Name: how ka and kb show acid-base strength
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 29 s
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Relative Strengths of Conjugate Acid-Base Pairs

The relationship between the dissociation constants of a conjugate acid-base pair can be expressed quantitatively.
For a weak acid, HA, its acid dissociation constant, Ka, is expressed as the hydronium ion concentration times the A ion concentration divided by the concentration of HA.
The base dissociation constant, Kb, for its conjugate base, A ion, is expressed as the HA concentration times the hydroxide ion concentration divided by the concentration of the A ion.
If the expressions for Ka and Kb are multiplied, the resulting equation is the equilibrium expression for Kw. This equation shows that the Ka for a weak acid and the Kb for its conjugate base are inversely proportional. A stronger acid with a higher Ka always has a proportionately weaker conjugate base with a lower Kb and vice versa.
As the value of KW is constant, this equation can be used to calculate Ka or Kb of a conjugate acid-base pair if either one of them is known. For example, if the Kb for a base is 1 &#215; 10&#8722;6, then the Ka for its conjugate acid can be calculated to be 1 &#215; 10&#8722;8.
The relationship between Ka, Kb, and KW can also be expressed in terms of their negative logarithms, pKa, pKb, and pKW.
For the equation, Ka times Kb is equal to Kw, when the negative logarithm of both sides is taken, the resulting equation is pKa plus pKb is equal to pKW, which is 14 at 25 &#176;C.
pKa and pKb can also be used to compare the strengths of weak acids and bases. The lower the value of the pKa, the stronger the acid. In the same manner, the lower the value of the pKb, the stronger the base. For example, an acid with a pKa of 2.1 is stronger than an acid with a pKa of 4.6.

relative strengths of conjugate acid-base pairs

Br&#248;nsted-Lowry acid-base chemistry is the transfer of protons; thus, logic suggests a relation between the relative strengths of conjugate acid-base pairs. The strength of an acid or base is quantified in its ionization constant, Ka or Kb, which represents the extent of the acid or base ionization reaction. For the conjugate acid-base pair HA / A&#8722;, the ionization equilibrium equations and ionization constant expressions are
<img alt="Chromatography column diagram; protein purification process; equipment setup; lab experiment." src="/files/ftp_upload/11398/11398_Equation_1.png" style="height: 90px;" data-altupdatedat="1761830439000" data-alt="Eq1" />
<img alt="Chemical reaction diagram; acid hydrolysis of disaccharide to monosaccharides; structural formula." src="/files/ftp_upload/11398/11398_Equation_2v2.png" style="height: 70px;" data-altupdatedat="1761830439000" data-alt="Eq2" />
Adding these two chemical equations yields the equation for the autoionization for water:
<img alt="Static equilibrium concept; &#931;Fx=0, MA=0 diagram; force balance, vector components, physics analysis." src="/files/ftp_upload/11398/11398_Equation_3.png" style="height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq3" />
As previously discussed, the equilibrium constant for a summed reaction is equal to the mathematical product of the equilibrium constants for the added reactions, and so
<img alt="Chemical equilibrium diagram, Haber process; N&#60;sub&#62;2&#60;/sub&#62; + 3H&#60;sub&#62;2&#60;/sub&#62; &#8652; 2NH&#60;sub&#62;3&#60;/sub&#62;, reaction pathway." src="/files/ftp_upload/11398/11398_Equation_4.png" style="height: 90px;" data-altupdatedat="1761830439000" data-alt="Eq4" />
This equation states the relation between ionization constants for any conjugate acid-base pair, namely, their mathematical product is equal to the ion product of water, KW. By rearranging this equation, a reciprocal relation between the strengths of a conjugate acid-base pair becomes evident:
<img alt="Chord progression diagram of twelve-bar blues showing cyclic pattern with roman numerals I, IV, V." src="/files/ftp_upload/11398/11398_Equation_5.png" style="height: 90px;" data-altupdatedat="1761830439000" data-alt="Eq5" />
The inverse proportional relation between Ka and Kb means the stronger the acid or base, the weaker its conjugate partner.
Taking the negative log of both sides of the equation, Ka &#215; Kb = KW yields
<img alt="Transcription initiation diagram, DNA to RNA synthesis, gene expression, promoter recognition." src="/files/ftp_upload/11398/11398_Equation_6.png" style="height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq6" />
then
<img alt="Chromatography process diagram showing sample separation on stationary phase; method TLC setup." src="/files/ftp_upload/11398/11398_Equation_7.png" style="height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq7" />
As pKW is 14 at 25 &#176;C, this equation can also be written as
<img alt="Spring-mass-damper system diagram, showing dynamic equilibrium with forces and damping analysis." src="/files/ftp_upload/11398/11398_Equation_8.png" style="height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq1" />
The pKa and pKb also represent the strength of acids and bases, respectively. Like pH and pOH, the higher the value pKa or pKb, the weaker the acid or base, respectively. 
<table style="margin-left: auto; margin-right: auto;">
	<tbody>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Acid</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Base</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Perchloric acid (HClO4)*</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Perchlorate ion (ClO4&#8722;)**</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Sulfuric acid (H2SO4)*</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydrogen sulfate ion (HSO4&#8722;)**</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydrogen iodide (HI)*</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Iodide ion (I&#8722;)**</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydrogen bromide (HBr)*</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Bromide ion (Br&#8722;)**</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydrogen chloride (HCl)*</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Chloride ion (Cl&#8722;)**</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Nitric acid (HNO3)*</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Nitrate ion (NO3&#8722;)**</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydronium ion (H3O+)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Water (H2O)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydrogen sulfate ion (HSO4&#8722;)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Sulfate ion (SO42&#8722;)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Phosphoric acid (H3PO4)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Dihydrogen phosphate ion (H2PO4&#8722;)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydrogen fluoride (HF)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Fluoride ion (F&#8722;)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Nitrous acid (HNO2)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Nitrite ion (NO2&#8722;)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Acetic acid (CH3CO2H)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Acetate ion (CH3CO2&#8722;)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Carbonic acid (H2CO3)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydrogen carbonate ion (HCO3&#8722;)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydrogen sulfide (H2S)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydrogen sulfide ion (HS&#8722;)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Ammonium ion (NH4+)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Ammonia (NH3)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydrogen cyanide (HCN)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Cyanide ion (CN&#8722;)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydrogen carbonate ion (HCO3&#8722;)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Carbonate ion (CO32&#8722;)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Water (H2O)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydroxide ion (OH&#8722;)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydrogen sulfide ion (HS&#8722;)&#8224;</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Sulfide ion (S2&#8722;)&#8225;</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Ethanol (C2H5OH)&#8224;</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Ethoxide ion (C2H5O&#8722;)&#8225;</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Ammonia (NH3)&#8224;</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Amide ion (NH2&#8722;)&#8225;</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydrogen (H2)&#8224;</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Hydride ion (H&#8722;)&#8225;</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Methane (CH4)&#8224;</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Methide ion (CH3&#8722;)&#8225;</td>
		</tr>
		<tr>
			<td colspan="2" style="color: black; border: 1px solid black; ">*Undergo complete acid ionization in water</td>
		</tr>
		<tr>
			<td colspan="2" style="color: black; border: 1px solid black; ">&#8224; Does not undergo acid ionization in water</td>
		</tr>
		<tr>
			<td colspan="2" style="color: black; border: 1px solid black; ">**Does not undergo base ionization in water</td>
		</tr>
		<tr>
			<td colspan="2" style="color: black; border: 1px solid black; ">&#8225; Undergo complete base ionization in water</td>
		</tr>
	</tbody>
</table>
The listing of conjugate acid&#8211;base pairs shown is arranged to show the relative strength of each species as compared with water. In the acid column, those species listed below water are weaker acids than water. These species do not undergo acid ionization in water; they are not Br&#248;nsted-Lowry acids. All the species listed above water are stronger acids, transferring protons to water to some extent when dissolved in an aqueous solution to generate hydronium ions. Species above water but below hydronium ion are weak acids, undergoing partial acid ionization, whereas those above hydronium ion are strong acids that are completely ionized in aqueous solution.
This text is adapted from<a href="https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases"> Openstax, Chemistry 2e, Section 14.3: </a><a href="https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases">Relative Strengths of Acids and Bases</a>.

Br&#248;nsted-Lowry acid-base chemistry is the transfer of protons; thus, logic suggests a relation between the relative strengths of conjugate acid-base ...

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Acids and Bases

KEY TERMS AND CONCEPTS

how ka and kb show acid-base strength

Brønsted-Lowry acid-base chemistry is based on proton transfer. That idea helps show how the strength of an acid is linked to the strength of its conjugate base, and vice versa. Acid and base strength is measured with ionization constants, Ka and Kb, which tell how far an ionization reaction goes.

For the conjugate pair HA and A−, the acid ionizes in water and the base reaction can be written for the partner species. When the two reactions are added, they give the autoionization of water. The equilibrium constant for the summed reaction is the product of the two individual constants, so Ka × Kb = Kw for any conjugate acid-base pair.

This relationship shows a reciprocal pattern. If one member of the pair is stronger, its conjugate partner is weaker. The stronger the acid, the weaker its conjugate base, and the stronger the base, the weaker its conjugate acid.

Taking the negative log of Ka × Kb = Kw gives the pKa and pKb form of the same idea. At 25 °C, pKw is 14, so the relationship can also be written as pKa + pKb = 14. Like pH and pOH, a larger pKa or pKb means a weaker acid or base.

The conjugate acid-base list is arranged to compare each species with water. Species below water in the acid column are weaker acids than water, so they do not undergo acid ionization in water and are not Brønsted-Lowry acids. Species above water transfer protons to water to some extent and form hydronium ions.

Among those acids, the ones above water but below hydronium ion are weak acids, so they ionize only partially in aqueous solution. The acids above hydronium ion are strong acids, and they are completely ionized in water. The list includes examples such as perchloric acid, sulfuric acid, hydrogen chloride, nitric acid, acetic acid, ammonia, ethanol, hydride, and methane with their conjugate partners.

How Ka and Kb Show Acid-Base Strength

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Water can act as both a Brønsted-Lowry acid and a Brønsted-Lowry base. When hydrogen fluoride dissolves in water, it donates protons to water. This is acid ionization. The products are hydronium ions and fluoride ions. Water can also accept protons from another substance. When pyridine dissolves in water, it accepts a proton from water molecules. This is base ionization. The products are hydroxide ions and pyridinium ions. These reactions show that water can play either role. A substance that...

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Brønsted-Lowry acid-base chemistry is based on proton transfer. That idea helps show how the strength of an acid is linked to the strength of its conjugate base, and vice versa. Acid and base strength is measured with ionization constants, Ka and Kb, which tell how far an ionization reaction goes. For the conjugate pair HA and A−, the acid ionizes in water and the base reaction can be written for the partner species. When the two reactions are added, they give the autoionization of water. The...

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Weak Base pH and Kb Calculations

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See how Ka, Kb, pKa, and pKb reveal the strength of conjugate acid-base pairs, including why stronger acids have weaker conjugate bases.

How Ka and Kb Show Acid-Base Strength | JoVE K12

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The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

PS.Chm.2.2

Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Lewis Symbols and the Octet Rule

Chemical bonds are complex interactions between two or more atoms or ions, which reduce the potential energy of the molecule. Gilbert N. Lewis developed a ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

The Born-Haber Cycle

Lattice Energy&#xA0;

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a ...

trends-in-lattice-energy-ion-size-and-charge

Trends in Lattice Energy: Ion Size and Charge

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Electronegativity

Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity.&#xA0;

Electronegativity values of ...

bond-polarity-dipole-moment-and-percent-ionic-character

Bond Polarity, Dipole Moment, and Percent Ionic Character

Bond Polarity

The absolute value of the difference in electronegativity (&Delta;EN) of two bonded atoms provides a rough measure of the polarity to be ...

lewis-structures-of-molecular-compounds-and-polyatomic-ions

Lewis Structures of Molecular Compounds and Polyatomic Ions

To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:


	Determine the ...

Resonance

The Lewis structure of a nitrite anion (NO2&minus;) may actually be drawn in two different ways, distinguished by the locations of the N-O and N=O ...

Formal Charges

In some cases, there are seemingly more than one valid Lewis structures for molecules and polyatomic ions. The concept of formal charges can be used to ...

Exceptions to the Octet Rule

Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures. These molecules fall into three categories:


	...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

PS.Chm.3.1

molecular-comparison-of-gases-liquids-and-solids

Molecular Comparison of Gases, Liquids, and Solids

Particles in a solid are tightly packed together (fixed shape) and often arranged in a regular pattern; in a liquid, they are close together with no ...

Construct an explanation to summarize the influences intermolecular forces have on the properties of chemical compounds.

PS.Chm.3.2

Analyze the structure of atoms and isotopes.

How Ka and Kb Show Acid-Base Strength

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