Objectives. Base Chemistry

Transcription

1 May 09, 2014 Objectives AcidAcid Base Chemistry Base Chemistry Chapter 19 Properties of Acids Aqueous solutions of acids taste sour. Change the color of acid/base indicators Many will conduct electrical currents. Acid Rxns with Metals 1. Al, Mg and Zn form hydrogen gas 2Al + 6HCl 2AlCl3 + 3H2 2. Metal carbonates form carbon dioxide CaCO3 + 2HCl CaCl2 + H2O + CO2 1) Acid/Base Theories 2) ph scale and ph calculations 3) Measuring ph, ph scale 4) Strength of Acids and Bases -Equilibrium (Honors) Three Acid and Base Theories 1. Arrhenius Theory 2. Bronsted-Lowry Theory 3. Lewis Theory 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. 1) Arrhenius Theory Svante Arrhenius published a paper about how acids, bases, and salts can conduct an electric current. Concluded that both acids and bases must release a charged particle in solution. Acids ionized to produce H and Bases ionized to produce hydroxide ions OH + - Examples of Arrhenius An acid in water ionizing to form a H+ ion HCl(aq) H+(aq) + Cl-(aq) A base in water dissociating to form hydroxide ion, OH- NaOH Na+(aq) + OH-(aq)

2 2) Bronsted-Lowry Theory This definition is an expansion on the Arrhenius acid definition. In a chemical reaction, any substance that donates a proton is an acid and any substance that accepts a proton is a base. Bronsted-Lowry acid: molecule or ion that is a proton donor Bronsted-Lowry base: molecule or ion that is a proton acceptor. Bronsted-Lowry Theory: General acid/base reaction Acid + Base Conjugate base + Conjugate Acid Conjugate base: particle that remains after a proton that is released by the acid. Conjugate acid: particle that remains after a base has acquired a proton from the acid. *Shows why ammonia is a base! Examples Identify the conjugate pairs. a) H2SO4(aq) + H2O(l) HSO4 - (aq) + H3O + (aq) b) H2O(l) + F - (aq) OH - (aq) + HF(aq) The third and final theory. 3) Lewis Theory He focused on electron behavior rather than proton behavior. Definitions: Acid: electron-pair acceptor Base: electron-pair donor Example BF 3(g) + NH3(g) F3BNH3(g)

3 Type Acid Base Amphoteric Compounds Any substance that can behave as an acid or as a base. Arrhenius H + or H 3 O + producer Bronsted- Lowry proton (H+) donor OH- producer proton (H+) acceptor The most common example is water. Lewis electron-pair acceptor electron-pair donor Ionization

4 ph and poh calculations Example ph 1. Find the ph of a solution with a 7.01Χ10-6 M H3O + concentration. [H3O + ]= 7.01Χ10-6 M ph= -log[h3o + ] ph= -log[7.01χ10-6 M] ph= ph is the power of the hydrogen [H + ] or [H 3 O + ] concentration ph= -log[h3o + ] ph Scale 2. Find the ph of a solution with Χ10-4 M H3O + concentration.

5 ph and poh table with H + and OH - values ph H + OH - 1x10-7 1x10-5 1x10-5 poh poh Scale poh of a solution is defined as the negative of the common logarithm of the hydroxide poh=-log[oh - ] As the concentration of the hydroxide ion increases, the ph increases and the poh decreases. The ph scale and poh scale are opposites of each other. 6 Relating ph and poh log([h 3O + ][OH - ])=log 1.00Χ10-14 log[h 3O + ]+log[oh - ]= -14 -log[h 3O + ]+(-log[oh - ])=14 ph + poh= 14 Measuring ph: Indicators 1. Indicators are organic bases and acids whose colors differ from their conjugate acids or bases. 2. Many organic substances can be made into solutions that 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.

6 Transition Interval A transition interval is the ph range at which the indicator is changing color. Not every indicator will work in the same ph range. ph meter: Makes a rapid, accurate ph measurement. This instrument is usually easier to read than liquid or ph indicator strips. Strength of Acids and Bases Strong vs. Weak Acids and Bases Ionization Constants K a : Quantitative expression for the strength of an acid K b : Quantitative expression for the strength of a base K w : Ionization constant for water Kw=[H3O + ][OH - ]=1.0Χ10-14

7 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 K b for the conjugate base of this acid? K w =K a K b (1.0x10-14 )=(1.4x10-4 )(Kb) Kb = 7.14x Many reactions do not run to completion, but the reaction concentrations may cease to change. The system contains a mixture of reactants and products which is called an equilibrium. -Equilibrium can be reached from either direction. Write the equilibrium reaction of the following: ja + kb lc + md -The equilibrium constant is a ratio of product to reactant concentrations using reversible arrows. Example: Write the equilibrium reaction of the following: 4NH 3 (g) + 7O 2 (g) 4NO 2 (g) + 6H 2 O(g) Special Notes: Products are always on the top, reactants are always on the 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!

8 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. 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: an acid that can only donate one proton per molecule. Polyprotic: is an acid that can donate more than one proton per molecule. (Diprotic can donate two protons.) (Triprotic can donate three protons.) HCl H 2 SO 4 Monoprotic, Diprotic and Triprotic An acid that contains "x" number of hydrogens/molecule able to dissociate in water.

9 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 (H 2CO 3) and bicarbonate (HCO 3 -) to maintain a ph between 7.35 and 7.45.