Friday, April 25 th : “A” Day Monday, April 28 th : “B” Day Agenda  Hand back/go over Ch. 14 Tests  Begin chapter 15: Acids & Bases  15.1: “What Are Acids & Bases?” Strong/weak acid, strong/weak base, Bronsted-Lowry acid/base, conjugate acid/base, amphoteric species  Homework: Concept Review: “What Are Acids and Bases?”

Ch. 14 Tests Chemical Equilibrium ClassAverage (out of 50) Percentage 3A 4B

Chapter 15: Acids and Bases  Vinegar, many fruit juices, colas and some other soft drinks are all acidic.  These substances can be recognized as acidic by their tart, sour, or sharp taste. (Of course, NEVER taste anything in the lab!)

Acid Solutions Conduct Electricity  Acids are electrolytes, so their solutions in water conduct electric current.  Like other electrolytes, hydrogen chloride dissociates to produce ions. HCl (g) + H 2 O (l)  H 3 O + (aq) + Cl − (aq)  The hydronium ion, H 3 O +, is able to transfer charge through aqueous solutions much faster than other ions do. The result is that acid solutions are excellent conductors of electricity.

Acids React With Many Metals  All metals that are above hydrogen in the activity series react with acids to produce hydrogen gas.  The reaction is caused by the hydronium ion, H 3 O +, present in the solution. 2 H 3 O + (aq) + Zn (s)  2 H 2 O (l) + H 2 (g) + Zn 2+ (aq)

Acids Generate Hydronium Ions (H 3 O + )  Because acids are electrolytes, they can be classified as strong or weak based on how much they dissociate into their ions.  Strong acid: an acid that ionizes completely in a solvent.  Nitric acid, HNO 3, is a strong acid: HNO 3 (l) + H 2 O (l)  H 3 O + (aq) + NO 3 - (aq)  No HNO 3 molecules are present in a solution of nitric acid. All of the HNO 3 dissociates into ions.

Acids Generate Hydronium Ions (H 3 O + )  Weak acid: an acid that releases few hydrogen ions in aqueous solution  When a weak acid is dissolved in water, only a small fraction of its molecules are ionized at any given time.  Hypochlorous acid, HOCl, is a weak acid. HOCl (l) + H 2 O (l) H 3 O + (aq) + ClO - (aq)  The arrows in this equation indicate equilibrium.

In some acids, a single molecule can react to form more than one hydronium ion.  The first one ionizes completely, and the other ionizes partially as a weak acid.  Sulfuric acid, H 2 SO 4, has 2 hydrogen atoms that can ionize:

Some Strong and Weak Acids

Arrhenius Definition of an Acid  The presence of a large number of hydronium ions identifies an aqueous solution as acidic.  Swedish chemist, Svante Arrhenius, was among the first to recognize this fact.  In 1890, he proposed that an acid be defined as any substance that, when added to water, increases the hydronium ion concentration.

Bases  Bases are also electrolytes. Unlike acids, which are usually liquids or gases, many common bases are solids.  Solutions of bases are slippery to the touch.  The slippery feel comes because bases react with oils in your skin, converting them into soaps.  This property of attacking oils and greases makes bases useful in cleaning agents.

Bases Generate Hydroxide Ions (OH - )  Because bases are electrolytes, they too can be classified as strong or weak based on how much they dissociate into ions.  Strong base: a base that ionizes completely in a solvent.  Sodium hydroxide is a strong base: NaOH (s)  Na + (aq) + OH − (aq)

Bases Generate Hydroxide Ions (OH - )  Weak base: a base that releases few hydroxide ions in aqueous solution.  Ammonia is a typical weak base:  NH 4 + is the ammonium ion. Because NH 3 is a weak base, most of the NH 3 molecules remain un-ionized.

Arrhenius Definition of a Base  Both strong and weak bases generate hydroxide ions when dissolved in water.  This property is the basis of the Arrhenius definitions of a base.  Alkali: a base that is very soluble in water. These solutions are said to be “basic” or “alkaline”.

Two Problems With Arrhenius Definitions 1.Arrhenius definitions are limited to aqueous solutions only. 2. Arrhenius definition cannot classify substances that sometimes act as acids and sometimes act as bases.

Bronsted-Lowry Classification  In 1923, Danish chemist Johannes Brønsted proposed a broader definition of acid.  The same year, British scientist Thomas Lowry happened to make exactly the same proposal independently.  Their idea was to apply the name acid to any species that can donate a proton. (H+)

Bronsted-Lowry Acids Donate Protons  Bronsted-Lowry acid: a substance that donates a proton to another substance.  A proton is a hydrogen atom that has lost its electron.  It is a hydrogen ion and can be represented as H +.  HCl is a Bronsted-Lowry acid because it donates a proton, H +, to water.

Bronsted-Lowry Bases Accept Protons  Bronsted-Lowry base: a substance that accepts a proton.  Ammonia, NH 3, is a Bronsted-Lowry base because it accepts a proton from water when it dissolves in water.

Bronsted-Lowry Recap  In the language of Brønsted and Lowry, an acid- base reaction is very simple: one molecule or ion passes a proton to another molecule or ion.  Whatever loses the proton is an acid, and whatever accepts the proton is a base. H+H+

Conjugate Acids and Bases  Conjugate acid: the acid that forms when a base gains a proton.  Conjugate base: the base that forms when an acid loses a proton. Base Acid Conjugate Acid Conjugate Base Always in pairs: Base/conjugate acid Acid/conjugate base

Conjugate Acid-Base Pairs

Amphoteric Species Are Both Acids and Bases  Some species are both an acid and a base and can both donate and accept protons.  Such species are described as amphoteric.  Amphoteric: describes a substance, such as water, that has the properties of an acid and the properties of a base.

Homework  Because the section review is very similar to the concept review (and I like the questions in the concept review better), you only have to complete the concept review for homework today…  You’re welcome! Next Time:  Sec quiz **Have fun and be safe if you’re going t prom!**