Name: __________ _________________________
CHEM 1212 Module 9 Reading Guide
Weak Acids and Bases
The relative strength of an acid or base is the extent to which it _____________ when dissolved in water. If the ionization reaction is essentially complete, the acid or base is termed _____________; if relatively little ionization occurs, the acid or base is _____________. Most acids are _____________ acids. Since the ionization is not complete, weak acids and bases are _____________ systems and the degree of ionization can be determined using equilibrium constants.
The ionization constants Ka and Kb are defined as the equilibrium constants for the ionization of a weak acid or weak base when it is dissolved in _____________. Ka values can be compared to determine which of two acids is _____________. A _____________ value of Ka indicates that the reaction is more favorable toward the products and will have a _____________ concentration of H3O+ and A? relative to the concentration of the nonionized acid, HA, in an equilibrium mixture. The ionization constant expression for the ionization of an acid or base does not include a term for the concentration of _____________ because pure liquids are never included in equilibrium-constant expressions.
Another measure of the strength of an acid is its _____________. In most cases of weak acid or weak base ionization, the extent of ionization is so small that the concentration of acid or base lost to ionization is a negligible amount (less than about 5% of the initial concentration). A comparison is made between the amount of _____________ ions produced at equilibrium and the maximum amount that could be produced if the acid ionized completely.
As equilibrium systems, determination of the pH of a weak acid or base will require an ICE table to write expressions for the _____________ concentrations. For an acid the equilibrium concentration of _____ will be x and pH = -log x. If the ionization of the weak acid is sufficiently small, you will be able to make the x is small approximation, otherwise you will need to solve a quadratic equation.
Weak bases are approached the same way. A _____________ Kb value indicates a stronger base, meaning it is better able to take on _____________ ions.
For a base the equilibrium concentration of _____ will be x and pOH = -log x.
It is not necessary to memorize any Ka values. These are found in tables of acid ionization constants. Sometimes pKa is reported instead of Ka, where . If comparing pKa values, _____________ pKa value means a stronger acid.
The strength of an acid and its conjugate base are easily related using the ionization constants, Ka x Kb = Kw. The inverse proportional relation between Ka and Kb means the stronger the acid or base, the _____________ its conjugate partner. This leads to the conjugate base of a strong acid not behaving as a base at all, because it is not strong enough to ionize with water. Hydronium ion is the strongest acid that may exist in water; any stronger acid will react completely with water to generate _____________ ions.
Practice Problems
47. Calculate [H3O+] and pH for a 0.317 M solution of HF; Ka = 6.8 × 10?4.
53. A 0.200 M solution of a weak base, represented as B, has a hydroxide ion concentration of 6.74 × 10?5 M. Calculate the value of Kb for B.
Polyprotic acids/bases
A _____________ acid is an acid that has two or more ionizable hydrogen atoms per formula unit. Each ionization has its own ionization constant, where each subsequent ionization has a much _____________ ionization constant than does the prior ionization for two reasons: (1) The prior ionization produces hydronium ions that repress further ionization, in accordance with _____________ principle, and (2) the conjugate base that results from the loss of the first ionizable hydrogen is usually a _____________ ion, and it is more difficult to remove a positive H+ from a negative ion. The hydrogen ions in a solution of a polyprotic acid therefore come mainly from the _____________ step in the ionization. Determining concentrations at equilibrium can be difficult for polyprotic acids unless the Ka values are several orders of magnitude apart in strength. Under those conditions the x is small approximation can be used with each ICE table to make it manageable.
Practice Problems
59. Oxalic acid, H2C2O4, is diprotic.
Write the two ionization reactions for oxalic acid.
Write the Ka1 and Ka2 expressions for oxalic acid.
Lewis acids
A third definition for acids and bases in the Lewis definition. This _____________ the scope of compounds that can be classified as acids or bases. A Lewis base is an electron pair _____________, and a Lewis acid is an electron pair _____________. A Brønsted acid refers to the entire molecule that is a _____________ donor such as HCl, but a Lewis acid is the donated proton itself. All Brønsted bases are also _____________ bases. Brønsted acids must be proton donors, but Lewis acids do not have to be able to donate a _____________, so Lewis acids include many species that are not Brønsted acids. Lewis acids must be able to accept an _____________, so Lewis acids must have an _____________ valence shell.
The reaction of a Lewis acid and a Lewis base forms a Lewis _____________, which is a combination of the acid and base as a single compound. In a Lewis adduct, a covalent bond forms in which both shared electrons come from _____________. This type of bond is called a _____________ covalent bond.
Another type of Lewis acid-base chemistry involves the formation of a complex ion (or a coordination complex) comprising a central atom, typically a transition metal cation, surrounded by ions or molecules called ligands. These ligands can be neutral molecules like H2O or NH3, or ions such as CN or OH. Often, the ligands act as Lewis bases, donating a pair of _____________ to the central atom.
Practice Problems
85. Classify the following species as Lewis acids or Lewis bases.
a. NH3
b. BH3
c. Al3+
d. S2?
e. CO2
Structural effects on acid strength
The stronger the chemical bond is that holds a hydrogen atom to the rest of the molecule, the _____________ likely that bond is to break and form H+ ionstherefore, the _____________ acidic the compound will be. There are two main considerations when comparing the relative acid strength of two similar compounds: the _____________ of the anion in a binary acid and the _____________ of the environment around the hydrogen atom.
As the anion size _____________ going down a group on the periodic table, so does the acid strength of the compound. The more _____________ the bond between hydrogen and its bonding partner, the more easily ionizable the hydrogen atom will be. Across a row in the periodic table, the acid strength of binary hydrogen compounds _____________ with increasing electronegativity of the nonmetal atom because the polarity of the H-A bond _____________. Hydrogen atoms in nonpolar bonds are not ionizable. Increasing the oxidation number of the central atom of an oxyacid increases the acidity of an oxyacid because this increases the attraction of the center atom for the electrons it shares with oxygen and thereby weakens the O-H bond. Any hydrogen atom in an oxyacid that is not bonded to an oxygen atom is a _____________ hydrogen atom.
Increase in acid strength with _____________ electronegativity of the atom to which the hydrogen is bonded is a direct result of the polarity of the bond between hydrogen and the other atom. The bond to an ionizable hydrogen in an acid is made _____________ polar by increasing the number of highly electronegative atoms near the hydrogen atom in the molecule; this is called an inductive effect.
Practice Problems
77. Rank the acid strength of HClO, HClO2, and HBrO from weakest to strongest.
82. Complete each of these sentences with increases or decreases. All other factors being equal,
a. as the strength of the bond to H increases, the strength of the acid .
b. as the polarity of the bond to H increases, the strength of the acid .
c. as the size of the anion bonded to H increases, the strength of the acid .
Text segments from OpenStax, Chemistry. OpenStax CNX. Sep 15, 2020 http://cnx.org/contents/85abf193-2bd2-4908-8563-90b8a7ac8df6@13.1.;
White, J. et al. Interactive General Chemistry. MacMillan, NY. 2019.;
Chemistry LibreTexts, Libretexts, [online] Available from: https://chem.libretexts.org/Courses/Heartland_Community_College/HCC:_Chem_161/5:_Thermochemistry/5.7:_Enthalpy_of_Formation.
Reading guide style Adapted from Ionic equations Study Guide by Montgomery College is licensed under CC BY 4.0
Document not to be reposted on the internet without express permission.
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