6.02 Acids - Bases. Dr. Fred Omega Garces. Chemistry 100. Miramar College Acids - Bases. January 10

Transcription

1 6.02 Acids - Bases Dr. Fred Omega Garces Chemistry 100 Miramar College 1 10 Acids - Bases

2 Acids-Bases Characteristics Acids (Properties) Taste Sour Dehydrate Substances Neutralize bases Dissolve metals Turns Blue Litmus Red Examples Juices: TJ, OJ, AJ Wine Banana Coffee Vitamin C Soda Base (Properties) Taste Bitter Denature Proteins Neutralize acids Turns metal g hydroxides Turns Red Litmus Blue Examples Lime water Milk of Magnesia Lye, Drano Ammonia blood Soap 2 10 Acids - Bases

3 Acids Dehydrating Properties Acids (Properties) Taste Sour Dehydrate Substances Neutralize bases Dissolve metals Turns Blue Litmus Red 3 10 Acids - Bases

4 Arrhenius Definition Svante Arrhenius ( ) Acid - Increases H + (H 3 O + ) concentration Base - Increases OH - concentration Arrhenius acids and bases are limited to aqueous solutions. Examples: Acids are substances that are able to ionize to form hydrogen ion and thereby increase the concentration of H + (aq) ions in aqueous solutions. HNO 3 (aq) g H + (aq) + NO - 3 (aq) Bases are substances that accept (react with) H + ions. Hydroxide ions, OH -, are basic because they readily react with H + ions to form water: H + (aq) + OH - (aq) g H 2 O (l) 4 10 Acids - Bases

5 Protons in water: Hydronium Hydronium ion: HCl (aq) g H + (aq) + Cl- (aq) Proton? H+ attacks water solvent to form hydronium H 3 O+ H 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 are used interchangeably Acids - Bases

6 Strong Acids Strong Acids Earlier we described acids and bases as electrolyte; that is, these substances ionizes (break up to ions) in solution. Strong electrolyte are substances which completely dissociates (100%). SA-Strong acids completely dissociates to H + and anion. HA g H + + A - Strong Acids; HX, H 2 SO 4, HNO 3, HClO 4 HClO 3 : All others are considered weak. HX: HCl, HBr, HI (all Halogens, except F ) H 2 SO 4 Sulfuric Acid HNO 3 Nitric Acid HClO 4 Perchloric Acid HClO 3 Chloric Acid Weak acids, incompletely dissociation in water. i.e., CH 3 COOH (acetic acid) only 1 in 100 dissociate 6 10 Acids - Bases

7 Strong - Weak Acids Strong vs Weak Acids 7 10 Acids - Bases

8 Strong Bases Strong Bases Substances which completely dissociate into cation and hydroxide. MOH g M + + OH - Strong Bases, M(OH) n ; NaOH, KOH, LiOH, Ca(OH) 2, Ba(OH) 2, Sr(OH) 2 All others are considered weak. Bases: 3 ways of OH - formation Extraction of H + from H 2 O: i.e.., reaction of NH 3 in water: NH 3 (aq) + H 2 O (l) D NH + 4 (aq) + OH - (aq) Extraction of H + from H 2 O by charge i.e.., reaction of CO 2-3 in water: CO 2-3 (aq) + H 2 O (l) D HCO - 3 (aq) + OH - (aq) Dissociation of ionic substance to OH - i.e.., dissociation of NaOH in water: NaOH (aq) D Na + (aq) + OH - (aq) 8 10 Acids - Bases

9 Strong - Weak Bases Strong vs Weak Bases 9 10 Acids - Bases

10 New Definition: Bronsted-Lowry Acids-Bases Bronsted - Lowry definition Acid - Proton H + (H 3 O + ) donor Base - Proton H + (H 3 O + ) acceptor. example: acids: HCl (aq) H + (aq) + Cl - (aq) Bases: NH 3 (aq) + H 2 O (l) NH + 4 (aq) + OH - (aq) HCl (aq) + NH 3 (aq) NH + 4 (aq) + Cl - (aq In an acid - base reaction, H + & OH - always combine together to form water and an ionic compound (a salt): HCl (aq) + NaOH (aq) g H 2 O (l) + NaCl (aq) Acids - Bases

11 Weak Acids and Bases Some Weak Acids in Water at 25 C Indicator of acid strength Acids - Bases

12 Polyprotic Acids Other examples of polyprotic acids Acids - Bases

13 Water: Acid-Base Properties Auto-ionization (Self-Ionization) of water Why does water have a ph of 7? Water is Amphoteric (it reacts with itself) 2 in 1 billion water molecules self-ionizes. H 2 O (l) + H 2 O (l) + Energy K w D H 3 O (aq) + OH - (aq) K eq = K w Endothermic reaction K w (ion-product constant) = see that, [H 3 O + ] = M [OH - ] = M The equation K w = [H 3 O + ][OH - ] is valid in pure water and in any aqueous solution. K w is temperature-dependent, the autoionization rxn is endothermic, so K w increases with temperature. C Kw Acids - Bases

14 K w & [H 3 O + ] How does K w dictate concentration of H 3 O + and OH -? K w = # = H 3 O + & # & $ % ' ( $ % OH '( at 25 C For pure water, If [H 3 O] + or [OH] - is concentration known, the other can be determine through the mass action expression and a iδe table. 2H 2 O (l)! OH - + H 3 O + i Excess 0 0 Δ - 2x + x + x e excess x x K w = = x 2 ph calculation to be discussed = x = [H 3 O] + = [OH] - ph g Acids - Bases

15 ph and the Concentration of Acids ph Scale 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 Acids - Bases

16 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: 2SO 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 Acids - Bases

17 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 Acids - Bases

18 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 > Acids - Bases

19 Buffer Buffer - A solution whose ph is resistant to change. Your body uses buffers to maintain the ph of your blood Blood ph Buffer system in body - 1. Proteins 2. Phosphates HPO 2-4 / H 2 PO - 4 : 1.6 / 1 3. Carbonates H 2 CO 3 / HCO - 3 : 10 / 1 Reaction: acid neutralization: H 3 O + + HCO 3 - D H 2 CO 3 + H 2 O H 2 CO 3 g H 2 O + CO 2 (exhale) base neutralization: OH - + H 2 CO 3 D HCO H 2 O Acids - Bases

20 Acidosis Blood ph ι 7.35 (ACIDOSIS) Depression of the acute nervous system or respiratory center in the medulla of the brain is affected by an accident or by depressive drugs. Symptoms: Depression of the acute nervous system Fainting spells Coma RIP Causes: 1. Respiratory Acidosis Difficulty Breathing (Hypo-ventilation) Pneumonia, Asthma anything which diminish CO 2 from leaving lungs. 2. Metabolic Acidosis Starvation or fasting Heavy exercise Mechanism : 1. Respiratory Acidosis: CO 2 doesn t leave lungs which result in the build up of H 2 CO 3 in the blood. 2. Metabolic Acidosis: If body doesn t have enough food then Fatty acids (Fat) are used. Fatty Acids g Acidic. Furthermore, exercise leads muscle to produce lactic acid Acids - Bases

21 Symptoms: Over simulation of the nervous system Muscle cramps Convulsion Death Causes: Alkalosis Blood ph h Respiratory Alkalosis Heavy rapid breathing (hyperventilation). Result from - fear, hysteria, fever, infection or reaction with drugs. 2. Metabolic Alkalosis Metabolic irregularities or by excess vomiting. (ALKALOSIS) Mechanism : 1. Respiratory Alkalosis Excessive loss of CO 2 lowers H 2 CO 3 and raise HCO 3 - level. (Can be remedied by breathing in a bag) 2. Metabolic Alkalosis Vomiting removes excess acidic material from stomach. (ph of stomach equals one) Acids - Bases

22 Buffer System at Work Buffer - System that resists change in ph when H 3 O + or OH - is added. Buffer solution may be prepared by a weak acid & its conjugate base. How it Works: A - g HA H 3 O + Buffer H 2 O Remember ph = Conc. of H 3 O + Your blood Rxn: HCO 3 - D H 2 CO 3 Excess H 3 O + + HCO 3 - g H 2 CO 3 + H 2 O H 3 O + CO 2 + H 2 O Excess OH - + H 2 CO 3 D HCO H 2 O OH Acids - Bases

23 Acid/Base Titration Titration A technique of chemical analysis to determine the amount of a substance in a sample. i.e., What is the acidic content of Lake Elsinor? A sample can be tested by titration. In a titration experiment, a known volume of a standard concentrated solution (the titrant) is used to analyze a sample (the analyte). One is usually an acid, the other a base. An indicator is added to the analyte to signal when the titration is complete. This is called the endpoint. When the moles of acid (H 3 O + ) and moles of base (OH - ) are equal in a titration experiment, the stoichiometric equivalent point is reached. This is called the equivalence point. Indicator changes endpoint moles titrant = moles equivalent point Acids - Bases

24 Acid- Base Indicators Some common acidbase 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 of phenolphthalein gives rise to a compound which is pink Acids - Bases

25 Titration Curve Titration curve between strong acid and strong base. Analyte is HCl and titrant is NaOH Acids - Bases

26 Strong Acid - Strong Base Calculation Titration of HCl with NaOH A titration is used to determine the concentration of HCl solution. Exactly 20.00mL of the acid solution was placed in a flask, with phenolphthalein added mL of M NaOH was needed to reach the endpoint. What is the conc. of the HCl? Rxn: NaOH g OH 100% H Cl g H+ 100% Net H + + OH - g H 2 O Conc H 3 O+: = mol OH ml 1 mol H+ 1 1 L 1mol OH ml [HCl] = M ml HCl ml end pt Acids - Bases

27 Summary The following summary list the important tools needed to solve problems dealing with acid-base equilibria. Equation / Concept Function 1 [H + ] [OH - ] = K w Permits the calculation of [H + ] or [OH - ] when the other is known. 2 p X = - log X This equation is the basis of the p-scale. 3 ph + poh = This equation shows the relationship between the ph and the poh 4 HA! H + + A - K a = [H+ ] [A ] [HA] 5 B + H 2 O! HB + OH - K b = [HB] [OH ] [B] 6 Percent ionization (α) amount ionized α = 100% initial amount This is the Mass Action Equation for the ionization of a weak acid in water. This equation yields the k a given the equilibrium concentration of all specie. The equation also yields the [H 3 O+] given the initial concentration of the weak base [HA] and the k a. This is the Mass Action Equation for the ionization of a weak base in water. This equation yields the k b given the equilibrium concentration of all specie. The equation also yields the [OH-] given the initial concentration of the weak base [B] and the k b. The percent ionization can be calculated from the initial concentration of the acid (or base) and the change in the concentration of the ions. Given the percent ionization (α) and the ph, the k a (or k b ) can be determined. 7 K a K b = K w This equation relates K a and K b for conjugate pairs in aqueous solution, 8 ID of the solute as : i) only a weak acid ii) only a weak base iii) a mixture of a weak acid and its conjugate base Identification of the function of the solute leads to the correct Mass Action expression and thereby leading to the correct equilibrium law. This is a critical first step to solve any acid-base equilibria 9 Identification of acidic cations and basic anions 10 Assumption which simplifies Mass Action 11 Reactions when H+ or OHare added to a buffer solution. Identification of function of cation and anion of a salt lead to ph of the salt solution. Given the k a or k b of the conjugates of these ions leads to the calculation of the ph or poh In order to simplify the math calculation of a Mass Action expression, assumption can be made base on the k a or k b value. Understanding the buffer reaction permits the determination of the effect of a strong acid or strong base on the ph of the solution. Adding H+ lowers the [A-] and raises [HA], adding OH- lowers [HA] and raises [A-] Acids - Bases