Chapter 2 Flashcards
Why can water accept a proton?
because it has two lone pairs, and either one can form a covalent bond with the proton.
Why can Cl- accept a proton?
because any one of its four lone pairs can form a covalent bond with a proton.
According to the Brønsted–Lowry definitions, what does any species with a hydrogen potentially act as?
an acid
According to the Brønsted–Lowry definitions, what does any species with a lone pair potentially act as?
a base
Why must both an acid and a base be present in an acid–base reaction?
because an acid cannot lose a proton unless a base is present to accept it.
Are most acid-base reactions reversible or not
reversible
How are reversible reactions represented in acid–base reactions?
by using two half-headed arrows.
What is the term for the species that results when an acid loses a proton?
the conjugate base of the acid.
What is the conjugate base of HCl?
Cl-
What is the term for the species that results when a base gains a proton?
the conjugate acid of the base
What is the conjugate acid of Cl-?
HCl
What is the conjugate base of H3O+?
H2O
What is the conjugate acid of H2O?
H3O+
In the reaction between ammonia and water, which substance is considered a base? Why?
water because it loses a proton.
In the reverse reaction between ammonium ion (+NH4) and hydroxide ion (OH-), which substance acts as an acid? Why?
Ammonium ion (+NH4) because it loses a proton.
Are the arrows for acid-base reactions the same length?
no
How is acidity and basicity defined in terms of affinity for a proton/proton loss?
*acidity is a measure of the tendency of a compound to lose a proton
*basicity is a measure of a compound’s affinity for a proton
What characterizes a strong acid versus a weak acid in terms of proton loss?
*A strong acid has a strong tendency to lose a proton.
*A weak acid has little tendency to lose its proton.
What characterizes the conjugate base of a strong acid?
weak because it has little affinity for the proton.
What characterizes the conjugate base of a weak acid? Why?
strong because it has a high affinity for the proton.
What relationship exists between the strength of an acid and the strength of its conjugate base?
The stronger the acid, the weaker its conjugate base.
Provide an example illustrating the relationship between an acid and its conjugate base.
HBr is a stronger acid than HCl, so Br- is a weaker base than Cl-.
What happens when a strong acid like hydrogen chloride is dissolved in water?
*almost all the molecules dissociate
*the products are favored at equilibrium
*the equilibrium lies to the right
What is the result when a much weaker acid, such as acetic acid, is dissolved in water?
*very few molecules dissociate
*the reactants are favored at equilibrium
*the equilibrium lies to the left
How is the equilibrium position indicated in terms of arrows?
A longer arrow is drawn toward the species favored at equilibrium.
What does Keq indicate about the dissociation of an acid (HA) in an aqueous solution?
Keq indicates the degree to which an acid (HA) dissociates in an aqueous solution.
In what type of solution is the degree of dissociation of an acid (HA) typically determined?
in a dilute solution.
Why is the concentration of water considered essentially constant in this context?
because the solution is dilute. (Always more water than everything else)
What is the result of combining the equilibrium constants Keq and H2O?
it allows the equilibrium expression to be rewritten using a new equilibrium constant, Ka, called the acid dissociation constant.
How do you get Ka from Keq of water?
the acid dissociation constant is the equilibrium constant multiplied by the molar concentration of water (55.5 M).
What does a larger acid dissociation constant indicate about the strength of an acid?
The larger the acid dissociation constant, the stronger the acid—that is, the greater its tendency to lose a proton.
How does the acid dissociation constant of hydrogen chloride compare to that of acetic acid?
Hydrogen chloride, with an acid dissociation constant of 10^7, is a stronger acid than acetic acid, with an acid dissociation constant of 1.74 * 10^-5.
What is used for convenience to indicate the strength of an acid?
by its pKa value rather than its Ka value.
How do you calculate pKa?
pKa = -log Ka
the ______ the acid, the _____ its pKa value.
the stronger the acid, the smaller its pKa value.
What does pH indicate?
The concentration of proton in a solution. [H+] or [H3O+]
equation of pH
pH=-log[H+]
What’s the difference between pH and pKa?
*The pH scale is used to describe the acidity of a solution, whereas the pKa indicates the tendency of a compound to lose its proton.
*Thus, the pKa is characteristic of a particular compound, much like a melting point or a boiling point.
What is the most common type of organic acids, and what functional group characterizes them?
carboxylic acids, characterized by having a COOH group.
Can you provide examples of carboxylic acids?
acetic acid and formic acid.
What is the typical range of pKa values for carboxylic acids, and what does this indicate about their acidity?
Carboxylic acids have pKa values ranging from about 3 to 5, indicating that they are weak acids.
What characterizes amines, and how do they typically behave in terms of acidity and basicity?
*Amines result from replacing one or more hydrogens bonded to ammonia with a carbon-containing substituent.
*They usually behave as bases rather than acids due to their high pKa values and are the most common organic bases.
*they are the most common organic bases
How can the strength of a base be assessed, and what does it involve considering?
by considering the strength of its conjugate acid, since the stronger the acid, the weaker its conjugate base.
What are protonated compounds, and how does their acidity compare (pka values)?
*Protonated compounds are compounds that have gained an additional proton.
*Protonated alcohols and protonated carboxylic acids are very strong acids, with pKa values less than 0.
What characterizes alcohols, and what functional group do they have?
compounds that have an OH group.
How do the acidity levels of alcohols compare to carboxylic acids, and what is their typical range of pKa values?
Alcohols are much weaker acids than carboxylic acids, with pKa values close to 16.
Can you provide examples of alcohols?
methyl alcohol and ethyl alcohol.
Why is the double bond on the protonated oxygen?
bcs it requires the proton
O and C are attached together by a double bond and O is more E than C (temporary negative charge on O) =more apt for a proton
Can alcohols behave both like an acid and a base?
yes, Alcohols:
*an acid and lose a proton
*as a base and gain a proton
Why do chemists use curved arrows in chemical reactions, and how are they distinguished from straight arrows?
*to indicate the bonds broken and formed in chemical reactions.
*They are called curved arrows to distinguish them from the straight arrows used to link reactants with products in the equation for a chemical reaction.
What does each curved arrow with a two-barbed arrowhead signify?
the movement of two electrons.
In an acid–base reaction, where does one of the arrows typically point, and what does it signify?
one of the arrows is drawn from a lone pair on the base to the proton of the acid. It signifies the donation of electrons from the base to the proton.
What does a second arrow in an acid–base reaction typically represent, and where does it point?
it is drawn from the electrons that the proton shared to the atom on which they are left behind. It signifies the movement of electrons to the atom where they remain.
What is the purpose of curved arrows in chemical reactions?
Curved arrows allow you to follow the electrons in a reaction, showing which bond is broken and which bond is formed.
Can a carboxylic acid act both as an acid and as a base?
Yes, a carboxylic acid can behave as both an acid (lose a proton) and as a base (gain a proton).
Can an amine behave both like an acid and a base?
yes
How can a carboxylic acid behave in terms of acidity and basicity? acid, base or both
both
Name the approximate pKa values of various compounds
It’s important to know pKa values for various compounds. They are remembered in increments of five, as shown in Table 2.1.
What are the pKa values for protonated alcohols, protonated carboxylic acids, and protonated water?
less than 0
What are the pKa values for carboxylic acids?
approximately 5.
What are the pKa values for protonated amines?
approximately 10.
What are the pKa values for alcohols and water?
approximately 15.
What is the key question in determining the most basic atom in a compound when acid is added to a solution?
Which atom of the compound is more apt to be protonated when an acid is added to a solution?
What strategy is suggested for solving the problem of determining the most basic atom in a compound?
*to look at the pKa values of the conjugate acids of the groups.
*Remember that the weaker acid has the stronger conjugate base, and the stronger base is more apt to be protonated.
Why is it recommended to consider the pKa values of conjugate acids when determining the most basic atom?
It helps identify the weaker acid, which in turn indicates the stronger conjugate base. The stronger base is more likely to be protonated.
In this question, which atom is the most apt to be protonated?
The conjugate acids have pKa values of ∼0 and ∼10. Because the + NH3 group is the weaker acid, the NH2 group is the stronger base, so it is the group more apt to be protonated.
For acids, higher pKa value means…
weaker the acid
What is the first step in predicting the outcome of an acid–base reaction
compare the pKa values of the reactants to determine which is the acid.
How is the outcome of an acid–base reaction predicted when comparing pKa values?
The reactant with the lower pKa value is predicted to be the acid, while the one with the higher pKa value is predicted to be the base.
How is the equilibrium position determined in terms of pKa values?
The equilibrium favors the formation of the weaker acid, which is the acid with the higher pKa value.
What is the equilibrium constant (Keq) for the reaction of acetic acid with ammonia, and how is it calculated?
pKeq = 4.8 - 9.4 = -4.6
Keq = 10^(-4.6) = 4.0 x 10^4
How is the stability of the conjugate base related to the acidity of the acid?
The strongest acid has the most stable (weakest) conjugate base.
How can the effect of electronegativity on acidity be observed when comparing alcohols and amines? Which one more acidic and why?
*Comparing the pKa values of alcohols and amines, the more electronegative atom bonded to hydrogen (oxygen in alcohols) makes the compound more acidic.
*Therefore, alcohols are more acidic than amines.
Are protonated alcohols or amines more acidic?
Again, because oxygen is more electronegative than nitrogen, a protonated alcohol is more acidic
than a protonated amine.
How does the electronegativity of an sp hybridized atom compare to an sp2 hybridized atom, and to an sp3 hybridized atom?
An sp hybridized atom is more electronegative than the same atom that is sp2 hybridized, and an sp2 hybridized atom is more electronegative than the same atom that is sp3 hybridized.
How does the acidity of ethyne compare to ethene and ethane, and what determines their acidity?
Ethyne is a stronger acid than ethene, and ethene is a stronger acid than ethane. The acidity is determined by having the hydrogen attached to the most electronegative atom.
How is the electron richness of the strongest base reflected in electrostatic potential maps?
The electrostatic potential maps show that the strongest base (the least stable) is the most electron-rich (the most red).
Which hydrogen halide is identified as the strongest acid, and why, despite the electronegativity trend in halogens?
pka values of common acids to learn by heart
HI is the strongest acid among the hydrogen halides because I- is the weakest, most stable base, even though iodine is the least electronegative of the halogens.
What is the determining factor for the strength of an acid when atoms are very different in size?
When atoms are very different in size, the strongest acid has its hydrogen attached to the largest atom.
Why is HI identified as the strongest acid among the hydrogen halides, and what ion contributes to its strength?
HI is the strongest acid among the hydrogen halides because I- is the most stable halide ion. The larger size of I- contributes to its increased stability.
What effect does inductive electron withdrawal have on the electron density around the oxygen carrying the negative charge in the conjugate base of a carboxylic acid?
It decreases the electron density around the oxygen carrying the negative charge in the conjugate base of a carboxylic acid, thereby stabilizing it.
How does the stabilization of the conjugate base influence the acidity of the carboxylic acid?
Stabilizing the conjugate base increases the acidity of the carboxylic acid.
Why is the fluoro-substituted compound identified as the strongest acid among the carboxylic acids mentioned?
The fluoro-substituted compound is the strongest acid because its conjugate base is the most stabilized by inductive electron withdrawal.
How does the distance between a substituent and the acidic proton affect the acidity of a compound, as illustrated below?
The effect a substituent has on the acidity of a compound decreases as the distance between the substituent and the acidic proton increases.
What is the typical pKa value for a carboxylic acid, and how does it compare to the pKa value of an alcohol?
*pKa carboxylic acid: approximately 5
pKa alcohol: around 15
*This indicates that carboxylic acids are much stronger acids compared to alcohols.
What structural difference between the conjugate bases of carboxylic acids and alcohols is responsible for inductive electron withdrawal, and how does it impact acidity?
*conjugate base of a carboxylic acid has a doubly bonded oxygen
*conjugate base of an alcohol has two hydrogens.
The presence of the doubly bonded oxygen in the carboxylic acid’s conjugate base induces electron withdrawal through inductive effects.
This reduces the electron density on the negatively charged oxygen, stabilizing the conjugate base and ultimately increasing the acidity of the carboxylic acid compared to an alcohol.
What are localized electrons? Where does the negative charge reside in the context of alcohols?
When an alcohol loses a proton, the electrons left behind reside on its single oxygen atom. These electrons are said to be localized because they belong to only one atom.
only on the oxygen atom
What does decreasing the electron density of an atom do to it? Explain how this is in link to the fact that CB of carboxylic acids are more stable than alcohols’.
stabilizes it
since alcohols’ oxygen’s negative charge is all carried by itself whereas in carboxylic acids, it is shared between three atoms, two oxygens and the carbon.
What are resonance contributors, and why do they not individually represent the actual structure of the conjugate base of a carboxylic acid?
Resonance contributors are two structures representing the potential arrangements of electrons in the conjugate base of a carboxylic acid. However, neither contributor alone represents the actual structure; instead, the actual structure is a resonance hybrid, a composite of the two contributors.
How do the two resonance contributors of the carboxylic acid’s conjugate base differ, and what is the significance of the double-headed arrow between them?
The two resonance contributors differ only in the location of their π electrons and lone-pair electrons, while all the atoms stay in the same place. The double-headed arrow signifies that the actual structure is a resonance hybrid, combining elements from both contributors.
What is the significance of the resonance hybrid in terms of electron distribution in the carboxylic acid’s conjugate base?
that the negative charge is distributed over both oxygens and the carbon. This delocalization of electrons contributes to the stability of the conjugate base.
How do you determine the site of protonation in a compound with delocalized electrons?
1) determine which atom in the compound is more apt to be protonated
(when a compound has two basic atoms, the one that is the stronger base is the one more apt to be protonated in an acidic solution. When a compound has delocalized electrons, you need to draw its resonance contributors to determine which atom is the stronger base)
2) One of the resonance contributors has a negative charge on one of the oxygens and a positive charge on the other.
Therefore, those oxygens in the resonance hybrid have a partial negative charge and a partial positive charge, respectively. The oxygen with the partial negative charge is the stronger base (proton acceptor), so it is the oxygen that is more apt to be protonated in an acidic solution.
Explain how the size and electronegativity of the atom to which the hydrogen is attached and affect the strength of an acid
*As the atom attached to the hydrogen increases in size (going down a column of the periodic table), the strength of the acid increases.
*Electronegativity:As the atom attached to the hydrogen increases in electronegativity (going from left to right across a row of the periodic table), the strength of the acid increases.
Explain how the hybridization of the hydrogen to which the hydrogen is attached affect the strength of an acid
The electronegativity of an atom changes with hybridization as follows:
sp > sp2 > sp3. Because an sp carbon is the most electronegative, a hydrogen attached to an sp carbon is the most acidic, and a hydrogen attached to an sp3 carbon is the least acidic.
Explain how inductive electron withdrawal affect the strength of an acid
An electron-withdrawing group increases the strength of an acid. As the electronegativity of the electron-withdrawing group increases or as it moves closer to the acidic hydrogen, the strength of the acid increases.
Explain how electron delocalization affect the strength of an acid
An acid whose conjugate base has delocalized electrons is more acidic than a similar acid whose conjugate base has only localized electrons.
What is the Henderson-Hasselbalch equation?
Why is the H-H equation very important? And what does it tell us?
because it tells us whether a compound exists in its acidic form (with its proton retained) or in its basic form (with its proton removed) at a particular pH.
It tells us that compounds exist primarily in their acidic forms in solutions that are more acidic than their pKa values and primarily in their basic forms in solutions that are more basic than their pKa values.
*when the pH of a solution equals the pK of the compound that undergoes dissociation, the a
pound in its basic form 1A 2 (because log 1 = 0).
*when the pH of the solution is less than the pK of the compound, the compound exists pri-
concentration of the compound in its acidic form (HA) equals the concentration of the com-
*when the pH of the solution is greater than the pKa of the compound, the compound exists primarily in its basic form.
How do you determine the structure of a compound (acidic or basic form) at a particular pH?
example:
*compare the pH of the solution with the pKa of the compound’s dissociable proton
*a. The pH of the solution is more acidic (5.5) than the pKa value of the compound (15.9). Therefore, the compound exists primarily as CH3CH2OH (with its proton).
b. The pH of the solution is more basic (5.5) than the pKa value of the compound (-2.5). Therefore, the compound exists primarily as CH3CH2OH (without its proton).
c. The pH of the solution is more acidic (5.5) than the pKa value of the compound (11.0). Therefore, the compound exists primarily as CH3N+ H3 (with its proton).
What happens to the distribution of a compound when the pH of the solution is one unit less than the compound’s pKa(= 5.2)(pH = 4.2)?
there is 10 times more compound present in the acidic form than in the basic form, as represented by the logarithmic relationship (log 10 = 1).
What happens to the compound’s distribution when the pH of the solution is greater than the pKa?
there is more compound in the basic form than the acidic firm
What is the modified H-H equation
What type of solution are these?
buffer solutions
What is the purpose of buffers? How?
maintain a constant pH when acids or bases are added to a solution
because the weak acid can give a proton to any HO- added to the solution and its conjugate base can accept any H + that is added to the solution.
What is Lewis definition of acids and bases?
an acid as a species that accepts a share in an electron pair and a base as a species that donates a share in an electron pair.
What else than compounds that lose protons are considered as lewis acids?
Compounds such as alu-minum chloride (AlCl3), ferric bromide (FeBr3), and borane (BH3) because they have unfilled valence orbitals that can accept a share in an electron pair.
These compounds react with a compound that has a lone pair, just as a proton does
