What happens when you mix substances together? If the substances are relatively unstable – there is an alternative arrangement for some electron(s) (individual electron, bonds, and/or lone pairs), a rearrangement will take place. A rearrangement of the electrons is a chemical change that results in a NEW substance forming. One common reaction that generally occurs in aqueous solution is known as an acid/base reaction.

Acid-Base Reactions An Arrhenius acid is one that ionizes in water to produce H + ions. An Arrhenius base is one that dissociates in water to produce OH – ions. HCl(g) H + (aq) + Cl – (aq) H2OH2O NaOH(s)Na + (aq) + OH – (aq) H2OH2O

Acid-Base Reactions A Brønsted acid is a proton donor. A Brønsted base is a proton acceptor. In these definitions, a proton refers to a hydrogen atom that has lost its electron—also known as a hydrogen ion (H + ). H 2 O is a Brønsted acid: donates a proton to become OH – NH 3 (g) + H 2 O(l)NH 4 (aq) + OH – (aq) + NH 3 is a Brønsted base: accepts a proton to become NH 4 +

Acid-Base Reactions Brønsted acids donate protons to water to form the hydronium ion (H 3 O + ). hydrogen ion (H + ) proton hydronium ion (H 3 O + ) Activity: consider the Bronsted definitions, and draw 3 Lewis structures for molecules that could be considered BOTH an acid and a base All refer to the same aqueous species

What happens when you mix substances together? Given time, reactions will reach equilibrium. Acid and bases may be strong or weak depending on the extent to which products are produced when the substance reacts with WATER The equilibrium may be approximately 100% complete (STRONG). The equilibrium may leave a significant quantity of atoms in the form of the reactants (WEAK).

Acid-Base Reactions There are only a few acids strong enough to react nearly 100% with water. Complete the equation for each reacting with water. 9.3

Acid-Base Reactions A weak acid is a weak electrolyte; it does not dissociate completely. Acetic acid, HC 2 H 3 O 2, is an example. The vast majority of acids are weak acids. HC 2 H 3 O 2 + H 2 O H 3 O + (aq) + C 2 H 3 O 2 (aq) – acidic proton

Acid-Base Reactions Strong bases react completely with water to convert water to OH -. Strong bases also include the hydroxides of Group 1A and heavy Group 2A metals, because they also provide 100% formation of OH, eventhough they do so by dissolving rather than reacting with water. Sodium hydroxide, NaOH, is an example. NaOH(s)Na + (aq) + OH (aq) H2OH2O NH 2 (aq) + H 2 O (l)NH 3 (aq) + OH (aq) –– – –

Acid-Base Reactions A monoprotic acid has one proton to donate. Hydrochloric acid is an example:

Acid-Base Reactions A polyprotic acid has more than one acidic hydrogen atom. Sulfuric acid, H 2 SO 4, is an example of a diprotic acid; there are two acidic hydrogen atoms. Polyprotic acids lose protons in a stepwise fashion: Step 1: Step 2: In H 2 SO 4, the second ionization occurs only to a small extent. In H 2 SO 4, the first ionization is strong.

Acid-Base Neutralization A neutralization reaction is a reaction between an acid and a base. ONLY When the base contains an (OH - ) does a neutralization reaction produces water and a salt. HCl(aq) + NaOH(aq) → H 2 O(l) + NaCl(aq) Activity: Draw 3 balanced reactions that do not produce a salt and water as products. acid basewater salt

Worked Example 9.4 Milk of magnesia, an over-the-counter laxative, is a mixture of magnesium hydroxide [Mg(OH) 2 ] and water. Because Mg(OH) 2 is insoluble in water (see Table 9.3), milk of magnesia is a suspension rather than a solution. The undissolved solid is responsible for the milky appearance of the product. When acid such as HCl is added to milk of magnesia, the suspended Mg(OH) 2 dissolves, and the result is a clear, colorless solution. Write balanced molecular and ionic equations for this reaction.

The pH Scale The acidity of an aqueous solution depends on the concentration of hydronium ions, [H 3 O + ]. The pH of a solution is defined as the negative base-10 logarithm of the hydronium ion concentration (in mol/L) In pure water at 25°C, [H 3 O + ] = log1.0 x 10 –7 pH = –log(1.0 x 10 –7 ) = 7.00 pH is a dimensionless quantity. pH = –log[H 3 O + ] [H 3 O + ] = 10 –pH 9.5

The pH Scale

Acid-Base Titrations Quantitative studies of acid-base neutralization reactions are most conveniently carried out using a technique known as a titration. A titration is a volumetric technique that uses burets. The point in the titration where the acid has been neutralized is called the equivalence point. 9.6

Aqueous Reactions and Chemical Analysis The equivalence point is usually signalled by a color change. The color change is brought about by the use of an indicator. Indicators have distinctly different colors in acidic and basic media. The indicator is chosen so that the color change, or endpoint, is very close to the equivalence point. Phenolphthalein is a common indicator.

Study Guide for Sections 9.3, 9.5, 9.6 DAY 23, Terms to know: Sections 9.3, 9.5, 9.6 Arrhenius acid/base definition, Brønsted acid/base definition, dissociation, strong versus weak, dynamic chemical equilibrium, monoprotic, diprotic, polyprotic, neutralization, pH, titration DAY 23, Specific outcomes and skills that may be tested on exam 4: Sections 9.3, 9.5, 9.6 Be able to identify a compound as a strong acids, weak acid, strong, base, or weak base Given an acid and base, be able to predict the products that would form from their reaction Given reactants and products, be able to identify the acids and bases on both sides of equation Be able to balance acid base reactions including mono-, di-, or polyprotic acids Be able to describe how pH affects a solution qualitatively Given a pH, be able to determine the H 3 O + molarity Given a H 3 O + molarity, be able to determine the pH Be able to describe what a neutralization process is, why the reaction occurs, and predict neutralization products Be able to describe what an acid/base titration is used and in general how it works

Extra Practice Problems for Sections 9.3, 9.5, 9.6 Complete these problems outside of class until you are confident you have learned the SKILLS in this section outlined on the study guide and we will review some of them next class period

Prep for Day 24 Must Watch videos: (Redox, crash course chemistry) (Redox, Bozeman) Other helpful videos: (Redox, Isaacs) (UC-Irvine, lectures 19) Read Section 9.4