Electrolytes, Acids and Bases

Transcription

1 Chapter 15 Acid(Latin acidus-sour): sour taste; turns plant dye litmusred; dissolves metals producing H 2 gas. Substances that are neither acids nor bases were called neutral substances. Electrolytes, Acids and Bases Electrolytes Nonelectrolytes Vinegar (HC 2 H 3 OH) Acetone (C 3 H 6 O) Hydrogen chloride (HCl) Carbon monoxide Baking soda (NaHCO 3 ) Ethanol (C 2 H 5 OH) Table salt (NaCl) Methane (CH 4 ) Milk of magnesia (Mg(OH) 2 ) Sucrose (C 12 H 22 O 11 ) Sulfuric acid (H 2 SO 4 ) Turpentine (C 10 H 22 ) Svante Arhenius hypothesized that ions come from the dissociation of the solute. An electrolyteis a solute that dissolves in water and dissociatesinto ions, yielding a solution that conducts electricity. Ionic compounds are usually metal plus nonmetal or group of nonmetals. Exceptions: HX (X-halogen) -polar covalent, produce acids in H 2 O; ammonium salts. Base: bittertaste; turns plant dye litmusblue; aqueous solutions feel slippery to touch. Electrolyteconducts electric current, non-electrolyte does not. Michael Faraday (early 1800) proposed that some mobile charged particles must exist in solution: ions. Radical, ridiculous Ph.D. thesis, Nobel prize later. A nonelectrolyteis a solute that dissolves in water without producing ions. The solution consists of intact solute molecules. Water soluble molecular substances usually consist entirely of nonmetals. Quick quiz: find electrolytes from the list: Al(NO 3 ) 3,(CH 3 ) 2 O, (NH 4 ) 2 SO 4, CH 3 OH, HBr

2 HCl and HF are both hydrogen halides. A strong electrolyteis one that completely dissociatesinto ions upon dissolving in water. A weak electrolyteis one that only partially dissociatesinto ions upon dissolving in water. Arhenius defined acid as an electrolyte that produces H + ions when dissolved in water. In fact, H + are always hydrated with a water molecule, it is more precise to say that the ions are H 3 O + ions. A strong acidis a water-soluble compound that dissociates extensivelyto produce a large number of H 3 O + (hydronium) ions. Depending on the maximum number of H 3 O + ions that can be produced from 1 mol of acid, it is referred to as monoprotic, diprotic or triprotic (1, 2 or 3 moles of H 3 O + ). Weak and Strong Electrolytes HCl(g) Dissolves in H 2 O H + (aq) + Cl - (aq) 100% dissociation All HCl molecules dissociate, no HCl molecules light bulb shines strongly. HF(g) Dissolves in H 2 O H + (aq) + F - (aq) Far more undissociated HF molecules than ions light bulb barely lights. Strong acids #H 3 O + Weak acids Hydrochloric (HCl) 1 1 Hydrofluoric (HF) Hydrobromic (HBr) 1 1 Hypochlorous (HClO) Hydroiodic (HI) 1 1 Acetic (HC 2 H 3 O 2 ) Nitric (HNO 3 ) 1 2 Carbonic (H 2 CO 3 ) Sulfuric (H 2 SO 4 ) 2 3 Phosphoric (H 3 PO 4 ) Polyprotic acids are not always stronger than monoprotic.

3 Dissociation constant, K eq, shows how many protons can be obtained from 1 mol of acid. Phosphoric acid is considered a weak acid because even the first dissociation step is weak. The 3 rd dissociation step is the weakest. Sulfuric acid is considered strong acid because of the first dissociation step. The second dissociation step is weak. Bases are opposite of acids. When added in a proper amount to an acidic solution, the acidic properties are destroyed; neutralization reaction produces salt and water. All bases given here are strong and completely dissociated. Bases can be monobasic, di- and tri- basic. Quick quiz: What do you get when you add HNO 3 (aq) to Ba(OH) 2 (aq)? Weak acid H 3 PO 4 + H 2 O H 3 O + + H 2 PO 4 - H 2 PO 4- + H 2 O H 3 O + + HPO 4 HPO 4 + H 2 O H 3 O + + PO 4 3- Strong acid H 2 SO 4 + H 2 O H 3 O + + HSO - 4 HSO 4- + H 2 O H 3 O + + SO 4 K eq = 7.5 x10-3 K eq = 6.2 x10-8 K eq = 4.2 x10-13 K eq > 1.0 x10 3 K eq = 1.2 x10-2 Arhenius definition: Any electrolyte that contains a metal ionand hydroxidegroup and produces hydroxide(oh - ) ions when dissolved in water. Lithium hydroxide, LiOH Sodium hydroxide, NaOH Potassium hydroxide, KOH Magnesium hydroxide, Mg(OH) 2 Calcium hydroxide, Ca(OH) 2 Barium hydroxide, Ba(OH) 2 HNO 3 (aq) + Ba(OH) 2 (aq) H 2 O(l) + Ba(NO 3 ) 2 (aq)

4 Arhenius theory cannot explain basicity of NH 3 as it has no OH - ions. Brønsted and Lowry independently explained it, defining a base as a substance that removes H 3 O +. To accept a proton (H + ) from the hydronium ion, the base must have a free electron pair. When a proton leaves H 3 O + ion, H 2 O stays behind. Base- anything that accepts a proton. Acid- anything that donates a proton. This new theory can explain acetyleneamide reaction, and formation of ammonium chloride in gas phase. Note that acetate and carbonate ions come from dissociation of weak acids! When a solution is Parenthesis mean: concentration acidic neutral basic [H 3 O + ] > = < [OH - ] Weak bases Ammonia, NH 3 K eq = 1.8 x10-5 Acetate ion, C 2 H 3 O - 2 K eq = 5.6 x10-10 Carbonate ion, CO 3 K eq = 2.1 x10-4 H 2 O + H 2 O H 3 O + + OH - Water also dissociates by the K eq = [H 3O + ] x[oh - ] K [H 2 O] x[h 2 O] w = K eq x[h 2 O] 2 = [H 3 O + ] x[oh - ] process called auto-ionization. constant

5 ph Scale Pure water is neutral because autoionization produces equal concentrations of H 3 O + and OH - ions: [H 3 O + ] = [OH - ] = 1 x10-7 M Acid solution: [H 3 O + ] > 1 x10-7 M. Basic solution: [OH - ] > 1 x10-7 M. When [H 3 O + ] rises, [OH - ] lowers to satisfy the ionic product of water, K w = 1 x Performing calculations with so small concentrations is tedious. Instead, scientists use logarithms. ph = -log [H 3 O + ] K w = K eq x[h 2 O] 2 = [H 3 O + ] x[oh - ] K w = [H 3 O] x[oh - ] = [1 x10-7 ] x[1 x10-7 ] K w = 1 x10-14 Taking a logarithm is asking to what power must I raise 10 to get the displayed number? Example: log 558 = 2.75, because = 558. Note that each onefold decrease in ph represents tenfold increase in acidity. To convert from ph to molar H 3 O + concentration, use the formula: [H 3 O + ] = 10 -(ph)

6 Reactions of Acids and Bases acid + metal hydrogen + ionic salt 2 HCl(aq) + Ca(s) H 2 (g) + CaCl 2 (aq) acid + base salt + water HBr(aq) + KOH(aq) KBr(aq) + H 2 O(l) acid + metal oxide salt + water 2 HCl(aq) + Na 2 O(aq) 2 NaCl(aq) + H 2 O(l) acid + carbonate salt + H 2 O + CO 2 2 HCl(aq) + Na 2 CO 3 (aq) 2 NaCl(aq) + H 2 O(l) + CO 2 (g) 2 KOH(aq) + 2 Al(s) + 6 H 2 O(l) 2 KAl(OH) 4 (aq) amphoteric hydroxides are capable of + 3 H 2 (g) reacting as a base or an acid Reactions of amphoteric hydroxides Zn(OH) 2 (s) + 2 HCl(aq) ZnCl 2 (aq) + 2 H 2 O(l) Zn(OH) 2 (s) + 2 NaOH(aq) Na 2 Zn(OH) 4 (aq) Reactions with certain metals 2 NaOH(aq) + Zn(s) + H 2 O(l) Na 2 Zn(OH) 4 (aq) + H 2 (g) Balanced eq.: NaOH(aq) + HCl(aq) H 2 O + NaCl(aq) moles of (known) base = moles of (unknown) acid mol NaOH = L NaOH x 1L NaOH x M HCl Titrations Stoichiometric procedure to determine solution concentrations. Done by acid / base neutralization. Needed: 1. buretwith known conc. of titrant 2. Known volume of the titrate in the Erlenmeyer flask 3. Indicator (usually phenolphtalein). Fill the buret with the titrant to 0.00 ml mark, drain it by opening the stopcock. Stop when indicator changes color and read the final volume of the titrant. M base x V base = M acid x V acid 1 mol HCl x 1 1 mol NaOH L HCl

7 Writing Net Ionic Equations HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) H + (aq) + Cl - (aq) + Na + (aq) + OH - (aq) Na + (aq) + Cl - (aq) + H 2 O(l) H + (aq) + OH - (aq) H 2 O(l) spectator ions cancel Formula eq. Total ionic eq. Net ionic eq. All three equations must have balanced number of atoms and electrical charges. Strong electrolytes are written in their ionic form. Weak electrolytes, nonelectrolytes, precipitates and gases are written in their molecular form. Net ionic equation includes only substances that have undergone a chemical change. 2 AgNO 3 (aq) + BaCl 2 (aq) Ba(NO 3 ) 2 (aq) + 2 AgCl(s) 2 Ag + (aq) + 2NO 3- (aq) + Ba 2+ (aq) + 2Cl - (aq) Ba 2+ (aq) + 2NO 3- (aq) + 2 AgCl(s) Ag + (aq) + Cl - (aq) AgCl(s) Try H 2 SO 4 + Ba(OH) 2 ; Mg + HCl, Na 2 CO 3 + H 2 SO 4. Colloidal Dispersions Colloidsare particles that are intermediate between true solution and suspension(e.g. fine sand in water, which settles down once the shaking stops). Colloids (e.g. milk) neither settle down, nor form a true solution; the colloidal particle sizes are between true solute ions or molecules, and particles of mechanical suspension (i.e. between 10-4 and 10-7 cm) with a diameter of ~ 500 nm. Thus, colloidal particle is 1000 times larger in diameter, and about 10 9 times larger in volume. AgCl precipitates, Ba 2+ and NO 3- are spectator ions. Ba 2+ + SO 4 BaSO 4 (s) 2 H + H 2 (g) 2H + + CO 3 H 2 CO 3 (aq) H 2 CO 3 (aq) H 2 O(l) + CO 2 (g) Colloidal particles can be removed by dialysis (as in artificial kidneys). Colloidal dispersion shows Tyndal effect (light scatter).

8 A buffer is a solution of a weak acid and its conjugate base. Addition of a strong acid or base to a buffer changes ph only slightly. Human blood (ph = 7.4) may not change by more than 0.4 ph units. Gastric juice has ph as low as 1.3. Only a fraction of a drop of gastric juice would change the ph of blood enough to kill you, if it were not bufferedby HCO 3- / CO 3 system. Each acidhas a conjugate base, and each basehas conjugate acid. The difference is in one proton. A bufferreplaces added strong acid with its weak acid, or added strong base with its weak base. OH - + HC 2 H 3 O 2 C 2 H 3 O + H 2 O Added base Weak acid in buffer Resisting ph Changes Buffer Solutions Conjugate weak base Neutral water H 3 O + + C 2 H 3 O HC 2 H 3 O 2 + H 2 O Added acid Weak base in buffer Conjugate weak acid Neutral water Weak acid conjugate has all properties of a base. Strong acid conjugate is neutral. In acetate buffer, the weak acid, HC 2 H 3 O 2 goes after any added OH - ions. Simultaneously, the acid conjugate, C 2 H 3 O goes after any added H 3 O +. No buffer entirely cancels out the effect of adding strong acid or base. ph still changes, but a lot less than in water. Buffer is exhausted when either weak acid or its conjugate gets used up.

9 HW Chp.15(p.359): 1, 3, 5, 17, 29, Identify the conjugate acid=base pairts in each of the following equations: NH 3 + H 2 O NH 4+ + OH - HC 2 H 3 O 2 + H 2 O C 2 H 3 O 2 -+ H 3 O + H 2 PO 4- + OH - HPO 4 + H 2 O HCl+ H 2 O Cl - + H 3 O + 3. Complete and balance these equations: Zn(s) + HCl(aq) Al(OH) 3 (s) + H 2 SO 4 (aq) Na 2 CO 3 (aq) + HC 2 H 3 O 2 (aq) Ca(HCO 3 ) 2 (s) + HBr(aq) KOH(aq) + H 3 PO 4 (aq) 5. For the first three of the formula equations in Q.3, write total and net ionic eq. 17.Calculate the [H + ] for: black coffee, ph = 5.0; black tea, ph = 6.8; green tea, ph = 7.9; laundry detergent, ph = Determine how many milliliters of MHClO 4 will be required to neutralize g Ca(OH) 2 according to the reaction: 2 HClO 4 (aq) + Ca(OH) 2 (s) Ca(ClO 4 ) 2 (aq) + 2 H 2 O (l) 39.Determine whether each of the following is a strong acid, weak acid, strong base or weak base. Then write an equation describing the process that occurs when the substance is dissolved in water: NH 3 ; HCl; KOH; HC 2 H 3 O 2.