Ch07. Exploring Acid Base Reactions. Measuring acidity. version 1.5. Nick DeMello, PhD

Transcription

1 Ch07 ph Exploring Acid Base Reactions. Measuring acidity. version 1.5 Nick DeMello, PhD

2 Ch07 ph & Reactions Water Ionization Reactions Water as an Acid-Base Metals (gas evolution) Amphoteric Property Carbonates (gas evolvution) Ion Product Constant Kw Hydroxides (neutralization) Relating [OH1-] and ph Scale ph defined examples Relating ph [H+] & [OH1-] Measuring ph Calculations with ph Calculator Use Significant Figures [H+] Titration Process Indicators & End Point Calculations

3 Ionization of Water Water is amphoteric it can act as an acid or a base. In water, H + is transferred from one H 2 O molecule to another one molecule acts as an acid, while another acts as a base equilibrium is reached between the conjugate acid base pairs H 2 O(l ) + H 2 O(l ) H 3 O + (aq) + OH (aq) 3

4 Ionization of Water Water is amphoteric it can act as an acid or a base. In water, H + is transferred from one H 2 O molecule to another one molecule acts as an acid, while another acts as a base equilibrium is reached between the conjugate acid base pairs H 2 O(l ) + H 2 O(l ) H 3 O + (aq) + OH (aq) 4

5 Ch07 ph & Reactions Water Ionization Reactions Water as an Acid-Base Metals (gas evolution) Amphoteric Property Carbonates (gas evolvution) Ion Product Constant Kw Hydroxides (neutralization) Relating [OH1-] and ph Scale ph defined examples Relating ph [H+] & [OH1-] Measuring ph Calculations with ph Calculator Use Significant Figures [H+] Titration Process Indicators & End Point Calculations

6 Writing the Ion Product Constant, K w In the equation for the dissociation of water, there is both a forward and a reverse reaction. H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH (aq) Base Acid Conjugate Conjugate Acid Base In pure water, the concentrations of H 3 O + and OH at 25 C are each M. [H 3 O + ] = [OH ] = M K w = [H 3 O + ] [OH ] K w = ( M) ( M) = (at 25 C) 6

7 Ion Product Constant, K w The ion product constant for water, K w, is defined as the product of the concentrations of H 3 O + and OH as equal to at 25 C (the concentration units are omitted) When the [H 3 O + ] and [OH ] are equal, the solution is neutral the [H 3 O + ] is greater than the [OH ], the solution is acidic the [OH ] is greater than the [H 3 O + ], the solution is basic

8 Using K w to Calculate [H 3 O + ], [OH ] If we know the [H 3 O + ] of a solution, we can use the K w to calculate the [OH ]. If we know the [OH ] of a solution, we can use the K w to calculate the [H 3 O + ].

9 Pure Water Is Neutral In pure water, the ionization of water molecules produces small but equal quantities of H 3 O + and OH ions. [H 3 O + ] = M [OH ] = M [H 3 O + ] = [OH ] Pure water is neutral.

10 Acidic Solutions Adding an acid to pure water increases the [H 3 O + ] causes the [H 3 O + ] to exceed M decreases the [OH ] Le Châtelier s Principle [H 3 O + ] > [OH ] The solution is acidic.

11 Basic Solutions Adding a base to pure water increases the [OH ] causes the [OH ] to exceed M decreases the [H 3 O + ] Le Châtelier s Principle [H 3 O + ] < [OH ] The solution is basic.

12 Comparison of [H 3 O + ] and [OH ]

13 Calculating [H 3 O + ] What is the [H 3 O + ] of a solution if [OH ] is M? 13

14 Milk of Magnesia If milk of magnesia has [OH ] of M, what is the [H 3 O + ] of the solution? 14

15 Ch07 ph & Reactions Water Ionization Reactions Water as an Acid-Base Metals (gas evolution) Amphoteric Property Carbonates (gas evolvution) Ion Product Constant Kw Hydroxides (neutralization) Relating [OH1-] and ph Scale ph defined examples Relating ph [H+] & [OH1-] Measuring ph Calculations with ph Calculator Use Significant Figures [H+] Titration Process Indicators & End Point Calculations

16 What ph means. Mixtures that have a high H + concentration have he properties of acids. They are acidic. Mixtures that have a low H + concentration, have a high OH 1- concentration. They have the properties of bases. They are basic. [H + ] = M = M [H + ] = M = 10-9 M [H + ] = M = 10-7 M [H + ] =.0001 M = 10-4 M [H + ] = 1 M = 10-1 M 16

17 What ph means. ph stands for pouvoir hydrogen (power of hydrogen). It s the negative power of the hydrogen ion concentration in a solution. ph is a measure of how acidic or basic that solution is. ph is defined as the negative log of the H + concentration. [H + ] = 10 -ph ph = -log [H + ] 17

18 The ph Scale The ph of a solution is used to indicate the acidity of a solution has values that usually range from 0 to 14 is acidic when the values are less than 7 is neutral at a value of 7 is basic when the values are greater than 7 18

19 ph related to [OH 1- ] As ph increases [H + ] Concentration goes up. [OH 1- ] Concentration goes down. K w = [H 3 O + ] [OH ] = 1.0 x

20 Acidic, Basic, or Neutral? HCl with a ph = 1.5 pancreatic fluid, [H 3 O + ] = M Sprite soft drink, ph = 3.0 ph = 7.0 [OH ] = M [H 3 O + ] = ACIDIC BASIC ACIDIC NEUTRAL ACIDIC BASIC

21 Ch07 ph & Reactions Water Ionization Reactions Water as an Acid-Base Metals (gas evolution) Amphoteric Property Carbonates (gas evolvution) Ion Product Constant Kw Hydroxides (neutralization) Relating [OH1-] and ph Scale ph defined examples Relating ph [H+] & [OH1-] Measuring ph Calculations with ph Calculator Use Significant Figures [H+] Titration Process Indicators & End Point Calculations

22 Acid-Base Indicators Some substances change color, depending on the ph of the solution they are in. A dye extracted from lichens, called litmus, is one example. Litmus turns red in acid. ph < 4.5 Litmus turns blue in base. ph > 8.3 Litmus is one of the oldest and most generally useful acid-base indicators. It was discovered in 1300 AD by the Spanish alchemist Arnaldus de Villa Nova. 22

23 Acid-Base Indicators Some substances change color, depending on the ph of the solution they are in. Phenolphthalein is another example: Is colorless in acid. ph < 7.0 Is light pink in base. ph > 7.0 Is bright pink in stronger base. ph >

24 Acid-Base Indicators Indicators are found in many common plants. The most useful indicators have more than one color change. Red cabbage for example produces an indicator with many different transitions. 24

25 Acid-Base Indicators Universal indicator is a mixture of four indicators that produces a complete spectrum. It s components are: methyl red bromothymol blue thymol blue phenolphthalein ph 1.0 = Cherry Red ph 2.0 = Rose ph 3.0 = Reddish-Orange ph 4.0 = Orange-Red ph 5.0 = Orange ph 6.0 = Yellow ph 7.0 = Yellowish-Green ph 8.0 = Green ph 9.0 = Bluish-Green ph 10.0 = Blue 25

26 ph Paper Litmus, Universal Indicator, and other indicators are often soaked into paper strips to make them more convenient to use. 26

27 ph Meters A ph meter is a device that directly measures the ph of a solution. It usually has a glass electrode plus a calomel reference electrode, or a combination electrode. Basic potentiometric ph meters simply measure the voltage between two electrodes and display the result converted into the corresponding ph value. Probes need to be kept clean of contamination as far as possible, and not touched by hand. Probes are best kept moist with a medium appropriate for the particular probe (distilled water, which can encourage diffusion out of the electrode, is undesirable) when not in use. For very precise work the ph meter should be calibrated before each measurement. For normal use calibration should be performed at the beginning of each day. Calibration should be performed with at least two standard buffer solutions that span the range of ph values to be measured. For general purposes buffers at ph 4.01 and ph are acceptable. 27

28 Ch07 ph & Reactions Water Ionization Reactions Water as an Acid-Base Metals (gas evolution) Amphoteric Property Carbonates (gas evolvution) Ion Product Constant Kw Hydroxides (neutralization) Relating [OH1-] and ph Scale ph defined examples Relating ph [H+] & [OH1-] Measuring ph Calculations with ph Calculator Use Significant Figures [H+] Titration Process Indicators & End Point Calculations

29 ph Calculations The ph scale is a logarithmic scale that corresponds to the [H3O + ] of aqueous solutions. is the negative logarithm (base 10) of the [H3O + ] ph = log [H3O + ] To calculate the ph, the negative powers of 10 in the molar concentrations are converted to positive numbers. If [H3O + ] is M ph = log [ ] = ( 2.00 ) =

30 ph Calculations If [H3O + ] is M ph = log [H3O + ] ph = log [ ] = ( 2.00 ) 1.0 EE or Exp +/ 2 log =

31 ph Calculations [H3O + ] = 10 -ph If ph is 7.0 [H3O + ] = / 2nd 10 x =

32 What is the concentration? HCl with a ph = 1.5 pancreatic fluid, [H 3 O + ] = M Sprite soft drink, ph = 3.0 ph = 7.0 [OH ] = M [H 3 O + ] = ACIDIC BASIC ACIDIC NEUTRAL ACIDIC BASIC

33 Ch07 ph & Reactions Water Ionization Reactions Water as an Acid-Base Metals (gas evolution) Amphoteric Property Carbonates (gas evolvution) Ion Product Constant Kw Hydroxides (neutralization) Relating [OH1-] and ph Scale ph defined examples Relating ph [H+] & [OH1-] Measuring ph Calculations with ph Calculator Use Significant Figures [H+] Titration Process Indicators & End Point Calculations

34 Significant Figures To determine the number of significant figures in the ph value, consider the following: The number of decimal places in the ph value is the same as the number of significant figures in the coefficient of [H3O + ]. The number to the left of the decimal point in the ph value is the power of 10 (this is determined by counting how how far we move the decimal point, it s not part of the significant figures of the ph measaurement) 34

35 What s the ph of Aspirin? Aspirin, which is acetylsalicylic acid, was the first nonsteroidal anti-inflammatory drug used to alleviate pain and fever. If a solution of aspirin has a [H3O + ] = M What is the ph of the solution? 35

36 What s the ph of Aspirin? Aspirin, which is acetylsalicylic acid, was the first nonsteroidal anti-inflammatory drug used to alleviate pain and fever. If a solution of aspirin has a [H3O + ] = M What is the ph of the solution? ph = log [H3O + ] ph = log 1.7 x EE or Exp +/ 3 ph = log = / 36 Exact Two SFs

37 Find the ph Find the ph of a solution with a [H3O + ] of ph = log [H3O + ] ph = log 5.6 x EE or Exp +/ 5 log +/ ph = = 4.25 Exact Two SFs 37

38 What s the [H + ]? Determine the [H3O + ] for solutions having each of the following ph values: ph = 3.0 For ph values that are whole numbers, the [H 3 O + ] can be written 1 10 ph. ph [H 3 O + ] = M ph = / 2nd 10 x = E 04 = 3.8 x

39 Red Wine A red wine has a ph of 3.2. What is the: (a) Concentration of [H3O + ]? (b) Concentration of [OH 1- ]? [H3O + ] = 10 -ph k w = [H30 + ][OH 1- ] = = x10-4 = 6 x10-4 (a) [OH 1- ] = kw / [H30 + ] = 1.0 x10-14 / [6.3 x10-4 ] = x10-11 = 1.6 x10-11 (b) 39

40 Ch07 ph & Reactions Water Ionization Reactions Water as an Acid-Base Metals (gas evolution) Amphoteric Property Carbonates (gas evolvution) Ion Product Constant Kw Hydroxides (neutralization) Relating [OH1-] and ph Scale ph defined examples Relating ph [H+] & [OH1-] Measuring ph Calculations with ph Calculator Use Significant Figures [H+] Titration Process Indicators & End Point Calculations

41 Reactions with Acid Acids react with metals to produce salt and hydrogen gas a reduction-oxidation reaction. bases to produce a salt and water a neutralization reaction. bicarbonate and carbonate ions to produce carbon dioxide gas a gas evolution reaction. A salt is an ionic compound that does not have H + as the cation or OH as the anion. Many of these types of reactions occur in the human body. Gastric acid contains HCl (aq) and is produced by parietal cells that line the stomach. When protein enters the stomach, HCl is secreted until the ph reaches 2, optimum for digestion. 41

42 Acids and Metals Acids react with active metals such as K, Na, Ca, Mg, Al, Zn, Fe, and Sn to produce hydrogen gas and the salt of the metal 2 K(s) + 2 HCl(aq) 2 KCl(aq) + H 2 (g) metal acid salt hydrogen Zn(s) + 2 HCl(aq) ZnCl 2 (aq) + H 2 (g) metal acid salt hydrogen 42

43 Acids and Metals Write balanced equations and label the metal, acid, and salt for the following reactions: Magnesium metal with HCl (aq) magnesium metal + hydrochloric acid magnesium chloride + hydrogen gas Mg(s) + Aluminum metal with HNO3 (aq) HCl(aq) MgCl 2 (aq) + H 2 (g) Mg(s) + 2 HCl(aq) MgCl 2 (aq) + H 2 (g) metal acid salt aluminum metal + nitric acid aluminum nitrate + hydrogen gas Al(s) + HNO 3 (aq) Al(NO 3 ) 3 (aq) + H 2 (g) Al(s) + 6 HNO 3 (aq) Al(NO 3 ) 3 (aq) + Al(s) + 6 HNO 3 (aq) 2 Al(NO 3 ) 3 (aq) + H 2 (g) H 2 (g) 2 Al(s) + 6 HNO 3 (aq) 2 Al(NO 3 ) 3 (aq) + H 2 (g) 43 2 Al(s) + 6 HNO 3 (aq) 2 Al(NO 3 ) 3 (aq) + 3 H 2 (g) metal acid salt

44 Acids, Carbonates, Bicarbonates Acids react with carbonates and hydrogen carbonates to produce carbon dioxide gas, a salt, and water 2 HCl(aq) + CaCO 3 (s) CO 2 (g) + CaCl 2 (aq) + H 2 O(l) acid carbonate carbon salt water dioxide HCl(aq) + NaHCO 3 (s) CO 2 (g) + NaCl(aq) + H 2 O(l) acid bicarbonate carbon salt water dioxide 44

45 Acids, Carbonates, Bicarbonates Write balanced equations for the following reactions: MgCO3(s) + HBr(aq) MgCO 3 (s) + HBr(aq) MgBr 2 (aq) + CO 2 (g) + H 2 O(l) MgCO 3 (s) + 2 HBr(aq) MgBr 2 (aq) + CO 2 (g) + H 2 O(l) HCl(aq) + NaHCO3(aq) HCl(aq) + NaHCO 3 (aq) NaCl(aq) + CO 2 (g) + H 2 O(l) 45

46 Acids and Hydroxides: Neutralization In a neutralization reaction, an acid reacts with a base to produce salt and water the salt formed is the anion from the acid and the cation from the base HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) acid base salt water 46

47 Ch07 ph & Reactions Water Ionization Reactions Water as an Acid-Base Metals (gas evolution) Amphoteric Property Carbonates (gas evolvution) Ion Product Constant Kw Hydroxides (neutralization) Relating [OH1-] and ph Scale ph defined examples Relating ph [H+] & [OH1-] Measuring ph Calculations with ph Calculator Use Significant Figures [H+] Titration Process Indicators & End Point Calculations

48 48 Questions?