1 Acids and Bases Original: L. Scheffler Modified: Swiftney

8.1 Theories of acids and bases 2 hours Assessment statementObjTeacher’s notes 8.1.1Define acids and bases according to the Brønsted–Lowry and Lewis theories. 1 TOK: Discuss the value of using different theories to explain the same phenomenon. What is the relationship between depth and simplicity? 8.1.2Deduce whether or not a species could act as a Brønsted–Lowry and/or a Lewis acid or base Deduce the formula of the conjugate acid (or base) of any Brønsted–Lowry base (or acid). 3 Students should make clear the location of the proton transferred, for example, CH 3 COOH/CH 3 COO – rather than C 2 H 4 O 2 /C 2 H 3 O 2 –. 2 Original: L. Scheffler Modified: Swiftney

3 Arrhenius Definition Arrhenius Acid - Substances in water that increase the concentration of hydrogen ions (H + ). Base - Substances in water that increase concentration of hydroxide ions (OH - ). Categorical definition – easy to sort substances into acids and bases Problem – many bases do not actually contain hydroxides Original: L. Scheffler Modified: Swiftney

4 Bronsted-Lowry Definition Acid- neutral molecule, anion, or cation that donates a proton. Base- neutral molecule, anion, or cation that accepts a proton. HA + :B  HB + + :A - Ex. HCl + H 2 O  H 3 O + + Cl - Acid Base Conj Acid Conj Base Operational definition - The classification depends on how the substance behaves in a chemical reaction. Original: L. Scheffler Modified: Swiftney

5 Conjugate Base-The species remaining after an acid has transferred its proton. Conjugate Acid-The species produced after base has accepted a proton. * HA & :A - -conjugate base/acid pair * :A - -conjugate base of acid HA * :B & HB + -conjugate acid/base pair * HB + -conjugate acid of base :B Conjugate Acid Base Pairs Original: L. Scheffler Modified: Swiftney

6 * Note: Water can act as acid or base (Amphoteric) AcidBase Conjugate Acid Conjugate Base HCl+ H 2 O  H 3 O + +Cl - H 2 PO H 2 O   H 3 O + + HPO 4 2- NH H 2 O   H 3 O + +NH 3 BaseAcid Conjugate Acid Conjugate Base :NH 3 + H 2 O   NH 4 + +OH - PO H 2 O   HPO OH - Examples of Bronsted- Lowry Acid Base Systems Original: L. Scheffler Modified: Swiftney

7 Lewis Acid-an electron pair acceptor Base-an electron pair donor *Note: form dative bonding G.N. Lewis Definition Original: L. Scheffler Modified: Swiftney

8 AcidBase Arrhenius: H + OH - (based on H 2 O) B-L:H + donorH + acceptor (e.g. NH 3 ) Lewis:Electron PairElectron Pair AcceptorDonor Acid Base Definition Summary Original: L. Scheffler Modified: Swiftney

9 8.2 Properties of acids and bases 1 hour Assessment statementObjTeacher’s notes 8.2.1Outline the characteristic properties of acids and bases in aqueous solution. 2 Bases that are not hydroxides, such as ammonia, soluble carbonates and hydrogen carbonates, should be included. Alkalis are bases that dissolve in water. Students should consider the effects on indicators and the reactions of acids with bases, metals and carbonates. Original: L. Scheffler Modified: Swiftney

10 Acids and Bases The concepts acids and bases were loosely defined as substances that change some properties of water. One of the criteria that was often used was taste. Substances were classified salty-tasting sour-tasting sweet-tasting bitter-tasting Sour-tasting substances would give rise to the word 'acid', which is derived from the Greek word oxein, which mutated into the Latin verb acere, which means 'to make sour' Vinegar is a solution of acetic acid. Citrus fruits contain citric acid. Original: L. Scheffler Modified: Swiftney

11 Acids React with certain metals to produce hydrogen gas. 2HCl + 2Na  2NaCl + H 2 React with carbonates and bicarbonates to produce carbon dioxide gas MgCO 3 + 2HCl  MgCl 2 + H 2 CO 3  H 2 O + CO 2 Have a bitter taste Feel slippery. Many soaps contain bases. Bases Original: L. Scheffler Modified: Swiftney

12 Properties of Acids þ Produce H + (as H 3 O + ) ions in water (the hydronium ion is a hydrogen ion attached to a water molecule) þ Taste sour þ Corrode metals þ Good Electrolytes þ React with bases to form a salt and water þ pH is less than 7 þ Turns blue litmus paper to red “Blue to Red A-CID” (BRA) Original: L. Scheffler Modified: Swiftney

13 Properties of Bases  Generally produce OH - ions in water  Taste bitter, chalky  Are electrolytes  Feel soapy, slippery  React with acids to form salts and water  pH greater than 7  Turns red litmus paper to blue “Basic Blue” Original: L. Scheffler Modified: Swiftney

Strong and weak acids and bases 2 hours Assessment statementObjTeacher’s notes 8.3.1Distinguish between strong and weak acids and bases in terms of the extent of dissociation, reaction with water and electrical conductivity. 2 Aim 8: Although weakly acidic solutions are relatively safe, they still cause damage over long periods of time. Students could consider the effects of acid deposition on limestone buildings and living things State whether a given acid or base is strong or weak. 1 Students should consider hydrochloric acid, nitric acid and sulfuric acid as examples of strong acids, and carboxylic acids and carbonic acid (aqueous carbon dioxide) as weak acids. Students should consider all group 1 hydroxides and barium hydroxide as strong bases, and ammonia and amines as weak bases Distinguish between strong and weak acids and bases, and determine the relative strengths of acids and bases, using experimental data. 2 Original: L. Scheffler Modified: Swiftney

15 Acid Base Dissociation Acid-base reactions are equilibrium processes. The ratio of the concentrations of the reactants and products is constant for a given temperature at equilibrium It is known as the Acid or Base Dissociation Constant. The stronger the acid or base, the larger the value of the dissociation constant. Original: L. Scheffler Modified: Swiftney

16 Acid Strength Strong Acid-Transfers all of its protons to water; - Completely ionized (dissociated); - Strong electrolyte; - The conjugate base is weaker and has a negligible tendency to be protonated. Weak Acid-Transfers only a fraction of its protons to water; - Partially ionized (dissociated); - Weak electrolyte; - The conjugate base is stronger, readily accepting protons from water  As acid strength decreases, base strength increases.  The stronger the acid, the weaker its conjugate base  The weaker the acid, the stronger its conjugate base Original: L. Scheffler Modified: Swiftney

17 Acid Dissociation Constants Dissociation constants for some weak acids Original: L. Scheffler Modified: Swiftney

18 Base Strength Strong Base-all molecules accept a proton; - Completely ionized (dissociated); - strong electrolyte; - conjugate acid is very weak, negligible tendency to donate protons. Weak Base-fraction of molecules accept proton; - Partially ionized (dissociated); - weak electrolyte; - the conjugate acid is stronger. It more readily donates protons.  As base strength decreases, acid strength increases.  The stronger the base, the weaker its conjugate acid.  The weaker the base the stronger its conjugate acid. Original: L. Scheffler Modified: Swiftney

19 Common Strong Acids/Bases Strong Bases Sodium Hydroxide Potassium Hydroxide *Barium Hydroxide *Calcium Hydroxide *While strong bases they are not very soluble Strong Acids Hydrochloric Acid Nitric Acid Sulfuric Acid Perchloric Acid Original: L. Scheffler Modified: Swiftney

8.4 The pH scale 1 hour Assessment statementObjTeacher’s notes 8.4.1Distinguish between aqueous solutions that are acidic, neutral or alkaline using the pH scale Identify which of two or more aqueous solutions is more acidic or alkaline using pH values. 2 Students should be familiar with the use of a pH meter and universal indicator State that each change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H + (aq)]. 1 Relate integral values of pH to [H + (aq)] expressed as powers of 10. Calculation of pH from [H + (aq)] is not required. TOK: The distinction between artificial and natural scales could be discussed Deduce changes in [H + (aq)] when the pH of a solution changes by more than one pH unit. 3 Aim 8: A study of the effects of small pH changes in natural environments could be included. Original: L. Scheffler Modified: Swiftney 20

Acid Base Equilibrium Strong acid Weak Acid HA(aq) H 3 O + (aq) + A - (aq) 0.1M pH = - log [H 3 O + ] e.g. HCl, HNO 3 HA(aq)H 3 O + ((aq) + A - (aq) 0.1 M <<0.1 M e.g. CH 3 COOH assume 100% dissociation ***The pH scale is used to describe the concentration of acid present in a solution pH is used to make an “ugly” number into something simple (between 0-14). Original: L. Scheffler Modified: Swiftney 21

22 The pH Scale * pH = - log [H 3 O + ] pH [H 3 O + ] [OH- ] pOH Original: L. Scheffler Modified: Swiftney

23 pH and acidity pH = - log [H 3 O + ] or pH = - log [H + ] The pH values of several common substances are shown at the right. Many common foods are weak acids Some medicines and many household cleaners are bases. Original: L. Scheffler Modified: Swiftney

pH of Common Substances Original: L. Scheffler Modified: Swiftney 24

25 pH and acidity 1.Acidity or Acid Strength depends on Hydronium Ion Concentration [H 3 O + ] 2.The pH system is a logarithmic representation of the Hydrogen Ion concentration [H + ] (or [OH - ]) as a means of avoiding using large numbers and powers. * pH = - log [H 3 O + ] = log(1 / [H 3 O + ]) * pOH = - log [OH - ] = log(1 / OH - ]) 3.In pure water, [H 3 O + ] = 1 x mol / L (at 20 o C)  pH = - log(1 x ) = - (0 - 7) = 7 4.pH range of solutions: pH 7 (Basic) [H 3 O + ] Original: L. Scheffler Modified: Swiftney

26 Calculating the pH pH = - log [H 3 O + ] Example 1: If [H 3 O + ] = 1 X pH = - log 1 X pH = - (- 10) pH = 10 Example 2: If [H 3 O + ] = 1.8 X pH = - log 1.8 X pH = - (- 4.74) pH = 4.74 Original: L. Scheffler Modified: Swiftney

27 pH Practice 1) pH of a solution containing 25 g HCl dissolved in 1.5 L of H 2 O? 2) pH of a solution containing 1.32 g of HNO 3 dissolved in 750 mL? 3) pH of a solution containing 1.2 moles of Nitric Acid and 1.7 moles of Hydrochloric acid dissolved in 1000 mL? 4) If a solution has a [H + ] of 4.5 x M, is this acidic or basic? Explain Original: L. Scheffler Modified: Swiftney

28 pH and acidity * Kw = [H 3 O + ] [OH - ] = 1.0 x In pure water * [H 3 O + ] = [OH - ] = 1.0 x10 -7 * pH + pOH = 14 Original: L. Scheffler Modified: Swiftney

29 Neutralization An acid will neutralize a base, giving a salt and water as products (THIS IS STOICHIOMETRY) Examples Acid Base Salt water HCl + NaOH  NaCl + H 2 O H 2 SO NaOH  Na 2 SO H 2 O H 3 PO KOH  K 3 PO H 2 O 2 HCl + Ca(OH) 2  CaCl H 2 O A salt is an ionic compound that is formed from the positive ion (cation) of the base and the negative ion (anion) of the acid Original: L. Scheffler Modified: Swiftney

30 Neutralization Calculations If the concentration of acid or base is expressed in Molarity or mol dm -3 then: --The volume in dm 3 multiplied by the concentration yields moles (mol). -- If the volume is expressed in cm 3 the same product yields millimoles (mmol) mol dm -3 x dm 3 = mole * mol dm -3 x cm 3 = (0.001) x mole = mmol Original: L. Scheffler Modified: Swiftney

31 Neutralization Problems The volume of solution in dm 3 multiplied by concentration in moles dm -3 will yield moles. If an acid and a base combine in a 1 to 1 ratio, the moles of acid will equal the moles of base. *** This is a DIFFERENT way of doing Acid/Base Stoichiometry Therefore the volume of the acid multiplied by the concentration of the acid is equal to the volume of the base multiplied by the concentration of the base. V acid C acid = V base C base If any three of the variables are known, it is possible to determine the fourth. Original: L. Scheffler Modified: Swiftney

32 Neutralization Problems Example 1: Hydrochloric acid reacts with potassium hydroxide according to the following reaction: HCl + KOH  KCl + H 2 O If cm 3 of M HCl exactly neutralizes cm 3 of KOH solution, what is the concentration of the KOH solution? Solution: V acid C acid = V base C base (15.00 cm 3 )(0.500 M) = (24.00 cm 3 ) C base C base = (15.00 cm 3 )(0.500 M) (24.00 cm 3 ) C base = M Original: L. Scheffler Modified: Swiftney

33 Neutralization Problems Whenever an acid and a base do not combine in a 1 to 1 ratio, a mole factor must be added to the neutralization equation n V acid C acid = V base C base The mole factor (n) is the number of times the moles the acid side of the above equation must be multiplied so as to equal the base side. (or vice versa) Example H 2 SO NaOH  Na 2 SO H 2 O The mole factor is 2 and goes on the acid side of the equation. The number of moles of H 2 SO 4 is one half that of NaOH. Therefore the moles of H 2 SO 4 are multiplied by 2 to equal the moles of NaOH. Original: L. Scheffler Modified: Swiftney

34 Neutralization Problems Example 2: Sulfuric acid reacts with sodium hydroxide according to the following reaction: H 2 SO NaOH  Na 2 SO H 2 O If cm 3 of M H 2 SO 4 exactly neutralizes cm 3 of NaOH solution, what is the concentration of the NaOH solution? Solution: In this case the mole factor is 2 and it goes on the acid side, since the mole ratio of acid to base is 1 to 2. Therefore 2 V acid C acid = V base C base 2 (20.00 cm 3 )(0.400 M) = (32.00 cm 3 ) C base C base = (2) (20.00 cm 3 )(0.400 M) (32.00 cm 3 ) C base = M Original: L. Scheffler Modified: Swiftney

35 Neutralization Problems Example 3: Phosphoric acid reacts with potassium hydroxide according to the following reaction: H 3 PO KOH  K 3 PO H 2 O If cm 3 of M KOH exactly neutralizes cm 3 of H 3 PO 4 solution, what is the concentration of the H 3 PO 4 solution? Solution: In this case the mole factor is 3 and it goes on the acid side, since the mole ratio of acid to base is 1 to 2. Therefore 3 V acid C acid = V base C base 3 (15.00 cm 3 )( C acid ) = (30.00 cm 3 ) (0.300 M) C acid = (30.00 cm 3 )(0.300 M) (3) (15.00 cm 3 ) C acid = M Original: L. Scheffler Modified: Swiftney

36 Neutralization Problems Example 4: Hydrochloric acid reacts with calcium hydroxide according to the following reaction: 2 HCl + Ca(OH) 2  CaCl H 2 O If cm 3 of M HCl exactly neutralizes cm 3 of Ca(OH) 2 solution, what is the concentration of the Ca(OH) 2 solution? Solution: In this case the mole factor is 2 and it goes on the base side, since the mole ratio of acid to base is 2 to 1. Therefore V acid C acid = 2 V base C base (25.00 cm 3 ) ( ) = (2) (20.00 cm 3 ) ( C base ) C base = (25.00 cm 3 ) (0.400 M) (2) (20.00 cm 3 ) C base = M Original: L. Scheffler Modified: Swiftney

37 Weak Acid Equilibria A weak acid is only partially ionized. Both the ion form and the unionized form exist at equilibrium HA + H 2 O   H 3 O + + A - The acid equilibrium constant is Ka = [H 3 O + ] [A - ] [HA] Ka values are relatively small for most weak acids. The greatest part of the weak acid is in the unionized form Original: L. Scheffler Modified: Swiftney

38 Water has the ability to act as either a Bronsted- Lowry acid or base. Autoionization – spontaneous formation of low concentrations of [H + ] and OH - ] ions by proton transfer from one molecule to another. Equilibrium Constant for Water Water as an Equilibrium System Original: L. Scheffler Modified: Swiftney

39 Amphoteric Solutions A chemical compound able to react with both an acid or a base is amphoteric. Water is amphoteric. The two acid-base couples of water are H 3 O + /H 2 O and H 2 O/OH - It behaves sometimes like an acid, for example And sometimes like a base : Hydrogen carbonate ion HCO 3 - is also amphoteric, it belongs to the two acid-base couples H 2 CO 3 /HCO 3 - and HCO 3 - /CO 3 2- Original: L. Scheffler Modified: Swiftney