Acids and Bases Chapter 20

Properties of…. Acids: Sour taste Change the color of indicators Electrolyte React with base to form water and salt React with certain metals to produce H2 gas.

Properties of…. Bases: Bitter taste, slippery feel Change the color of indicators Electrolyte React with acid to form water and salt

Names and Formulas of Acids Acids have the general formula HX, where X is a monatomic or polyatomic anion. When the compound HCl(g) dissolves in water, to form HCl(aq), it is named as an acid, hydrochloric acid. How are acids named?

Three rules for naming acids:

Name the following acids: HNO3 (hydrogen nitrate) is called… Nitric acid H2SO3 (hydrogen sulfite) is called… Sulfurous acid HF (hydrogen fluoride) is called… Hydrofluoric acid

Naming bases A base produces hydroxide ions when dissolved in water. NaOH is called sodium hydroxide. What would you call Ca(OH)2?

The Self-Ionization of Water When water molecules collide, a hydrogen ion is occasionally transferred from one molecule to another in the following reaction, the self-ionization of water: H2O + H2O  H3O+ + OH- H3O+ is named the hydronium ion.

The Self-Ionization of Water This reaction can also be written as… H2O  H+ + OH- (Basically, the two equations are the same, but the first is more realistic, because H+ always becomes H3O+ in water.)

The Ion-Product Constant for Water (Kw) The product of [H3O+] and [OH] is always constant at 25oC. When the concentrations are multiplied, the product is 1.0x10-14. This is called the… Ion-product constant for water, Kw. [H3O+] x [OH] = 1.0x10-14 = Kw at 25oC

Concentrations of [H3O+] and [OH] In pure water, H2O + H2O  H3O+ + OH- the [H3O+] will always be equal to the [OH-].

In Pure Water… and so, Kw = 1.0x10-14 = [H3O+] x [OH-] [H3O+] = [OH-] and Kw = 1.0x10-14 = [H3O+] x [OH-] so, [H3O+] = [OH-] = 1.0x10-7M

water is neither acidic or basic. In Water In pure water, since [H3O+] = [OH-], water is neither acidic or basic. Water is neutral. [H3O+] = [OH-] = 1.0x10-7M

In Acids and Bases… [H3O+] > 1.0 x 10-7M [H3O+] < 1.0 x 10-7M In acidic solutions, the [H3O+] is greater than 1.0 x 10-7M [H3O+] > 1.0 x 10-7M In basic solutions, the [H3O+] is less than 1.0 x 10-7M [H3O+] < 1.0 x 10-7M

Ion Concentrations

Determine the [H3O+]… Kw = 1.0x10-14 = [H3O+] x [OH-] What is the [H3O+] in a solution if the [OH-] = 6.30 x 10-5M? Is the solution acidic, alkaline (basic) or neutral? Kw = 1.0x10-14 = [H3O+] x [OH-] 1.0x10-14 / [OH-] = [H3O+] 1.0x10-14 / 6.30 x 10-5M = [H3O+] Answer: [H3O+] = 1.59 x 10-10M, acidic because [H3O+] < 1 x 10-7M

Determine the [OH-]… Kw = 1.0x10-14 = [H3O+] x [OH-] What is the [OH-] in a solution if the [H3O+] = 3.50 x 10-11M? Kw = 1.0x10-14 = [H3O+] x [OH-] 1.0x10-14 / [H3O+] = [OH-] 1.0x10-14 / 3.50 x 10-11M = [OH-] Answer: [OH-] = 2.86 x 10-4M

To understand pH and pOH Objectives To understand pH and pOH To learn to find pH and pOH for various solutions To use a calculator to find pH To learn methods for measuring pH of a solution To learn to calculate the pH of strong acids   

The pH Scale pH is the negative log of the hydrogen ion concentration. pH = –log [H+] The “p scale” is used to express small numbers (“p” means –log).

Determine the pH… What is the pH of a solution that is 1.0 x 10-3M [H+] ? pH = –log [H+] = –log(1.0 x 10-3M) Answer: pH = 3.00 (really, no calculator required for this one. When the coefficient is 1.0, you can figure this without a calculator.)

Determine the pH… What is the pH of a solution that is 5.6 x 10-8M [H+] ? pH = –log [H+] = –log(5.6 x 10-8M) Answer: pH = 7.25 (there is no unit, because this is a log)

Significant figures for logarithms

Determine the [H+]… What is the [H+] if the pH of a solution is 9.00? pH = –log [H+] 9.00 = –log [H+] – 9.00 = log [H+] (Move the negative sign) 10–9 .00 = [H+] (Take the inverse log of both sides) Answer: [H+] = 1.0 x 10-9M (Again, no calculator is needed if the pH is a whole number.)

Using a Calculator to Determine the [H+] from pH

Determine the [H+]… What is the [H+] if the pH of a solution is 2.75? pH = –log [H+] 2.75 = –log [H+] – 2.75 = log [H+] (Move the negative sign) 10–2.75 = [H+] (Take the inverse log of both sides) Answer: [H+] = 1.8 x 10-3M

The pH Scale Because the pH scale is a log scale based on 10, the pH changes by 1 for every power of 10 change in the [H+].

The pH Scale Acidic solutions have a pH that is less than 7. Neutral solutions have a pH of 7. Alkaline (basic) solutions have a pH that is greater than 7.

(The negative exponent makes this confusing.) The pH Scale In acidic solutions, [H3O+] > 1.0 x 10-7M, so the pH < 7 In neutral solutions, [H3O+] = 1.0 x 10-7M, so the pH = 7 In basic solutions, [H3O+] < 1.0 x 10-7M, So the pH > 7 (The negative exponent makes this confusing.)

The pOH Scale pOH = log [OH] pOH is just as valid as pH, but it is not as commonly used, except in Chemistry. pOH = log [OH]

Determining pH from pOH You can take the “p” (or –log) of each part of the Kw expression, and you get… Kw = 1.0x10-14 = [H3O+] x [OH-] pKw = 14.00 = pH + pOH so, pH + pOH = 14.00 You can easily convert pOH to pH using pKw.

Determine the pH… pH + pOH = 14.00 14.00 – 11.60 = pH What is the pH of a solution if the pOH is 11.60? pH + pOH = 14.00 14.00 – 11.60 = pH Answer: pH = 2.40

Determine the pH… What is the pH if the [OH-] = 5.74 x 10-4M? pOH = -log [OH-] = -log (5.74 x 10-4M) pOH = 3.241 pH = 14 - 3.241 Answer: pH = 10.759 (Remember, you need 3 sig figs, which are places to the right of the decimal for logs.)

Acid Base Indicators Indicators – substances that exhibit different colors in acidic and basic solutions In an acid solution the indicator will be in the HIn form. In a basic solution the indicator will be in the In form.

Acid Base Indicators One common indicator is litmus paper. Red litmus paper turns blue in base. Blue litmus paper turns red in acid.

Acid Base Indicators Another common indicator is bromothymol blue. Bromothymol blue is blue in base and yellow in acid.

Acid Base Indicators Another common indicator is phenolphthalein. Phenolphthalein is bright pink in base, and clear in acid.

Objectives To learn about two models of acids and bases To understand the relationship of conjugate acid-base pairs To understand the concept of acid strength To understand the relationship between acid strength and the strength of the conjugate base To learn about the ionization of water

The Arrhenius Model Svante Arrhenius defined acids and bases: Arrhenius Acid – produces hydrogen ions in aqueous solution Example: HCl  H+ + Cl- Arrhenius Base – produces hydroxide ions in aqueous solution Example: KOH  K+ + OH-

Monoprotic acids have one ionizable hydrogen (proton). (Example: HCl) Diprotic acids have 2 ionizable hydrogens. (Example: H2SO4) Triprotic acids have 3 ionizable hydrogens. (Example: H3PO4) (Diprotic and triprotic acids are called polyprotic acids.)

Is HC2H3O2 (acetic acid, found in vinegar) a monoprotic, diprotic, or triprotic acid?

The only hydrogen that is ionizable is the one attached to the oxygen (so it is monoprotic). This H is ionizable because it is in a very polar bond, so this hydrogen is slightly positive and attracted to water.

The Arrhenius model does not explain why ammonia (NH3) is a base. A new model was created by Brønsted and Lowry. This model explains why NH3 and other molecules are bases, even though they contain no hydroxide.

The Brønsted-Lowry Model Brønsted Acid – Hydrogen ion donor Brønsted Base – Hydrogen ion acceptor

Brønsted Equations NH3 + H2O  NH4++ OH- In the reaction, The NH3 is accepting H+ to form NH4+, so the NH3 is a base. The H2O is donating H+ to form OH-, so the water is an acid.

NH3 + H2O  NH4++ OH- Base Acid Now, in the reverse reaction, the NH4+ acts as an acid and the OH- acts as a base. We call these the conjugate acid and conjugate base.

Conjugate Acid-Base Pairs NH3 + H2O  NH4++ OH- Base Acid Conjugate Conjugate acid base The base always forms the conjugate acid and the acid always forms the conjugate base. NH3 and NH4+ are a conjugate acid-base pair. They differ from each other by one H+ ion.

Identify the base, acid, the conjugate acid and the conjugate base: HNO3 + H2O  NO3- + H30+ Acid Base CB CA Here, water is acting as a base.

Identify the conjugate acid-base pairs: HCl + H2O  H30+ + Cl-

Water: Is it an acid or a base? Water is amphiprotic. It can behave as either an acid or a base, by donating or accepting H+. (Some books use the term amphoteric to describe the same property.)

B. Acid Strength Strong acid – completely ionized or completely dissociated

B. Acid Strength Weak acid – most of the acid molecules remain intact

B. Acid Strength A strong acid has a relatively weak conjugate base.

B. Acid Strength Common strong acids are Sulfuric acid, H2SO4 Hydrochloric acid, HCl Nitric acid, HNO3 Perchloric acid, HClO4

To learn about acid-base titrations Objectives To learn about acid-base titrations

Acid-Base Titrations Titration – (volumetric analysis) delivering a measured volume of a solution of known concentration into the solution being analyzed Equivalence point – when just enough base has been added to react with all of the acid solution being analyzed (where moles of acid = moles of base)

Acid-Base Titrations Buret – device used for accurate measurement of the delivery of a liquid

Acid-Base Titrations Titration curve (pH curve) – plot of the data (pH vs volume) for a titration