Unit Nine Notes N C U9

Transcription

1 Unit Nine Notes N C U9 I. AcidBase Theories A. Arrhenius Acids and Bases 1. Acids contain hydronium ions (H O ) commonly referred to as hydrogen ions (H ) that dissociate in water a. Different acids release different numbers of H, known as protons since the hydrogen loses its electron, resulting in only one proton (positive charge) Common Acids and Types Acid Number of H ions per mole Type of acid HNO (nitric acid) HC 2 H O 2 (acetic acid) HCl (hydrochloric acid) HBr (hydrobromic acid) 1 Monoprotic HF (hydrofluoric acid) HI (hydroiodic acid) H 2 SO 4 (sulfuric acid) H 2 CO (carbonic acid) 2 Diprotic H (phosphoric acid) H PO (phosphorous acid) Triprotic b. Not all compounds containing hydrogen are acids c. Not all hydrogens in an acid will necessarily dissociate in water d. Dissociation only occurs when very polar bonds are present because the hydrogen ions are stabilized by dissolving in solution (i.e., forming hydronium ions in solution) 2. Bases contain hydroxide ions (OH ) that dissociate in water a. Differences in solubility in water 1. High solubility: KOH, NaOH, hydroxides with Group 1A elements 2. Low solubility: Ca(OH) 2, Mg(OH) 2, hydroxides with Group 2A elements b. React with acids to produce salt and water via doublereplacement reaction NaOH(aq) HCl(aq) NaCl(aq) H 2 O(l) B. BronstedLowry Acids and Bases 1. Some bases do not give off hydroxide ions but are still basic (i.e., NH, Na 2 CO ) 2. Acid: hydrogenion (H ) donor; Base: hydrogenion (H ) acceptor. Conjugate acid: what makes the solution acid; Conjugate base: what makes the solution basic 4. Conjugate acidbase pair related by the loss or gain of a single hydrogen ion page 1 N C U9

2 5. Examples NH H2O( aq) NH 4 aq OH Ammonia Water Ammonium ion Hydroxide ion hydrogen ion hydrogen ion Conjugate acid Conjugate base acceptor; Bronsted donor; Bronsted since this makes Lowry base Lowry acid the solution basic H 2O l HO aq Cl ( HCl aq Hydrochloric acid Water Hydronium ion ) Chloride ion hydrogen ion hydrogen ion Conjugate acid Conjugate base donor; Bronsted acceptor; Bronsted since this makes the Lowry acid Lowry base solution acidic 6. Amphoteric substance a. Substance that can act as either an acid or a base b. Example: water C. Lewis Acids and Bases 1. Lewis Acid: can accept a pair of electrons to form a covalent bond 2. Lewis Base: can donate a pair of electrons to form a covalent bond D. Summary of acids and bases AcidBase Definitions Type Acid Base Arrhenius H producer OH producer BronstedLowry H donor H acceptor Lewis Electronpair acceptor Electronpair donor II. Strengths of Acids and Bases A. Strong and Weak Acids and Bases 1. Strong acids completely ionize in water 2. Weak acids only slightly ionize in water B. Acid dissociation constant [acid] H 2O l HO aq Cl ( HCl aq [H ] ) [conjugate base] K a H conjugate base acid since the concentration of water is a constant page 2 N C U9

3 1. Calculation done at equilibrium 2. The smaller the constant, the less likely the acid will ionize in water. The smaller the constant, the weaker the acid 4. Each hydrogen ionizing in water has a different ionization constant C. Base dissociation constant NH H2O( aq) NH 4 aq OH [base] [conjugate acid] [OH ] K b OH conjugate acid base since the concentration of water is a constant 1. Calculation done at equilibrium 2. The smaller the constant, the less likely the base will ionize in water. The smaller the constant, the weaker the base Relative Strengths of Common Acids and Bases Substance Formula Relative Strength Hydrochloric acid Nitric acid Sulfuric acid Phosphoric acid Ethanoic acid Carbonic acid Hydrosulfuric acid Hypochlorous acid Boric acid HCl HNO H2SO4 HPO4 CHCOOH H2CO H2S HclO HBO Strong acids Increasing strength of acid Neutral solution Sodium cyanide Ammonia Methylamine Sodium silicate Calcium hydroxide Sodium hydroxide Potassium hydroxide NaCN NH CHNH2 Na2SiO Ca(OH)2 NaOH KOH Increasing strength of base Strong bases page N C U9

4 III. Neutralization Reactions A. AcidBase Reactions 1. An acid reacts with a base to produce water and salt (generally) 2. Examples (strong acids reacting with strong bases) a. HCl(aq) NaOH(aq) NaCl(aq) H 2 O(l) b. H 2 SO 4 (aq) 2KOH(aq) K 2 SO 4 (aq) H 2 O(l). Solubilities of salts dictated by the Solubility Rules for Ionic Compounds (p. 227) 4. Neutralization reaction a. Reaction in which an acid and a base react in an aqueous solution to produce a salt and water b. A doublereplacement reaction c. Neutralization will not necessarily occur between weak acids and/or weak bases d. Type of reaction to prepare pure samples of salt (i.e., NaCl from the reaction shown above) B. Titration 1. Process of adding a known amount of solution of known concentration to determine the concentration of another solution 2. Steps a. A measured volume of an acid solution of unknown concentration is added to a flask. b. Several drops of an indicator are added to the solution. c. Measured volumes of a base of known concentration are mixed into the acid until the indicator just barely changes color and maintains that color. This occurs at the end point.. Examples for strong acids and strong bases 4. When titration is complete at the end point, the contents of the flask are only salt and water. Example 1: Solution 1: How many moles of sulfuric acid are required to neutralize 0.75 mol of potassium hydroxide? It helps to first know the equation of neutralization. A reaction between sulfuric acid (a strong acid) and potassium hydroxide (a strong base) is a doublereplacement reaction. H 2 SO 4 (aq) 2KOH(aq) K 2 SO 4 (aq) 2H 2 O(aq) Since there is 1 mole of H 2 SO 4 for every 2 moles 1 mol of KOH, H the factor by which to 2 SO 4 multiply the moles of potassium hydroxide is 2 mol KOH. This means the following: 0.75 mol KOH 1 mol H 2SO mol KOH 0.8 mol H 2SO 4 Example 2: What is the molarity of sodium hydroxide if 20.0 ml of the solution is neutralized by 17.4 ml of 1.00M phosphoric acid? page 4 N C U9

5 It helps to first know the equation of neutralization. A reaction between phosphoric acid (a strong acid) and sodium hydroxide (a strong base) is a double replacement reaction. H NaOH Na H 2 O First, you need to convert the volume of acid to the number of moles of acid using the molarity. Then use can use your mole ratio, mol NaOH 1 mol H to find moles of your sodium hydroxide. Once you know the number of moles of the sodium hydroxide, you can divide by how much volume of sodium hydroxide 17.4 ml H you have to get the molarity. L H 1.00 mol H PO 4 mol NaOH ml H L H 1 mol H L NaOH 2.61M NaOH C. Titration Curves 1. StrongAcid; StrongBase Titration a. The end point (equivalence point) has a ph at or very close to ph = 7. b. Both acid and base completely ionize and therefore do not create hydrolyzing salts (salts produced that remove hydrogen ions from water or donate hydrogen ions to water) c. Titration curve for a strongacid, strongbase titration 2. StrongAcid; WeakBase Titration a. The end point (equivalence point) has a ph < 7. b. Hydrolyzing salts that are produced from the titration will donate hydrogen ions to the water, increasing the concentration of page 5 N C U9

6 hydrogen ions ([H ]) and therefore decrease the ph of the solution comparatively. c. Titration curve for a strongacid, weakbase titration. WeakAcid; StrongBase Titration a. The end point (equivalence point) has a ph > 7. b. Hydrolyzing salts that are produced from the titration will remove hydrogen ions to the water, decreasing the concentration of hydrogen ions ([H ]) and therefore increase the ph of the solution comparatively. c. Titration curve for a weakacid, strongbase titration 4. Indicator selection for titration page 6 N C U9

7 a. The indicator s point of color change must be taken into consideration. i. Bromythol blue works for strongacid, strongbase titrations because it will change to green right at or close to ph = 7. Phenolphthalein has an end point at ph = 8 but is often used because it works even if one is colorblind. Also, the ph changes so rapidly near the end point (4 to 10 with one drop of base) that it will work well. ii. Methyl red works for strongacid, weakbase titrations because it will change color at ph = 5. Other indicators would not show a change or would show it too iii. prematurely. Phenolphthalein works well for weakacid, strongbase titrations because of its end point in the upper ph range. Alizarin yellow R works also as its color will change in the range of 10 < ph < 12. b. ph meters i. Often used in industry for more precise measurement of endpoint ii. Helpful in creating a titration curve 5. Summary of titrations and solutions a. Strong acid Strong base = Neutral solution b. Strong acid Weak base = Acidic solution c. Weak acid Strong base = Basic solution page 7 N C U9