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Acid-Base Equilibria. Arrhenius acids increase [H + ] when dissolved in water acids can be classified as monoprotic, diprotic or triprotic bases increase.

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Presentation on theme: "Acid-Base Equilibria. Arrhenius acids increase [H + ] when dissolved in water acids can be classified as monoprotic, diprotic or triprotic bases increase."— Presentation transcript:

1 Acid-Base Equilibria

2 Arrhenius acids increase [H + ] when dissolved in water acids can be classified as monoprotic, diprotic or triprotic bases increase [OH - ] when dissolved in water bases can be classified as monobasic, dibasic, or tribasic

3 A-B Strength strong acids & bases ionize completely and are strong electrolytes 7 acids & 8 bases (memorize them!!!) diprotic acids & dibasic bases do NOT ionize completely, only their first H + or OH - ionizes completely. Strong acid + strong base = neutral salt strong A-B are not equilibrium expressions, but all other A-B are reversible

4 Bronsted-Lowry A-B restricts definition to H + – acids donate H + – bases accept H + allows the classification of less traditional A-B conjugate A-B pairs = 2 formulas in an equation whose formulas differ by a H +

5 What is the acid, base, and the conjugates? HClO +H 2 O H 3 O + + ClO - CO H 2 O OH - + HCO 3 -

6 Wait, water can go both ways? amphoteric substances can behave as either an acid or base depending on what they react with. water and anions with protons (H + ) attached are most common amphoterics

7 Weak A-B only partially ionize in water and are weak electrolytes can be written as equilibrium expressions with a K a or K b value K value indicates how much the acid or base will ionize (high K = higher ionization) larger K values indicate a stronger acid or base For di- and tri- protic/basic, there will be 2 K values (one for the first ionization and one for the second)

8 Autoionization of Water H 2 O + H 2 O OH - + H 3 O + reversible equilibrium where water can donate a proton to itself K w = 1.0 x at room temp. For any conjugate A-B pair, K w = K a x K b What is the K a value for NH 4 + ?

9 Example Is an aqueous solution of Na 2 HPO 4 acidic or basic?

10 pH scale pH = -log[H + ] works for pH ranges from 2-12 and approximates pH outside that window The exponent on the [H + ] is an indicator of approximate pH.

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12 Strong A-B no equilibrium b/c all acid/base ionizes use original acid concentration to calculate pH Calculate the [H + ] and pH in a solution of 0.37M hydrochloric acid. Calculate the [OH - ] and pH in a 0.58M solution of NaOH

13 Weak Acids use RICE to find equilibrium concentrations: RHA + H 2 O H 3 O + + A - II00 C-x+x+x EI-xxx b/c K a for most weak acids is less than 10 -3, I-x is about equal to I, so K a = x 2 /I

14 Example What is the pH of a 0.20M solution of acetic acid (CH 3 COOH)?

15 Weak Bases similar calculations as acids (replace H 3 O + with OH - ) RB + H 2 O OH - + HB + II00 C-y+y+y EI-yyy b/c K b for most weak bases is less than 10 -3, I-y is about equal to I, so K b = y 2 /I

16 Example What is the pH of a 0.68M solution of aqueous ammonia?

17 Classify the following as weak/strong acids/bases: chloric acid ammonium chloride calcium hydroxide ethyl amine sodium cyanide

18 Example Measurements show that the pH of a 0.10M solution of acetic acid is What is the K b of potassium acetate?

19 A-B properties of salt solutions for the most part anions are slightly basic (because they attract protons) and cations are slightly acidic (because they can donate protons) ions from strong A-B are the only neutral ions To determine if a salt is acidic or basic, look at the ions it forms: – ignore any neutral ions – if anion is left, salt is basic – if cation is left, salt is acidic – if both cation & anion are neutral, salt is neutral – if both cation & anion are not neutral, the A-B-ness can’t be determined from the formula

20 Classify the following salts as acidic, basic, or neutral: NaNO 2 CH 3 NH 3 Cl NaCl MgSO 4 Al 2 (SO 3 ) 3

21 A-B-ness & chemical structure 3 factors affect attraction of electrons (acidity increases with stronger attraction of electrons) 1.ionic charge – when comparing similar atoms, more positive ions are stronger acids 2.oxidation # on central atom – when comparing similar formulas with the same central atom, the higher the ox#, the stronger the acid 3.electronegativity – when comparing similar formulas with different central atoms, the higher the EN, the stronger the acid

22 Common Base Reactions Strong bases also include hydrides (H - ), nitrides (N 3- ), and carbides (C 2 2- ) – NaH + H 2 O  H 2 + Na + + OH - – Mg 3 N 2 + 6H 2 O  2NH 3 + 3Mg OH - – Ca 2 C 2 + 2H 2 O  C 2 H 2 + Ca OH - strong bases also include oxides of groups 1&2 metals – Li 2 O + H 2 O  2Li + + 2OH - – CaO + H 2 O  Ca OH -

23 Common Acid Reactions nonmetal oxides (aka. acid anhydrides) turn into acids when placed in water – SO 2 + H 2 O H 2 SO 3 – CO 2 + H 2 OH 2 CO 3 – Cl 2 O 7 + H 2 O  2H + + 2ClO 4 -


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