Weak base - Citizendia

In chemistry, a weak base is a chemical base that does not ionize fully in an aqueous solution. In Computer science, ACID ( Atomicity Consistency Isolation Durability) is a set of properties that guarantee that Database transactions are In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and Acid-base extraction is a procedure using sequential Liquid-liquid extractions to purify Acids and bases from mixtures based on their chemical properties Acid-base homeostasis is the part of Human homeostasis concerning the proper balance between Acids and bases, in other words the PH. An acidity function is a measure of the Acidity of a medium or solvent system usually expressed in terms of its ability to donate protons to (or accept protons from a For an individual weak acid or weak base component see Buffering agent. pH is the measure of the acidity or alkalinity of a Solution. The proton affinity, E pa of a Anion or of a neutral Atom or Molecule is a measure of its gas-phase basicity. The self-ionization of water (also autoionization of water, and autodissociation of water) is the chemical reaction in which two water molecules react to produce a In Computer science, ACID ( Atomicity Consistency Isolation Durability) is a set of properties that guarantee that Database transactions are A mineral acid is an Acid derived by Chemical reaction from inorganic Minerals as opposed to Organic acids These have Hydrogen An organic acid is an Organic compound with Acidic properties A Strong acid is an Acid that Ionizes completely in an Aqueous solution (not in the case of Sulfuric acid as it is diprotic A superacid is an Acid with an Acidity greater than that of 100% Sulfuric acid, which has a Hammett acidity function ( H 0 A weak acid is an Acid that does not completely donate all of its hydrogens when dissolved in water In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and An organic base is an Organic compound which acts as a base. Organic bases are usually but not always proton acceptors In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and In Chemistry, a superbase is an extremely strong base. There is no commonly accepted definition for what qualifies as a superbase but most chemists would accept As the name suggests a non-nucleophilic base is an organic base that is a very Strong base but at the same time a poor Nucleophile. A chemical substance is a Material with a definite chemical composition. In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and Ionization is the physical process of converting an Atom or Molecule into an Ion by adding or removing charged particles such as Electrons As Bronsted-Lowry bases are proton acceptors, a weak base may also be defined as a chemical base in which protonation is incomplete. In chemistry protonation is the addition of a proton ( H[[Cation +]] to an Atom, Molecule, or Ion. This results in a relatively low pH level compared to strong bases. pH is the measure of the acidity or alkalinity of a Solution. In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and 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:

$\mbox{pH} = -\log_{10} \left[ \mbox{H}^+ \right]$

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. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive In Chemistry, concentration is the measure of how much of a given substance there is mixed with another substance 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.

$\mbox{pOH} = -\log_{10} \left[ \mbox{OH}^- \right]$

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

$K_a \times K_b = {[H_3O^+][NH_3]\over[NH_4^+]} \times {[NH_4^+][OH^-]\over[NH_3]} = [H_3O^+][OH^-]$

Since $K w = [H 3 O +][O H -]$ then, $K_a \times K_b = K_w$

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

l o g K a + l o g K b = l o g K w

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

p K a + p K b = p K w = 14. 00

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. In a Chemical process, chemical equilibrium is the state in which the chemical activities or Concentrations of the reactants and products have no net change A weak acid is an Acid that does not completely donate all of its hydrogens when dissolved in water For example, when ammonia is put in water, the following equilibrium is set up:

$\mathrm{K_b={[NH_4^+][OH^-]\over[NH_3]}}$

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. The self-ionization of water (also autoionization of water, and autodissociation of water) is the chemical reaction in which two water molecules react to produce a ) 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). Diethylamine is a Secondary amine with the molecular structure CH3CH2NHCH2CH3 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.

1 Percentage protonated
2 A typical pH problem
3 Examples
4 See also
5 References
6 External links

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:

$B(aq) + H_2O(l) \leftrightarrow HB^+(aq) + OH^-(aq)$

B represents the base.

$Percentage\ protonated = {molarity\ of\ HB^+ \over\ initial\ molarity\ of\ B} \times 100\% = {[{HB}^+]\over [B]_{initial}} {\times 100\%}$

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:

$\mathrm{H_2O(l) + C_5H_5N(aq) \leftrightarrow C_5H_5NH^+ (aq) + OH^- (aq)}$

$K_b=\mathrm{[C_5H_5NH^+][OH^-]\over [C_5H_5N]}$

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	$K_b=\mathrm {1.8 \times 10^{-9}} = {x \times x \over .20-x}$
We can assume that x is so small that it will be meaningless by the time we use significant figures.	$\mathrm {1.8 \times 10^{-9}} \approx {x^2 \over .20}$
Solve for x.	$\mathrm x \approx \sqrt{.20 \times (1.8 \times 10^{-9})} = 1.9 \times 10^{-5}$
Check the assumption that x << . 20	$\mathrm 1.9 \times 10^{-5} \ll .20$ ; so the approximation is valid
Find pOH from pOH = -log [OH^-] with [OH^-]=x	$\mathrm pOH \approx -log(1.9 \times 10^{-5}) = 4.7$
From pH = pK_w - pOH,	$\mathrm pH \approx 14.00 - 4.7 = 9.3$
From the equation for percentage protonated with [HB⁺] = x and [B]_initial = . 20,	$\mathrm percentage \ protonated = {1.9 \times 10^{-5} \over .20} \times 100\% = .0095\%$

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. Alanine (abbreviated as Ala or A) is an α- Amino acid with the Chemical formula HO2CCH(NH2CH3 Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor Methylamine is the Chemical compound with a formula of CH3NH2 Pyridine is a Chemical compound with the formula C5[[Hydrogen H5]] N. In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and

References

Atkins, Peter, and Loretta Jones. In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and A weak acid is an Acid that does not completely donate all of its hydrogens when dissolved in water Chemical Principles: The Quest for Insight, 3rd Ed. , New York: W. H. Freeman, 2005.

External links

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Contents

Percentage protonated

A typical pH problem

Examples

See also

References

External links