Chesapeake Campus – Chemistry 112 Laboratory
Lab #9 – Determination of the Ka and Kb of Weak Acids and Bases
Objectives
- Determine the pH and pOH for a solution of a weak acid or weak base.
- Calculate the [H+] in a solution.
- Determine the value of weak acids and base equilibrium constants (Ka and Kb)
- Compare if or how the [H+] and Ka values change with weak acid concentration.Introduction
Perhaps one of the most important chemical systems in chemistry deals with acid base equilibrium. Many chemical processes in the body involves acid base chemistry. Proteins are large biomolecules that are the backbones of tissues, biological catalyst such as enzymes, and oxygen transporters such as hemoglobin and myoglobin which escort oxygen to the cells of many tissues and muscles. Proteins are made of individual units of amino acids which contain acidic and basic moieties. In the electron transport chain where the body uses oxygen to generate energy, an acidic H+ pump is needed for the formation of water when oxygen receives electrons generated from the breakdown of macronutrients such as proteins, fatty acids and carbohydrates. In addition, the digestion of food in the stomach is aided by HCl (hydrochloric acid) which is considered a strong acid. This acid assists in breaking down proteins and other complex food nutrients. The intestine aid in further food adsorption and the pH in this vital organ is more basic. Finally, the blood has a pH of 7.4 and is considered a very strong buffer that resist dramatic changes in its pH.
When defining acids and bases, there are three descriptions for these types of molecules. The three types of acids and bases are Arrhenius, Bronsted-Lowry and Lewis. According to Arrhenius, an acid is a substance that yields a proton (H+) in water and a base is a substance that yields a hydroxide (OH-1). Under this description, a substance like HCl (g) can be classified as an Arrhenius Acid and NaOH an Arrhenius Base as illustrated below:
𝐻𝐶𝑙 → 𝐻+ +𝐶𝑙−
(𝑠) 𝐻2𝑂 (𝑎𝑞)
𝑁𝑎𝑂𝐻 → 𝑁𝑎+ (𝑠) 𝐻2𝑂 (𝑎𝑞)
(𝑎𝑞)
Bronsted-Lowry definition of acid and bases are based on the ability of a substance to donate and accept an H+ respectfully. In this chemical system, the acid and base are codependent as the proton donor (Bronsted-Lowry Acid) gives up an H+ to the proton acceptor (Bronsted-Lowry Base). The resulting reaction between the Bronsted-Lowry Acid / Base yields a set of products known as conjugate acids and bases. The reaction below between H2SO4 and water depicts the Bronsted- Lowry Acid / Base equilibrium:
+ 𝑂𝐻− (𝑎𝑞)
HSO +HOHO+ +HSO−
2 4(𝑎𝑞) 2 (𝑙) 3 (𝑎𝑞) (𝑎𝑞)
In the reaction above, the sulfuric acid (H2SO4) is losing an H to water as an H+; therefore, sulfuric acid is behaving as the Bronsted-Lowry acid and water is behaving as the Bronsted-Lowry base. On the product side, hydronium ion (H3O+) and the bisulfate ion are produced. The hydronium ion is called the conjugate acid and it is formed when the base, H2O, on the reactant side accepts a proton (H+). The bisulfate ion (HSO4-1) is the conjugated base and it is produced when the reactant, H2SO4, loses a proton.
Finally, acids and bases can be defined by the Lewis definition. According to Lewis, an acid is any substance that accepts an electron pair to form a covalent bond and a base is any substance that can donate an electron pair to form a covalent bond. Analogous to the Bronsted-Lowry system, Lewis acids and bases are also codependent. However, for Lewis acid base reactions, the resulting product is generally a one component complex.
𝐵𝐶𝑙+∶𝑁𝐻→𝐵𝐶𝑙− 𝑁𝐻+ 3 3 3⃛⃛3
In the above example, the BCl3 is the Lewis acid and the NH3 is the Lewis base. Substances such as BCl3, AlCl3, and M#+ (where M is a metal ion) can be Lewis acids since they are capable of accepting a pair of electrons. These types of acids have an empty orbital that can accommodate a pair of electrons. Substances that contains an atom with lone pair electrons (i.e., N, O, S, and P) or negative ions ( i.e., OH-1, I-1, Br-1, Cl-1, and F-1) are Lewis Bases. Generally speaking, Lewis Acids and Bases are more applicable to organic and bio-inorganic chemical processes.
In addition to the above definitions of acids and bases, acids and bases can be classified as strong or weak. For the sake of this experiment, we will define a strong and weak acids / bases from a Bronsted-Lowry perspective. Strong acids are substances that donates 100% or nearly 100% of its protons in a chemical reaction and a strong base can be considered a substance that has a very high affinity for protons. Both strong acids and bases have K values that are very large (>> 1000). A weak acid is a substance that partially or sparingly dissociate protons and weak bases will have a low affinity for protons. The K values for these systems are small (<<<< 1)
For this experiment, the students will quantitatively measure the value of the equilibrium constants (Ka and Kb) for a couple of acids (acetic and phosphoric) and a weak base (ammonia). The equilibrium constant will be obtained by measuring the pH of various concentrations of these acids and bases and from these pH, determining the concentrations of either hydronium ion (H3O+) or hydroxide (OH-1) ions. A pH electrode will be used to measure the pH of the acids and base solution. The general equilibrium expressions for a weak acid and base are depicted below:
𝐻𝐴 +𝐻𝑂 ↔𝐻𝑂+ +𝐴− 𝐾=[𝐻3𝑂+][𝐴−]
(𝑎𝑞) 2 (𝑙) 3 (𝑎𝑞) (𝑎𝑞) 𝑎
[𝐻𝐴]
CHM 112 Lab 9 Page 2
Materials
:𝐵 +𝐻𝑂 ↔𝐵𝐻+ +𝑂𝐻−
𝐾=[𝐵𝐻+][𝑂𝐻−]
𝑏
(𝑎𝑞)
2 (𝑙)
(𝑎𝑞) (𝑎𝑞)
𝑏
𝐾 =[𝐵𝐻][𝑂𝐻−]
:𝐵− (𝑎𝑞)
+𝐻𝑂
2 (𝑙)
↔𝐵𝐻 +𝑂𝐻− (𝑎𝑞) (𝑎𝑞)
[:𝐵] [:𝐵−]
To determine Ka, the H3O+, A-1, and HA must be provided at equilibrium; while, the OH-1, BH+ (BH) and :B( or:B-1) must be obtained at equilibrium for Kb. To get the needed values for Ka determination, the H3O+ can be determined by using Equation 1 and 2 below. Since the weak acid is monoprotic, the [A-] (conjugate base) concentration will be equivalent to the H3O+ concentration and the HA concentration at equilibrium will be the measure HA concentration – the calculated concentration of H3O+.
𝑝𝐻 = − log[𝐻3𝑂+] Equation 1 [𝐻3𝑂+] = 10−𝑝𝐻 Equation 2
To obtain the needed concentration values of the chemical species for Kb determination, the (OH-1) can be determine by calculating the pOH and the (OH-1) concentration as expressed in equations 2 and 3. The conjugate acid (:BH+/:B) will be equivalent to the OH-1 concentration and the base (:B or :B-1) concentration at equilibrium is the measure base concentration – the calculate OH-1 concentration.
𝑝𝐻 + 𝑝𝑂𝐻 = 14.00 𝑝𝑂𝐻 = 14.00 − 𝑝𝐻
𝑝𝑂𝐻 = − log[𝑂𝐻−] [𝑂𝐻−] = 10−𝑝𝑂𝐻
Equation 3 Equation 4
Equation 5 Equation 6