15.1 Acids - Bases in Water

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1 15.1 more equilibrium Dr. Fred Omega Garces Chemistry 201 Miramar College 1

2 Acids-Bases Characteristics Acids (Properties) Taste Sour Dehydrate Substances Neutralizes bases Dissolves metals Examples: Juices: TJ, OJ, AJ Wine Banana Coffee Vitamin C Soda Base (Properties) Taste Bitter Denatures Proteins Neutralizes acids Turns metal g hydroxides Examples: Milk of Magnesia Lime water Lye, Drain Ammonia blood Soap 2

3 Practice Naming Acids HNO 2 HI HF Nitrous Acid Hydroiodic Acid Hydrofluoric Acid Oxy-anions Add H+ Oxy-acids H 3 PO 4 H 3 PO 3 Phosphoric Acid Phosphorous Acid Add H+ HClO 2 HClO Chlorous Acid Hypochlorous Acid Add H+ H 2 CO 3 HCN Carbonic Acid Hydrocyanic Acid Add H+ HC 2 H 3 O 2 Acetic Acid H 2 Cr 2 O 7 Dichromic Acid HClO 4 Perchloric Acid 3

4 Arrhenius Definition Svante Arrhenius ( ) Acid - increases H + (H 3 O + ) concentration Svante Arrhenius, a Swedish scientist, was the first to recognize that acids, bases, and salts, when dissolved in water are dissociated into ions Bases - increases OH - concentration Arrhenius acids and bases are limited to aqueous solutions. Behavior in water Acids are substances that have H in its formula and dissociates in water to yield yield, H + (actually H 3 O + ) H 2 SO 4(aq) g H + (aq) + HSO 4 - (aq) Bases are substances that have OH in its formula and dissociates in water to yield OH -. Hydroxide ions, OH -, are specie which readily react with H + ions to form water: H + (aq) + OH - (aq) g H 2 O (l) 4

5 Factors Affecting Acid/Base Strength What determines the strength of acids and Base? Dissociation property- Electrolyte - Substances which dissociate in water. Strong electrolyte completely dissociates to ions Weak electrolyte undergoes partial dissociation. Acid Examples : HClO 4 + H 2 O g H 3 O + + ClO 4-100% Dissociation (Strong Acid) HNO 3 + H 2 O g H 3 O + + NO 3-100% Dissociation (Strong Acid) H 2 S + H 2 O D H 3 O + + HS - Less 100% Dissociation (Weak Acid) Base Examples : NaOH + H 2 O g OH - + Na + 100% Dissociation (Strong Base) Ca(OH) 2 + H 2 O g 2OH - + Ca % Dissociation (Strong Base) NH 3 + H 2 O D OH - + NH + 4 Less 100% Dissociation (Weak Base) 5

6 Strong Acids Strong Acids Strong Electrolyte that ionizes (break up to H + ions) in solution 100 % of time. Examples : * H 2 SO 4, HClO 4, HClO 3, HNO 3, HX (X = Cl, Br, I) Only strong acids dissociates 100% in water, K a >>1 These are Strong Acids. Rxn: HClO 4 + H 2 O g H 3 O + + ClO % Dissociation HNO 3 + H 2 O g H 3 O + + NO % Dissociation H 2 S + H 2 O D H 3 O + + HS - Less 100% Dissociation *All other acids under goes less than 100 % Dissociation. 6

7 Strong Acids; indicative by acid dissociation constant, Ka The K a values shown are the acid dissociation constant for monoprotic acids. The K a is the equilibrium constant for the ionization of the acid, HA D H + + A. Strong acids do not have meaningful K a values and are left out from this table. 7

8 Strong Base Strong Base: Strong Electrolyte that ionizes (break up to OH - ions) in solution 100 % of time. (Remember solubility Rules) Examples : Group-1 hydroxide: LiOH, KOH, NaOH, RbOH Heavy Grp-2 hydroxides: Ca(OH) 2, Sr(OH) 2, Ba(OH) 2, Ionic metal oxides: Li 2 O, Na 2 O, K 2 O, CaO These are bases which dissociate 100% in water (or in the case of oxides, react with water) to produce hydroxides. Rxn: KOH g OH - + K + 100% Dissociation Ca(OH) 2 g 2OH - + Ca % Dissociation Li 2 O + H 2 O g 2OH - + Li + 100% Dissociation CaO + H 2 O g Ca(OH) 2 strong base produced NH 3 + H 2 O D OH - + NH 4 + Less 100% Dissociation *All other bases undergo less than 100 % Dissociation. 8

9 Chemistry of Acids and Bases Acids Base Strong Acid Weak Acid 9

10 Protons in water: Hydronium Hydronium ion: HCl (aq) g H + (aq) + Cl- (aq) Proton? H H + attacks water solvent to form hydronium H 3 O + e - p Proton in H 2 O In reality: H + + H 2 O g H 3 O + H+ is always associated with solvent. H + protons & H 3 O + Hydronium ion H + (aq) D H 3 O + considered the same. These terms are used interchangeably 10

11 Water: Acid-Base Properties Autoionization (Self-Ionization) of water Why does water have a ph of 7? Water is Amphoteric (it reacts with itself) 2 in 1 billion self-ionize. H 2 O (l) + H 2 O (l) + E D H 3 O (aq) + OH- (aq) Temperature Dependency of Kw Endothermic reaction ΔH rxn = kj, K eq = K w K w (ion-product constant) = 1 25 C The equation K w = [H 3 O + ][OH - ] is valid in pure water. In any aqueous solution. K w is temperature-dependent, the auto-ionization rxn is endothermic, so K w increases with temperature. C Kw

12 Consequences of Autoionization A change in [H 3 O + ] causes an inverse change in [OH - ] & vice versa. Higher [H 3 O + ] g lower [OH - ] Higher [OH - ] g lower [H 3 O + ] Both ions are present in all aqueous systems. Acidic solution g [H 3 O + ] > [OH - ] Neutral solutions g [H 3 O + ] = [OH - ] Basic Solutions g [OH - ] > [H 3 O + ] Kw = [H 3 O + ] [OH - ] or pkw = ph + poh (as shown later) 12

13 K w & [H 3 O + ] How does K w dictate concentration of H + and OH -? K w = = [ H 3 O + ] [ OH ] at 25 C For pure water, solve the ice-table to determine the concentration of [H 3 O] + & [OH] -. 2H 2 O (l)! OH - (aq) + H 3 O + (aq ) i constant 0 0 Δ - 2x + x + x [e] constant x x K w = = x M = x =[H 3 O] + =[OH] - 13

14 ph Calculation For pure water [H 3 O + ] = M Since M = M Concentration is so small, it is much more convenient to use a scale called p-h (power of hydrogen) M g ph = 7 ph = - log [H 3 O + ] or [H 3 O + ] = [ph] Example: Calculate: ph for [H 3 O + ] = M [H 3 O + ] of ph = Calculator Sequence: 1.67 EE 5 + / - log + / / - 10 x Answer :

15 ph Scale ph and the Concentration of Acids Conc [H 3 O + ] Exp [H 3 O + ] ph poh 1M M M M M M M M M M M M M M M

16 Other p Scale p scale is the negative log Convenient for expressing magnitude of small numbers i. [H 3 O + ] ph = - log [H 3 O + ] and [H 3 O + ] = [ph] ii. [OH - ] poh = - log [OH - ] and [OH - ] = [poh] ii. K eq i.e., K w, K a & K b pk eq = - log [K eq ] and [K eq ] = [pkeq] pk w = - log [K w ] pk a = - log [K a ] pk b = - log [K b ] and [K w ] = [pkw] and [K a ] = [pka] and [K b ] = [pkb] 16

17 Determining ph, poh, [OH - ], [H 3 O + ] Use this chart to determine acid and base concentration at (25 C) K w = & pk w = 14 [H 3 O + ] [OH - ] = K w & ph + poh = pk w K w / [H 3 O + ] [OH - ] [H 3 O + ] [OH - ] = K w -log [H 3 O + ] K w / -log 10 -ph ph ph + poh = pk w = poh 10 -poh 17

18 Kw, ph and poh ph measures the concentration of [H 3 O + ] Kw = = [H 3 O + ] [OH - ] pkw = ph + poh 14 = ph + poh Example: What is the ph of a solution having a [OH - ] = M Way of Multiplying Way of adding / subtracting Using the formula: Using the formula: = [H 3 O + ] [OH - ] 14 = ph + poh = [H 3 O + ] [OH - ] 14 = ph + poh =[H 3 O + ] [ ] 14 = ph [H 3 O + ] = M ph = 1.48 ph =

19 ph Values ph Values for various samples: Smaller the ph value the greater the acidity larger the ph value the lower the acidity Acid: ph < 7 Neutral: ph = 7 Base: ph > 7 20

20 Rainwater ph (H 2 0) not 7.0 Rain water isn t neutral Dissolving CO 2 CO 2(g) + H 2 O (g) g H 2 CO 3 (aq ) carbonic acid H 2 CO 3 (aq) ionization: H 2 CO 3 (aq) g H + (aq) + HCO 3 - (aq) Overall: CO 2(g) + H 2 O (l) g H + (aq) + HCO 3 - (aq) Other sources of Acidic Rain: SO 2(g) + O 2(g) g 2 SO 3(g) & N 2 g) + O 2 g) g 2 NO 2(g) from burning fossil fuels and from volcanoes from car combustion 2 SO 3(g) + H 2 O (l) g H 2 SO 4(aq) sulfuric acid 2 NO 2(g) + H 2 O (l) g HNO 3(aq) + HNO 2 (aq) nitric acid nitrous acid 21

21 Rainwater and Acid Rain Affects of Acid Rain on Historical Artifacts and the Environment a) Details of the marble that is part of the Washington Square Arch in Washington Square Park completed on July 17, b) The same statue some 50 years later (June 1994) after the destructive effect of acid rain. The combustion of fossil fuels produces CO 2 ; automobile contain NO 2 ; and burning lowgrade coal releases SO 2. All these chemicals contribute to acid rain. A forest damaged by acid rain 22

22 Measuring ph in the Laboratory ph range for various indicators Some common acid-base indicators. The color changes occur over a range of ph values. Notice that a few indicators have two color changes over two different ph ranges. Mechanism for phenolphthalein indicator. At Low ph phenolphthalein is colorless and has a structure in which there is a five membered ring. In the presence of excess acid the five membered ring is broken and the resulting change in conformation gives rise to a compound which is pink. 23

23 Acid Base in Water Summary Pure water has a low conductivity because it autoionizes to a small extent. This process is described by an equilibrium reaction whose equilibrium constant is the ion-product constant for water, K w ( at 25 C). Thus, [H 3 O + ] and [OH - ] are inversely related. In acidic solution, [H 3 O + ] is greater than [OH-], the reverse is true in basic solution, and the two are equal in neutral solution. To express small values of [H 3 O + ] more simply, we use the ph scale (ph = -log [H 3 O + ]). A high ph represents a low [H 3 O + ]. Similarly, poh = -log [OH - ], and pk = - log K. At 25 C, in acidic solutions, ph < 7.00, in basic solutions, ph > 7.00; and in neutral solutions, ph = 7.0. The sum of the ph and poh equals pk w (14.00 at 25 C) 24

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