Weak Base

Information about Weak Base

Acid-base reaction theories
pH
Self-ionization of water
Buffer solutions
Systematic naming
Redox reactions
Electrochemistry
Acids:

Bases:

Strong bases
Weak bases

In chemistry, a weak base is a chemical base that does not ionize fully in an aqueous solution. As Bronsted-Lowry bases are proton acceptors, a weak base may also be defined as a chemical base in which protonation is incomplete. This results in a relatively low pH level compared to strong bases. Bases range from a pH of greater than 7 (7 is neutral, like pure water) to 14 (though some bases are greater than 14). The pH level has the formula:

$\text{[math]}$

Since bases are proton acceptors, the base receives a hydrogen ion from water, H₂O, and the remaining H⁺ concentration in the solution determines the pH level. Weak bases will have a higher H⁺ concentration because they are less completely protonated than stronger bases and, therefore, more hydrogen ions remain in the solution. If you plug in a higher H⁺ concentration into the formula, a low pH level results. However, the pH level of bases is usually calculated using the OH^- concentration to find the pOH level first. This is done because the H⁺ concentration is not a part of the reaction, while the OH^- concentration is.

$\text{[math]}$

By multiplying a conjugate acid (such as NH₄⁺) and a conjugate base (such as NH₃) the following is given:

$\text{[math]}$

Since $\text{[math]}$ then, $\text{[math]}$

By taking logarithms of both sides of the equation, the following is reached:

$\text{[math]}$

Finally, multipying throughout the equation by -1, the equation turns into:

$\text{[math]}$

After acquiring pOH from the previous pOH formula, pH can be calculated using the formula pH = pK_w - pOH where pK_w = 14.00.

Weak bases exist in chemical equilibrium much in the same way as weak acids do, with a Base Ionization Constant (K_b) (or the Base Dissociation Constant) indicating the strength of the base. For example, when ammonia is put in water, the following equilibrium is set up:

$\text{[math]}$

Bases that have a large K_b will ionize more completely and are thus stronger bases. As stated above, the pH of the solution depends on the H⁺ concentration, which is related to the OH^- concentration by the Ionic Constant of water (K_w = 1.0x10^-14) (See article Self-ionization of water.) A strong base has a lower H⁺ concentration because they are fully protonated and less hydrogen ions remain in the solution. A lower H⁺ concentration also means a higher OH^- concentration and therefore, a larger K_b.

NaOH (s) (sodium hydroxide) is a stronger base than (CH₃CH₂)₂NH (l) (diethylamine) which is a stronger base than NH₃ (g) (ammonia). As the bases get weaker, the smaller the K_b values become. The pie-chart representation is as follows:

purple areas represent the fraction of OH- ions formed
red areas represent the cation remaining after ionization
yellow areas represent dissolved but non-ionized molecules.

Percentage protonated

As seen above, the strength of a base depends primarily on the pH level. To help describe the strengths of weak bases, it is helpful to know the percentage protonated-the percentage of base molecules that have been protonated. A lower percentage will correspond with a lower pH level because both numbers result from the amount of protonation. A weak base is less protonated, leading to a lower pH and a lower percentage protonated.

The typical proton transfer equilibrium appears as such:

$\text{[math]}$

B represents the base.

$\text{[math]}$

In this formula, [B]_initial is the initial molar concentration of the base, assuming that no protonation has occurred.

A typical pH problem

Calculate the pH and percentage protonation of a .20 M aqueous solution of pyridine, C₅H₅N. The K_b for C₅H₅N is 1.8 x 10^-9.

First, write the proton transfer equilibrium:

$\text{[math]}$

The equilibrium table, with all concentrations in moles per liter, is


	C₅H₅N	C₅H₆N⁺	OH^-
initial normality	.20	0	0
change in normality	-x	+x	+x
equilibrium normality	.20 -x	x	x

Substitute the equilibrium molarities into the basicity constant	$\text{[math]}$
We can assume that x is so small that it will be meaningless by the time we use significant figures.	$\text{[math]}$
Solve for x.	$\text{[math]}$
Check the assumption that x << .20	$\text{[math]}$ ; so the approximation is valid
Find pOH from pOH = -log [OH^-] with [OH^-]=x	$\text{[math]}$
From pH = pK_w - pOH,	$\text{[math]}$
From the equation for percentage protonated with [HB⁺] = x and [B]_initial = .20,	$\text{[math]}$

This means .0095% of the pyridine is in the protonated form of C₅H₆N⁺.

Examples

Alanine, C₃H₅O₂NH₂
Ammonia, NH₃
Methylamine, CH₃NH₂
Pyridine, C₅H₅N

Other weak bases are essentially any bases not on the list of strong bases.

References

Atkins, Peter, and Loretta Jones. Chemical Principles: The Quest for Insight, 3rd Ed., New York: W.H. Freeman, 2005.

External links

ACID (Atomicity, Consistency, Isolation, Durability) is a set of properties that guarantee that database transactions are processed reliably. In the context of databases, a single logical operation on the data is called a transaction.
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In chemistry, a base is most commonly thought of as a substance that can accept protons. This refers to the Brønsted-Lowry theory of acids and bases. Alternate definitions of bases include electron pair donors (Lewis), and as sources of hydroxide anions (Arrhenius).
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An acid-base reaction is a chemical reaction that occurs between an acid and a base. Several concepts exist which provide alternative definitions for the reaction mechanisms involved and their application in solving related problems.
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pH is a measure of the acidity or alkalinity of a solution. Aqueous solutions at 25 ℃ with a pH less than seven are considered acidic, while those with a pH greater than seven are considered basic (alkaline). The pH of 7.
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The self-ionization of water is the chemical reaction in which two water molecules react to produce a hydronium (H₃O⁺) and a hydroxide ion (OH⁻):

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Buffer solutions are solutions that resist change in Hydronium ion and the hydroxide ion concentration (and consequently pH) upon addition of small amounts of acid or base, or upon dilution.
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Redox (shorthand for reduction/oxidation reaction) describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed.

This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide, or the
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Electrochemistry is a branch of chemistry that studies the reactions which take place at the interface of an electronic conductor (the electrode composed of a metal or a semiconductor, including graphite) and an ionic conductor (the electrolyte).
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A strong acid is an acid that dissociates completely in an aqueous solution (not in the case of sulfuric acid as it is diprotic), or in other terms, with a pK_a
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A weak acid is an acid that does not ionize in solution to a significant extent; that is, if the acid was represented by the general formula HA, then in aqueous solution a significant amount of undissociated HA still remains.
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Ionization is the physical process of converting an atom or molecule into an ion by changing the difference between the number of protons and electrons. This process works slightly differently depending on whether an ion with a positive or a negative electric charge is being
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aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations as a subscript (aq). The word aqueous means pertaining to, related to, similar to, or dissolved in water.
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Protonation is the addition of a hydron (H⁺) to an atom, molecule, or ion. A Bronsted-Lowry acid is defined as a chemical substance that protonates another substance.
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Proton

The quark structure of the proton.
Composition: 2 up, 1 down
Family: Fermion
Group: Quark
Interaction: Gravity, Electromagnetic, Weak, Strong
Antiparticle: Antiproton
Discovered: Ernest Rutherford (1919)
Symbol: p+
Mass: 1.
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In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of
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chemical equilibrium is the state in which the chemical activities or concentrations of the reactants and products have no net change over time. Usually, this state results when the forward chemical process proceeds at the same rate as their reverse reaction.
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The self-ionization of water is the chemical reaction in which two water molecules react to produce a hydronium (H₃O⁺) and a hydroxide ion (OH⁻):

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Diethylamine is a secondary amine with the molecular structure CH₃CH₂NHCH₂CH₃. It is a flammable, strongly alkaline liquid. It is miscible with water and ethanol.
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Alanine (abbreviated as Ala or A)^[1] is an α-amino acid with the chemical formula HO₂CCH(NH₂)CH₃. The L-isomer is one of the 20 proteinogenic amino acids, i.e. the building blocks of proteins.
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Ammonia is a compound with the formula NH₃. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of the planet as a precursor to foodstuffs and fertilizers.
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Methylamine is the chemical compound with a formula of CH₃NH₂. It is a derivative of ammonia, wherein one H atom is replaced by a methyl group. It is the simplest primary amine.
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Pyridine is a chemical compound with the formula C₅H₅N. It is a liquid with a distinctively putrid, fishy odour. Pyridine is a simple and fundamentally important heterocyclic aromatic organic compound that is structurally related to benzene, wherein one CH
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Weak Base

Information about Weak Base

Percentage protonated

A typical pH problem

Examples

See also

References

External links