Chapter 16-17: Acid-Base Equilibria
Base-dissociation constant
(Kb) The ratio of the concentration of the conjugate acid times the concentration of the hydroxide ion to the concentration of the conjugate base.
Common-ion effect
A decrease in the solubility of an ionic compound caused by the addition of a common ion For instance, take the following two equations as an example: NaCH3COO -> Na+ + CH3COO- CH3COOH <-> H+ + CH3COO- Given those two equations, notice they share the acetate (CH3COO-) ion. Therefore, in a solution of acetic acid dissolved in water, consider what may happen if sodium acetate is added. Following Le Chatelier's Principle, adding sodium acetate would mean more production of the acetate ion. If the concentration of the acetate ion increases, this forces the reaction for acetic acid to shift left, decreasing H+ production and lowering ionization rate. In general, if a weak electrolyte and a strong electrolyte are together in a solution and contain a common ion, the weak electrolyte will ionize less than it normally would if alone in the solution.
pH titration curve
A graph of pH as a function of added titrant volume
Brønsted-Lowry base
A molecule or ion that is a proton acceptor, working in conjunction with the acid which provides the proton to accept.
Titration
A procedure in which one reactant is slowly added into a solution of another reactant, while equilibrium concentrations along the way are monitored. The benefits of such a procedure are to identify the concentration of one of the reactants or to find the equilibrium constant for the reaction.
What occurs in an acid-base titration?
A solution of known concentration of base is slowly added to an acid or vice versa until an indicator changes color at the equivalence point.
Amphiprotic
A species that can either accept or donate a proton. It acts as a base when combined with something more strongly acidic than itself and as an acid when combined with something more strongly basic than itself. An example would be water, as it can act as either an acid or a base depending on the reaction.
Brønsted-Lowry acid
A substance that donates a proton (H+ ion) to another substance
Arrhenius acid
A substance that, when dissolved in water, increases the concentration of H+ ions in the solution.
Arrhenius base
A substance that, when dissolved in water, increases the concentration of OH- ions.
When [H+] > [OH-], the solution is
Acidic
Strong acid
An acid that completely transfers its protons in water, leaving essentially no undissociated molecules in solution. Its conjugate base will have negligible basicity (negligible tendency to accept protons in aqueous solution).
Negligible acid
An acid that contains hydrogen but does not demonstrate any acidic behavior in water. The conjugate base is strong and reacts completely with water to form OH- ions.
Polyprotic acid
An acid that has more than one ionizable H atom.
Weak acid
An acid that only partially dissociates in aqueous solution and therefore exists in the solution as a mixture of undissociated acid and its conjugate base. The conjugate base will be weak.
How is the value of Ka related to acid strength?
As Ka increases, the strength of the acid increases
What is the relationship between [H+] and pH?
As [H+] increases, pH decreases and becomes more acidic
What is the relationship between [OH-] and pH?
As [OH-] increases, pH increases and becomes more basic
What is the trend in Ka values for a polyprotic acid?
As more H+ ions are removed, Ka becomes successively smaller since the electronegative ions that retain H+ are more difficult to extricate from
When [H+] < [OH-], the solution is
Basic
Properties of bases
Butter taste, slippery feel, turns litmus paper from red to blue, pH is greater than 7
How do cations influence pH?
Cations are conjugate acids of weak bases and form hydronium ions in water, decreasing pH.
How is percent ionization calculated?
Concentration of ionized HA divided by original concentration of HA times 100
Conjugate acid-base pair
Consists of two substances related to each other by the donating and accepting of a single hydrogen ion
What is true of directionality in every acid-base reaction?
Equilibrium favors transfer of the proton from the stronger acid to the stronger base to form the weaker acid and the weaker base. Given HA + H2O <-> H3O+ + A- Assuming HA is the strong acid, A- must have negligible basicity. H2O is more basic than A- so the reaction is forced to the right. If HA is weak, then A- will be more basic than H2O, forcing the reaction left.
Strong acid-strong base titration curve
Equivalence point is 7. It is important to recognize that initial pH is very low. At the molecular level, only the acid is present initially. When the base is added, the H+ ions from the acid are consumed by the OH- ions, leaving a solution of entirely just cations from the base, some of the acid is left, and the conjugate base from the acid. At the equivalence point, only the conjugate base and the cation from the strong base are left. After the equivalence point, some OH- ions will then be introduced into the solution. Note that, often, if the cation and anion are present they may form a salt due to neutralization.
Weak acid-strong base titration curve
Equivalence point is greater than 7, has a halfway equivalence point, change near the equivalence point is smaller than in string acid-strong base titrations
Hydronium ions
H3O+ ions, which form when an acid dissolves in water and H+ ions interact with water. Recall that a water molecule has two unshared pairs of electrons, so its high electronegativity is responsible for the attraction between the protons from the acid and itself.
If HCl is dissociating in water, it must actually react with water. Model an equation for this.
HCl + H2O -> Cl- + H3O+
Notice that, since the Brønsted-Lowry definitions are not contingent on aqueous states, we can model other equations with substances in other phases. Write an equation for the reaction between HCl and NH3 and determine bases and acids.
HCl + NH3 -> NH4+ + Cl- HCl donates its proton to NH3, making HCl the acid and NH3 the base. NH4+ is, therefore, the conjugate acid and Cl- is the conjugate base.
Using the Brønsted-Lowry definition of acids and bases, determine the acid and base in the following reaction: HCl + H2O -> Cl- + H3O+
HCl is the acid because it donates a proton to H2O. This means that, by accepting the proton, H2O must be the base. Therefore, Cl- is the conjugate base and H3O+ is the conjugate acid.
What are the strong acids?
HCl, HBr, HI, HNO3, H2SO4, HClO4, HClO3
What are the seven most common strong acids?
HCl, HBr, HI, HNO3, HClO3, HClO4, H2SO4
What charged molecule forms as H+ ions are added to an aqueous solution?
Hydronium ions
How do anions that are conjugate bases of an acid affect pH?
If the acid is strong, then the conjugate base is negligible in basicity. If the acid is weak, the anion's contribution to forming OH- must be represented.
How can the concentration of H+ be determined for a strong acid in aqueous solution?
It is equal to the initial concentration of acid. For instance, if the solution was a 0.20 M solution of HNO3, there are 0.20 M of H+ and 0.20 M of NO3-.
How can the concentration of OH- be determined for a strong base in aqueous solution?
It is equal to the initial concentration of base. For instance, if the solution was a 0.30 M solution of NaOH, there are 0.30 M of Na+ and 0.30 M of OH-. Note, this will vary for the alkaline earth metals where the concentration of OH- is two times the initial concentration of base.
Acid-dissociation constant
Ka = [H+][A-]/[HA], represents dissociation of weak acids
How might the ion-product constant for water be calculated given the acid and base dissociation constants?
Ka x Kb = Kw
What is the formula for Kb?
Kb = [HB+][OH-]/[B]
What is the equilibrium constant for the autoionization of water?
Kc = [H3O+][OH-]
What are the strong bases?
LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2 OH- bonded to an alkali metal or an alkaline earth metal Or even O-2 ionic metal compounds like CaO
What are the strong bases?
LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2, O2-, H-, and CH3-
The conjugate base of a strong acid will have
Negligible basicity
When the [H+] = [OH-] the solution is
Neutral
How do HCl and NaOH differ in terms of their Arrhenius definitions?
Since HCl dissociates into H+ and Cl- while NaOH dissociates into Na+ and OH- in water, it is evident that HCl must be an Arrhenius acid and NaOH must be an Arrhenius base for their contributions of H+ and OH- to the solution, respectively.
Properties of acids
Sour taste, ability to dissolve many metals and form hydrogen gas, ability to neutralize bases, change blue litmus paper to red, pH is less than 7
The strength of the conjugate base of an acid with negligible acidity is
Strong
Strong electrolytes
Strong acids or bases that exist in aqueous solution entirely as ions.
If the reaction is favored to the right, what is the general Kc value?
The equilibrium constant is greater than 1
If the reaction is favored to the left, what is the general Kc value?
The equilibrium constant is less than 1
How can percent ionization represent strength of an acid?
The greater the percent ionization, the greater the strength of the acid
How are significant figures determined in a logarithm?
The numbers to the right of the decimal point are the significant figures in a logarithm. The number of significant figures in the original concentration is the number of decimal places for the pH.
Leveling effect
The observation that strong acids all have the same strength in water and are completely converted into solutions of H3O+ ions; strong bases are likewise leveled in water and are completely converted into solutions of OH- ions
What occurs if the salt contains an anion and a cation both capable of reacting with water?
The pH depends on the relative abilities of the ions to react with water
Conjugate acid
The particle formed when a base gains a hydrogen ion
Conjugate base
The particle that remains when an acid has donated a hydrogen ion
Percent ionization
The percent of a substance that undergoes ionization on dissolution in water. The term applies to solutions of weak acids and bases and represents the strengths of each.
Half equivalence point
The point in a titration at which exactly half the molar equivalence of reactant is consumed by the titrant being added. At this point in an acid-base titration, the pKa of the unknown solution is revealed. This can be found by dividing the volume of titrant at the equivalence point by 2 and finding the pH at that volume.
Equivalence point
The point in a titration where the number of moles of hydrogen ions equals the number of moles of hydroxide ions
Ion-product constant for water
The product of the concentrations of hydrogen ions and hydroxide ions in water Kw = [H3O+ or H+][OH-] At 25 degrees Celsius, Kw = 1.0 x 10^-14
How does the titration for a weak acid-strong base differ from a strong acid-strong base?
The solution of the weak acid has a higher initial pH than a solution of a strong acid of the same concentration. The pH change in the rapid-rise portion of the curve near the equivalence point is smaller for the weak acid than for the strong acid. The pH at the equivalence point is above 7.00 for the weak acid titration.
What is the relationship between the strength of an acid or base and it's conjugate base or acid?
The stronger an acid, the weaker its conjugate base and the stronger a base, the weaker its conjugate acid. In other words, the more easily a base accepts a proton, the less easily its conjugate acid gives up the proton.
How can conjugate acid-base pairs be identified?
There are two methods: 1. When an acid donates a proton, the substance left over after H+ is donated is the conjugate base. When a base accepts a proton, the substance left over with an extra H+ is the conjugate acid. 2. Starting on the right side (products side) of the equation, work backwards, the substance that donates a proton to form a reactant that is the base is the conjugate acid. The product that receives that proton forms the acid on the reactants side and is the conjugate base. Since these reactions go both ways in a state of equilibrium, they may be analyzed in both ways. For reference, consider HNO2 + H2O -> NO2- + H3O+ where HNO2 is the acid, H2O is the base, NO2- is the conjugate base, and H3O+ is the conjugate acid. Clearly, the reaction works both ways.
The conjugate base of a weak acid will be a
Weak base
Titrations of polyprotic acids
When a polyprotic acid is titrated with a base, there is an equivalence point for each dissociation. Using the henderson-hasselbalch equation we can see that half way to each equivalence point gives us the pKa for that step
Hydrolysis
When ions react with water to form H+ or OH- ions
Autoionization of water
When pure water reacts with itself to form hydronium and hydroxide ions
What is the formula for percent ionization?
[H+] equilibrium/[HA] initial times 100
What expression may be used to model pKw?
pH + pOH = 14.00
What expression can be used to model pH?
pH = -log[H+]
What occurs if the salt contains a cation that reacts with water to produce hydronium ions and an anion that does not react with water?
pH is acidic Cation is conjugate acid of a weak base or has a charge of +2 or greater
What occurs if the salt contains an anion that reacts with water to produce hydroxide ions and a cation that does not react with water?
pH is basic Occurs when the anion is the conjugate base of a weak acid and the cation is from group 1A or is Ca+2, Sr+2, or Ba+2
What occurs if a salt contains an anion that does not react with water and a cation that does not react with water?
pH is neutral This occurs when the anion is a conjugate base of a strong acid and the cation is form group 1A or is Ca+2, Sr+2, or Ba+2.
How are pKa and pKb related?
pKa + pKb = 14 = pKw
What is pKa and pKb equal to?
pKa = -logKa and pKb = -logKb
What expression can be used to model pOH?
pOH = -log[OH-]