Title: Does Size Matter?: A student investigation of the inverse relationship between molecule size and per-unit-volume concentrations.

Transcription

1 Title: Does Size Matter?: A student investigation of the inverse relationship between molecule size and per-unit-volume concentrations. Author: Eric Kolb Date: May 5, 2003 Background: When comparing two different solutes in equal percent-by-weight solutions, the relative sizes of the solute molecules will determine the concentration of solute molecules in a given volume such that a larger solute molecule (thus larger mass) will result in a more diluted concentration-by-volume solution. In this lesson, students will explore this relationship in three related experiments. Experiment 1 is largely a thought experiment in which molecular models of acetic acid (larger molecule size dimension) and sodium hydroxide (smaller molecule size dimension) are used to determine their relative concentration in a given volume: as a class, the students will determine that the number of acetic acid molecules that can fit into a given volume such as a beaker or coffee cup will be less than the number of sodium hydroxide molecules that can fit into the same volume. In Experiment 2, the students (in groups of two) will validate the empirical relationship developed in Experiment 1 by conducting a simple NaOH and acetic acid neutralization experiment: for a 1% by-weight-concentration of NaOH and acetic acid, the number of drops of acetic acid (less concentrated by volume) required to neutralize the NaOH (more concentrated by volume) will be significantly more than the number of NaOH required to neutralize the acetic acid. In Experiment 3, the students will use the molecule size relations and experimental techniques developed in Experiments 1 and 2 to devise and conduct an experiment that will determine the identity of an unknown acid or base solution. The teaching methods of this lesson were adapted and modified from those outlined in the Science Education for Public Understanding Program (SEPUP) lab module Chemical Survey & Solutions and Pollution published by University of California at Berkeley. Pre-Lesson Knowledge: o Conceptual knowledge of molecules o Basic knowledge of chemical solution components (solute and solvent) o Basic knowledge of chemical concentration and dilution o How to read simple chemical formulas o General understanding of acids and bases as well as acid/base neutralization

2 Objectives: o Students will learn about variables that affect the chemical concentration of a solution. o Using molecule models, the students will relate the mass of the molecule to its size dimensions and gain understanding of how the mass of a molecule effects the concentration of said molecules present within a given volume. o Students will analyze the physical properties of chemical solutions using ph indicators and acid/base neutralization techniques. o Students will relate the ph of a solution to the size of the solute molecules within the solution. The student will then devise an experiment that will use this relationship to identify an unknown chemical solution. Standards: Standard 1: Science as Inquiry 1SC-E1. Identify a question, formulate a hypothesis, control and manipulate variables, devise experiments, predict outcomes, compare and analyze results, and defend conclusions Conduct an experiment using a scientific method Analyze the results of an experiment Defend conclusions drawn from the analysis 1SC-E2. Create a model (e.g., a computer simulation, a stream table) to predict change Construct a model that demonstrates change within a system Describe variables that cause change Explain cause and effect of variables within a system 1SC-E3. Organize and present data gathered from their own experiences, using appropriate mathematical analyses and graphical representations Construct a representation of data (e.g., histogram, stem-and-leaf plot, scatter plot, circle graph, flow chart) Interpret patterns in collected data Standard 5: Physical Science 5SC-E1. Examine, describe, compare, measure, and classify objects and mixtures of substances based on common physical and chemical properties (e.g., states of matter, mass, volume, electrical charge, density, boiling points, ph, magnetism, solubility) Classify objects and mixtures of substances based on physical and chemical properties Analyze physical and chemical properties of objects and mixtures Materials: o Molecular model kit (e.g., o 500 ml beaker o Eight 30-mL bottles of 1% and 5% by-weight-concentration acetic acid solution 1% and 5% by-weight-concentration HCL solution 1% and 5% by-weight-concentration sodium hydroxide solution 1% and 5% by-weight-concentration sodium or calcium carbonate solution distilled water o For each group: Universal Indicator (ph 4-10)

3 Time: 2-3 days Neutral Litmus paper Eyedropper Stirrer Solution tray. Can be a water color tray or other tray device that contains at least several small cups to hold liquid Paper towels Grade Level: 7-8 th grade Procedure: Using molecule models, the students will relate the mass of the molecule to its size dimensions and gain understanding of how the mass and size of a molecule effects the concentration per unit volume. 1. Show molecule models of NaOH and acetic acid. 2. Tell the students that the relative size proportions between the molecules are represented in the models. 3. While showing the 500-mL volume container, ask the students to estimate how many NaOH molecules can fit into the container. Ask the students to estimate how many of the acetic acid molecules can fit into the container. 4. Have the student answer questions 1 and 2 on Worksheet A. Students will analyze the physical properties of the acid/base solutions using neutralization techniques. Done in groups of two. For each group, distribute the solution tray, stirrer, eyedropper, universal indicator, bottles of 1% by-weightconcentration acetic acid (solution A) and sodium hydroxide (solution B), distilled water. Have the students do the following: 5. Place ten drops of solution B in Cup 1 and ten drops of distilled water in Cup 2. Add one drop of indicator to Cup 1 and Cup 2. Next, add Solution A, one drop at a time, to Solution B in Cup 1, stirring after each drop until the same color is obtained as in the ten drops of distilled water and universal indicator in Cup 2. Have the students record the number of drops of Solution A needed to neutralize Solution B in Cup 1 on Worksheet A, Question Have the students do a similar experiment by placing ten drops of Solution A and one drop of universal indicator in Cup 3 and then adding Solution B, one drop at a time, in order to see how many drops of Solution B are needed to neutralize the ten drops of Solution A. The students are to record the number of drops needed to neutralize Solution A in Cup 3 on Worksheet A, Question 4. Students will relate the ph of a solution to the relative number of molecules present in the acid and base solutions. The relative size of the molecules can thus be inferred.

4 7. Tell the students that neutralization is the result of mixing a given number of solute particles of acid with the same number of solute particles of base so that the acid and base properties cancel each other out. 8. As a class, notate and compare on the chalkboard and on Worksheet A, Question5, the number of drops required to neutralize both solutions that each group determined. Ask them why it took less drops of Solution B to neutralize Solution A than it took drops of Solution A to neutralize Solution B. 9. Guide the discussion such that the issue of the relative size of the molecules is reflected in the students answer. Have them write their answer on Worksheet A, Question 6. Tell the students that Solution A is acetic acid and Solution B is NaOH, both of which are 1% by-weight-concentration. The students will devise an experiment that will use this ratio relationship technique to identify an unknown chemical solution. The setup: a lab technician made a 5% byweight solution of one of the solutes. Trouble is, the technician forgot which solute was put into solution. It s the students opportunity to make things right by designing and conducting an experiment that can identify the unknown solute using their newly acquired knowledge. Done in groups of four. 10. Have each group determine only one of the four 5% by-weight-concentration unknown acid or base solutions. 11. Litmus paper may be used by the students to determine if the unknown is an acid or a base. 12. In order to identify the correct unknown, the ratio of drops required to neutralize each of the 1% by-weight-concentration acids and bases must first be determined. Then, the students should determine through experimentation using the techniques of Experiment 2, the ratio of drops of 1% by-weightconcentration acids or bases required to neutralize the unknown solution. The student will then need to divide the experimental 5% by-weight-concentration neutralization ratio by 5 and compare this quotient with the known 1% byweight-concentration neutralization ratios in order to determine which of the known ratios most closely matches that of their experimental ratio value.

5 Worksheet A Name: 1. The concentration of a solution is defined by how many particles of solute are present within a given volume. Imagine that the volume of the container represents the volume of one drop of solution obtained from an eyedropper. If you obtained one drop of HCl and one drop of acetic acid solution from an eyedropper (container), based on the size of the molecules, which drop of solution has a higher concentration of solute? 2. In the following diagrams, let a circle represent a drop from an eyedropper and a triangle represent a given number of particles. Suppose, as shown, each drop of solution (each containing a different solute) contains the same number of particles and the solute particles of solution A and B are the same size. Solution A Solution B In this example, the concentrations of both solutions are the same (three solute particles per drop of solution). Using for Solution A and for Solution B and the circles below, draw an example in which each drop of solution B contains twice as many particles as each drop of solution A. 3. In the neutralization experiment, how many drops of Solution A were required to neutralize Solution B? 4. In the neutralization experiment, how many drops of Solution B were required to neutralize Solution A?

6 5. Using the chart below, notate how many drops of solution were required for each group to neutralize the remaining solution Number of drops of Solution A needed to neutralize Solution B Number of drops of Solution B needed to neutralize Solution A 6. Using the bar charts of Question 5 and your results as noted in Questions 3 and 4, are there more solute particles of acid in a drop of Solution A or more solute particles of base in a drop of Solution B? How did you determine your answer? What other variables may account for the different numbers of drops required for neutralization as shown in the graphs of Question If each drop of acid contains three times as many particles as a drop of base, how many drops of base would be needed to neutralize 25 drops of acid?