2O(l) H+(aq) + OH¯(aq) Applying the equilibrium law to the second equation gives Kc = [H+(aq)] [OH¯(aq)] [ ] is the equilibrium concentration in mol dm-3 [H2O(l)] As the dissociation is small, the water concentration is very large compared with the dissociated ions and any changes to its value are insignificant; its concentration can be regarded as constant. This “constant” is combined with (Kc) to get a new constant (Kw). Kw = [H+(aq)] [OH¯(aq)] mol2 dm-6 = 1 x 10-14 mol2 dm-6 (at 25°C) Because the constant is based on an equilibrium, Kw VARIES WITH TEMPERATURE

Ionic product of water - Kw The value of Kw varies with temperature because it is based on an equilibrium. Temperature / °C 0 20 25 30 60 Kw / 1 x 10-14 mol2 dm-6 0.11 0.68 1.0 1.47 5.6 H+ / x 10-7 mol dm-3 0.33 0.82 1.0 1.27 2.37 pH 7.48 7.08 7 6.92 6.63 What does this tell you about the equation H2O(l) H+(aq) + OH¯(aq) ?

Ionic product of water - Kw The value of Kw varies with temperature because it is based on an equilibrium. Temperature / °C 0 20 25 30 60 Kw / 1 x 10-14 mol2 dm-6 0.11 0.68 1.0 1.47 5.6 H+ / x 10-7 mol dm-3 0.33 0.82 1.0 1.27 2.37 pH 7.48 7.08 7 6.92 6.63 What does this tell you about the equation H2O(l) H+(aq) + OH¯(aq) ? • Kw gets larger as the temperature increases • this means the concentration of H+ and OH¯ ions gets greater • this means the equilibrium has moved to the right • if the concentration of H+ increases then the pH decreases • pH decreases as the temperature increases

Ionic product of water - Kw The value of Kw varies with temperature because it is based on an equilibrium. Temperature / °C 0 20 25 30 60 Kw / 1 x 10-14 mol2 dm-6 0.11 0.68 1.0 1.47 5.6 H+ / x 10-7 mol dm-3 0.33 0.82 1.0 1.27 2.37 pH 7.48 7.08 7 6.92 6.63 What does this tell you about the equation H2O(l) H+(aq) + OH¯(aq) ? • Kw gets larger as the temperature increases • this means the concentration of H+ and OH¯ ions gets greater • this means the equilibrium has moved to the right • if the concentration of H+ increases then the pH decreases • pH decreases as the temperature increases Because the equation moves to the right as the temperature goes up, it must be an ENDOTHERMIC process

Relationship between pH and pOH Because H+ and OH¯ ions are produced in equal amounts when water dissociates [H+] = [OH¯] = 1 x 10-7 mol dm-3 their concentrations will be the same. Kw = [H+] [OH¯] = 1 x 10-14 mol2 dm-6 take logs of both sides log[H+] + log[OH¯] = -14 multiply by minus - log[H+] - log[OH¯] = 14 change to pH and pOH pH + pOH = 14 (at 25°C)

Relationship between pH and pOH Because H+ and OH¯ ions are produced in equal amounts when water dissociates [H+] = [OH¯] = 1 x 10-7 mol dm-3 their concentrations will be the same. Kw = [H+] [OH¯] = 1 x 10-14 mol2 dm-6 take logs of both sides log[H+] + log[OH¯] = -14 multiply by minus - log[H+] - log[OH¯] = 14 change to pH and pOH pH + pOH = 14 (at 25°C) N.B. As they are based on the position of equilibrium and that varies with temperature, the above values are only true if the temperature is 25°C (298K) Neutral solutions may be regarded as those where [H+] = [OH¯]. Therefore a neutral solution is pH 7 only at a temperature of 25°C (298K) Kw is constant for any aqueous solution at the stated temperature

Buffer solutions - Brief introduction Definition “Solutions which resist changes in pH when small quantities of acid or alkali are added.” Acidic Buffer (pH < 7) made from a weak acid + its sodium or potassium salt ethanoic acid sodium ethanoate Alkaline Buffer (pH > 7) made from a weak base + its chloride ammonia ammonium chloride Uses Standardising pH meters Buffering biological systems (eg in blood) Maintaining the pH of shampoos

What should you be able to do? REVISION CHECK What should you be able to do? Recall the definition of acids and bases in the Brønsted-Lowry system Recall the definition of acids and bases in the Lewis system Recall and explain the difference between strong and weak acids Recall and explain the difference between strong and weak bases Recall the definition of pH Recall the definition of the ionic product of water Explain how and why pH varies with temperature Recall the relationship between pH, [H+], [OH¯], pOH and Kw CAN YOU DO ALL OF THESE? YES NO

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