Materials Included In Kit

Additional Materials Required

Prelab Preparation

To prepare 1 L of a 1 M NaOH solution, accurately weigh approximately 40 g of sodium hydroxide and place in a 1 L volumetric flask. Fill the volumetric flask one-third to one-half full with distilled or deionized water. Swirl the flask until the sodium hydroxide has dissolved (Caution: the dissolution of sodium hydroxide in water is highly exothermic and may require the use of a water bath to prevent excessive heat from building up). Fill up to the mark with distilled or deionized water. Mix thoroughly. Transfer the sodium hydroxide solution into a 1 L plastic bottle and label it with the concentration.

Safety Precautions

Phenolphthalein indicator solution is an alcohol-based solution and is therefore flammable; it is toxic by ingestion. Do not use near flames or other sources of ignition. Dilute sodium hydroxide solution is slightly toxic by ingestion and inhalation, irritating to the body tissues and a lachrymator. Solid sodium hydroxide is a corrosive solid that evolves large amounts of heat when added to water. Can cause skin burns and is very dangerous to eyes. 1-octanol is a flammable liquid; it is toxic by ingestion. Do not use near flames or other sources of ignition. Acetic acid is corrosive. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron or lab coat. Remind students to wash their hands thoroughly with soap and water before leaving the lab. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Excess acetic acid solution may be neutralized according to Flinn Suggested Method #24a. Excess sodium hydroxide solution may be neutralized according to Flinn Suggested Disposal Method #10. Unused phenolphthalein solution may be saved for future use. The titrated solutions may be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b. The 1-octanol solutions may be handled according to Flinn Suggested Disposal Method #18b.

Lab Hints

• This laboratory activity was specifically written, per teacher request, to be completed in one 50-minute class period. It is important to allow time between the Prelab Homework Assignment and the Lab Activity. Prior to beginning the homework, show the students the chemicals and equipment that will be available to them on lab day. Alternatively, you could provide the students with a list of the chemicals and equipment. For more advanced groups, you could include some additional supplies that are not required to successfully complete the lab.
• In order to save time, no rough titration was done and care was taken to ensure that only two titrations were needed for each section.
• Guide students toward having an equal volume of acetic acid solution and 1-octanol in the separating flask.
• If the appropriate chemicals are available, students may standardize the NaOH. Standardization is incorporated in Acidity of Beverages—Blended Inquiry Lab for AP^® Chemistry (Flinn Catalog No. AP7645).
• Students should rinse the buret with the titrant, and ensure that there are no air bubbles below the stopcock.
• Remind students to read the volume in a buret from the top-down. A buret is marked every 0.1 mL, and thus the volume may be measured to an accuracy of 0.05 mL (see Figure 4).
  {12325_Hints_Figure_4}
• Magnetic stir bars can be used during the titration if available.

Teacher Tips

• Extraction represents a different type of chemical equilibria to those most commonly studied in the classroom.
• Entropy provides all chemicals with a natural tendency to mix. For this reason, material is not solely found in the layer that it has the highest affinity for.
• This is an excellent inquiry opportunity as it enables students to directly measure and study a system at equilibrium. Furthermore, the study can be expanded by investigating the effect other solutes have on the distribution ratio of acetic acid.

Further Extensions

Alignment to the Curriculum Framework for AP^® Chemistry 

Enduring Understanding and Essential Knowledge
Atoms are conserved in physical and chemical processes. (1E)
1.E.2: Conservation of atoms makes it possible to compute the masses of substances involved in physical and chemical processes. Chemicals processes result in the formation of new substances, and the amount of these depends on the number and the types and masses of elements in the reactants, as well as the efficiency of the transformation.

Forces of attraction between particles (including the noble gases and also different parts of some large molecules) are important in determining many macroscopic properties of a substance, including how the observable physical state changes with temperature. (2B)
2.B.1: London dispersion forces are attractive forces present between all atoms and molecules. London dispersion forces are often the strongest net intermolecular force between large molecules.
2.B.2: Dipole forces result from the attraction among the positive ends and negative ends of polar molecules. Hydrogen bonding is a strong type of dipole-dipole force that exists when very electronegative atoms (N, O, and F) are involved.
2.B.3: Intermolecular forces play a key role in determining the properties of substances, including biological structures and interactions.

Chemical changes are represented by a balanced chemical equation that identifies the ratios with which reactants react and products form. (3A)
3.A.2: Quantitative information can be derived from stoichiometric calculations that utilize the mole ratios from the balanced chemical equations. The role of stoichiometry in real-world applications is important to note, so that it does not seem to be simply an exercise done only by chemists.

Chemical reactions can be classified by considering what the reactants are, what the products are, or how they change from one into the other. Classes of chemical reactions include synthesis, decomposition, acid–base, and oxidation–reduction reactions. (3B) 3
.B.2: In a neutralization reaction, protons are transferred from an acid to a base.

Electrostatic forces exist between molecules as well as between atoms or ions, and breaking the resultant intermolecular interactions requires energy. (5D)
5.D.3: Noncovalent and intermolecular interactions play important roles in many biological and polymer systems.

Chemical or physical processes are driven by a decrease in enthalpy or an increase in entropy, or both. (5E)
5.E.1: Entropy is a measure of the dispersal of matter and energy.

Chemical equilibrium is a dynamic, reversible state in which rates of opposing processes are equal. (6A)
6.A.3: When a system is at equilibrium, all macroscopic variables, such as concentrations, partial pressures and temperature, do not change over time. Equilibrium results from an equality between the rates of the forward and reverse reactions, at which point Q=K.

Chemical equilibrium plays an important role in acid-base chemistry and in solubility. (6C)
6.C.3: The solubility of a substance can be understood in terms of chemical equilibrium.

Learning Objectives
1.18 The student is able to apply conservation of atoms to the rearrangement of atoms in various processes.
1.20 The student can design, and/or interpret data from, an experiment that uses titration to determine the concentration of an analyte in a solution.
2.13 The student is able to describe the relationships between the structural features of polar molecules and the forces of attraction between particles.
2.15 The student is able to explain observations regarding the solubility of ionic solids and molecules in water and other solvents on the basis of particle views that include intermolecular interactions and entropic effects.
3.4 The student is able to relate quantities (measured mass of substances, volumes of solutions, or volumes and pressures of gases) to identify stoichiometric relationships for a reaction, including situations involving limiting reactants and situations in which the reaction has not gone to completion.
6.5 The student can, given data (tabular, graphical, etc.) from which the state of a system at equilibrium can be obtained, calculate the equilibrium constant, K.

Science Practices
1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.
1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
2.2 The student can apply mathematical routines to quantities that describe natural phenomena.
4.2 The student can design a plan for collecting data to answer a particular scientific question.
4.3 The student can collect data to answer a particular scientific question.
6.1 The student can justify claims with evidence.
6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
7.1 The student can connect phenomena and models across spatial and temporal scales.

Answers to Prelab Questions

1. A student investigates the distribution of an iodine solution between water and cyclohexane. They start with a solution of iodine water which they titrate with Na₂S₂O₃, using starch as the end-point indicator, to determine the initial concentration. They then extract the iodine water with an equal volume of hexane and observe that during the extraction the cyclohexane layer turns purple. They then titrate the post extraction aqueous layer with Na₂S₂O₃, again using starch as the endpoint indicator. The table below contains the data they collected during their experiment, use it to determine the distribution ratio of iodine between water and cyclohexane.
   {12325_PreLabAnswers_Table_1}
   Students should find that the initial concentration of the iodine solution was 0.405 M and that the concentration after extraction was 0.147 M. This means that the concentration of iodine in the cyclohexane layer is 0.258 M. And the distribution ratio is: D = [I₂]org/[I₂]aq = 1.76
2. The distribution ratio of I₂ between water and cyclohexane can be shifted in favor of the aqueous layer by the addition of KI. Draw a diagram showing the organic and aqueous layers as well as all equilibria that involve I₂.
   {12325_PreLabAnswers_Figure_5}
3. Assuming that an acetic acid solution is 12% by mass and that the density of the solution is 1.00 g/mL, what volume of 1 M NaOH is needed to fully neutralize a 10 mL aliquot of the acetic acid solution? 10 mL of a 12% acetic acid solution contains 1.2 g of acetic acid which gives it a concentration of 1.998 M. It would take 19.98 mL of a 1 M NaOH solution to fully neutralize this solution.
4. Acetic acid is a weak acid and sodium hydroxide is a strong base, would you expect the equivalence point for this titration to be above pH 7, at pH 7, or below pH 7? Explain why this is the case. The equivalence point of the titration between a weak acid and a strong base is above pH 7. This is because the equivalent point will be halfway between the weak acid’s buffer pH and the final strong base pH.
5. Figure 3 shows the intermolecular forces present in 1-octanol. Identify and label these forces.
   {12325_PreLabAnswers_Figure_3}
6. Sketch a diagram showing the intermolecular forces between acetic acid and water.
   {12325_PreLabAnswers_Figure_6}
7. Sketch a diagram showing the intermolecular forces between acetic acid and 1-octanol.
   {12325_PreLabAnswers_Figure_7}
8. Would you expect acetic acid to be more soluble in water or 1-octanol? Explain your choice. Acetic acid is more soluble in water than 1-octanol. This is because the large non-polar regions of 1-octanol reduce the acetic acids affinity for this solvent.
9. Water and 1-octanol are both able to hydrogen bond, however the two are immiscible. Comment on why you think this is. The long non-polar backbone of 1-octanol means that the majority of the molecule is only able to engage in London dispersion force interactions. Therefore, although there is a small polar region that can hydrogen bond, it is thermodynamically more favorable for the water to remain separate.
10. At the phase boundary between the water and 1-octanol layers acetic acid molecules will be constantly crossing back and forth. Write an equilibrium equation for this process.
    {12325_PreLabAnswers_Equation_1}
11. Look up the densities of water and 1-octanol. Which do you expect to be the top layer and which the lower? Water has a density of 1.00 g/mL, and 1-octanol has a density of 0.824 g/mL. Therefore, the upper layer is 1-octanol, and the lower layer is water.

Sample Data

The organic concentration was determined by subtracting the average post-extraction aqueous concentration from the average preextraction aqueous concentration. Dividing the post-extraction organic concentration by the post-extraction aqueous concentration gives a distribution ratio of 0.39.

Answers to Questions

Example Procedure

1. Obtain 50 mL of acetic acid solution.
2. Titrate two 10 mL aliquots of the acetic acid solution with the 1 M NaOH solution, using phenolphthalein as an indicator, to determine the concentration of the acetic acid.
3. Obtain a clean dry 125 mL separating funnel, and check that the stopcock is closed.
4. Set up a support stand with an iron ring.
5. Pipet 25 mL of the acetic acid solution into the separating funnel.
6. Pipet 25 mL of 1-octanol into the separating funnel and insert the stopper (this should be a different pipet from the one used with the acetic acid).
7. Invert the funnel once so the stopcock is now pointing up into the air, and open the stopcock to release any pressure that has built up. Caution: Ensure that the funnel is pointing away from yourself and other students. This can be done into a fume hood if needed.
8. Close the stopcock, and invert the funnel twice more.
9. Once again carefully open the stopcock to release any pressure build up.
10. Close the stopcock. Tip the separating funnel back and forth to ensure adequate mixing, pausing every now and again torelease pressure.
11. Place the separating funnel into the iron ring and remove the stopper.
12. Once the solution inside the funnel separates into two clearly defined layers, draw off the lower layer, taking care not to take any of the upper layer.
13. Titrate two 10 mL aliquots of the lower layer with the 1 M NaOH solution, again using phenolphthalein as an indicator, to determine the concentration of the acetic acid.
14. Calculate the distribution ratio of acetic acid between the two layers.

References

AP^® Chemistry Guided-Inquiry Experiments: Applying the Science Practices; The College Board: New York, NY, 2013.

Introduction

Capture the concepts and hit the ground running on exam day with this lab! Encompassing multiple Big Ideas, this lab involves extraction, which is a commonly used method for isolating and separating organic substances. Through the use of two immiscible solvents (usually water and an organic solvent), different materials can be isolated based on their affinity for one or the other of these layers. However, due to a variety of factors, some material will still remain in the other layer. In this investigation, the distribution of acetic acid between water and 1-octanol will be examined. A prelab homework assignment guides you through the necessary concepts to ensure success on lab day. You will find it fun, engaging and challenging!

Concepts

• Intermolecular forces
• Equilibrium
• Extraction
• Neutralization
• Titration

Background

Solutions are homogeneous mixtures of two or more substances. One of these substances is described as the solvent, and the others as solutes. All chemicals exhibit a natural tendency towards mixing. However, intermolecular forces can render certain substances practically immiscible or insoluble.

Liquid–liquid extraction is a commonly used chemistry technique during the purification steps following a reaction. During a liquid–liquid extraction, two immiscible solvents are used to separate different materials into their preferred environments (see Figure 1). At the interface between the two solvents, molecules are able to move back and forth as part of a dynamic equilibrium, with the equilibrium constant favoring the preferred solvent.

For example, hexane and water are not miscible and as such the less dense hexane floats above the more dense water, rather than mixing. This inability to mix is driven by the water layer having hydrogen bonding, dipole–dipole, and London dispersion force interactions; whereas the hexane layer only has London dispersion force interactions. Solutes in this two-phase system will tend to spend more time in the layer that has intermolecular forces most similar to theirs. That is, polar molecules tend to be found primarily in the water layer and non-polar molecules primarily in the hexane layer. The difference in polarity between the two layers can be made even more pronounced by dissolving a strong electrolyte, such as sodium chloride, in the water layer (see Figure 2). The distribution ratio is the ratio of the total analytical concentration in the extract phase to the total analytical concentration of the solute in the other phase. The distribution ratio is most commonly expressed as the concentration in the organic layer divided by the concentration in the aqueous layer as shown in Equation 1. Because this ratio is related to the analytical concentration, it ignores other competing equilibria and is related to the detectable amount of solute regardless of its form. For example, aqueous iodine solutions contain both I₂ and I₃^–, due to the presence of KI. Since both of these species react with thiosulfate, the combined concentration determined by titration is used in the calculation of the distribution ratio. The distribution ratios of water and 1-octanol systems are commonly examined in the pharmaceutical industry, as these ratios provide a good indication of a drug’s ability to distribute between the lipid bilayers and blood serum in humans and other animals.

Experiment Overview

The purpose of this activity is to complete the homework assignment prior to lab to promote conceptual understanding of intermolecular forces, the liquid-liquid extraction technique and equilibria. You will need to consider the equipment and chemicals that are being made available for you, and then using the prelab questions as a guide design an experiment that will enable you to calculate the distribution ratio of acetic acid between water and 1-octanol. On lab day you will complete the experiment and then report your distribution ratio.

Prelab Questions

See the Student PDF for the Prelab Homework Assignment

Student Worksheet PDF

12325_Student1.pdf