8.1 Understanding Acids and Bases Properties of Acids and Bases Acids- (Arrhenius Theory definition) A compound that ionizes in water to form hydrogen ions (H+) Sour tasting, water-soluble substances Very reactive and can combine with many other substances Very good conductors of electricity when dissolved in water Must be treated with care because they are corrosive Chemical formulas begin with Hydrogen Turn blue litmus paper red Neutralize bases React with metals to produce hydrogen gas Some metals react with acids to produce Hydrogen Gas Zinc (s) + Hydrochloric acid ---- Hydrogen (g) + Zinc chloride Acid reacts with compounds that contain carbonate and hydrogen carbonate (bicarbonate) groups Bases - (Arrhenius Theory definition) An ionic hydroxide (OH) that dissociates in water to produce hydroxide(OH-) ions Bitter-tasting, water-soluble substances Feel slippery when dissolved in water Good conductors of electricity when dissolved in water Often described as alkaline React with Proteins and break them down into smaller molecules Formulas end in OH Table 1 shows Pure liquid samples of bases and ionic compounds conduct electricity Aqueous solutions of Ionic compounds, Bases and Acids conduct electricity Acids do not conduct electricity in their pure states (solid or liquid) but do conduct electricity in their aqueous state Acids Ionize in water to produce H+ Ionization A reaction in which electrically neutral molecules (or atoms) produce ions When an acid molecule reacts with water a proton (H+ ion) is transferred from the acid molecule to a water molecule leaving behind the rest of the acid molecule as an anion (negative non-metal ion) For acids assume that they are aqueous unless otherwise stated Example Ionization equation: HCl(g) + H2O(l) -> H3O+(aq) + Cl-(aq) Chemists simplify the ionization equation by canceling out a water molecule from both sides of the equation to produce the simplified net ionic equation Simplified Ionization Equation: HCl(g) -> H+(aq) + Cl-(aq) Bases Dissociate in water to produce OHDissociation reactions When ionic compounds separate into their positive and negative ions Example o NaCl(aq) --> Na+ + Clo Ba(OH)2 Ba2+ + 2OH- Sample problem Write dissociation or ionization equations for the dissolving of the following chemicals in water a. Potassium Chloride b. Hydroiodic acid Types of acids Strong acids An acid that ionizes almost completely (>99%) in water to form aqueous hydrogen Ions Ex. Sulfuric acid, Nitric acid, hydrochloric acid Weak Acids An acid that ionizes only partially (< 50%) in water to form aqueous Hydrogen ions Example Carbonic acid, acetic acid and nitric acid To communicate the percentage ionization and the strength of an acid we write the Percentage ionization over the chemical equation arrow Sample problem The following acidic solutions were tested (at equal concentration and temperature) for electrical conductivity. Write ionization equations to explain the relative conductivity of each acid a. Hydrobromic acid (aqueous hydrogen bromide): high conductivity b. Hydrofluoric acid (aqueous hydrogen fluoride): low conductivity Hydrochloric, nitric, and sulfuric acid are strong. At this point assume all others are weak Also assume all bases are hydroxides and strong. 8.2 pH of a Solution pH is a way of indicating the concentration of hydrogen ions present in a solution. Concentration is the amount of a substance (moles) in a given volume of solution. Molar concentration of H+ ions is extremely important in chemistry. The higher the concentration of H+ ions the more acidic a solution is. Any aqueous solution can be classified as acidic, neutral, or basic using a scale based on the hydrogen ion concentration. Square brackets [ ] around an element, molecule, or ion indicate Molar concentration (mol/L) Neutral solution, [H+(aq)] = 1 x 10-7 mol/L = a pH of 7 Acidic Solution, [H+(aq)] > 1 x 10-7 mol/L = a pH of 0 - 6.99 Basic Solution, [H+(aq)] < 1 x 10-7 mol/L = a pH of 7.1 – 14 [H+(aq)] = 10-pH Example pH of 2 is the same as a [H+(aq)] of 1 x 10-2 mol/L [H+] of 1 x 10-11 means pH is 11. When pH changes by one unit [H+(aq)] changes by a factor of ten. Example pH changes from 5 to 6 the Hydrogen ion concentration has been decreased by a factor of ten (10-5 to 10-6) How much more acidic is a pH of 2 than a pH of 5? pH can be changed by dilution If pH changes by one unit the [H+] concentration has decreased by a factor of 10 If the [H+] concentration changes by a factor of 100 pH changes by a factor of 2. Dilute an acid pH increases dilute a base pH decreases Sample problem What is the pH of each of the following solutions? a. 1 x 10-2 mol/L hydrogen ion concentration in Vinegar (HC2H3O2) b. [H+(aq)] = 1.0 x 10-12 mol/L in household ammonia Sample problem What is the Hydrogen ion concentration for the following solutions? a. A carbonated beverage with a pH of 3.0 b. An antacid solution for which pH = 10.00 Neutralization A reaction between an acid and a base that results in a pH closer to 7 Acid + Base Ionic salt + Water Removing H+ ions is much more effective at changing pH than dilution. Diluting a solution makes it less hazardous but neutralizing is more effective. Calculating PH and Hydrogen Ion Concentration pH = -log10[H+] [H+] = 10-pH Sample problem An antacid solution has a hydrogen ion concentration of 4.7x10-11 mol/L. What is its pH? Sample Problem The pH reading of a solution is 12.22. What is the hydrogen ion concentration?