Acids and Bases Chapter 19.

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Presentation transcript:

Acids and Bases Chapter 19

Describing Acids and Bases

Mini-Project Work with a partner. Organize the following formulas into two groups with four formulas in each group: HNO3, NaOH, H2SO4, H2CO3, Ca(OH) 2, KOH, H8PO4, Mg(OH) 2 One way to organize them into groups is: Group One Group Two HNO3 NaOH H2SO4 Ca(OH) 2 H2CO3 KOH H8PO4 Mg(OH) 2 Group One formulas represent acids. Group Two formulas represent bases.

ACIDS Electrolytes in solution Taste sour (lemon, vinegar) React with metal (corrosion) React with carbonates (makes bubbles of CO2 Turns blue litmus RED In Water forms Hydrogen ION Water HCl H+ + Cl-

BASES Electrolyte in solution Taste Bitter (soap, tonic water) Feel Slippery (soap) Turns Red Litmus Blue React with Acids to make water Water NaOH Na+ + OH-

Why are Some Solutions Acid & Others Base? Acid solutions contain more H+ ions than OH- ions. Base solutions contain more OH- ions than H+ ions. Water is the standard for Acid/Base and is defined as NEUTRAL Water has equal amounts of H+ and OH- ions

Arrhenius Model of Acids/Bases Substance is an acid if it contains hydrogen and dissociation causes hydrogen ions to form in solution Substance is a base if it contains a hydroxide and dissociates to produce hydroxide ions in solution

Bronsted-Lowry Model Acid is a proton (hydrogen ion) donor Base is a proton (hydrogen ion) receptor This is a broader definition than Arrhenius model because there are substances that cause donation or reception without having hydrogen in them.

Example When an acid dissolves in water, it donates an H+ ion to a water molecule forming H3O+. The water molecule acts as a base and accepts the H+ ion HX + H2O ⇄ H3O+ + X- Conjugate acid = species produced when a base accepts a hydrogen ion from an acid Conjugate base = species produces when an acid donates a hydrogen ion to a base Conjugate base pair = 2 substances related to each other by donating and accepting a single hydrogen ion

Historical views on acids O (e.g. H2SO4) was originally thought to cause acidic properties. Later, H was implicated, but it was still not clear why CH4 was neutral. Arrhenius made the revolutionary suggestion that some solutions contain ions & that acids produce H3O+ (hydronium) ions in solution. Ionization + H O H O Cl Cl H + + The more recent Bronsted-Lowry concept is that acids are H+ (proton) donors and bases are proton acceptors

The Bronsted-Lowry concept In this idea, the ionization of an acid by water is just one example of an acid-base reaction. + Cl H O acid base conjugate acid conjugate base conjugate acid-base pairs Acids and bases are identified based on whether they donate or accept H+. “Conjugate” acids and bases are found on the products side of the equation. A conjugate base is the same as the starting acid minus H+.

Practice problems Identify the acid, base, conjugate acid, conjugate base, and conjugate acid-base pairs: HC2H3O2(aq) + H2O(l)  C2H3O2–(aq) + H3O+(aq) acid base conjugate base conjugate acid conjugate acid-base pairs OH –(aq) + HCO3–(aq)  CO32–(aq) + H2O(l) base acid conjugate base conjugate acid conjugate acid-base pairs

Answers: question 18 (a) HF(aq) + SO32–(aq)  F–(aq) + HSO3–(aq) acid base conjugate base conjugate acid conjugate acid-base pairs (b) CO32–(aq) + HC2H3O2(aq)  C2H3O2–(aq) + HCO3–(aq) base acid conjugate base conjugate acid (c) conjugate acid-base pairs H3PO4(aq) + OCl –(aq)  H2PO4–(aq) + HOCl(aq) acid base conjugate base conjugate acid conjugate acid-base pairs

More Amphoteric = substances like water that can act like either an acid or a base

Monoprotic and Polyprotic Acids Monoprotic – acids based on formula that can donate only one hydrogen ions CH3COOH + H2O ⇄ H3O+ + CH3COO- Polyprotic – acids that can donate multiple hydrogen ions H3PO4 + H2O ⇄ H3O+ + H2PO4+ H2PO4+ + H2O ⇄ H3O+ + HPO4+2 HPO4+2 + H2O ⇄ H3O+ + PO4+3 Anhydride = oxides that can become acids or bases by adding elements contained in water

Acid Rain Acid rain comes from rain collecting gasses from the air to create acids: Carbon Dioxide = carbonic acid Sulfur oxides = sulfuric acid Nitrogen oxides = nitric acid Damages statues, buildings, kills forests, kills fish

Acids and Bases in Solution Chapter 19.2

Acid/Base Strength In strong acids, almost all molecules ionize. In weak acids, fewer molecules ionize.

Conjugate Pairs Strength If an acid is a strong acid, its conjugate pair base is a weak base Why? If HX is strong acid, it ionizes completely. The conjugate base must be a weak base because it has a greater attraction to the H+ than HX The reaction equilibrium lies far to the right of the equation.

Conjugate Pair Strength For a weak acid, the equation equilibrium lies to the right (reactant side) Conjugate base (Y-) has a stronger attraction for the H+ ion than the base H2O HY + H2O H3O+ + Y-

Acid Ionization Constants An “ionization constant” is the tendency of an item to make ions in solution. Higher the constant, the higher the amount of ions. Acid ionization constant is value of the equilibrium constant expression for a weak acid Value Ka indicates whether reactants or products are favored at equilibrium Weak acids have low Ka values

Acid Ionization Constants Substance Formula Ka Acetic Acid HC2H3O2 1.7 x 10-5 Boric Acid H3BO3 5.9 x 10-10 Carbonic Acid H2CO3 4.3 x 10-7 HCO3- 4.8 x 10-11 Hydrogen Sulfide H2S 8.9 x 10-8 HS- 1.2 x 10-13 Hypochlorous Acid HClO 3.5 x 10-8 Nitrous Acid HNO2 4.5 x 10-4 Oxalic Acid H2C2O4 5.6 x 10-2 HC2O4- 5.1 x 10-5 Phosphoric Acid H3PO4 6.9 x 10-3 HSO3 6.3 x 10-8

Base Ionization Constant Same Basic Principle as Acid Measures OH- concentrations

pH Scale Chapter 19.3

Ionization Constant for Water The ionization constant for water is: 1.0 x 10-14 [Ka][Kb] = [1.0x10-7] [1.0 x 10-7] Experiments show that the product of [H+] and [OH-] always equals 1.0 x 10-14 at 298°C pH scale is a way of showing this relationship of ionization constants

The pH Scale pH stands for ‘per Hydrion’ Low pH is Acid High pH is base Water is neutral (7.0)

pH There are many ways to consider acids and bases. One of these is pH. [H+] is critical in many chemical reactions. A quick method of denoting [H+] is via pH. By definition pH = – log [H+], [H+] = 10-pH The pH scale, similar to the Richter scale, describes a wide range of values An earthquake of “6” is 10 as violent as a “5” Thus, the pH scale condenses possible values of [H+] to a 14 point scale (fig. 2, p370) Also, it is easier to say pH = 7 vs. [H+] = 1 x 10–7

pH pH = -log [H+] [H+] = 10-pH pOH = -log [OH-] pH + pOH = 14

Calculations with pH Q: What is the pH if [H+]= 6.3 x 10–5? pH = – log [H+] ‘(-)’, ‘log’, ‘6.3’, ’10x’, ‘(-)’, ‘5’, ‘)”, ‘)”, ‘ENTER’) Ans: 4.2 Q: What is the [H+] if pH = 7.4? [H+] = 10–pH mol/L (’10x’, ‘(-)’, ‘7.4’, “)” ‘ENTER‘) 3.98 x 10–8 M

Calculating pH from Strong Acid Solutions Strong acids are 100% ionized For monoprotic acids, concentration of the acid IS the concentration of the H+ ion Use Acid concentration as substitute for H+ ion concentration. Use Base concentration as substitute for OH- concentration

Calculating pH from Strong Acid Solutions Example: What is the pH of a 0.1M solution of HCl? 0.1 M HCl = 1 x 10-1 M Calculate pH = 1 Example: What is pH of solution that is 7.5 x 10-4 M Ca(OH)2? (7..5 x 10-4) x 2 = 1.5 x 10-3M There are 2 OH- ions per molecule Calculate pOH = -log[OH-] = 2.8 pH = 14-2.8 = 11.2

Calculating Molarity from pH Example: what is the molarity of an acid solution with a pH of 2.37? [H+] = 10-pH [H+] = 10-2.37 = 4.27 x 10-3 M

Neutralization Chapter 15.4

Acid-Base Reactions HCl + NaOH H2O + Na+ + Cl- Neutralization reaction is a reaction between an acid and a base Makes Water + Salt Solution becomes Neutral (not acid or base) HCl + NaOH H2O + Na+ + Cl-

Acid-Base Reactions Mg(OH)2 + 2 HCl → MgCl2 + 2H2O Note: Cation from base (Mg) is combined with anion from acid (Cl) The salt is MgCl2 The H+ and OH- always combine to form water

Acid-Base Titration Acid/Base Titration is the stoichiometry of acid/base reactions. Titration is a method for determining the concentration of a solution by using another solution of known concentration Uses an INDICATOR to show when the acid/base reaction is complete (neutral) Indicator is a chemical that changes color as determined by acid or base conditions There are many indicators with different pH points.

Acid/Base Titration Curve

pH Indicators Name Acid Color pH Range of Color Change Base Color Methyl violet Yellow 0.0 - 1.6 Blue Thymol blue Red 1.2 - 2.8 Methyl orange 3.2 - 4.4 Bromocresol green 3.8 - 5.4 Methyl red 4.8 - 6.0 Litmus 5.0 - 8.0 Bromothymol blue 6.0 - 7.6 8.0 - 9.6 Phenolphthalein Colorless 8.2 - 10.0 Pink Thymolphthalein 9.4 - 10.6 Alizarin yellow R 10.1 - 12.0

Calculating Molarity from Titration Write the balanced equation Calculate the number of moles used in the ‘known’ solution Use the mole ratio from the balanced equation to calculate moles of reactant in the ‘unknown’ solution Calculate the molarity of the ‘unknown’ solution based on moles used and liters used.

Salt Hydrolysis When you put salts in water, the resulting solution can be either acid, base, or neutral Salts will dissolve to form ions The anions will accept hydrogens from water The cations will accept hydroxides from water Which way it goes depends upon the strength of the conjugate acids/bases If conjugate acid is strong, it will be acid If conjugate base is strong, it will be base If both are strong, it will be neutral

What are Buffers? Buffers are solutions that resist changes in pH when limited amounts of acid or base are added. Buffer is a weak acid and its conjugate base or a weak base and it’s conjugate acid Buffer can accept or donate Hydrogen ions and shift its equilibrium point left or right Buffers have limits, but the work for a while Heavily used in human body, especially blood

Acid Names 1. Binary or hydrohalic acids – HF, HCl, HBr, HI, etc. “hydro____ic acid” are usually strong acids If name ends in ‘-ide’ Acid name will be “hydro ____ic acid” HF and H2S are weak hydrohalic acid. Although the H-F bond is very polar, the bond is so strong (due to the small F atom) that the acid does not completely ionize.

Acid Naming 2. Oxyacids – contain a polyatomic ion Most common form (MCF) “ic” ending – strong acids (contain 2 oxygen per hydrogen) If chemical name ends in “-ate” Acid name will be “___IC Acid” HNO3 – nitric from nitrate H3PO4 - phosphoric from phosphate H2SO4 - sulfuric from sulfate HClO3 - chloric from chlorate

Acids with 1 less oxygen than the MCF “ous” ending- weaker acids Chemical name ends in “-ite” Acid name is “___OUS Acid” HNO2 – nitrous from nitrite H3PO3 - phosphorous from phosphite H2SO3 - sulfurous from sulfite HClO2 - chlorous from chlorite   c. Acids with 2 less oxygen than the MCF “hypo___ous” – very weak acids HNO - hyponitrous H3PO2 - hypophosphorus HClO - hypochorous

Organic acids – have carboxyl group -COOH - usually weak acids d. Acids with 1 more oxygen than the MCF “per______ic” – very strong acids HClO4 – perchloric acid HNO4 - pernitric acid Organic acids – have carboxyl group -COOH - usually weak acids Acid names are based on the base organic name or common name HC2H3O2 - acetic acid C7H5COOH - benzoic acid