Topic 18 Topic 18

Although taste is not a safe way to classify acids and bases, you probably are familiar with the sour taste of acids. Properties of Acids and Bases Acids and Bases: Basic Concepts Lemon juice and vinegar, for example, are both aqueous solutions of acids. Bases, on the other hand, taste bitter. Topic 18 Topic 18

Properties of Acids and Bases Acids and Bases: Basic Concepts Topic 18 Topic 18 Bases have a slippery feel. Like taste, feel is not a safe chemical test for bases, but you are familiar with the feel of soap, a base, on the skin.

Acids and bases cause certain colored dyes to change color. The most common of these dyes is litmus. Litmus Test and Other Color Changes Acids and Bases: Basic Concepts When mixed with an acid, litmus is red. Topic 18 Topic 18

Litmus Test and Other Color Changes Acids and Bases: Basic Concepts Topic 18 Topic 18 When added to a base, litmus is blue. Therefore, litmus is a reliable indicator of whether a substance is an acid or a base.

Reactions with Metals and Carbonates Acids and Bases: Basic Concepts This is why acids corrode most metals. Topic 18 Topic 18 Another characteristic property of an acid is that it reacts with metals that are more active than hydrogen.

Reactions with Metals and Carbonates Acids and Bases: Basic Concepts Topic 18 Topic 18 Bases do not commonly react with metals. Click box to view movie clip.

Reactions with Metals and Carbonates Acids and Bases: Basic Concepts Topic 18 Topic 18 Another simple test that distinguishes acids from bases is the reaction of acids with ionic compounds that contain the carbonate ion,, to form carbon dioxide gas, water, and another compound.

The submicroscopic behavior of acids when they dissolve in water can be described in several ways. Submicroscopic Behavior of Acids Acids and Bases: Basic Concepts Topic 18 Topic 18 The simplest definition is that an acid is a substance that produces hydronium ions when it dissolves in water. A hydronium ion, H 3 O +, consists of a hydrogen ion attached to a water molecule.

When HCl dissolves in water, it produces hydronium ions by this reaction: Submicroscopic Behavior of Acids Acids and Bases: Basic Concepts Topic 18 Topic 18

The reaction of an acid with water is a transfer of a hydrogen ion, H +, from an acid to a water molecule. Acidic Hydrogen Atoms Acids and Bases: Basic Concepts Topic 18 Topic 18 This transfer forms the positively charged hydronium ion, H 3 O +, and a negatively charged ion. Any hydrogen atom that can be transferred to water is called an acidic hydrogen.

To help distinguish acids from other hydrogen-containing molecules, acidic hydrogens are written first in the formula. Acidic Hydrogen Atoms Acids and Bases: Basic Concepts Topic 18 Topic 18 Any time hydrogen is the first element in a formula of a compound, the substance is an acid.

The behavior of bases is also described at the molecular level by the interaction of the base with water. Submicroscopic Behavior of Bases Acids and Bases: Basic Concepts Topic 18 Topic 18 A base is a substance that produces hydroxide ions, OH –, when it dissolves in water. There are two mechanisms by which bases produce hydroxide ions when they dissolve in water.

The simplest kind of base is a water-soluble ionic compound, such as sodium hydroxide, that contains the hydroxide ion as the negative ion. Simple Bases: Metal Hydroxides Acids and Bases: Basic Concepts Topic 18 Topic 18

Two related classes of compounds do not fit the previous models of acids and bases, but they still act as acids or bases. Other Acids and Bases: Anhydrides Acids and Bases: Basic Concepts Topic 18 Topic 18 These compounds are both oxides, which are compounds containing oxygen bonded to just one other element. These oxides are called anhydrides, which means that they contain no water.

Anhydrides differ, depending upon whether the oxygen is bonded to a metal or a nonmetal. Other Acids and Bases: Anhydrides Acids and Bases: Basic Concepts Topic 18 Topic 18 Nonmetal oxides form acids when they react with water and are called acidic anhydrides. Metal oxides, on the other hand, react with water to form bases and are called basic anhydrides.

Other Acids and Bases: Anhydrides Acids and Bases: Basic Concepts Topic 18 Topic 18 In both of these reactions, water is an active reactant. Click box to view movie clip.

These metal oxides are predominantly those of sodium, potassium, and calcium. Basic Anhydrides Acids and Bases: Basic Concepts Topic 18 Topic 18 These metal oxides are ionic so they are solids, even at the high temperature of a roaring fire. They are the major component of the ash that is left when the fire burns out.

Sodium hydroxide, NaOH, is a strong base because when NaOH dissolves in water, all NaOH formula units dissociate into separate sodium and hydroxide ions. Strong Bases Acids and Bases: Basic Concepts Topic 18 Topic 18 The dissociation of the base is complete.

The strength of a base is based on the percent of units dissociated, not the number of OH – ions produced. Some bases, such as Mg(OH) 2, are not very soluble in water, and they don’t produce a large number of OH – ions. Strong Bases Acids and Bases: Basic Concepts Topic 18 Topic 18

HCl is a strong acid because no HCl molecules are in a water solution of HCl. Strong Acids Acids and Bases: Basic Concepts Topic 18 Topic 18 Because of the strong attraction between the water molecules and HCl molecules, every HCl molecule ionizes.

Acetic acid, HC 2 H 3 O 2, is a good example of a weak acid. The molecular structure of a weak acid determines the extent to which the acid ionizes in water. A solution of weak acid contains a mixture of un-ionized acid molecules, hydronium ions, and the corresponding negative ions. Weak Acids Acids and Bases: Basic Concepts Topic 18 Topic 18

Weak Acids Acids and Bases: Basic Concepts Topic 18 Topic 18 The concentration of the un-ionized acid is always the greatest of the three concentrations.

Ammonia is a weak base because most of its molecules don’t react with water to form ions. Weak Bases Acids and Bases: Basic Concepts Topic 18 Topic 18 Other examples of bases that produce so few OH – ions that they are considered to be weak bases are Al(OH) 3, and Fe(OH) 3.

Although the terms weak and strong are used to compare the strengths of acids and bases, dilute and concentrated are terms used to describe the concentration of solutions. Strength Is Not Concentration Acids and Bases: Basic Concepts Topic 18 Topic 18 The combination of strength and concentration ultimately determines the behavior of the solution.

For example, it is possible to have a concentrated solution of a weak acid or weak base or a dilute solution of a weak acid or weak base. Strength Is Not Concentration Acids and Bases: Basic Concepts Topic 18 Topic 18 Similarly, you can have a concentrated solution of a strong acid or strong base, as well as a dilute solution of a strong acid or strong base.

In most applications, the observed range of possible hydronium or hydroxide ion concentrations spans 10 –14 M to 1M. The pH Scale Acids and Bases: Basic Concepts Topic 18 Topic 18 This huge range of concentrations presents a problem when comparing different acids and bases.

pH is a mathematical scale in which the concentration of hydronium ions in a solution is expressed as a number from 0 to 14. The pH Scale Acids and Bases: Basic Concepts Topic 18 Topic 18 A scale of 0 to 14 is much easier to work with than a range from 1 to 10 –14 (100 to 10 –14 ).

Think about the pH numbers 0 to 14 and the hydronium ion concentration range. The pH Scale Acids and Bases: Basic Concepts Topic 18 Topic 18 Pure water has a hydronium ion concentration of 10 –7 M, and a pH of 7. A solution with a hydronium ion concentration of 10 –11 M has a pH of 11. A solution with a pH of 4 has a hydronium ion concentration of 10 –4 M.

pH is convenient because there are simple methods for measuring it in the lab or in the field. Measuring pH Acids and Bases: Basic Concepts Topic 18 Topic 18 Indicators register different colors at different pHs. pH meters are instruments that measure the exact pH of a solution.

pH of 7 is neutral. A pH less than 7 is acidic, and a pH greater than 7 is basic. Interpreting the pH Scale Acids and Bases: Basic Concepts Topic 18 Topic 18 As the pH drops from 7, the solution becomes more acidic. As pH increases from 7, the solution becomes more basic.

The reaction of an acid and a base is called a neutralization reaction. Types of Acid-Base Reactions Acids and Bases: Basic Concepts Topic 18 Topic 18 Click box to view movie clip.

A solution of hydrochloric acid, HCl, is added to exactly the amount of a solution of basic sodium hydroxide, NaOH, that will react with it. Neutralization Reactions Acids and Bases: Basic Concepts Topic 18 Topic 18 Click box to view movie clip.

A typical type of acid-base reaction is one in which both the acid and base are strong. Strong Acid + Strong Base Acids and Bases: Basic Concepts Topic 18 Topic 18 The reaction of aqueous solutions of hydrochloric acid and sodium hydroxide is a good example of this type of reaction. HCl is the acid; NaOH is the base. The products are NaCl, which is a salt, and water. Now take a closer look at these reactants and their products.

You also know that sodium hydroxide in water completely dissociates into sodium ions and hydroxide ions because NaOH is a strong base. Strong Acid + Strong Base Acids and Bases: Basic Concepts Topic 18 Topic 18

Instead of an overall equation, an ionic equation, in which substances that primarily exist as ions in solution are shown as ions, can be written. Strong Acid + Strong Base Acids and Bases: Basic Concepts Topic 18 Topic 18

Sodium and chloride ions are important components of the overall equation, but they do not directly participate in the chemical reaction. Spectator Ions and the Net Ionic Reaction Acids and Bases: Basic Concepts Topic 18 Topic 18 They are called spectator ions because they are present in the solution but do not participate in the reaction.

Just as in a mathematical equation, items common to both sides of the equation can be subtracted. Spectator Ions and the Net Ionic Reaction Acids and Bases: Basic Concepts Topic 18 Topic 18 This process simplifies the equation so that the reactants and products that actually change can be seen more clearly.

When ions common to both sides of the equation are removed from the equation, the result is called the net ionic equation for the reaction of HCl with NaOH. Spectator Ions and the Net Ionic Reaction Acids and Bases: Basic Concepts Topic 18 Topic 18

A Broader Definition of Acids and Bases Acids and Bases: Basic Concepts Topic 18 Topic 18 In the more inclusive Brønsted-Lowry model, an acid is a hydrogen-ion donor and a base is a hydrogen-ion acceptor. When a Brønsted-Lowry acid donates a hydrogen ion, a conjugate base is formed. When a Brønsted-Lowry base accepts a hydrogen ion, a conjugate acid is formed. Two substances related to each other by the donating and accepting of a single hydrogen ion are a conjugate acid-base pair.

Identifying Conjugate Acid-Base Pairs Acids and Bases: Basic Concepts Topic 18 Topic 18 Identify the conjugate acid-base pairs in this reaction. A hydrogen ion is donated by HClO 2, which is the Brønsted-Lowry acid in the forward reaction. The resulting conjugate base is ClO 2 –. The base in the forward reaction is H 2 O, which accepts a hydrogen ion to form the conjugate acid H 3 O +.

Identifying Conjugate Acid-Base Pairs Acids and Bases: Basic Concepts Topic 18 Topic 18 As in the case of the weak base-strong acid reaction, mixing equal moles of acid and base does not produce a neutral solution. Because the final pH in this case is basic, more hydroxide than hydronium ions must be present in the final reaction mixture.

Basic Assessment Questions Question 1 Find the pH of the following solution. Topic 18 Topic 18 The hydronium ion concentration equals: 10 –2 M.

Basic Assessment Questions Answer pH = 2 Topic 18 Topic 18

Basic Assessment Questions Question 2 Find the pH of the following solution. Topic 18 Topic 18 The hydronium ion concentration equals: 10 –8 M.

Basic Assessment Questions Answer pH = 8 Topic 18 Topic 18

Basic Assessment Questions Question 3 Write the net ionic equation for sulfuric acid, H 2 SO 4, and strontium hydroxide, Sr(OH) 2. Topic 18 Topic 18

Basic Assessment Questions Answer Topic 18 Topic 18

Acids and Bases: Additional Concepts Topic 18 Topic 18 Additional Concepts

Acids and Bases: Additional Concepts Buffers Defined A buffer is a solution that resists changes in pH when moderate amounts of acids or bases are added. Topic 18 Topic 18 Click box to view movie clip.

Acids and Bases: Additional Concepts Buffers Defined It contains ions or molecules that react with OH – or H + if one of these ions is introduced into the solution. Topic 18 Topic 18 Buffer solutions are prepared by using a weak acid with one of its salts or a weak base with one of its salts

Acids and Bases: Additional Concepts Buffers Defined For example, a buffer solution can be prepared by using the weak base ammonia, NH 3, and an ammonium salt, such as NH 4 Cl. Topic 18 Topic 18 If an acid is added, NH 3 reacts with the H +.

Acids and Bases: Additional Concepts Buffers Defined If a base is added, the NH 4 + ion from the salt reacts with the OH –. Topic 18 Topic 18

Acids and Bases: Additional Concepts Buffers Defined These pH changes are insignificant when you compare them to the changes that occur in the unbuffered solution. Topic 18 Topic 18 These two buffer systems are common ones used in many laboratories. Blood has many buffer systems to maintain its constant pH of 7.4.

Acids and Bases: Additional Concepts Acid-Base Titrations The general process of determining the molarity of an acid or a base through the use of an acid-base reaction is called an acid-base titration. Topic 18 Topic 18 Click box to view movie clip.

Acids and Bases: Additional Concepts Acid-Base Titrations Topic 18 Topic 18 The known reactant molarity is used to find the unknown molarity of the other solution. Solutions of known molarity that are used in this fashion are called standard solutions. In a titration, the molarity of one of the reactants, acid or base, is known, but the other is unknown.

Acids and Bases: Additional Concepts Acid-Base Titrations Topic 18 Topic 18 You know that NaOH and HCl react completely. You know the concentration of the NaOH solution, so it is your standard solution.

Acids and Bases: Additional Concepts Acid-Base Titrations Topic 18 Topic 18 You can use the reaction, the volumes of acid and base used, plus the molarity of the base to determine the molarity of the unlabeled HCl.

Acids and Bases: Additional Concepts Determining Concentration: Using Stoichiometry Topic 18 Topic 18 From the balanced equation for the reaction, you know that one mole of HCl reacts with one mole of NaOH. Therefore, the number of moles of HCl in 20.0 mL of the HCl solution equals the number of moles of NaOH in 19.9 mL of 0.100M NaOH solution.

Acids and Bases: Additional Concepts Determining Concentration: Using Stoichiometry Topic 18 Topic 18 Now, use the factor label method to solve this solution stoichiometry problem, just as you used it to solve other stoichiometry problems. Because you know the concentration of the NaOH solution, first find the number of moles of NaOH involved in the reaction.

Acids and Bases: Additional Concepts Determining Concentration: Using Stoichiometry Topic 18 Topic 18 Next, examine the balanced equation for the reaction and determine that, because their coefficients are the same, equal numbers of moles of NaOH and HCl react.

Acids and Bases: Additional Concepts Determining Concentration: Using Stoichiometry Topic 18 Topic 18 Because 1.99 x 10 –3 mol NaOH react, 1.99 x 10 –3 mol HCl present in solution also react. Finally, use the volume to find the molarity of the acid.

Acids and Bases: Additional Concepts Determining Concentration: Using Stoichiometry Topic 18 Topic 18 Based on your single titration, the molarity of the HCl solution is M. However, before you put this value on the label, you probably would repeat the titration for several additional trials in order to verify your analysis and be more confident of the value on the label.

Acids and Bases: Additional Concepts Finding Molarity Topic 18 Topic 18 A 15.0-mL sample of a solution of H 2 SO 4 with an unknown molarity is titrated with 32.4 mL of 0.145M NaOH to the bromothymol blue endpoint. Based upon this titration, what is the molarity of the sulfuric acid solution?

Acids and Bases: Additional Concepts Finding Molarity Topic 18 Topic 18 Because the molarity of the NaOH solution is known, the number of moles of NaOH involved in the titration can be calculated. The corresponding number of moles of H 2 SO 4 can then be determined, and this figure can be used to calculate the molarity of the acid.

Acids and Bases: Additional Concepts Finding Molarity Topic 18 Topic 18 Write the balanced equation for the reaction. Remember that sulfuric acid is a diprotic acid. Because the concentration of the NaOH solution is known, find the number of moles of NaOH used in the titration.

Acids and Bases: Additional Concepts Finding Molarity Topic 18 Topic 18 Using the balanced equation, find the number of moles of H 2 SO 4 that react with 4.70 x 10 –3 mol NaOH. Find the concentration of the H 2 SO 4.

Acids and Bases: Additional Concepts Calculating ion concentrations from pH Topic 18 Topic 18 When the pH of a solution is known, you can determine the concentrations of H + and OH –. First, recall the equation for pH. Multiply both sides of the equation by –1.

Acids and Bases: Additional Concepts Calculating ion concentrations from pH Topic 18 Topic 18 Now take the antilog of both sides of the equation. Rearrange the equation. A similar relationship exists between [OH – ] and pOH.

Acids and Bases: Additional Concepts Calculating [H + ] from pH Topic 18 Topic 18 What is the [H + ] in an antacid solution with a pH of 9.70? Use pH to find [H + ]. Use a log table or calculator to find that the antilog of – 9.70 is 2.0 x 10 –10.

Additional Assessment Questions A 0.100M LiOH solution was used to titrate an HBr solution of unknown concentration. At the endpoint, 21.0 mL of LiOH solution had neutralized 10.0 mL of HBr. What is the molarity of the HBr solution? Question 1 Topic 18 Topic 18

Additional Assessment Questions 0.210M HBr Answer Topic 18 Topic 18

Additional Assessment Questions Calculate the pH of aqueous solutions having the following ion concentrations. Question 2 Topic 18 Topic 18

pH: Answer 2 Question 2 Additional Assessment Questions Topic 18 Topic 18