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Acid Base Chemistry Objectives Properties of Acids Properties Of May 09, 2014 Objectives 1) Acid/Base Theories 2) pH scale and pH calculations 3) Measuring pH, pH scale Acid Base Chemistry 4) Strength of Acids and Bases Acid Base Chemistry -Equilibrium (Honors) Chapter 19 Properties of Acids Three Acid and Base Theories • Aqueous solutions of acids taste sour. Change the color of acid/base indicators • 1. Arrhenius Theory • Many will conduct electrical currents. Acid Rxns with Metals 1. Al, Mg and Zn form hydrogen gas 2. Bronsted-Lowry Theory 2Al + 6HCl 2AlCl3 + 3H2 2. Metal carbonates form carbon dioxide 3. Lewis Theory CaCO3 + 2HCl CaCl2 + H2O + CO2 Properties of Bases • Aqueous solutions have a bitter taste. • Change the color of acid/base indicators • Many react with acids to form salts in water. • They often feel slimy to the touch. • Solutions that are basic are often called alkaline. Examples of Arrhenius 1) Arrhenius Theory An acid in water ionizing to form a H+ ion Svante Arrhenius published a paper + - about how acids, bases, and salts can HCl(aq) H (aq) + Cl (aq) conduct an electric current. Concluded that both acids and bases A base in water dissociating to form hydroxide ion, OH- must release a charged particle in solution. Acids ionized to produce H+ and + - Bases ionized to produce hydroxide NaOH Na (aq) + OH (aq) ions OH- May 09, 2014 2) Bronsted-Lowry Theory Bronsted-Lowry Theory: General acid/base reaction This definition is an expansion on the Arrhenius acid definition. Acid + Base Conjugate base + Conjugate Acid In a chemical reaction, any substance that donates a proton is an acid and any substance that accepts a Conjugate base: particle that remains after a proton proton is a base. that is released by the acid. Bronsted-Lowry acid: molecule or ion that is a proton Conjugate acid: particle that remains after a base has donor acquired a proton from the acid. Bronsted-Lowry base: molecule or ion that is a proton *Shows why ammonia is a base! acceptor. Examples Identify the conjugate pairs. - + a) H2SO4(aq) + H2O(l) HSO4 (aq) + H3O (aq) - - b) H2O(l) + F (aq) OH (aq) + HF(aq) 3) Lewis Theory The third and final theory. He focused on electron behavior rather than proton behavior. Definitions: Acid: electron-pair acceptor Base: electron-pair donor Example BF3(g) + NH3(g) F3BNH3(g) May 09, 2014 Amphoteric Compounds Type Acid Base Any substance that can behave as an acid or as a base. + + The most common example is water. Arrhenius H or H3O OH- producer producer Bronsted- proton (H+) donor proton (H+) acceptor Lowry Lewis electron-pair acceptor electron-pair donor Ionization May 09, 2014 • pH and pOH calculations Example pH pH Scale -6 + pH is the power of the 1. Find the pH of a solution with a 7.01Χ10 M H3O + + concentration. hydrogen [H ] or [H3O ] + -6 concentration • [H3O ]= 7.01Χ10 M + • pH= -log[H3O ] + • pH= -log[7.01Χ10-6 M] pH= -log[H3O ] • pH= 5.154 -4 + 2. Find the pH of a solution with 5.025 Χ10 M H3O concentration. May 09, 2014 pH and pOH table with H+ and OH- values pOH Scale pOH of a solution is defined as the negative of the + - pH H OH pOH common logarithm of the hydroxide 1x10-7 pOH=-log[OH-] 1x10-5 As the concentration of the hydroxide ion increases, the pH increases and the pOH decreases. 1x10-5 The pH scale and pOH scale are opposites of each other. 6 Relating pH and pOH Measuring pH: Indicators 1. Indicators are organic bases + - -14 and acids whose colors differ log([H3O ][OH ])=log 1.00Χ10 from their conjugate acids or + - log[H3O ]+log[OH ]= -14 bases. + - -log[H3O ]+(-log[OH ])=14 2. Many organic substances can be made into solutions that pH + pOH= 14 can be used to identify acids and bases. 3. Example: Red cabbage will be red if it is acidic and green if it is basic. May 09, 2014 Transition Interval A transition interval is the pH range at which the pH meter: Makes a rapid, accurate pH measurement. indicator is changing color. This instrument is usually easier to read than liquid or pH indicator strips. Not every indicator will work in the same pH range. Strength of Acids and Bases Strong vs. Weak Acids and Bases Ionization Constants Ka: Quantitative expression for the strength of an acid Kb: Quantitative expression for the strength of a base Kw: Ionization constant for water + - -14 Kw=[H3O ][OH ]=1.0Χ10 May 09, 2014 Equilibrium/Ionization Constants We can relate the three values to find either the Ka or Kb value. Ka for lactic acid, CH3CHOHCO2H is 1.4Χ10-4. What is the Kb for the conjugate base of this acid? Kw=KaKb -14 -4 (1.0x10 )=(1.4x10 )(Kb) -11 Kb = 7.14x10 Chemical Equilibrium Chemical Equilibrium -Many reactions do not run to completion, but the reaction -Equilibrium can be reached from either direction. concentrations may cease to change. The system contains a Write the equilibrium reaction of the following: mixture of reactants and products which is called an equilibrium. jA + kB lC + mD -The equilibrium constant is a ratio of product to reactant concentrations using reversible arrows. Chemical Equilibrium Chemical Equilibrium Example: Special Notes: Write the equilibrium reaction of the following: • Products are always on the top, reactants are always on the 4NH3(g) + 7O2(g) 4NO2(g) + 6H2O(g) bottom. • K does vary with temperature. • K is typically expressed without units. We are usually more interested in the value of the number than in the units. NOTE: We do not include liquids or solids in equilibrium expressions! May 09, 2014 Chemical Equilibrium Chemical Equilibrium Sometimes: We do not know all of the concentrations of products and reactants at equilibrium, so if we know initial concentrations and K, we can calculate the others. To do this we use an ICE table. Chemical Equilibrium Ionization of polyprotic acids Protons can only be donated one at a time to water. As a result each hydrogen ion will leave the acid in its own reaction step. The first proton leaves in the first ionization, the second proton is removed by the second ionization, etc.) • One proton=one reaction step • Two protons=two reaction steps • Three protons=three reaction steps Monoprotic and Polyprotic Acids Monoprotic, Diprotic and Triprotic Monoprotic: an acid that can only donate one proton per An acid that contains "x" number of hydrogens/molecule molecule. able to dissociate in water. Polyprotic: is an acid that can donate more than one proton per molecule. HCl (Diprotic can donate two protons.) H2SO4 (Triprotic can donate three protons.) May 09, 2014 Neutralization/Titration Titration: a way to determine the amount of acid/base needed to completely neutralize a substance. (Honors: More notes to follow) Buffers • A buffer system is a solution that can absorb moderate amounts of acid or base without a significant change in its pH • Example: Enzymes Blood: buffer of carbonic acid (H2CO3) and - bicarbonate (HCO3 ) to maintain a pH between 7.35 and 7.45..
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