Integrating Self-Assembly into the Integrating Self-Assembly into the study of Ionic Crystalsstudy of Ionic Crystals

Rob SnyderRob SnyderSTEM Education InstituteSTEM Education Institute

July 2013July 2013

Forming simple sodium chloride crystals can be integrated into a wide range of middle school and high school STEM topics that include determining the:

• Affect of the rate of evaporation of the size and structure of ionic crystals.

• Role of forces and energy in the dissolving and formation of ionic crystals.

• Enthalpy of solution (heat of solution) of sodium chloride that can be compared with values for other solutes.

• Affect of the size of crystals on the rate at which a solute dissolves.

• Solubility product constant for sodium chloride, Etc

Evaporation of salt water is the oldest method of “table salt” production. It is practical in warm climates where evaporation rates exceed precipitation rates for extended periods where there are steady prevailing winds

A large area San Francisco Bay is used for commercial salt production that include household sea salts.

Sodium chloride is also mined from deposits formed as water very slowly evaporated in ancient shallow seas. Those deposits were then buried deep beneath Earth’s surface by geologic processes.

This salt mine is 1200 feet below the city of Detroit.

Source: http://apps.detnews.com/apps/history/index.php?id=17

Sodium chloride crystals with cubic cleavage form when ions slowly form a crystalline lattice structure.

Students can form sodium chloride crystals dimensions that can be measured.

Crystals of NaCl can show cubic cleavage due to the ratio of diameters of sodium and chloride ions that is < 1.

r+ = cation radius r- = anion radius

Source: http://wikis.lib.ncsu.edu/index.php/Halite-NaCl

Lithium iodide (also an alkali metal halide) comes closest to adopting a truly close packed cubic crystalline structure.

Students can be asked to describe the roles that forces and energy play as an ionic solute dissolves

in a polar solvent to form a solution.

The rate at which a solution forms depends on:

• Temperature• Agitation• Degree of unsaturation• Solute particle size.

Dissolution, dissociation, and solvation are terms often used to describe the “dissolving” process.

Animations are available that illustrate the interactions between water and an ionic solute.

Examples have been developed by:

The American Chemical Society http://www.middleschoolchemistry.com/multimedia/

chapter5/lesson3#sodium_chloride_dissolving http://www.inquiryinaction.org/chemistryreview/solids/

The Virtual Chem Book http://www.elmhurst.edu/~chm/vchembook/

171solublesalts.html

Today

Make (or be given) a saturated solution of NaCl.

Use watch glasses or shallow dishes to evaporate the water from the solution.

Develop a strategy to produce very small, regularly shaped crystals as water evaporates. (Hot plates, warming trays and sunny or shaded spaces are available in the lab rooms.)

The next several days

Monitor and manage the formation of small crystals during the next several days. Hot plates and sunny windowsill are available.

Use a USB microscope to measure and record dimensions (in cm) of small regularly shaped NaCl crystals.

Store NaCl crystals in a covered Petri dish.

Crystal dimensions made using a USB microscope can be recorded on a data table. The total surface and area of

crystals can also be recorded.

Crystal

#Length

(cm)Width(cm)

Height(cm)

Surface Area(cm2)

Volume(cm2)

Data for the last column will be discussed on Friday..

Students can be also asked to describe the roles that energy and forces play in the self-assembly of sodium chloride crystals as water evaporates from a solution.

Na+ Cl-

Earlier in the week you explored P/T Ratios in two and three dimensional structures where: P = the number of cards or wood blocks on the perimeter of a structure. T = the total number of cards or wood blocks in a structure.

You could have also determined Surface Area to Volume Ratios (SA/V) for different arrays of

wood blocks where:

SA = The Total Surface Area of a Structure V = The Volume of the Structure

A comparison of P/T and SA/V Ratios for arrays of wood cubes with 2.54 cm (0.0254 m) edges

Wood Blocks P/T Ratio SA/V Ratio (cm)

SA/V Ratio(m)

50 x 50 x 50 0.115 0.047 4.73

10 x 10 x 10 0.488 0.236 23.7

5 x 5 x 5 0.928 0.473 48.4

1 1 2.36 237

• The units associated with the ratio (cm2/cm3) cannot be used to mathematically calculate a rate of a chemical or physical change.

• SA/V Ratios can be used to correlate rates of chemical or physical processes. However, changing measurement units would change that correlation.

A graph illustrates how the SA/V Ratio decreases as the size of a cube increases. Each point on the graph above represents the surface area and volume for cubes that are increasing by one unit in length. The graph also illustrates that the volume increases faster than the surface area as object size increases.

Small organisms, like amoebas, have a large SA/V Ratios and can accomplish respiration by simple diffusion across its body surface. A large organism, like a pig, has a much smaller SA/V Ratio, so it needs special respiratory organs such as lungs for taking in oxygen and removing carbon dioxide.

It would be quite a challenge to determine the P/T Ratios for the very small NaCl crystals that you have formed.

You can, however, calculate the SA/V Ratios.

You have, hopefully, used a USB microscope to measure the dimensions of a very small NaCl crystal

and can now determine a SA/V Ratio for a crystal.

Crystal

Length

(cm)

Width

(cm)

Height

(cm)

Total Surface Area (cm2)

Volume (cm3)

SA/V Ratio

Surface Areas to Volume Ratios Identified as one of the “big ideas” (Stevens, Sutherland, Schank, & Krajcik, 2007) and a potential threshold concept (Park & Light, in press) of nanoscale science, “surface area to volume ratio” (SA/V) explains many properties (such as reactivity and melting point) that exhibit different behaviors at the nanoscale, a unique feature that characterizes nanoscale science and technology. As such, a sophisticated understanding of SA/V can be regarded as the prerequisite to learning size-dependent properties and other advanced nanoscience concepts.

  Source: Unpacking Student Conceptions of Surface Area to Volume Ratio in the Nanoscience Context: http://rees2009.pbworks.com/f/rees2009_submission_96.pdf

What would be the SA/V ratio for a nanoscale NaCl crystal?

An On-Line Calculator is available at: (http://www.cod.edu/people/faculty/chenpe/sa-ratio.html)

Dimensions of a nanoscale NaCl crystal 10.6 x 10-7 cm x 3.4 x 10-7 cm x 4.7 x 10-7 cm

What might be an interesting property of a nanoscale NaCl crystal?

SA/V ratio = 1.20 x 107

Some challenge questions for students

How would you design an experiment to determine how the surface area to volume ratios affect the rate of a chemical reaction?

What strategy could you use to determine the number of ions in a sodium chloride crystal that you formed?

How many Na+ and Cl- ions are required to form a crystal with a nanoscale dimension?

What are the advantages and disadvantages of using SA/V Ratios to correlate changes in the dimensions of a structure with chemical or physical properties?

How is the formation of sodium chloride crystals (as water evaporates) an example of self-assembly?

• Structural components are mobile. • The goal is a low energy equilibrium state.• Ordered structures result from a less ordered system.• Assembly is a result of attractive or repulsive forces

between the components.• An environment is selected to induce designed interaction.• Components retain physical identity through and after.

• The process is reversible or adjustable..

Whitesides & Boncheva (2002)