How does one define acids and bases? In chemistry, acids and bases have been defined differently by three sets of theories. Also, the Lewis theory of acids and bases states that acids are electron pair acceptors while bases are electron pair donors. Acids and bases can be defined by their physical and chemical observations.
This scale is shown by the following formula:. The pH scale is often measured on a 1 to 14 range, but this is incorrect see pH for more details. Something with a pH less than 7 indicates acidic properties and greater than 7 indicates basic properties. A pH at exactly 7 is neutral. The Lewis theory of acids and bases states that acids act as electron pair acceptors and bases act as electron pair doners. This definition doesn't mention anything about the hydrogen atom at all, unlike the other definitions.
It only talks about the transfer of electron pairs. To demonstrate this theory, consider the following example. This is a reaction between ammonia NH 3 and boron trifluoride BF 3.
Since there is no transfer of hydrogen atoms here, it is clear that this is a Lewis acid-base reaction. In this reaction, NH 3 has a lone pair of electrons and BF 3 has an incomplete octet, since boron doesn't have enough electrons around it to form an octet.
Because boron only has 6 electrons around it, it can hold 2 more. BF 3 can act as a Lewis acid and accept the pair of electrons from the nitrogen in NH 3 , which will then form a bond between the nitrogen and the boron.
This is considered an acid-base reaction where NH 3 base is donating the pair of electrons to BF 3. A special property of acids and bases is their ability to neutralize the other's properties. Another product of a neutralization reaction is an ionic compound called a salt.
Therefore, the general form of an acid-base reaction is:. Titrations are performed with acids and bases to determine their concentrations. At the equivalence point, the number of moles of the acid will equal the number of moles of the base. This indicates that the reaction has been neutralized. For instance, 30 mL of 1. The concentration of HCl needs to be determined. At the eqivalence point:. Solution : There are 6 strong acids and all other acids are considered weak.
We recommend downloading the newest version of Flash here, but we support all versions 10 and above. If that doesn't help, please let us know. Unable to load video. Please check your Internet connection and reload this page. If the problem continues, please let us know and we'll try to help. An unexpected error occurred. Previous Video Anions that are the conjugate base of strong acids, like chloride formed by the dissociation of hydrochloric acid, are too weak to accept a proton from water.
Therefore, chloride ions are pH neutral; that is, they are neither acidic nor basic. In contrast, anions formed by weak acids, like acetate, the conjugate base of acetic acid, acts as a weak base as it can accept a proton from water. Cations that are the conjugate acid of strong bases, like sodium ions formed by sodium hydroxide, cannot accept protons and, therefore, are also pH neutral. In contrast, cations produced by weak bases, like ammonium, the conjugate acid of the ammonia, acts as a weak acid as it can donate protons to water.
When they ionize, salts can produce acidic and basic solutions if they contain a conjugate acid or a conjugate base of a weak acid or base. Ammonium bromide produces ammonium and bromide ions in water.
Bromide ions are pH neutral, whereas ammonium ions act as a weak acid as they can donate protons. When sodium acetate is dissolved in water, the sodium ions do not react with water; however, the acetate ion can accept a proton—forming a basic solution. Salts that contain pH neutral cations and anions form neutral solutions.
For example, sodium chloride dissociates into sodium ions and chloride ions when dissolved in water. As both of these ions can neither accept nor donate protons, they form a neutral solution. Small and highly charged metal ions, like iron III and aluminium III , can also act as weak acids when they become hydrated. When aluminum III is hydrated, it acts as a weak acid and transfers protons from its waters of hydration to the free water molecules, leading to the production of hydronium ions.
The smaller the metal ion and the higher the charges present on it, the greater is its tendency to act as an acid. Salts are ionic compounds composed of cations and anions, either of which may be capable of undergoing an acid or base ionization reaction with water.
The reaction can be written as follows:. Autoprotolysis of water : The self-ionization of water produces hydronium and hydroxide ions in solution. Like all equilibrium reactions, this reaction has an equilibrium constant. Because this is a special equilibrium constant, specific to the self-ionization of water, it is denoted K W ; it has a value of 1.
If we write out the actual equilibrium expression for K W , we get the following:. Here we have the reason why neutral water has a pH of 7. This relationship always holds true for any aqueous solution, regardless of its level of acidity or alkalinity.
Utilizing this equation is a convenient way to quickly determine pOH from pH and vice versa, as well as to determine hydroxide concentration given hydrogen concentration, or vice versa. Self-ionization of Water : Explanation of self-ionization of water and the formation of hydronium and hydroxide ions.
The acid dissociation constant K a is the measure of the strength of an acid in solution. The acid dissociation constant K a is a quantitative measure of the strength of an acid in solution. K a is the equilibrium constant for the following dissociation reaction of an acid in aqueous solution:.
The K a expression is as follows:. Acid dissociation constants are most often associated with weak acids, or acids that do not completely dissociate in solution. This is because strong acids are presumed to ionize completely in solution and therefore their K a values are exceedingly large.
Due to the many orders of magnitude spanned by K a values, a logarithmic measure of the acid dissociation constant is more commonly used in practice. The logarithmic constant pK a is equal to -log 10 K a. The larger the value of pK a , the smaller the extent of dissociation. A weak acid has a pK a value in the approximate range of -2 to 12 in water. Acids with a pK a value of less than about -2 are said to be strong acids.
A strong acid is almost completely dissociated in aqueous solution; it is dissociated to the extent that the concentration of the undissociated acid becomes undetectable. Acetic acid dissociation : The acetic acid partially and reversibly dissociates into acetate and hydrogen ions.
What is the pK a for acetic acid? This reaction has a special equilibrium constant denoted K W , and it can be written as follows:. Here we have the reason that neutral water has a pH of 7. This relationship will always apply to aqueous solutions.
It is a quick and convenient way to find pH from pOH, hydrogen ion concentration from hydroxide ion concentration, and more. The pH and pOH Scale : Relation between p[OH] and p[H] brighter red is more acidic, which is the lower numbers for the pH scale and higher numbers for the pOH scale; brighter blue is more basic, which is the higher numbers for the pH scale and lower numbers for the pOH scale.
Generically, this p-notation can be used for other scales. However, because these values are often very small for weak acids and weak bases, the p-scale is used to simplify these numbers and make them more convenient to work with. What is an acidic solution? What is a basic or alkaline solution? Chemical reactions in the body, the food we eat, medication we take, and the effects of some diseases can add or remove hydrogen or hydroxyl ions in or from our body fluids.
Then, all cells in our body depend on homeostatic regulation of acid-base balance to maintain pH within optimal living conditions. There are several homeostatic mechanisms to maintain pH within optimal conditions. It can be regulated by the internal availability of substances chemicals , by adjusting breathing rate, and by eliminating chemicals in urine.
Chemical buffers in the body are substances that can absorb extra hydrogen ions preventing a change in pH.
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