1. © The Royal Society of Chemistry 2010-11 Acids & Bases What is this resource and who is it for? This is an Open Educational Resource designed to summarise key concepts in acid-base chemistry that are required for entry to an undergraduate course. The resource is designed for 1 st year undergraduates and revisits ideas from A-level. It also begins to show how they are applied early in a University course.Open Educational Resource It is therefore anticipated that you will have some knowledge of A-level chemistry before undertaking the activity but even with good subject knowledge, there should also be some material here you are unfamiliar with. This resource is not designed to support any specific A-level specification but may also be used in schools alongside other resources where appropriate. Learning Objectives On completion of this activity you should be able to … Use appropriate language to describe acid-base behaviour Use K w, K a and relevant assumptions to calculate pH or [H + ] for strong and weak acids. Appreciate some of the factors that determine the pK a of organic acids and bases. Calculate the pH of a buffer solution Related Resources This resource builds on some of the ideas presented in the activities on dynamic equilibria and mechanism basics     Who made it? Declan Fleming worked at the University of Bath as a Teacher Fellow 2010-2011 as part of The Royal Society of Chemistry’s work under the National HE STEM Programme. Part of his remit was looking into ways that e-learning can be used to support students at the KS5/HE interface.National HE STEM Programme Declan’s work is copyright The Royal Society of Chemistry.

2. © The Royal Society of Chemistry 2010-11 Hydronium Ion: H 3 O + forms when H + ions in aqueous solution accept a dative bond from a water molecule Defining some terms Arrhenius 1884: Acids are substances that form H + ions in solution, bases form OH - ions. Brønsted and Lowry 1923: Acids are proton donors, bases are proton acceptors Lewis, 1923: Acids are electron pair acceptors, bases are electron pair donors. The dissociation of water is an example where all three definitions of acid / base behaviour can be applied. Acid Conjugate Base Base Conjugate Acid We can define acid- base pairs (known as conjugates) in a reaction Out of copyright

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4. pH is a way of measuring acidity and uses the log scale. A log scale allows us to “compress” very large or small numbers into numbers that are easier to understand. Logs and pH According to B-L theory, acidity can be represented by the concentration of protons (H + ) in the solution. This concentration is very small so logs are used. pH = -log 10 [H + ] -log 10 ( x ) e.g. if [H + ] = 1x10 -3 mol dm -3 log 10 (1x10 -3 ) = -3 So pH = 3

5. © The Royal Society of Chemistry 2010-11 The equilibrium lies well to the left so the equilibrium concentration of water is effectively unchanged. K w, the ionic product of water K c = [H + ] [OH - ] [H 2 O] K w = [H + ] [OH - ] = 1 x 10 -14 mol 2 dm -6 In pure water at 298K, the concentration of H + is 1 x 10 -7 mol dm -3. The concentration of OH - is the same. As long as there is the same number of each, the solution is neutral. H 2 O H + + OH - +58 kJ mol -1

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7. Strong Acid Complete Dissociation Dilute Let’s assume we have HCl, 0.25M on the left and 0.75M on the right. What would the pH be? Strong Acids Strong Acid Complete Dissociation Concentrated pH = -log 10 [H + ] pH = -log 10 (0.25 + 0.0000001) For strong acids and bases, assume complete dissociation and ignore the [H + ] from dissociation of water insignificant = 0.60 © RSC / Keith Taber

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9. Let’s assume we have CH 3 COOH, 0.25M on the left and 0.75M on the right. What would the pH be? (pK a of ethanoic acid = 4.76) Weak Acids Weak Acid Small amount of dissociation Concentrated Still much larger than H + from water K a = [H + ] [A - ] [HA] Weak Acid Small amount of dissociation Dilute Concentration at eqm virtually unchanged because of weak dissociation Assume same as [H + ] 1.74x10 -5 = [H + ] 2 0.25 K a = 10 -4.76 = 1.74 x 10 -5 [H + ] = 2.1 x 10 -3 pH = -log 10 [2.1 x 10 -3 ] pH = 2.7 For weak acids, assume (i) only small amount of dissociation so [HA] is unchanged, (ii) negligible [H + ] from dissociation of water, (iii) [H + ] = [A - ] © RSC / Keith Taber

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11. The larger K a is, the stronger the acid. If K a is larger, it follows that the equilibrium has moved to the right (dissociated) side. Factors affecting K a HA [H + ] + [A - ] K a = [H + ] [A - ] [HA] In order for the equilibrium to sit more to the right, we need to slow the backwards reaction down and one way of achieving this is to increase the stability of A - The ethanoate anion is stabilised by distributing its negative charge over more than one atom. It could be represented by the diagram on the right. Negative charge distributed over 3 atoms

12. © The Royal Society of Chemistry 2010-11 If we’re able to spread this charge over a larger area, the anion will be more stable and K a will rise. Factors affecting K a Likewise if more negative charge is pushed into the area supporting the negative charge, this will destabilise the anion. -- -- -- pK a = 5.03pK a = 0.64 HA [H+] + [A - ] K a = [H + ] [A - ] [HA]

13. © The Royal Society of Chemistry 2010-11 Image by Benjah-bmm27 Benjah-bmm27

14. © The Royal Society of Chemistry 2010-11 Pyridine Factors affecting K a Pyrrole pK a = 5.21 pK a = -4 + H + Pyridine acts as a base, pyrrole does not. To understand why, we need to think why the conjugate acid of pyrrole is so unstable compared to that of pyridine 6 electrons in p-orbitals (just like benzene) including the 2 from the lone pair. The compound is aromatic. If the lone pair were to act as a base and become a bonding pair, the aromaticity would be lost. The electronic structure of pyrrole is similar to that of benzene Image by Benjah-bmm27Benjah-bmm27 Image by Jiří JanoušekJiří Janoušek

15. © The Royal Society of Chemistry 2010-11 Pyrrole pK a = 5.21pK a = -4 + H + Pyridine Pyridine acts as a base, pyrrole does not. To understand why, we need to think why the conjugate acid of pyrrole is so unstable compared to that of pyridine 6 electrons in p-orbitals (just like benzene) NOT including the 2 from the lone pair. The compound is aromatic. The lone pair is free to bond to Lewis acids without disrupting the aromaticity The electronic structure of pyridine is also similar to that of benzene but here the lone pair is not delocalised into the ring so is free to bond to Lewis acids Factors affecting K a Image by Benjah-bmm27Benjah-bmm27 Image by Jiří JanoušekJiří Janoušek

16. © The Royal Society of Chemistry 2010-11 Buffers A healthy diet contains lots of chemicals that could alter the pH of our blood. Given the sensitivity of enzymes to pH, we must maintain the pH of the blood to as narrow a range as possible. We can make use of mixtures of chemicals known as buffer solutions. These can resist changes in pH. HA [H + ] + [A - ] K a = [H + ] [A - ] [HA] A buffer solution consists of a weak acid and its salt. If acid is added, the salt can absorb excess H + If base is added, the weak acid can dissociate to replace any H + that was absorbed Blood plasma contains a buffer consisting of … Carbonic Acid: H 2 CO 3 Bicarbonate:HCO 3 - Image by EuthmanEuthman

17. © The Royal Society of Chemistry 2010-11 Still much larger than H + from water K a = [H + ] [A - ] [HA] Concentration at eqm virtually unchanged because of weak dissociation No longer the same as [H + ] – the vast majority will come from the addition of the salt For buffers, assume (i) [HA] still equals [acid], (ii) negligible [H + ] from H 2 O, (iii) [A - ] = [salt] (but not [H + ]) Buffers (contd.) [H + ] = K a x [acid] [conj base] [CH 3 COOH] = [CH 3 COO - ] CH 3 COOH H 2 O CH 3 COO - Na + H 2 O CH 3 COO -, Na + OH -, H 2 O pH Volume of NaOH added [H + ] = K a pH = pK a Image by MappesMappes

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19. Buffer Calculation Walkthrough [H + ] = K a x [acid] [conj. base] [H + ] = a b KaKa 0.158 = a b Initial moles of Et-NH 2 : 0.200 a = 0.200-b 0.158b = 0.200-b a = 0.158b b = 0.173 a = 0.0274 0.0274moles / 0.1mol dm -3 = 0.274dm 3 = 274cm 3 10 -11.5 10 -10.7 Click for a slightly shorter, better version … Et-NH 2 Et-NH 3 +

20. © The Royal Society of Chemistry 2010-11 Chemguide.co.uk:http://www.chemguide.co.uk/basicorg/acidbase/acids.html#tophttp://www.chemguide.co.uk/basicorg/acidbase/acids.html#top Section 6.2 in Chemistry3 : introducing inorganic, organic and physical chemistry, Burrows, Price et al – Oxford University Press 2009 Summary pH = -log 10 [H + ] K w = [H + ] [OH - ] = 1 x 10 -14 mol 2 dm -6Acid Conjugate Base Base Conjugate Acid For strong acids and bases, assume complete dissociation and ignore the [H + ] from dissociation of water For weak acids, assume (i) only small amount of dissociation so [HA] is unchanged, (ii) negligible [H + ] from dissociation of water, (iii) [H + ] = [A - ] The strength of an acid or base is determined by the stability of its conjugate base / acid. For example, electron withdrawing groups can stabilise carbanions resulting from the dissociation of a carboxylic acid. Further Reading For buffers, assume (i) only small amount of dissociation so [HA] is unchanged, (ii) negligible [H + ] from dissociation of water, (iii) [A - ] = amount of salt / conj base added to mixture

21. © The Royal Society of Chemistry 2010-11 Image Credits By KiyokKiyok Other images public domain or self-produced © RSC / Keith Taber – edited from image in Chemical misconceptions – prevention, diagnosis and cure, Keith Taber, RSC 2002 By WJ PilsakWJ Pilsak By organickorganick By Ben MillsBen Mills By EuthmanEuthman By MappesMappes By JiříJiří Janoušek By JiříJiří Janoušek Out of copyright Out of copyright Out of copyright By Benjah-bmm27 By Benjah-bmm27 The content of this resource, together with The Royal Society of Chemistry’s name, is subject to a Creative Commons licence on an “Attribution, Non-Commercial, Share-Alike” basis. The HE STEM Programme name and logo are the name and registered marks of the University of Birmingham. To the fullest extent permitted by law the University of Birmingham reserves its rights I its name and marks which may not be used except with its prior written permission. This resource was created by Declan Fleming.