1 lab period; work in pairs. Complete the Preparation page before coming to lab.
Goals
Background
According to the Bronsted view, an acid is a substance capable of donating a proton, H+, to a base. Thus an acid is a proton donor, and a base is a proton acceptor. Reaction between a generic acid, HA (note that we represent the donatable proton explicitly) and a generic base, B, is shown in equation 1:
| HA + | :B ---> | BH+ + | A- | (1) |
| acid | base | acid | base |
B uses a non-bonding pair of electrons to attach the proton; this pair is shown in the equation. The reverse of reaction 1 also involves proton transfer, with BH+ serving as the acid and A- as the base. The pair of species, HA and A- are related by gain or loss of H+. As such they are related and are therefore called a conjugate acid-base pair. B and BH+ are also a conjugate acid-base pair.
Our primary interest in acids in this experiment involves their reaction with water, shown in equation 2.
HA(aq) + H2O ---> H3O+(aq) + A-(aq) (2)
In this reaction, water functions as a Bronsted base. The extent of reaction 2 is indicated quantitatively using the equilibrium constant, Keq. The equilibrium constant expression for 2 is given in 3:
Keq = Ka = [H3O+(aq)][A-(aq)]/[HA(aq)] (3)
The equilibrium constant for reaction of an acid with water is usually symbolized Ka, to remind us of the type of reaction being dealt with. The reactant water, since it is present in huge concentration and is thus essentially a pure liquid, is not included in the Ka expression. The strength of an acid in aqueous solution is defined in terms of the magnitude of Ka for reaction 2. Strong acids have Ka values larger than 1; weak acids have Ka values less than 1. The common strong acids are HCl (hydrochloric), HBr (hydrobromic), HI (hydroiodic), HNO3 (nitric), H2SO4 (sulfuric), HClO3 (chloric) and HClO4 (perchloric). All other acids that we will commonly encounter are weak; i.e., their reaction with water according to 2 occurs to only a minor extent.
In this experiment, we will explore the manner in which the ratio of conjugate base concentration to conjugate acid concentration, R = [A-]/[HA], is related to and affected by pH. We can get at this through a rearrangement of equation (3):
We can draw several conclusions from this equation:
The manner in which the species, A-, is distributed between the deprotonated (A-) and protonated (HA) forms can be depicted visually using a distribution diagram. A distribution diagram is a plot of the fraction of A- present in each of the 2 forms as a function of pH. The distribution diagram for a weak acid with pKa = 6 is shown in the figure.
In this experiment, you will develop a distribution diagram for a conjugate acid/base pair assigned to you. This can be accomplished using pH measurements, as we will see.
Focus Questions
As you carry out the experiment, keep the following Focus Questions in mind. When you have finished the experiment, provide answers to them.
Equipment and Materials
Safety
Safety goggles must be worn at all times in the laboratory. Dilute solutions of acids and bases are irritating to the skin. In the event of skin contact, rinse thoroughly with plenty of water.
Experimental
Record all data in your notebook.
Your team will be assigned a pH meter and a particular acid-base pair from among the following to investigate:
BE SURE YOU CAN IDENTIFY THE ACID MEMBER AND BASE MEMBER OF EACH PAIR!
0.1 M stock solutions of all acids and bases are available in the laboratory. Obtain supplies of your assigned acid and base in clean, dry, appropriately LABELLED beakers. You will prepare a series of at least 9 solutions, each requiring at most 10 mL of acid solution and at most 10 mL of base solution, so take only a little more of the stock solutions than you will need. Record the exact concentrations of the stock solutions. Keep the reservoir beakers covered with watch glasses at all times to retard evaporation and exclude CO2 from the air.
Before beginning, calibrate the pH meter using standard buffer solutions of pH 4 and pH 7, or pH 7 and pH 10. Your instructor will provide instructions for doing the calibration and will tell you which two buffers to use.
Prepare a series of solutions of your conjugate acid and base as follows. For the first solution, pipet 1.0 mL of acid solution and 9.0 mL of base solution into a product vial, and stir to mix. Prepare 8 more solutions in which the ratio of acid volume to base volume is systematically varied, such that the total volume taken is always 10.0 mL. Your final solution should contain 9.0 mL of acid solution and 1.0 mL of base solution.
Rinse and dry the pH electrode and submerge it in the first solution. Measure and record the pH of the solution. Repeat this procedure for each of your remaining eight solutions.
Transfer a 2.5-mL aliquot of solution 5 to a product vial, and add a small volume of distilled water. Measure the initial pH. Then titrate the solution with standard 0.10 M NaOH until the pH has changed by a total of 0.2 units. Record the volume of NaOH required to do this.
Transfer a 2.5-mL aliquot of solution 5 to a product vial, and add a small volume of distilled water. Measure the initial pH. Then titrate the solution with standard 0.10 M HCl until the pH has changed by a total of 0.2 units. Record the volume of HCl required to do this, and the initial and final pH values.
Pipet 2.5 mL of distilled water to a small product vial, and measure the initial pH. Add 1 drop of 0.10 M NaOH, and record pH again.
Clean-up. When you have finished all of your work:
Disposal
Spent solutions may be flushed down the drain with plenty of water.
Preparation
Equilibrium: Distribution
Diagrams
Read
Problems
1. Hypochlorous acid, HOCl, reacts with water as follows: