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Nitric acid, HNO3, is a common strong acid (“Common Strong Acids and Bases” table), and therefore is a strong electrolyte. Because most acids are weak acids, ...
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**Acids are substances that ionize in aqueous solutions to form hydrogen ions H
(** aq ). As a hydrogen atom consists of a proton and an electron, H
is simply a proton. Thus, acids are also called proton donors.
Acetic acid (CH 3 COOH) is the primary component in vinegar and is a monoprotic acid. It has four hydrogens but only one of them, the H in the COOH group, is ionized in water. The three other hydrogens are bound to carbon and do not break their C-H bonds in water. CH 3 COOH ( aq ) ⟶ **H
(** aq) + CH 3 COO
- (aq) Acetic acid, CH 3 COOH Nitric acid, HNO 3 Hydrochloric acid, HCl ! FIGURE 4.6 Molecular models
**Bases are substances that accept H
ions. Bases produce hydroxide ions OH**
- ( aq ) when they dissolve in water. NH 3 ( aq ) + H 2 O ⟶ **NH 4
(** aq) + OH - (aq) Ionic hydroxide compounds, such as NaOH, KOH, and Ca(OH) 2 , are among the most common bases. When dissolved in water, they release OH
ions into the solution. Compounds that do not contain OH
ions can also be bases. For example, ammonia (NH 3 ) is a common base. When added to water, it accepts an H
ion. SECTION 4.3 Acids, Bases, and Neutralization Reactions 125 in component of citrus H 2 O NH 3 OH! " " NH 4
An H 2 O molecule acts as a proton donor (acid), and NH 3 acts as a proton acceptor (base). Only a fraction of the NH 3 molecules
PRACTICE EXERCISE (Comparing Acid Strengths)
G I V E I T S O M E T H O U G H T Why isn’t Al(OH) 3 classified as a strong base? SAMPLE EXERCISE 4.5 Comparing Acid Strengths The following diagrams represent aqueous solutions of acids HX, HY, and HZ, with water molecules omitted for clarity. Rank the acids from strongest to weakest. SOLUTION Analyze We are asked to rank three acids from strongest to weakest, based on schematic drawings of their solutions. Plan We can determine the relative numbers of uncharged molecular species in the diagrams. The strongest acid is the one with the most ions and fewest undissociated molecules in solution. The weakest acid is the one with the largest number of undissociated molecules. Solve The order is. HY is a strong acid because it is totally ionized (no HY molecules in solution), whereas both HX and HZ are weak acids, whose solutions consist of a mixture of molecules and ions. Because HZ contains more ions and fewer molecules than HX, it is a stronger acid. PRACTICE EXERCISE Imagine a diagram showing 10 ions and 10 ions. If this solution were mixed with the one pictured above for HY, what species would be present in a diagram that represents the combined solutions after any possible reaction? Answer: The diagram would show 10 ions, 2 ions, 8 ions, and 8 H 2 O molecules.
Na+ OH- Y- Na+ OH- H+ HY 7 HZ 7 HX H+
G I V E I T S O M E T H O U G H T Why isn’t Al(OH) 3 classified as a strong base? SAMPLE EXERCISE 4.5 Comparing Acid Strengths The following diagrams represent aqueous solutions of acids HX, HY, and HZ, with water molecules omitted for clarity. Rank the acids from strongest to weakest. SOLUTION Analyze We are asked to rank three acids from strongest to weakest, based on schematic drawings of their solutions. Plan We can determine the relative numbers of uncharged molecular species in the diagrams. The strongest acid is the one with the most ions and fewest undissociated molecules in solution. The weakest acid is the one with the largest number of undissociated molecules. Solve The order is. HY is a strong acid because it is totally ionized (no HY molecules in solution), whereas both HX and HZ are weak acids, whose solutions consist of a mixture of molecules and ions. Because HZ contains more ions and fewer molecules than HX, it is a stronger acid. PRACTICE EXERCISE Imagine a diagram showing 10 ions and 10 ions. If this solution were mixed with the one pictured above for HY, what species would be present in a diagram that represents the combined solutions after any possible reaction? Answer: The diagram would show 10 ions, 2 ions, 8 ions, and 8 H 2 O molecules.
Na+ OH- Y- Na+ OH- H+ HY 7 HZ 7 HX H+
G I V E I T S O M E T H O U G H T Why isn’t Al(OH) 3 classified as a strong base? SAMPLE EXERCISE 4.5 Comparing Acid Strengths The following diagrams represent aqueous solutions of acids HX, HY, and HZ, with water molecules omitted for clarity. Rank the acids from strongest to weakest. SOLUTION Analyze We are asked to rank three acids from strongest to weakest, based on schematic drawings of their solutions. Plan We can determine the relative numbers of uncharged molecular species in the diagrams. The strongest acid is the one with the most ions and fewest undissociated molecules in solution. The weakest acid is the one with the largest number of undissociated molecules. Solve The order is. HY is a strong acid because it is totally ionized (no HY molecules in solution), whereas both HX and HZ are weak acids, whose solutions consist of a mixture of molecules and ions. Because HZ contains more ions and fewer molecules than HX, it is a stronger acid. PRACTICE EXERCISE Imagine a diagram showing 10 ions and 10 ions. If this solution were mixed with the one pictured above for HY, what species would be present in a diagram that represents the combined solutions after any possible reaction? Answer: The diagram would show 10 ions, 2 ions, 8 ions, and 8 H 2 O molecules.
Na+ OH- Y- Na+ OH- H+ HY 7 HZ 7 HX H+
HX HY HZ
It is possible to make predictions about the electrolytic strength of a great number of water-soluble substances. To classify a soluble substance as strong electrolyte, weak electrolyte, or non-electrolyte, we work our way down and across this table. We first ask whether the substance is ionic or molecular. If it is ionic, it is a strong electrolyte. Summary of the Electrolytic Behavior of Common Soluble Compounds Strong Electrolyte Weak Electrolyte Nonelectrolyte Ionic All None None Molecular Strong acids (see table) Weak acids, weak bases All other compounds
The properties of acidic solutions are quite different from those of basic solutions. Acids have a sour taste, whereas bases have a bitter taste. In addition, acidic and basic solutions differ in chemical properties in several other important ways. When an acid and a base are mixed, a neutralization reaction occurs. The products of the reaction have none of the characteristic properties of either the acidic solution or the basic solution. E.g. When hydrochloric acid is mixed with a solution of sodium hydroxide, the reaction is HCl ( aq ) + NaOH ( aq ) ⟶ H 2 O ( l ) + NaCl (aq) acid base water salt Water and table salt, NaCl, are the products of the reaction. By analogy to this reaction, the term salt has come to mean any ionic compound whose cation comes from a base (for example, Na
from NaOH) and whose anion comes from an acid (for example, Cl
from HCl). In general, a neutralization reaction between an acid and a metal hydroxide produces water and a salt.
PRACTICE EXERCISE (Writing Chemical Equations for a Neutralization Reaction)