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

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

2 Acid & Bases 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... Navigation is achieved by... either clicking on the grey arrows at the foot of each page orusing the left and right arrow keys on the keyboard

3 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 K w Relation between pH and pOH Introduction to buffer solutions Check list Acid & Bases

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

5 BRØNSTED-LOWRY THEORY ACIDproton donorHCl ——> H + (aq) + Cl¯(aq) BASEproton acceptor NH 3 (aq) + H + (aq) ——> NH 4 + (aq) ACIDS AND BASES

6 BRØNSTED-LOWRY THEORY ACIDproton donorHCl ——> H + (aq) + Cl¯(aq) BASEproton acceptor NH 3 (aq) + H + (aq) ——> NH 4 + (aq) Conjugate systems Acids are related to bases ACID PROTON + CONJUGATE BASE Bases are related to acids BASE + PROTON CONJUGATE ACID ACIDS AND BASES

7 BRØNSTED-LOWRY THEORY ACIDproton donorHCl ——> H + (aq) + Cl¯(aq) BASEproton acceptor NH 3 (aq) + H + (aq) ——> NH 4 + (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 CH 3 COO¯ + H 2 O CH 3 COOH + OH¯ base acid acid base ACIDS AND BASES

8 LEWIS THEORY ACIDlone pair acceptorBF 3 H + AlCl 3 BASElone pair donorNH 3 H 2 O ACIDS AND BASES LONE PAIR DONOR LONE PAIR ACCEPTOR LONE PAIR DONOR LONE PAIR ACCEPTOR

9 STRONG ACIDS completely dissociate (split up) into ions in aqueous solution e.g. HCl ——> H + (aq) + Cl¯(aq) MONOPROTIC1 replaceable H HNO 3 ——> H + (aq) + NO 3 ¯(aq) H 2 SO 4 ——> 2H + (aq) + SO 4 2- (aq) DIPROTIC 2 replaceable H’s STRONG ACIDS AND BASES

10 STRONG ACIDS completely dissociate (split up) into ions in aqueous solution e.g. HCl ——> H + (aq) + Cl¯(aq) MONOPROTIC1 replaceable H HNO 3 ——> H + (aq) + NO 3 ¯(aq) H 2 SO 4 ——> 2H + (aq) + SO 4 2- (aq) DIPROTIC 2 replaceable H’s STRONG BASES completely dissociate into ions in aqueous solution e.g. NaOH ——> Na + (aq) + OH¯(aq) STRONG ACIDS AND BASES

11 Weak acids partially dissociate into ions in aqueous solution e.g. ethanoic acid CH 3 COOH(aq) CH 3 COO¯(aq) + H + (aq) When a weak acid dissolves in water an equilibrium is set upHA(aq) + H 2 O(l) A¯(aq) + H 3 O + (aq) The water stabilises the ions To make calculations easier the dissociation can be written... HA(aq) A¯(aq) + H + (aq) WEAK ACIDS

12 Weak acids partially dissociate into ions in aqueous solution e.g. ethanoic acid CH 3 COOH(aq) CH 3 COO¯(aq) + H + (aq) When a weak acid dissolves in water an equilibrium is set upHA(aq) + H 2 O(l) A¯(aq) + H 3 O + (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 K a or pK a values The dissociation constant for the weak acid HA is K a = [H + (aq)] [A¯(aq)] mol dm -3 [HA(aq)] WEAK ACIDS

13 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 NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH¯ (aq) as in the case of acids it is more simply written NH 3 (aq) + H + (aq) NH 4 + (aq) WEAK BASES

14 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 NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH¯ (aq) as in the case of acids it is more simply written NH 3 (aq) + H + (aq) NH 4 + (aq) The weaker the basethe less it dissociates the more the equilibrium lies to the left The relative strengths of bases can be expressed as K b or pK b values. WEAK BASES

15 Hydrogen ion concentration [H + (aq) ] Introductionhydrogen 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.

16 Hydrogen ion concentration [H + (aq) ] pHhydrogen ion concentration can be converted to pH pH = - log 10 [H + (aq)] to convert pH into hydrogen ion concentration [H + (aq)] = antilog (-pH) pOHAn equivalent calculation for bases converts the hydroxide ion concentration to pOH pOH = - log 10 [OH¯(aq)] in both the above, [ ] represents the concentration in mol dm -3 STRONGLY ACIDIC pH OH¯ [H + ] WEAKLY ACIDIC NEUTRAL STRONGLY ALKALINE WEAKLY ALKALINE

17 Ionic product of water - K w Despite being covalent, water conducts electricity to a very small extent. This is due to the slight ionisation...H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH¯(aq) or, more simply H 2 O(l) H + (aq) + OH¯(aq)

18 Ionic product of water - K w Despite being covalent, water conducts electricity to a very small extent. This is due to the slight ionisation...H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH¯(aq) or, more simply H 2 O(l) H + (aq) + OH¯(aq) Applying the equilibrium law to the second equation gives K c = [H + (aq)] [OH¯(aq)] [ ] is the equilibrium concentration in mol dm -3 [H 2 O(l)]

19 Ionic product of water - K w Despite being covalent, water conducts electricity to a very small extent. This is due to the slight ionisation...H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH¯(aq) or, more simply H 2 O(l) H + (aq) + OH¯(aq) Applying the equilibrium law to the second equation gives K c = [H + (aq)] [OH¯(aq)] [ ] is the equilibrium concentration in mol dm -3 [H 2 O(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 (K c ) to get a new constant (K w ). K w = [H + (aq)] [OH¯(aq)] mol 2 dm -6 = 1 x mol 2 dm -6 (at 25°C) Because the constant is based on an equilibrium, K w VARIES WITH TEMPERATURE

20 Ionic product of water - K w The value of K w varies with temperature because it is based on an equilibrium. Temperature / °C K w / 1 x mol 2 dm H + / x mol dm pH What does this tell you about the equation H 2 O(l) H + (aq) + OH¯(aq) ?

21 Ionic product of water - K w The value of K w varies with temperature because it is based on an equilibrium. Temperature / °C K w / 1 x mol 2 dm H + / x mol dm pH What does this tell you about the equation H 2 O(l) H + (aq) + OH¯(aq) ? K w 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

22 Ionic product of water - K w The value of K w varies with temperature because it is based on an equilibrium. Temperature / °C K w / 1 x mol 2 dm H + / x mol dm pH What does this tell you about the equation H 2 O(l) H + (aq) + OH¯(aq) ? K w 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

23 Relationship between pH and pOH Because H + and OH¯ ions are produced in equal amounts when water dissociates[H + ] = [OH¯] = 1 x mol dm -3 their concentrations will be the same. K w = [H + ] [OH¯] = 1 x mol 2 dm -6 take logs of both sideslog[H + ] + log[OH¯] = -14 multiply by minus- log[H + ] - log[OH¯] = 14 change to pH and pOHpH + pOH = 14 (at 25°C)

24 Relationship between pH and pOH Because H + and OH¯ ions are produced in equal amounts when water dissociates[H + ] = [OH¯] = 1 x mol dm -3 their concentrations will be the same. K w = [H + ] [OH¯] = 1 x mol 2 dm -6 take logs of both sideslog[H + ] + log[OH¯] = -14 multiply by minus- log[H + ] - log[OH¯] = 14 change to pH and pOHpH + 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) K w is constant for any aqueous solution at the stated temperature

25 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 UsesStandardising pH meters Buffering biological systems (eg in blood) Maintaining the pH of shampoos

26 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 K w CAN YOU DO ALL OF THESE? YES NO

27 You need to go over the relevant topic(s) again Click on the button to return to the menu

28 WELL DONE! Try some past paper questions

29 ACIDS AND BASES THE END © 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING


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