Acids and Bases HW: read CH 16

Acids and Bases Importance Commonly found in all aspects of daily life: car batteries, cleaners, fertilizers, detergents, and plastics

Litmus Indicator: place solution onto strip of litmus paper and it will turn color (acid red, base blue)

Defining Acids & Bases Arrhenius Definitions According to Arrhenius, acids donate hydrogen ion (H + ) in water, while bases donate hydroxide ions (OH - ) in water acid: HCl + H 2 O  H 3 O + + Cl - base: NH 3 + H 2 O  NH OH - This definition is not completely accurate as it only works with solutions in water and was not accurate for ammonia (NH 3 )

Defining Acids & Bases Bronsted-Lowery Definitions Bronsted-Lowery acid donates a proton (H + ion), while a Bronsted-Lowery base accepts a proton (H + ion) (What does donate mean???) HCl + NH 3  NH Cl - acid base NH 3 + HNO 3  NH NO 3 - base acid

Acids may donate more than one H + ion monoprotic - donate only 1 H + (HCl) diprotic - donate 2 H + (H 2 SO 4 ) triprotic - donate 3 H + (H 3 PO 4 )

Water as an Acid and a Base Water is amphoteric because it can act as both an acid and a base HCl + H 2 O  H 3 O + + Cl - accept NH 3 + H 2 O  NH OH - donate

Conjugate Acid-Base Pairs Conjugate Acid-Base Pairs (Bronsted-Lowery definition) All reactions may proceed in a forward or reverse direction (trying to reach equilibrium) Two substances are related by the loss of or gain of a single hydrogen ion; conjugate acid and base pairs. When an acid loses a H+, it becomes a base in the reverse direction (so named conjugate base) HCl + H 2 O  H 3 O + + Cl - acid base c.a. c.b. NH 3 + H 2 O  NH OH - base acid c.a. c.b. Song 2 min

Conjugate Acid-Base YOU TRY! Label the following A) NH H 2 O  H 3 O + + NH 3 B) H 2 PO H 2 O  HPO H 3 O + C) CO H 2 O  OH - + HCO 3 -

Acid & Base Strength Strong and Weak Acids Acid strength depends on how easily H + ions will dissociate to form H 3 O + in water The easier the ions dissociate, the stronger the acidity Strong acids show equation with 1 arrow (less likely to reach equilibrium) while weak acids show 2 arrows. STRONG: HCl + H 2 O  H 3 O + + Cl - WEAK: HC 2 H 3 O 2 + H 2 O  H 3 O + + C 2 H 3 O 2 -

Acid & Base Strength Strong & Weak Bases Same ideas apply except the base strength depends on how strongly they want the H + ion Strong acids have weak conjugate bases; strong bases have weak conjugate acids

Strong Acids and Bases Strong Acids HCl HBr HI HNO 3 (nitric) H 2 SO 4 (sulfuric) HClO 4 (perchloric) Strong Bases Hydroxides or oxides of group IA or IIA metals (except Be and Mg) (NaOH, CaO, etc)

Logarithims and pH scale pH [H3O+] actual amount M M M M M M M M M M M M M M M ACIDACID BASEBASE neutral

pH scale The pH scale is a mathematical scale used to express the hydronium ion concentration [H 3 O+] or [H+]; scale ranges from 0 (most acidic) to 14 (most basic) This scale was founded by Soren Sorensen who developed a relationship between the tiny ion concentration in Molarity and an easy number pH = - log [H + ] pOH = - log [OH - ] (these formulas will be given) NOTE: pH + pOH = 14

pH scale Steps for calculating pH on a calculator (same works for pOH) 1. enter (-) 2. Press the log key 3. enter the given [H + ] Example: Human blood has a concentration of H + of 3.72 x M. What is the pH? Is this acidic or basic? pH = - log [H + ]pH = - log [3.72 x ] pH = - (-7.43)pH = 7.43BASIC

pH scale Example: In household bleach, the concentration of OH- ions is 5.0 x M. What is the pH? Is this acidic or basic? FIRST FIND OH, THEN SUBTRACT FROM 14 pOH = - log [OH - ] pOH = -log[5.0 x M] pOH = 1.30 pH + pOH = 1414 – 1.30 = pH = 12.70BASIC

YOU TRY!! A) Calculate the pH for a solution where [H + ] = 1.0 x M B) Calculate the pH for a solution where [OH - ] = 5.0 x M C) Calculate the pH and pOH for a solution where [H + ] = 3.6 x M

pH scale If given a pH you can find the ion concentration by using the inverse log Inverse log (-pH) = [H + ] Steps on a calculator Steps on a calculator 1) inverse log by using 2 nd log keys (for most calculators) 2) enter (-) 3) enter the pH

pH scale EXAMPLE: The pH of rainwater in a polluted area was found to be What is the [H + ] for this rainwater? 3.2 x M YOU TRY! A) the pH of a solution is What is the [H + ] of the solution? B) The pOH of a liquid drain cleaner was found to be What is the [OH-] for this cleaner? The pH? The [H+]?

Calculating Ion Concentrations The pH of pure water is 7 and at this value the amount of acids and bases are equal The concentration of H + and OH - ions in an aqueous solution at 25 o C is 1.0 x K w = [H 3 O + ][OH - ] = 1.0 x K w is the ion product constant NOTE: this equation is simplified to K w = [H + ][OH - ] = 1.0 x (this formula will be given)

Example: Calculate [H + ] in a solution in which [OH - ] = 2.0 x M. Is this solution acidic, neutral, or basic? [H + ] = 1.0 x [OH - ] [H + ] = 1.0 x [2.0 x ] [H + ] = 5.0 x M pH = -log 5.0 x M = = BASIC Example: What is the concentration of OH- ions in lime water if [H + ] = 3.98 x M? Is lime water acidic, basic, or neutral? [OH - ] = 1.0 x [3.98 x ] [OH - ] = 2.51 x M pH = -log 3.98 x M = = BASIC

YOU TRY!! Calculate the [H + ] or [OH - ] as required for each of the following at 25 o C and state whether the solution is neutral, acidic, or basic. A) [H + ] = 3.4 x M B) [H + ] = 2.6 x M C) [OH - ] = 6.2 x M

Titration (lab technique) This is a neutralization reaction that occurs when an acid and a base react completely Through this technique, you gradually add one reactant to another until a color change signals equivalent amounts of reactant have reacted The point when the color completely changes is the end point of titration The color change should be very close to the point when the amount of acid and base is equal, the equivalence point By measuring the amount of reactant added to reach neutralization, you can calculate the concentration of an unknown solution

Titration 7 min

Buffers Many solutions can only exist at specific pH levels If a little acid is added to a solution it becomes more acidic (and vice versa with base added) BUFFERS are in many of these solutions and they help maintain the pH Buffers are made of a weak acid and base This mixture is able to release or absorb H+ ions (protons) to keep the solution’s pH constant Buffers can only do so much - if tons of acid is added, it may reach its buffer capacity and be unable to maintain the pH