ACID Properties, Definitions & Finding H+ for Strong and Weak Acids and Bases Søren Sørensen

Properties of acids pH < 7 How do we recognize acids? pH < 7 Turn blue litmus paper red (Blue to Red = A-CID!) Taste sour Corrosive (gradual destruction of a substance, usually metal, by chemical reaction) React with active metals to form hydrogen gas Conduct electricity React with hydroxides (bases) to form water and a salt 2

Properties of bases pH > 7 Turn red litmus blue (Basic Blue) How do we recognize bases? pH > 7 Turn red litmus blue (Basic Blue) Taste bitter, chalky Feel slippery Caustic (capable of burning/destroying living tissue) Conduct electricity React with acids to form water and a salt 3

Acid/Base Definitions Arrhenius Model- Acids produce hydrogen ions in aqueous solutions Bases produce hydroxide ions in aqueous solutions Bronsted-Lowery Model- Acids are proton donors Bases are proton acceptors Lewis Acid Model- Acids are electron pair acceptors Bases are electron pair donors

The H atom loses an electron Acid- Base Theory 1st-What does a H atom look like? 1 Proton and 1 electron 2nd- When a Hydrogen ion, H+, forms, what happens? The H atom loses an electron So H+ = a proton BUT In an aqueous solution, A H+ or proton rides piggy back on a water molecule as H3O+, a HYDRONIUM ion. . Soo…. PROTONS, HYDROGEN IONS AND HYDRONIUM IONS are ALL interchangeable.

Let’s play a game called: WATCH THE PROTON!! Bronsted-Lowry Theory: Acids donate protons, and Bases accept protons. Proton donor HA + B  HB + A- acid base Conjugate acid Conjugate base Proton acceptor Why is HB the conjugate acid? Proton donor HA + B  HB + A- Conjugate acid Conjugate base Consider the reverse reaction: Proton acceptor

How about Bases? Ex. NaOH, LiOH, KOH Strong Bases are Alkali metals combined with OH- Ex. NaOH, LiOH, KOH WEAK BASES have Carbon and Hydrogen combined with N, NH, or NH2 Ex. C6H6NH2, CH3NH2

Again-WATCH THE PROTON between water and methyl amine (a WEAK base) acid Proton donor Water donates a proton. Conjugate base H2O + CH3NH2  CH3NH3+ + OH- base Proton acceptor METHYL AMINE accepts the proton and therefore is a BASE! Conjugate acid

You try… 1. In the following reaction, identify the acid –base pairs! HBr + NH3  NH4+ + Br – 2. What is the conjugate base of H2S? 3. What is the conjugate acid of NO3 -? HS- HNO3

KNOW: THE STRONGER THE ACID, THE WEAKER IT’S CONJUGATE BASE and vice versa! Question What is the strongest base in this reaction? HNO3 + H2O  NO3- + H3O+ ANS: H2O is the strongest base. Strong acids, such as HNO3 have weak conjugate bases So NO3- is a weak base. NO3- and H2O compete for the H+ and water WINS most of the time as this reaction goes to completion.

Of the following acids, determine A) the strongest acid B) the acid that produces the lowest concentration of hydronium ions per mole of acid C) The acid with the strongest conjugate base D) The strong acid E) The acid with the weakest conjugate base Iodic H2O2 H2O2 ALL of these are WEAK! Iodic

Amphoteric Compounds… … behave either as an acid or a base. Water is the most common amphoteric compound.

Thus- the Equilibrium constant for water = Kw AUTOIONIZATION H2O can function as both an ACID and a BASE. http://media.wwnorton.com/college/chemistry/chemtours/chapter_17/self_ionization/interface.swf Thus- the Equilibrium constant for water = Kw Kw = [H3O+] [OH-] = 1.00 x 10-14 at 25 oC

Thus, for ANY Aqueous Solution, [H3O+][OH-]= 1 x 10-14 Use this relationship to find [OH-] in an acid solution Use this relationship to find [H+] in a basic solution Variation of Kw with Temperature

Acid is electron-pair acceptor. Base is an electron-pair donor. Lewis Theory Gilbert N. Lewis - 1923 – U.S. Acid is electron-pair acceptor. Base is an electron-pair donor. BF3 + NH3  BF3NH3 To make bond F H F H F-B + :N-H  F - B – N-H Acid base Electron pair

Two for You: In the following reaction, which substance is the Lewis acid and which is the Lewis base? Cl- + BCl3BCl4 ? Which of the following would make the best Lewis base? HNO3 NH4+ PH3 LiOH

SUMMARY:- there are Three models of acids: l. ARRHENIUS MODEL Basis for the model--action in water * acid definition: produces H+ in water solution * base definition: produces OH1- in water solution 2. BRONSTED-LOWRY MODEL Basis for the model-- proton transfer * acid definition: donates a proton ( H+ ) * base definition: accepts a proton * conjugate acid definition: the acid becomes the conjugate base after it donates the proton because it can now accept it back. conjugate base definition: the base becomes the conjugate acid after it accepts the proton because it can now donate it back. 3. LEWIS MODEL Basis for model--electron pair transfer * acid definition: accepts a pair of electrons * base definition: donates a pair of electrons

ALL OTHER ACIDS ARE WEAK!!!

ALL OTHER BASES ARE WEAK ALL OTHER BASES ARE WEAK!!! Organic Weak Bases generally have -NH2, -NH, or -N

Acid Ionization

Dissociation of Strong Acids Strong acids are strong electrolytes that, for practical purposes, are assumed to ionize completely in water ANIMATION Youtube HCl(aq ) + H2O(l ) -> H3 O+ (aq ) + Cl-1 (aq ) HNO3 (aq ) + H2O(l ) -> H3 O+ (aq ) + NO3-1 (aq ) HClO4 (aq ) + H2O(l ) -> H3 O+ (aq ) + ClO4-1(aq ) H2SO4 (aq ) + H2O(l ) -> H3 O+ (aq ) + HSO4-1(aq )

Acid Dissociation Acid Proton Conjugate base For a STRONG acid HA  H+ + A- Acid Proton Conjugate base For a STRONG acid NO HA exists, therefore Ka is INFINITE! Alternately, H+ may be written in its hydrated form, H3O+ (hydronium ion)

FINDING the [H+] Concentration of a STRONG ACID Because a strong acid- 100% dissociates, the [H+] concentration is the SAME as the concentration of the ACID!! 0.05M HCl [H+] = 0.05 M .55 M HNO3 [H+] = 0.55 M 1.5 M H2SO4 [H+] = 3.0 M Bottom line, for strong acids [acid] = [H+]

Dissociation of Weak Acids Weak acids, only ionize to a limited extent in water. FORM AN EQUILIBRIUM. Large Ka or small Ka? ANIMATION Video HNO2(aq) + H2O(l) H3O+(aq) + NO2-1(aq)

Dissociation Constants for Weak Acids Formula Conjugate Base Ka  Iodic   HIO3   IO3-   1.7 x 10-1   Oxalic   H2C2O4   HC2O4-   5.9 x 10-2   Sulfurous   H2SO3   HSO3-   1.5 x 10-2   Phosphoric   H3PO4   H2PO4-   7.5 x 10-3   Citric   H3C6H5O7   H2C6H5O7-   7.1 x 10-4   Nitrous   HNO2   NO2-   4.6 x 10-4   Hydrofluoric  HF   F-   3.5 x 10-4   Formic   HCOOH   HCOO-   1.8 x 10-4   Benzoic   C6H5COOH   C6H5COO-   6.5 x 10-5   Acetic   CH3COOH   CH3COO-   1.8 x 10-5   Carbonic   H2CO3   HCO3-   4.3 x 10-7   Hypochlorous   HClO   ClO-   3.0 x 10-8   Hydrocyanic   HCN   CN-   4.9 x 10-10  The larger Ka, the Stronger WEAK acid it is! Also, the larger Ka, The Weaker It’s Conjugate base

Finding the [H+] of a WEAK Acid What is the [H+] of a 0.50 M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ? Step #1: Write Equilibrium Step #2: ICE it! HC2H3O2  C2H3O2- + H+ I C E 0.50 - x + x + x 0.50 - x x x

FINDING [H+] FOR WEAK ACID SUBSTITUTE INTO Ka EXPRESSION ASSUME X is small compared to 0.5 SOLVE FOR X [H+] = 3.0 x 10-3 M

Percent Ionization Percent Ionization = 3 x 10-3M /.5M X 100% = 0.6% What this means is-- if I had 100 molecules of this weak acid, .6 would ionize or exist as ions in aqueous solution. A VERY, VERY WEAK ACID!

What about BASES you ask? Base Ionization

Dissociation of Strong Bases MOH(s)  M+(aq) + OH-(aq) Strong bases are soluble metallic hydroxides Group I hydroxides (NaOH, KOH) are very soluble Group II hydroxides (Ca, Ba,Sr) are less soluble All weak bases are generally INSOLUBLE! NH3 is most soluble Weak Base

The [OH-] of strong bases is calculated directly from the concentration of the base in solution (just like strong acids) The [OH-] Concentration of a STRONG BASE Solution is EQUAL to the Concentration of the STRONG BASE. ACID [OH-] 0.15 M NaOH 0.15M 1.43 M KOH 1.43 M .0017M Ca(OH)2 0.0034 M

Reaction of Weak Bases with Water The base reacts with water, producing its conjugate acid and hydroxide ion: CH3NH2 + H2O  CH3NH3+ + OH- Kb = 4.38 x 10-4 NOTE: For a Basic disssociation, you MUST have H2O as a reactant!!

Reaction of Weak Bases with Water The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion: B + H2O  BH+ + OH- (Yes, all weak bases do this – DO NOT endeavor to make this complicated!)

Kb for Some Common Weak Bases Many students struggle with identifying weak bases and their conjugate acids. What patterns do you see that may help you? Base Formula Conjugate Acid Kb Ammonia   NH3  NH4+  1.8 x 10-5   Methylamine  CH3NH2  CH3NH3+  4.38 x 10-4   Ethylamine  C2H5NH2  C2H5NH3+  5.6 x 10-4   Diethylamine  (C2H5)2NH  (C2H5)2NH2+  1.3 x 10-3   Triethylamine   (C2H5)3N   (C2H5)3NH+  4.0 x 10-4   Hydroxylamine  HONH2   HONH3+    1.1 x 10-8   Hydrazine H2NNH2  H2NNH3+    3.0 x 10-6   Aniline  C6H5NH2   C6H5NH3+    3.8 x 10-10   Pyridine  C5H5N   C5H5NH+    1.7 x 10-9 

A Weak Base Equilibrium Problem What is the [H+] of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #1: Write the equation for the reaction NH3 + H2O  NH4+ + OH-

A Weak Base Equilibrium Problem What is the [H+] of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #2: RICE it! NH3 + H2O  NH4+ + OH- 0.50 I C E - x +x +x x x 0.50 - x

A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #3: Set up Kb expression NH3 + H2O  NH4+ + OH- E 0.50 - x x x Step #3b: Make an assumption! As2

A Weak Base Equilibrium Problem What is the [H+] of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #4: Solve for x, which is also [OH-] NH3 + H2O  NH4+ + OH- E 0.50 - x x x [OH-] = 3.0 x 10-5 M [OH-] [H+]= 1 x 10-14 M [3.0 x 10-5] [H+]= 1 x 10-14 M [H+] = 3.3 x 10-10 M

pH + pOH = 14 pH = -log10(H3O+) pOH = -log10(OH-) [H3O+] = 10-pH Calculating pH, pOH pH = -log10(H3O+) pOH = -log10(OH-) Relationship between pH and pOH pH + pOH = 14 Finding [H3O+], [OH-] from pH, pOH [H3O+] = 10-pH [OH-] = 10-pOH

pH and pOH Calculations

A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ? Step #5: Convert [OH-] to pH NH3 + H2O  NH4+ + OH- E 0.50 - x x x

Graphic: Wikimedia Commons user Slower

Let’s Practice!

Answers Notice(comes in handy!!): pH is always 1 less than power of ten of [H+] pOH is always 1 less than power of ten of [OH-] Always check: [H+][OH-] should equal 1 x 10-14 or 10 x 10-15!

NOW Another Typical Weak BASE Equilibrium Problem What is the pH of a 0.50 M solution of aniline, C6H5NH2, Kb = 7.4 x 10-10 ? Step #1: Write the dissociation equation C6H5NH2 + H2O  C6H5NH3+ + OH- NOTE: For a Basic disssociation, you MUST have H2O as a reactant!!

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of aniline, C6H5NH2, Kb = 7.4 x 10-10 ? Step #2: ICE it! C6H5NH2 C6H5NH3+ OH- I C E 0.50 - X +X +X 0.50 - X X X

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of aniline, C6H5NH2, Kb = 7.4 x 10-10 ? Step #3: Set up the law of mass action C6H5NH2 + H2O  C6H5NH3+ + OH- E 0.50 - x x x

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of aniline, C6H5NH2, Kb = 7.4 x 10-10 ? Step #4: Solve for x, which is also [OH-] C6H5NH2 + H2O  C6H5NH3+ + OH- E 0.50 - x x x [OH-] = 1.92 x 10-5 M

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of aniline, C6H5NH2, Kb = 7.4 x 10-10 ? Step #5: Convert [OH-] to pOH Convert pOH to pH!! DONE!!!!