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

Rob SnyderJuly 2012

A study of the formation of 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 size of crystals on the rate at which a solute dissolves.

• 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 compares with values for other solutes.

• Solubility product constant for sodium chloride.• Etc.

Studies of geometric forms are included in a geometry curriculum as well as in a study of the arrangement of

atoms and ions in molecules and in ionic crystals.

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.

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 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

Table salt is sometimes mined from “salt domes”.

Pressure exerted on forced some salt deposits up through Earth’s rock layers from depths as great as 30,000 or 40,000 feet.

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.

Questions to pose to students either before or during a crystal growing activity.

• How would you make a saturated or supersaturated solution of sodium chloride?

• How would you recover the solute from a sodium chloride solution?

• How would you manage the evaporation process to form very small sodium chloride crystals?

Students can be asked to describe the forces that play a role as an ionic solute dissolves in a polar

solvent to form a solution.

The rate at which a solution forms depends on:

• Temperature• Mixing• 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/chapt

er5/lesson3#sodium_chloride_dissolving

• http://www.inquiryinaction.org/chemistryreview/solids/

The Virtual Chem Book• http://www.elmhurst.edu/~chm/vchembook/171solublesal

ts.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 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.

Crystal

#l

Length

(cm)

Width

(cm)

Height

(cm)

Data for the last three columns will be discussed on Thursday.

Students can be asked to describe the role that energy plays in forming crystals of sodium chloride

as water evaporates from a solution.

Na+ Cl-

Surface Area to Volume Ratios(SA/V)

On Tuesday, you explored the influence of SA/V Ratios on the rate of a chemical reaction.

The calculation of SA/V Ratios can be an optional extension of the basic process of forming simple ionic crystals.

Other examples of the influence of SA/V Ratios includes how rapidly cubes of ice melt, paint dries, butter melts and solutes dissolve in a solvent.

Total surface areas, volumes and SA/V Ratios for crystals can now be added to your data table.

Crystal

Length

(cm)

Width

(cm)

Height

(cm)

Total Surface Area (cm2)

Volume (cm3)

SA/V Ratio

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

Two important comments about units

Changes in SA/V ratios depends on units used.

The SA/V Ratio is a numerical expression area per unit volume and is used only to correlate rates of a process with numerical values for the ratio. Changing units would change the correlation.

Would this NaCl crystals have a nanoscale dimension?

1 picometer (pm) = 1 x 10-12 m = 1 x 10-10 cm

The grey half-spheres in the data table are neutral atoms.

The length of an edge was 167 + 154 + 167 = 488 pm

488 pm = 488 x 10-12 pm = 4.88 x 10-10 m = 4.88 x 10-8 cm

1.0 nanometer = 1.0 x 10-9 m = 1.0 x 10-7 cm

The crystal would have sub-nanoscale dimensions.

What is the SA/V Ratio of the simple NaCl crystal?

• Area of one face = (4.88 x 10-8 cm)2 = 2.38 x 10-16 cm2

• Total area = 6 x (2.38 x 10-16 cm2) = 1.428 x 10-15 cm2

• V = L x W x D = (4.88 x 10-8 cm)3 = 1.162 x 10-22 cm3

• SA/V Ratio = 1.428 x 10-15 cm2 ÷ 1.162 x 10-22 cm3

• SA/V Ratio = 1.229 x 107

How does this SA/V ratio compare with crystals that you formed?

Some challenge questions for students

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

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

• What would be the challenges in designing an experiment to determine the affect of SA/V Ratio on the rate at which a solute dissolves?

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

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

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

between the components.between the components.• An An environment is selectedenvironment is selected to induce designed to induce designed

interaction.interaction.• Components Components retain physical identityretain physical identity through and through and

after.after.• The process is The process is reversible or adjustablereversible or adjustable..

Whitesides & Boncheva (2002)