What is the ph of a 1.0 L 1M HCl solu7on that has reacted with 1.0L of a 1M NaOH solu7on?

Transcription

1 What is the ph of a 1.0 L 1M HCl solu7on that has reacted with 1.0L of a 1M NaOH solu7on? Today fundamentals of 7tra7on Calculate expected ph resul7ng from 7tra7on of a strong acid with a strong base Explain func7on of indicators

2 Indicators An indicator is a substance that whose op7cal proper7es are a func7on of ph What does that mean? It changes color in response to changes in ph Typically an indicator only works in a specific range of ph values

3 Indicators and ph In an acidic solu7on, a good indicator will act as a Brønsted base, and accept a proton This changes its op7cal proper7es, and gives an inves7gator a visual indica*on of the [H+] In basic solu7ons, a good indicator will act as a Brønsted acid, and donate H+ ions to OH- present in the solu7on

4 indicators The ph range an indicator is effec7ve in is limited, called its transi'on interval A good indicator at a low ph is one that ionizes well, and so will only accept protons from strong acids A good indicator at a higher ph (but s7ll an acidic ph) is one that weakly ionizes, but s7ll strongly enough so that it can accept protons from weak acids

5 Examples of indicators

6 Role of indicators Not to tell what the ph is of a solu7on (for that we use a ph meter, which measures voltage between electrodes placed in the solu7on) Indicator is tell us the equivalence point when an acid and base react

7 Equivalence point The is the point at which the concentra7on of [H +] equals the concentra7on of protonated conjugate We use this representa7on HA --- H+ + A- Where HA is the Brønsted acid, and A- is the conjugate base The equivalance point is when [A- ] = [HA] What are these values for water?

8 7tra7on The process of determining an acids equivalence point by slowly adding a base in the presence of an indicator As base is added, ph changes why?

9 Acid equilibrium constant Really, this is only useful for weak acids (that do not ionize completely) Symbolized by K a This is the ph at which [H3O+][A- ] [HA] These values are molar concentra7on numbers, just like ph we can write this figure as an inverse log

10 What is [H+] when the ph is 6.32? Today titrations Determine the molarity of a strong acid solution titrated to equivalence point by a strong base

11 What 7tra7on looks like An indicator is used to show you when the equivalence point is reached

12 diagrams Visual representa7on of a weakly acidic solu7on Strongly acidic solu7on Titra7on with a strong base Ques7ons: what happens to concentra7on of [HA] for a weak acid as it is 7trated with a strong base? [H+]? [A- ]?

13 Titra7on calcula7ons We begin with this formula: M base X V base = M acid X V acid What does this look like? We also need to consider mole ra7os, which means we need to begin with a balanced chemical reac7on for the 7tra7on

14 Example In a 7tra7on, 27.4 ml of a M Ba(OH) 2 is added to a 20.0 ml sample of HCl of unknown concentra7on un7l an equivalence point is reached. What was the molarity of the HCl? Step 1: write out balanced equa7on Step 2: M base V base = M acid V acid Step 3: determine mole ra7o from reac7on Step 4: mul7ply known M by mole ra7o so units cancel, remaining units will be in terms of molarity of unknown Solu7on on next slide

15 solu7on Ba(OH) 2 + 2HCl BaCl 2 + 2H 2 O Need common units for volume (because M is moles/liter), so convert volume units to L Mole ra7o 2 mol HCl/1 mol Ba(OH) mol Ba(OH) 2 /L X 1L/1000ml = 4.22X10-4 mol Ba(OH) 2 2 mol HCl/1 mol Ba(OH) 2 X 4.22X10-4 mol Ba(OH) 2 = 8.44 X 10-4 mol HCl 8.44 X 10-4 mol HCl X 1000ml/L = 4.22 X 10-2 mol HCl

16 What does the answer mean? In that case, you didn t know the ini7al molarity of the HCl solu7on, so the calcula7on allowed you to find that

17 Addi7onal examples A 15.5 ml sample of M KOH solu7on required 21.2 ml of aqueous ace7c acid solu7on to reach equivalence. What was the molarity of the ace7c acid solu7on? By 7tra7on, a 17.6 ml aqueous H 2 SO 4 solu7on neutralized 27.4 ml of M LiOH. What was the molarity of the acid?

18 What does this show?

19 K a Defined is [H 3 O + ][A - ] [HA] pka = - logka Describes the ph value when

20 Review: conjugate bases

21 What happens to rela7ve ra7o of acid to conjugate base in a 7tra7on? HW page 523 # 15, 16, 21, 36 Predict equivalence point for 7tra7ons Explain Ka and how it used

22 Titra7on curve

23 Other 7tra7ons We ve looked at strong acid with a strong base equivalence point will be at about ph 7.0 Strong acid + weak base equivalence point will be < 7; why? Weak acid + strong base equivalence point will be > 7; why? What about weak acid + weak base? depends on K a of each

24 K a Defined is [H 3 O + ][A - ] [HA] What would a LARGE K a mean? What would a small K a mean?

25 Ka problems What would the ph be of a 0.12 M ace7c acid solu7on (Ka of ace7c acid is 1.74 X 10-5 ) What does that K a tell you? Set up: ka = ([H 3 O + ][A - ])/[HA] [H 3 O + ] and [A - ] will always be equal! Why? Simplify [H 3 O + ][A - ] to [H + ] 2 Rearrange to get [H + ] 2 = K a [HA] So [H+] is the square root of K a [HA], now plug in what you know: K a and [HA] [H+] = sqrt(1.74x10-5 X 0.12 M) = 1.44 X 10-3 So ph = 2.8

26 K b Example reac7on: NH 3 + H 2 O NH OH - So we have K b = [HB + ][OH - ]/[B] The [dissocia7on products] divided by [non- dissociated base] This describes how many hydroxide ions are produced from the dissocia7on of a base in water Higher Kb = more OH- This is a way to find poh! So rela7vely what would the ph be of a solu7on with of substance with a high Kb?

27 Example of a Kb problem What is ph of a M solu7on of ammonia (K b = 1.77 X 10-5 ) Strategy: same as with Kb, we are considering a base that dissociates and gives you a single OH-, so [BH + ] [OH - ] is essen7ally [OH - ] 2 Rearranging Kb equa7on looks just like Ka: You will find [OH- ] = sqrtkb X B Plug in and solve: [OH- ] = sqrt {1.77X10-5 X M} Remember this lets you find poh, so subtract this from 14 and you have ph

28 What does Ka tell you about an acid? Lab tomorrow equip not available today Today: prac7ce 7tra7on and equilibrium calcula7ons instead

29 Review: K a Ka = [H 3 O + ][A - ] [HA] BUT this is making an assump7on the only source of H+ ions is the acid dissolved in water. Is there another source of H+? YES the water! Also what happens to the molarity of HA when it is dissolved? Does it stay the same? NO! it decreases by [A- ] (this is a small number for weak acids!) So is it possible to accurately know ph to many decimal places?

30 Example two solu7ons A M solu7on of a generic weak acid (HA) has a ph of Determine the Ka. Solu7on 1: use the equa7on you know, solve for Ka Solu7on 2: use this equa7on (which in some books/websites is the proper equa7on) Ka = [H + ][A - ] [HA]- [A - ] What s the difference? Does the method maxer? Some%mes!

31 Today green books!! Page 285 Look at 1, 2, 3, 5 and 6 P 293 1&2 P296 #1 P299 #1