CHAPTER 15: ACIDS AND BASES15.1 Brønsted Acids and Bases- Conjugate acid-base pair: the conjugate base of a Brønsted acid is the species that remains when one proton has been removed from the acido HCl Cl-- It is acceptable to represent the proton in aqueous solution as either H+ or H3O+15.2 The Acid-Base Properties of Water- Water acts as a proton donor (acid) in basic solutions, and a proton acceptor (base) in basic solutions- An H+ and OH- ion in chemical equilibrium with H2O is sometimes called the autoionization of water- Kw: the ion-product constant, which is the product of the molar concentrations of [H+] and [OH-] ions at a particular temperature.o Kw = 1.0 x 10^-14- At 25ºC [H+] = [OH-] = 1.0 x 10^-7o pH = 7- In an acidic solution, [H+] > [OH-]- In a basic solution, [H+] < [OH-]15.3 pH—A Measure of Acidity- The pH of a solution is defined as a negative logarithm of the hydrogen ion concentration (in mol/L)o pH = -log [H+]o pOH = -log[OH-]- pH + pOH = 14.0015.4 Strength of Acids and Bases- Strong acids are strong electrolytes that are assumed to ionize completelyin water. Most strong acids are inorganic acidso HClo HIo HBro HNO3o HClO4o H2SO4 (diprotic)- Most acids are weak acids, which only ionize to a limited extent in water- Like strong acids, strong bases are strong electrolytes that ionize completely in water- All alkali metal hydroxides are solubleo KOHo NaOHo Ba(OH)2 is also soluble from the alkaline earth metal group- As acid strength increases, the conjugate base strength decreaseso HClO4; strong acid — ClO4-; weak baseo HF; medium acid — F-; medium baseo NH3; weak acid—NH2; strong base- If we know the relative strengths of two acids, we can predict the positionof equilibrium between one of the acids and the conjugate base of the other15.5 Weak Acids and Acid Ionization Constants- Ka is the acid ionization constant, which is the equilibrium constant of an acido At a given temperature, the strength of the acid HA is measured quantitatively by the magnitude of Ka- One way to determine Ka is to measure the pH of the acid solution of known concentration at equilibrium15.6 Weak Bases and Base Ionization Constants- Kb is the base ionization constant, which is the equilibrium constant of a base15.7 The Relationship Between the Ionization Constants of Acids and Their Conjugate Bases- For any conjugate acid-base pair it is always true that Ka*Kb = Kw15.8 Diprotic and Polyprotic Acids- These acids ionize in a stepwise manner (lose one proton at a time)o Ionization constants decrease markedly for the second and third stages.- Ionization of H3PO4 (phosphoric acid)o H2PO4, Ka = 7.5 x 10^-3o HPO4-, Ka = 6.2 x 10^-8o PO4(2-), Ka = 4.8 x 10^-1315.9 Molecular Structure and the Strength of Acids- Acid strength depends ono Properties of the solvento Temperatureo Molecular structure- Acid strength is measured by its tendency to ionizeo Which is why HF, although being more diverse in terms of electronegativity than HCl, HI, and HBr, is considered a weak acid It’s ability to form hydrogen bonds (a strong bond) serves itgreater intermolecular forces, thereby reducing its ability toionize Essentially: HF has a greater bond enthalpy than its hydrohalic acid counterparts HF << HCl < HBr < HIo A high degree of polarity characterizes a stronger acid- Oxoacids contain hydrogen, oxygen, and one other element Z, which occupies a central positiono If Z is highly electronegative, or is in a high oxidation state, it will attract more electrons, making the Z-O bond more covalent and the O-H bond more polar Consequently, the tendency for the hydrogen to be donated increaseso As for oxoacids having different central atoms from the same group of the periodic table and that have the same oxidation number, acid strength increases with increasing electronegativity HClO3 > HBrO3- As for oxoacids having same central atoms but different number of attached groups, the acid strength increases as the oxidation number of the central atom increaseso Oxidation number of Cl increases: HClO4 (+7) > HClO3 (+5) > HClO2 (+3) > HClO (+1)o In this case, chlorine’s ability to draw electrons away from the OH group increases (thus making the O-H bond more polar) Thus, HClO4 is the strongest because it is attached to thelargest number of O atoms15.12 Lewis Acids and Bases- A Lewis base is a substance that can donate a pair of electrons- A Lewis acid is a substance that can accept a pair of electrons- When referring to acids and bases, the Brønsted definition is more commonly usedo The term “Lewis acid” is typically reserved for substances that can accept a pair of electrons but do not contain ionizable hydrogen atomsCHAPTER 16: ACID-BASE EQUILIBRIA AND SOLUBILITY EQUILIBRIA16.2 The Common Ion Effect- According to Le Châtelier’s Principle, the addition of CH3COO- ions from CH3COONa to a solution of CH3COOH will suppress the ionization of CH3COOH, thereby decreasing the hydrogen ion concentration- The common ion effect: a shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance16.3 Buffer Solutions- A buffer solution is a solution of o 1) a weak acid or a weak base o 2) its salt- Such a solution has the ability to resist changes in pH upon the addition ofsmall amounts of either acid or baseo If an acid is added to a solution in the buffer-zone of a titration curve, the H+ ions will be consumed by the conjugate base in the buffer- Buffer capacity depends on amounto The greater amount of acid and conjugate base in the buffer, the greater capacity- To prepare a buffer solution with a specific pH, we choose an acid whose pKa is closed to the desired pH16.4 Acid-Base Titrations- Strong acid + strong baseo Equivalence point at pH = 7- Pages 733-736 in the textbook provide three sample calculations for measuring pH at different points of the titration. There are four stages overall, and I suggest you memorize the steps on pages 737-739.- Weak acid + strong baseo Equivalence point at pH > 7o OH- ions are in excesso Steps for measuring pH on pages 737-739 in the textbook- Strong acid + weak baseo Equivalence point at pH < 7o H+ ions are in excesso Steps for measuring pH on pages 737-739 in the textbook16.6 Solubility Equilibria- Ksp is known as the solubility producto The product of molar concentrations of the constituent ions, each raised to the power of its stoichiometric coefficient in the equilibrium equationo The
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