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1.5: Acids, Bases, and Salts

  • Page ID
    6976
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    Learning Objectives

    • Explain the properties of water
    • Describe inorganic and organic acids
    • Describe inorganic bases
    • Explain pH, acids, and bases
    • Describe buffers
    • Outline Ionic compounds and chemical reactions

    ​​​​​Water is the most abundant and important inorganic compound on earth. It makes up 60 to 89-percent of the volume of most living cells, and it possesses several properties that make it vital to life. Water has a high heat capacity. Water absorbs and releases large amounts of heat before changing appreciably in temperature. This property prevents sudden changes in temperature caused by external factors, such as sun or wind exposure, or by internal conditions that release heat rapidly, such as vigorous muscle activity. As part of blood system or the environment, water redistributes heat among adjacent structures ensuing the temperature remains homeostatic.

    When water evaporates, or vaporizes, water changes from a liquid to a gas, water vapor. The transformation requires that large amounts of heat be absorbed to break hydrogen bonds that hold water molecules together. This property is beneficial because as water evaporates from an object or organism large amounts of heat are removed providing efficient cooling. This property is referred to as high heat of vaporization.

    Water is the best solvent in nature. It is called the universal solvent. Biological molecules do not react chemically unless they are in solution, and virtually all chemical reactions that occur in the living cells depend on water’s solvent properties. Water molecules are referred to as being polar. They orient with their slightly negative ends toward the positive ends of the solutes. First attracting the solute molecules, and then surrounding them. This characteristic is called polarity, and it explains the reason that ionic compounds and other small reactive molecules, such as acids and bases, dissociate in water, where their ions separating from each other and become evenly scattered in the water forming a true solution. Water is a polar solvent.

    Water also forms layers of water molecules, called hydration layers, around large charged molecules such as protein, shielding them from the effects of other charged substances in the areas and preventing them from settling out of solution. Such protein water mixtures are biological colloids. Water is also the major transport medium because it is an excellent polar solvent. Nutrients, gases, and metabolic wastes are carried dissolved in water-based fluids. Wastes are excreted form living organisms in watery fluids. Specialized molecules that lubricate organisms also use water as the dissolving medium.

    Water is an important reactant in many chemical reactions. Nutrients are decomposed by adding a water molecule to each chemical bond that is broken. Decomposition reactions are more specifically referred to as hydrolysis reactions. When large carbohydrates or protein molecules are synthesized from smaller molecules, a water molecule is removed for every bond formed, a reaction that is called dehydration synthesis.

    Water forms resilient cushions, cushioning around certain biological structures providing protection from physical trauma.

    When inorganic salts such as sodium chloride (NaCl) are dissolved in water, they undergo ionization or dissociation. They break apart into ions. Substances labeled acids and bases demonstrate similar behavior.

    An acid can be defined as a substance that dissociates into one or more hydrogen ions (H+) and one or more negative ions (anions) an acid is also called a proton donor (H+). A base dissociates into one or more positive ions (cations) that can accept or combine with protons. Sodium hydroxide (NaOH) is a base because it dissociates to release OH-, has a strong attraction for protons. Bases are among the most important proton acceptors in chemistry. A salt is a substance that dissociates in water into cations and anions, neither of which is H+ or OH-.

    Acids

    Salts, acids, and bases are electrolytes. They ionize and dissociate in water and can conduct an electrical current.

    Acids have a sour taste, and they can react with or dissolve metals. The definition of an acid is a substance that releases hydrogen ions (H+) in measurable amounts. Acids are also characterized as being proton donors because a hydrogen ion is a hydrogen nucleus, or a single proton.

    When acids dissolve in water, they release hydrogen ions (protons) and anions, negative charged particles. The concentration of the protons determines the acidity of the solution. The anions have little or no effect on the acidity of the solution. Hydrochloric acid (HCl) dissociates into a proton and a chloride ion:

    • H+, proton + Cl-, anion → HCl

    Living organisms maintain a constant balance of acids and bases. If a particular acid or base concentration is too high or too low, enzymes change in shape and are no longer effective. In aqueous environments, acids dissociate into hydrogen ions and anions. Bases dissociate into hydroxide ions and cations. The more hydrogen ions that are free in a solution, the more acidic the solution. The more hydroxide ions that are free in a solution the more basic or alkaline is the solution.

    Biochemical reactions are sensitive to small changes in the acidity or alkalinity of the environment in which they occur. H+ and OH- are involved in almost all biochemical processes, and any deviation from a narrow band of the normal H+ and OH- concentration dramatically modifies the systems’ functioning. Acids and bases that are formed in living systems must be kept in balance.

    It is convenient to express the amount of H+ in a solution by a logarithmic pH scale that ranges from 0 to 14. The term pH means potential of hydrogen ion concentration. On a log scale, a change of one whole number has 100 times more hydrogen ions than a solution of pH 2, and a pH of 2 has 100 times more hydrogen ions than a solution of pH 3.

    Acidic solutions contain more H+ than OH- and have a pH lower than 7. If a solution has more OH- than H+, it is a basic or alkaline solution. In pure water, a small percentage of molecules are dissociated into H+ and OH- ions so that it has a pH of 7. When the ion concentrations of a solution are equal, the solution has a pH of 7 and is considered to be neutral.

    Concentrations of H+ and OH- at differing pH levels
    Figure \(\PageIndex{1}\): Depiction demonstrating the concentrations of H+ and OH- at differing pH levels – Image by COC OER is licensed under CC BY 4.0

    The pH of a solution can be changed. When substances that will increase the concentration of hydrogen ions are added to a solution, then the pH will decrease. In systems, where buffers are present, the pH does not fluctuate as much as in systems that contain no buffers. Buffers prevent the pH of a solution from changing drastically. The pH in the environment can be altered by waste products from organisms, pollutants from industry, and fertilizers used in agricultural fields or gardens. When bacteria are grown in a laboratory, they excrete waste products such as acids that can alter the pH. If acid production were to continue, the medium where the bacteria are growing will become acidic enough to inhibit bacterial enzymes and kill the bacteria. To prevent this problem, pH buffers are added to solutions or to the natural environment to prevent changes in pH. Buffers resist a change in pH.

    Bases

    Bases have a bitter taste, feel slippery, and are proton acceptors. They take up hydrogen ions in measureable amounts. Common inorganic bases include the hydroxides like magnesium hydroxide and sodium hydroxide. Lie acids, hydroxides dissociate when dissolved in water. In this case, hydroxyl ions (OH-) and cations are released. Ionization of sodium hydroxide produces a hydroxyl ion and a sodium ion. The hydroxyl ion binds to a proton present in the solution. This reaction produces water and simultaneously reduces the acidity of the solution by taking up free H+ ions:

    • NaOH → Na+ + OH-
    • OH- + H+ → H2O

    The term pH is used to express the intensity of an acid or alkaline solution.

    PH Scale- Acidic vs. Basic (Alkaline)
    Figure \(\PageIndex{2}\): pH scale from 0 to 14 demonstrating acid, neutral, and baseImage by Heinrich-Böll-Stiftung is licensed under CC BY-SA 2.0

    Salts and Neutralization

    The more hydrogen ions in a solution, the more acidic the solution. The greater the concentration of hydroxyl ions, the more basic, or alkaline, the solution.

    When acids and bases are mixed, they react with each other in displacement reactions to form water and a salt. For example, when hydrochloric acid and sodium hydroxide interact, sodium chloride (salt) and water are formed.

    • HCl (acid) + NaOH (base) → NaCl (salt) + H2O (water)

    This type of reaction is called a neutralization reaction because the joining of H+ and OH- to form water neutralizes the solution. The salt produced is dissolved in the aqueous solution and disassociated into Na+ and Cl- ions.

    Salts are ionic compounds containing cations other than H+ and anions other than the hydroxyl ion, OH-. When salts are dissolved in water, they dissociate into their component ions. Sodium sulfate, Na2SO4, dissociates into two Na+ ions and one SO4-2. The salt dissociates because the ions are formed. The water overcomes the attraction between the oppositely charged ions, and they disassociate.

    All ions are electrolytes which are substances that conduct an electrical current in solution. Salts dissociate in aqueous solutions into ions, and the most common salts are sodium salts. In their ionized form, salts play a vital role in nature and aqueous solutions.

    Buffers

    Organisms are extremely sensitive to slight changes in the pH of the environment. In high concentrations, acids and bases are damaging to cells. Homeostasis of acid-base balance is regulated by chemical systems called buffers.

    Buffers resist abrupt and large changes in the pH of a solution by releasing hydrogen ions when the pH begins to rise and by binding hydrogen ions when the pH drops.

    Chemical buffer systems react by binding hydrogen ions or by releasing hydrogen ions. The acidity of a solution reflects the free hydrogen ions and not the hydrogen ions bound to anions. Acids that dissociate completely and irreversibly in water are called strong acids. They can dramatically change the pH of a solution. Acids that do not dissociate completely, like carbonic acid (H2CO3) and acetic acid, are weak acids. Undissociated acids do not affect pH, so that acetic acid solutions are much less acidic than hydrochloric acid solution. Weak acids disassociate in predictable ways, and molecules of the intact acid are in dynamic equilibrium with the dissociated ions.

    For this reason, when a strong acid is added to a solution of a weak acid the equilibrium will shift to the left and some H+ will recombine to form acetic acid. On the other hand, if a strong base is added and the pH begins to rise, the equilibrium shifts to the right and more acetic molecules disassociate to release H+ ions. This characteristic of weak acids allows them to play a role in the chemical buffer systems found in nature.

    • H+ acetic acid ↔ H+ + Acetic-

    Bases are proton acceptors. Strong bases are those bases, like hydroxides, that dissociate easily in water and quickly bind H+ ions. Sodium bicarbonate ionizes incompletely and reversibly. Since it accepts relatively few protons, its released bicarbonate ion is considered to be a weak base.

    One of the buffer systems that helps to maintain the pH in aqueous solutions is the carbonic acid-bicarbonate system. Carbonic acid dissociates reversibly in aqueous solutions releasing bicarbonate ions and protons, H+. The chemical equilibrium between carbonic acid, a weak acid, and bicarbonate ion, a weak base, resists changes in pH by shifting to the right or the left as H+ ions are added to or removed from the solution:

    • H2CO3 (drop in pH) ↔ H+ + HCO3- (rise in pH)

    As the pH rises and becomes more alkaline, the equilibrium shifts to the right, forcing more carbonic acid to dissociate. Similarly, as the pH begins to drop, the equilibrium shifts to the left as more bicarbonate ions begin to bind with protons. Strong bases are replaced by a weak base, bicarbonate ion. Protons are released by strong acids and are tied up in weak acids, carbonic acid. As a result, the pH changes are much less than they would be in the absence of the buffering system.

    Review Questions

    1. What properties of water make it essential for life?
    2. What are acids and bases?
    3. Define a buffer.

    Chapter Quiz

    1. ___________ is the most abundant and important inorganic compound on earth.
      1. Salt
      2. Water
      3. Bicarbonate
      4. Oxygen
    2. A(n) ___________ can be defined as a substance that dissociates into one or more hydrogen ions (H+) and one or more negative ions (anions) an acid is also called a proton donor (H+).
      1. Base
      2. Buffer
      3. Acid
      4. Salt
    3. ___________ are added to solutions or to the natural environment to prevent changes in pH.
      1. Base
      2. Buffer
      3. Acid
      4. Salt
    4. ___________ have a bitter taste, feel slippery, and are proton acceptors. They take up hydrogen ions in measureable amounts.
      1. Bases
      2. Buffers
      3. Acids
      4. Salts
    5. When acids and bases are mixed, they react with each other in displacement reactions to form water and a(n) ___________.
      1. Base
      2. Buffer
      3. Acid
      4. Salt
    6. Nutrients are decomposed by adding ___________ to each chemical bond that is broken.
      1. A salt
      2. Water
      3. Bicarbonate
      4. Oxygen
    7. HCl + NaOH → NaCl + H2O: ___________ is a base.
      1. NaOH
      2. HCl
      3. NaCl
      4. H2O
    8. HCl + NaOH → NaCl + H2O: ___________ is an acid.
      1. NaOH
      2. HCl
      3. NaCl
      4. H2O
    9. HCl + NaOH → NaCl + H2O: ___________ is a salt.
      1. NaOH
      2. HCl
      3. NaCl
      4. H2O
    10. ___________ system that helps to maintain the pH in aqueous solutions is the carbonic acid-bicarbonate system.
      1. A base
      2. A buffer
      3. An acid
      4. A salt

    1.5: Acids, Bases, and Salts is shared under a not declared license and was authored, remixed, and/or curated by John Rowe.

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