CHAPTER 7 Acid Base Equilibria

Transcription

1 1 CHAPTER 7 Acid Base Equilibria Learning Objectives Acid base theories Acid base equilibria in water Weak acids and bases Salts of weak acids and bases Buffers Logarithmic concentration diagrams

2 2 ACID BASE THEORIES Arrhenius theory: H + and OH Theory of solvent systems: solvent cations and anions Brønsted-Lowry theory: taking and giving protons (conjugate pairs) Lewis theory: taking and giving electrons In general: Acid is a substance that increases the concentration of H 3 O + (hydronium ion). Conversely, a base decreases the concentration of H 3 O + in aqueous solution. A more general definition of acids and bases given by Brønsted and Lowry is that an acid is a proton donor and a base is a proton acceptor.

3 3 ACID BASE EQUILIBRIA IN WATER Hydrochloric acid is a strong acid, and in water, its ionization is complete: Acetic acid is a weak acid, which ionizes only partially in water (a few percent): We can write an equilibrium constant for this reaction:

4 4 ACID BASE EQUILIBRIA IN WATER Pure water ionizes slightly, or undergoes autoprotolysis: The equilibrium constant for this is

5 5 THE ph SCALE ph = - Log [H + ] In general, panything = log Anything, and it is a mathematical function to express the very low quantities.

6 6 THE ph SCALE Blood ph is 7.4 at 37 C. The ph of blood must be measured at body temperature to accurately reflect the status of blood buffers.

7 7 Weak Acids and Bases, K a and K b Salts of weak acids are bases (conjugate bases) and have K b = K w / K a Also salt of weak base is a conjugate acid and has K a = K w / K b For a diprotic acid base pair like ethylenediamine, NH 2 C 2 H 4 NH 2, K b1 = K w /K a2 and K b2 = K w /K a1

8 8 Weak Acids and Bases, K a and K b

9 9 Buffers Buffer is a solution that resists changes in ph when a small amount of acid or base is added or when the solution is diluted. Buffer solution is usually made of (weak acid with its conjugated base) or (weak base with its conjugated acid). Examples: CH 3 COO - and CH 3 COOH HCN and CN - C 6 H 5 COOH and C 6 H 5 COONa NH 3 and NH 4 Cl (C 2 H 5 ) 3 N and (C 2 H 5 ) 3 NH +

10 10 Buffers, Henderson-Hasselbalch equation

11 11 Buffers, Henderson-Hasselbalch equation

12 12 Buffers, Henderson-Hasselbalch equation

13 13 Buffers, Henderson-Hasselbalch equation Calculating the ph of a buffer when strong acid or base is added when strong acid is added to the buffer: when strong base is added to the buffer:

14 14 Buffers, Henderson-Hasselbalch equation Example: Find the ph of a solution of M HA (Ka =8.4x10-9 ) plus M NaA? Answer: ph=8.614 Example: For 1L of the above solution, calculate the ph after addition of 12mL of 1.0M HCl. HCl is an acid, it will react with the basic content (A - ) of the buffer solution and forming more of HA, so it will affect the acid/base content of the solution, NaA will decrease and HA will increase. Amount of change is equal to the moles of HCl (0.012Lx1.0M) - mol. A (1x0.1026) - (0.012x1.0) ph pka Log Log mol. HA (1x ) (0.012x1.0) ph Log Log(2.1757)

15 15 Buffers, Henderson-Hasselbalch equation Added base will react with the acid content of the buffer, this will decrease the concentration (or moles) of acid and increase the concentration (or moles) of base content of the buffer. Example: For 1L of the previous buffer solution, calculate the ph after addition of 12mL of 1.0M NaOH. NaOH will affect the acid/base content of the solution, HA will decrease and NaA will increase. Amount of change is equal to the moles of NaOH (0.012Lx1.0M) ph pk a - mol. A (1x0.1026) (0.012x1.0) Log Log mol. HA (1x ) - (0.012x1.0) ph Log Log(6.4966)

16 16 Acid-Base Titration Titration is used to determine the concentration of analyte volumetrically, depending on the stoichiometric ratio between analyte and titrant. Examples: HCl + NaOH NaCl + H 2 O (ratio 1:1) H 2 SO 4 + 2NaOH Na 2 SO 4 + 2H 2 O (ratio 1:2) 2CH 3 COOH +Ca(OH) 2 (CH 3 COO) 2 Ca + 2H 2 O (ratio 2:1) H 2 SO 4 + Ca(OH) 2 CaSO 4 + 2H 2 O (ratio 1:1)

17 17 ph curve During the acid-base titration, the ph will change. If the analyte is acid, ph will be low and start increasing through titrant addition (base). Example: (titration 50 ml of 0.10M HCl with 0.10M NaOH) Volume of NaOH added ph equiv. point Acid excess [H][OH]=K w Base excess

18 18 ph curve Titration strong base with strong acid Titration diprotic acid with strong base

19 19 ph curve Question 50 ml of 0.10M HCl was titrated with 0.24M NaOH. Calculate the ph after addition of 0, 5, 20, 25 and 30 ml of NaOH. Question 17.4mL of M HCl was needed to titrate 25.0mL of NaOH with unknown concentration. Calculate the concentration of NaOH.

20 20 CHAPTER 8 Acid Base Titrations Learning Objectives Calculating acid base titration curves Indicators Mixtures of Acids or Bases Derivatives of a Titration Curve

21 21 TITRATION Strong acid with strong base (monoprotic acid with monohydroxy base) Four stages a, b, c, d At the beginning (a) only acid is present, concentration if [H + ] = [acid] Before the equivalence point (b), acid in excess [H ] mole of acid - mole of base totalsolution volume (L) At the equivalence point (c), base reacts completely with the acid and H + comes only from water hydrolysis [H + ] = 1x10-7 After the equivalence point (d), base in excess [OH - mole of base - mole of acid ] totalsolution volume (L) [H Kw ] [OH - ]

22 22 TITRATION Strong base with strong acid (monohydroxy base with monoprotic acid ) Four stages a, b, c, d At the beginning (a) only base is present, concentration if [H + ] = K w /[base] Before the equivalence point (b), base in excess [OH - ] mole of base - mole of acid totalsolution volume (L) [H Kw ] [OH - ] At the equivalence point (c), acid reacts completely with the base and H + comes only from water hydrolysis [H + ] = 1x10-7 After the equivalence point (d), acid in excess mole of acid - mole of base [H ] totalsolution volume (L)

23 23 TITRATION

24 24 TITRATION Detection of the End Point: Indicators The point at which the reaction is observed to be complete is called the end point. The difference between the equivalence point and the end point is referred to as the titration error. Indicator is added to the solution for visual detection of color change which indicates a ph change. An indicator for an acid base titration is a weak acid or weak base that is highly colored. Choose an indicator with a pka near the equivalence point ph.

25 25 TITRATION Weak Acid versus Strong Base Weak Base versus Strong Acid

26 26 TITRATION Weak acid HA with strong base (monoprotic acid with monohydroxy base) Four stages a, b, c, d At the beginning (a) only acid is present, concentration of [H ] [HA].K a Before the equivalence point (b), buffer is formed ph pk a - moles of A ( moles of base added) Log moles of HA ( started moles of HA- moles of base added) Special case: in the middle region of (b) when volume of NaOH =1/2 equivalence volume, ph=pk a At the equivalence point (c), hydrolysis of A Kw [OH ] [A ].K b [H ] - [OH ], - started moles of HA [A ] total volume (L) After the equivalence point (d) - mole of base - mole of acid [OH ] totalsolution volume (L) [H Kw ] [OH - ]

27 27 TITRATION Weak-acid titrations require careful selection of the indicator.

28 28 TITRATION

29 29 TITRATION Mixtures of Acids or Bases One acid should be at least 10 4 times weaker than the other to titrate separately. The stronger acid will titrate first and will give a ph break at its equivalence point. If two strong acids are titrated together, there will be no differentiation between them, and only one equivalence point break will occur. For H 2 SO 4, the first proton is completely dissociated and the second proton has a Ka of about Therefore, the second proton is ionized sufficiently to titrate as a strong acid, and only one equivalence point break is found. For Phosphoric acid, the first proton titrates as a strong acid, followed by titration of the second proton to give a second equivalence point; the third proton is too weakly ionized to be titrated.

30 30 TITRATION Equivalence Points from Derivatives of a Titration Curve Bases In general, derivatives reveal subtle features that are not always observable on the original plots. The rate at which ph changes with added titrant is the greatest at the equivalence point. For an acid sample, titrated with a base, this rate of change is the first derivative of the ph vs. V B plot, or d ph /d VB