Chemistry
Weak Acids and Bases
Weak acids and bases are substances that only partially dissociate in water, resulting in a lower concentration of ions compared to strong acids and bases. This means they have a lower tendency to donate or accept protons. Weak acids and bases play a crucial role in maintaining the pH balance in biological systems and are important in various chemical reactions.
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12 Key excerpts on "Weak Acids and Bases"
- Frederick Bettelheim, William Brown, Mary Campbell, Shawn Farrell(Authors)
- 2019(Publication Date)
- Cengage Learning EMEA(Publisher)
All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 8.1 Acids and Bases ● By the Arrhenius definitions , acids are substances that produce H 3 O 1 ions in aqueous solution. ● Bases are substances that produce OH 2 ions in aqueous solution. 8.2 Defining the Strength of Acids and Bases ● A strong acid reacts completely or almost completely with water to form H 3 O 1 ions. ● A strong base reacts completely or almost completely with water to form OH 2 ions. 8.3 Conjugate Acid–Base Pairs ● The Brønsted–Lowry definitions expand the definitions of acid and base beyond water. ● An acid is a proton donor; a base is a proton acceptor. ● Every acid has a conjugate base , and every base has a conjugate acid . The stronger the acid, the weaker its conjugate base. Conversely, the stronger the base, the weaker its conjugate acid. ● An amphiprotic substance , such as water, can act as either an acid or a base. 8.4 The Position of Equilibrium in an Acid–Base Reaction ● In an acid–base reaction, the position of equilibrium favors the reaction of the stronger acid and the stronger base to form the weaker acid and the weaker base. 8.5 Acid Ionization Constants ● The strength of a weak acid is expressed by its ioniza-tion constant , K a . ● The larger the value of K a , the stronger the acid. ● p K a 5 2 log K a . 8.6 Properties of Acids and Bases ● Acids react with metals, metal hydroxides, and metal oxides to give salts , which are ionic compounds made up of cations from the base and anions from the acid. ● Acids also react with carbonates, bicarbonates, ammonia, and amines to give salts.
eBook - PDF- Ageetha Vanamudan(Author)
- 2023(Publication Date)
- Delve Publishing(Publisher)
Strong bases may be made from sodium hydroxide and potassium hydroxide (KOH). • Weak Base - This sort of base entirely dissolves in water and leaves no trace. A conjugate acid and base are examples of an aqueous solution. • This molecule forms a covalent bond with a neutral acid. • The fourth and last Super Base. Because it is made up of an alkali metal and its conjugate acid, this sort of base is more powerful than a strong base. The phrase “super base” (NaH) is used to describe sodium hydride (NaH). • Its solid base benefits gaseous acids and anion exchange resins. The solid-state is the place to be. Solid bases such as silicon dioxide and sodium hydroxide are used to cover alumina (Metz et al., 2014). 13.16 INTERACTION BETWEEN ACID AND BASE When an acid and a base combine, a chemical reaction known as an acid- base reaction occurs. This method may be used to calculate the pH of a solution. Several acid-base theories provide distinct viewpoints on reaction processes and how they might be used to tackle associated challenges. One such example is the Bronsted–Lowry acid-base theory. When the acidic or basic nature of a gaseous or liquid species is not immediately observable, the usefulness of acid-base interactions becomes obvious. Take the acid-base reaction models as a collection of assumptions. Introduction to Molecular Science 232 The Bronsted-Lowry theory is a subset of the current Lewis model, which gives the most detailed explanation of acids and bases. In contrast, the Arrhenius hypothesis is the most restricted. They are unaccountable in any manner. Because a weak base solution is a buffer solution, it is difficult to measure the addition of acid to it. Buffering can also be accomplished with a slightly acidic solution, such as citric acid (Mendoza & Núñez, 2009). When a weak acid and a weak base are mixed, an equilibrium mixture is generated. HPO 2 , a hydrogen phosphate ion, can react with adenine, which has the symbol AH.
eBook - ePub- Jeffrey Gaffney, Nancy Marley(Authors)
- 2017(Publication Date)
- Elsevier(Publisher)
Table 5.1 ). Also, weak acids produce strong conjugate bases and weak bases produce strong conjugate acids.• Strong acid → Weak conjugate base• Strong base → Weak conjugate acid• Weak acid → Strong conjugate base• Weak base → Strong conjugate acidSince a weak acid has a weaker tendency to give up its proton and the conjugate base it produces has a stronger tendency to accept its proton, a competition results between the acid and the conjugate base for the proton. So, only a fraction of the weak acid molecules will donate a proton to the base, giving rise to a significant concentration of both the acid and conjugate base forms in solution. The reaction is said to be in chemical equilibrium , the state in which both reactants and products are present in concentrations which have no further tendency to change with time. This equilibrium is represented in the chemical equation by the presence of a double arrow between products and reactants, indicating that the reaction can go in both directions and both products and reactants are present;HA + B ⇌ A + HBThe concentrations of all the species depend on the strengths of the acids and bases.The Brønsted-Lowry definition of acids and bases also describes more clearly what actually happens when an acid dissolves in water. From this point of view, the dissolution of acids, such as HCl, in aqueous solution is seen to involve an acid-base reaction between the acid and water. Although HCl is a covalent compound, it can donate a proton to a water molecule to form the cation H3 O+ ;HCl aq +H 2O l →H 3O +aq +Cl −aqThe common name for this cation is the hydronium ion . It belongs to a family of ions called oxonium ions, which is any oxygen cation containing three-bonded species. Other oxonium ions have three organic groups bonded to the oxygen and will be discussed in Chapter 13
eBook - PDF- F. John Gennari, Horacio J. Adrogue, John H. Galla, Nicolaos Maddias, F. John Gennari, Horacio J. Adrogue, John H. Galla, Nicolaos Maddias(Authors)
- 2005(Publication Date)
- CRC Press(Publisher)
1 Acid–Base Chemistry and Buffering F. John Gennari University of Vermont College of Medicine, Burlington, Vermont, U.S.A. John H. Galla Department of Medicine, University of Cincinnati College of Medicine, Cincinnati, Ohio, U.S.A. INTRODUCTION Acid–base biochemistry encompasses the physical chemistry of the constitu-ents of biological solutions that influence the dissociation of, and therefore the concentration of, hydrogen ions (H þ ) in those solutions. In the biologi-cal solutions that comprise the body fluids, these constituents include elec-trolytes that are essentially completely dissociated at the solute strength that exists in the body fluids, termed ‘‘strong ions’’ (1,2), a wide variety of weak acids and, most importantly, the volatile weak acid H 2 CO 3 (carbonic acid). Central to an understanding of acid–base homeostasis is knowledge of the chemistry of weak acids and, in particular, carbonic acid. In this chapter, we review the physical chemistry that underlies acid–base homeostasis, incorporating the concepts of Brønsted and Lowry, who defined acids as H þ donors and bases as H þ acceptors (3,4). The additional role of strong ions, which are regulated independently of the dictates of acid–base home-ostasis but influence [H þ ] is discussed at the end of this chapter. Van Slyke (5) revolutionized our ability to approach and deal with the acid–base status of biological solutions, using the concepts of Brønsted and Lowry to focus on [H þ ] through evaluation of a single weak acid, H 2 CO 3 (carbonic acid). Stewart (1,2) added the constraints of electroneutrality to the assessment of [H þ ] and separated the quantities that can be manipulated 1 external to the solution, i.e., the concentrations of strong ions, buffer content, and PCO 2 , from the quantities that are dependent on the nature of the solution, i.e., [H þ ] and [HCO 3 ].
eBook - PDFChemistry
An Atoms First Approach
- Steven Zumdahl, Susan Zumdahl, Donald J. DeCoste, , Steven Zumdahl, Steven Zumdahl, Susan Zumdahl, Donald J. DeCoste(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
Kevin Steele/Aurora Creative/Getty Images PROBLEM-SOLVING STRATEGY Solving Acid–Base Problems 1. List the major species in solution. 2. Look for reactions that can be assumed to go to completion—for example, a strong acid dissociating or H 1 reacting with OH 2 . (Box continues on the following page) 549 13.5 Calculating the pH of Weak Acid Solutions Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 13.6 Bases According to the Arrhenius concept, a base is a substance that produces OH 2 ions in aqueous solution. According to the Brønsted–Lowry model, a base is a proton acceptor. The bases sodium hydroxide (NaOH) and potassium hydroxide (KOH) fulfill both criteria. They contain OH 2 ions in the solid lattice and, be- having as strong electrolytes, dissociate completely when dissolved in aqueous solution: NaOHssd ¡ Na 1 saqd 1 OH 2 saqd leaving virtually no undissociated NaOH. Thus a 1.0-M NaOH solution really contains 1.0 M Na 1 and 1.0 M OH 2 . Because of their complete dissociation, NaOH and KOH are called strong bases in the same sense as we defined strong acids. All the hydroxides of the Group 1A elements (LiOH, NaOH, KOH, RbOH, and CsOH) are strong bases, but only NaOH and KOH are common laboratory reagents, because the lithium, rubidium, and cesium compounds are expensive. The alkaline earth (Group 2A) hydroxides—Ca(OH) 2 , Ba(OH) 2 , and Sr(OH) 2 —are also strong bases. For these compounds, 2 moles of hydroxide ion are produced for every mole of metal hydroxide dissolved in aqueous solution.
- Peter V. Hobbs(Author)
- 2000(Publication Date)
- Cambridge University Press(Publisher)
5 Acids and bases Very early in the history of chemistry many substances were designated as acids, bases, and salts. Acids have a sour taste (e.g., citric acid gives lemon juice its sour taste); they dissolve certain metals; and they also dissolve carbonate minerals to produce carbon dioxide. Bases have a bitter taste (e.g., sodium carbonate); they feel slippery when touched; and they react with many dissolved metal salts to form precipitates. However, the most striking characteristic of bases is their ability to neutralize the properties of acids; when a base reacts with an acid a salt is produced. The French chemist Lavoisier thought that all acids contain oxygen (the word oxygen means acid former in Greek). However, it was sub-sequently found that many acids contain no oxygen (e.g., hydrochloric acid, HC1), but that they all contain hydrogen. Acids and bases figure prominently in the equilibrium of aqueous solu-tions, where they significantly enhance the electrical conductivity of water. In this chapter, we will explore some of the important properties of acids and bases in aqueous solutions. This will lead us to a discussion of several theories of acids and bases. 5.1 Some definitions and concepts Equation (4.22) is valid for aqueous solutions as well as for pure water. A solution for which [H + (aq)] = [OH~(aq)] is said to be neutral; thus, pure water is neutral. If [H + (aq)] > [OH~(aq)] the solution is said to be acidic. If [OH(aq)] > [H + (aq)] the solution is said to be basic. Exercise 5.1. If 0.02 mole of hydrochloric acid is dissolved in 1L 1 of water, what are the concentrations of H + (aq) and OH(aq) ions I in the solution at 25°C? I 83 84 Acids and bases Solution. Hydrochloric acid is a gas under normal conditions, but it is a strong electrolyte that dissolves in water to form equal numbers of H + (aq) and Q(aq).
eBook - PDF- Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2015(Publication Date)
- Openstax(Publisher)
782 Chapter 14 | Acid-Base Equilibria This OpenStax book is available for free at http://cnx.org/content/col11760/1.9 At equilibrium, a solution of a weak base in water is a mixture of the nonionized base, the conjugate acid of the weak base, and hydroxide ion with the nonionized base present in the greatest concentration. Thus, a weak base increases the hydroxide ion concentration in an aqueous solution (but not as much as the same amount of a strong base). For example, a solution of the weak base trimethylamine, (CH 3 ) 3 N, in water reacts according to the equation: (CH 3 ) 3 N(aq) + H 2 O(l) ⇌ (CH 3 ) 3 NH + (aq) + OH − (aq), giving an equilibrium mixture with most of the base present as the nonionized amine. This equilibrium is analogous to that described for weak acids. We can confirm by measuring the pH of an aqueous solution of a weak base of known concentration that only a fraction of the base reacts with water (Figure 14.10). The remaining weak base is present as the unreacted form. The equilibrium constant for the ionization of a weak base, K b , is called the ionization constant of the weak base, and is equal to the reaction quotient when the reaction is at equilibrium. For trimethylamine, at equilibrium: K b = [(CH 3 ) 3 NH + ][OH − ] [(CH 3 ) 3 N] Figure 14.10 pH paper indicates that a 0.1-M solution of NH 3 (left) is weakly basic. The solution has a pOH of 3 ([OH − ] = 0.001 M) because the weak base NH 3 only partially reacts with water. A 0.1-M solution of NaOH (right) has a pOH of 1 because NaOH is a strong base. (credit: modification of work by Sahar Atwa) The ionization constants of several weak bases are given in Table 14.3 and in Appendix I.
eBook - PDF- Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
Figure 11.9 shows the different conductivities of aqueous solutions containing HCl and CH 3 COOH. This behaviour shows the different strengths of the acids. We say that HCl is a strong acid, whereas both HF and CH 3 COOH are weak acids. A general definition of these terms is as follows. • A strong acid reacts completely with water to give quantitative formation of H 3 O + . • A weak acid reacts incompletely with water to form less than stoichiometric amounts of H 3 O + . There are analogous definitions for bases. • A strong base reacts completely with water to give quantitative formation of OH - . • A weak base reacts incompletely with water to form less than stoichiometric amounts of OH - . Note that these definitions strictly apply only to Brønsted–Lowry acids in aqueous solution. The use of the words ‘strong’ and ‘weak’ is somewhat unfortunate, as it implies that there is a distinct cut-off point between strong and weak acids. This is not the case. Our definition of strong acids and bases implies that they are completely dissociated in water and should therefore have comparable strengths — this is true only if we restrict ourselves to relatively dilute aqueous solutions. However, Weak Acids and Bases show an enormous range of strengths in aqueous solution; rather than relying on the qualitative concept ‘weak’ to describe their behaviour, it is more informative to quantify their ability to either donate or accept a proton by looking at the value of the equilibrium constant for their reaction with water, and we will do this in section 11.4. We will first look at the nature of strong acids and bases in aqueous solution. 510 Chemistry FIGURE 11.9 Electrical conductivity of solutions of strong and weak acids at equal concentrations. The wires lead to a battery (not shown), which provides electrical power to light the bulb. (a) HCl reacts completely with water to give H 3 O + and Cl - , resulting in high conductivity, thus enabling the light to glow brightly.
eBook - PDF- William R. Robinson, Edward J. Neth, Paul Flowers, Klaus Theopold, Richard Langley(Authors)
- 2016(Publication Date)
- Openstax(Publisher)
748 Chapter 14 | Acid-Base Equilibria This OpenStax book is available for free at http://cnx.org/content/col12012/1.7 At equilibrium, a solution of a weak base in water is a mixture of the nonionized base, the conjugate acid of the weak base, and hydroxide ion with the nonionized base present in the greatest concentration. Thus, a weak base increases the hydroxide ion concentration in an aqueous solution (but not as much as the same amount of a strong base). For example, a solution of the weak base trimethylamine, (CH 3 ) 3 N, in water reacts according to the equation: (CH 3 ) 3 N(aq) + H 2 O(l) ⇌ (CH 3 ) 3 NH + (aq) + OH − (aq), giving an equilibrium mixture with most of the base present as the nonionized amine. This equilibrium is analogous to that described for weak acids. We can confirm by measuring the pH of an aqueous solution of a weak base of known concentration that only a fraction of the base reacts with water (Figure 14.10). The remaining weak base is present as the unreacted form. The equilibrium constant for the ionization of a weak base, K b , is called the ionization constant of the weak base, and is equal to the reaction quotient when the reaction is at equilibrium. For trimethylamine, at equilibrium: K b = [(CH 3 ) 3 NH + ][OH − ] [(CH 3 ) 3 N] Figure 14.10 pH paper indicates that a 0.1-M solution of NH 3 (left) is weakly basic. The solution has a pOH of 3 ([OH − ] = 0.001 M) because the weak base NH 3 only partially reacts with water. A 0.1-M solution of NaOH (right) has a pOH of 1 because NaOH is a strong base. (credit: modification of work by Sahar Atwa) The ionization constants of several weak bases are given in Table 14.3 and in Appendix I.
eBook - PDF- Edward J. Neth, Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2019(Publication Date)
- Openstax(Publisher)
A reciprocal relation exists between the strengths of a conjugate acid-base pair: the stronger the acid, the weaker its conjugate base. Water exerts a leveling effect on dissolved acids or bases, reacting completely to generate its characteristic hydronium and hydroxide ions (the strongest acid and base that may exist in water). The strengths of the binary acids increase from left to right across a period of the periodic table (CH 4 < NH 3 < H 2 O < HF), and they increase down a group (HF < HCl < HBr < HI). The strengths of oxyacids that contain the same central element increase as the oxidation number of the element increases (H 2 SO 3 < H 2 SO 4 ). The strengths of oxyacids also increase as the electronegativity of the central element increases [H 2 SeO 4 < H 2 SO 4 ]. 14.4 Hydrolysis of Salts The ions composing salts may possess acidic or basic character, ionizing when dissolved in water to yield acidic or basic solutions. Acidic cations are typically the conjugate partners of weak bases, and basic anions are the conjugate partners of weak acids. Many metal ions bond to water molecules when dissolved to yield complex ions that may function as acids. 14.5 Polyprotic Acids An acid that contains more than one ionizable proton is a polyprotic acid. These acids undergo stepwise ionization reactions involving the transfer of single protons. The ionization constants for polyprotic acids decrease with each subsequent step; these decreases typically are large enough to permit simple equilibrium calculations that treat each step separately. 14.6 Buffers Solutions that contain appreciable amounts of a weak conjugate acid-base pair are called buffers. A buffered solution will experience only slight changes in pH when small amounts of acid or base are added. Addition of large amounts of acid or base can exceed the buffer capacity, consuming most of one 14 • Summary 715 conjugate partner and preventing further buffering action.
eBook - PDF- Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2019(Publication Date)
- Openstax(Publisher)
A reciprocal relation exists between the strengths of a conjugate acid-base pair: the stronger the acid, the weaker its conjugate base. Water exerts a leveling effect on dissolved acids or bases, reacting completely to generate its characteristic hydronium and hydroxide ions (the strongest acid and base that may exist in water). The strengths of the binary acids increase from left to right across a period of the periodic table (CH 4 < NH 3 < H 2 O < HF), and they increase down a group (HF < HCl < HBr < HI). The strengths of oxyacids that contain the same central element increase as the oxidation number of the element increases (H 2 SO 3 < H 2 SO 4 ). The strengths of oxyacids also increase as the electronegativity of the central element increases [H 2 SeO 4 < H 2 SO 4 ]. 14.4 Hydrolysis of Salts The ions composing salts may possess acidic or basic character, ionizing when dissolved in water to yield acidic or basic solutions. Acidic cations are typically the conjugate partners of weak bases, and basic anions are the conjugate partners of weak acids. Many metal ions bond to water molecules when dissolved to yield complex ions that may function as acids. 14.5 Polyprotic Acids An acid that contains more than one ionizable proton is a polyprotic acid. These acids undergo stepwise ionization reactions involving the transfer of single protons. The ionization constants for polyprotic acids decrease with each subsequent step; these decreases typically are large enough to permit simple equilibrium calculations that treat each step separately. 14.6 Buffers Solutions that contain appreciable amounts of a weak conjugate acid-base pair are called buffers. A buffered solution will experience only slight changes in pH when small amounts of acid or base are added. Addition of large amounts of acid or base can exceed the buffer capacity, consuming most of one 14 • Summary 739 conjugate partner and preventing further buffering action.
eBook - PDF- John A. Olmsted, Gregory M. Williams, Robert C. Burk(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
Other Acids A number of small molecules that are acids do not fall into the categories mentioned above. These acids have no clear patterns in their structures, so it is best simply to learn their names and structures. Three hydrogen halides, HCl, HBr, and HI, are strong acids, but the fourth, HF, is a weak acid. Hydrogen sulphide (H 2 S) and hydrocyanic acid (HCN) are weak acids that also happen to be quite poisonous. Table 15.2 summarizes the acid–base properties of these and other relatively small weak acids, and Example 15.9 provides some practice in identifying acids. Polyprotic acids (acids having more than one acidic hydrogen) are dealt with in Section 15.7. 1 5.4 Recognizing Acids and Bases 749 Weak Bases Besides water, the most common weak base is ammonia (NH 3 ). Many other weak bases are derivatives of ammonia called amines. In these organic compounds, one, two, or three of the N H bonds in ammonia have been replaced with N C bonds. The nitro- gen atom in an amine, like its counterpart in ammonia, has a lone pair of electrons that can form a bond to a proton. Water does not protonate an amine to an appreciable extent, so all amines are weak bases. Table 15.3 lists several examples of bases derived from ammonia. Methylamine Dimethylamine Trimethylamine Pyridine Aniline H H H N N H H N Hydrazine H H H H N N Hydroxylamine H H H N O N N Example 15.10 shows how the acid–base properties of a molecule can be determined from formulas and structures. Practice Exercise 15.9 Decide if each of the following formulas represents a strong acid, a weak acid, or neither. Justify your conclusions. (a) HF; (b) HCO 2 H; (c) (CH 3 ) 2 O; and (d) H 2 CO 3 .
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