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Molecular Packing - Does it add up? (Bob Fiero)

Question

Add alcohol (high proof or concentration) to water and predict if the volumes add as expected? That is when you add a volume of alcohol with a volume of water, why is the combined volume less?

Author(s)

Bob Fiero

Principles Illustrated

Packing of molecules creating a varying of density with the same mass.

Our knowledge about the geometry and electronic structure of molecules in the liquid phase is very limited. Not only do the molecular arrangements change rapidly—on the scale of picoseconds to femtoseconds—but the properties of the individual molecules are also constantly changing, and interactions between the molecules cannot be neglected. Thus it is not surprising that there is still much to learn about common and simple liquids.

Standards

Biological, chemical, and physical properties of matter result from the ability of atoms to form bonds from electrostatic forces between electrons and protons and between atoms and molecules. As a basis for understanding this concept:

a.

 

Students know atoms combine to form molecules by sharing electrons to form covalent or metallic bonds or by exchanging electrons to form ionic bonds.

b.

 

Students know chemical bonds between atoms in molecules such as H2, CH4, NH3, H2CCH2, N2, Cl2, and many large biological molecules are covalent.

c.

 

Students know salt crystals, such as NaCl, are repeating patterns of positive and negative ions held together by electrostatic attraction.

d.

 

Students know the atoms and molecules in liquids move in a random pattern relative to one another because the intermolecular forces are too weak to hold the atoms or molecules in a solid form.

e.

 

Students know how to draw Lewis dot structures.

f.

*

Students know how to predict the shape of simple molecules and their polarity from Lewis dot structures.

g.

*

Students know how electronegativity and ionization energy relate to bond formation.

h.

*

Students know how to identify solids and liquids held together by Van der Waals forces or hydrogen bonding and relate these forces to volatility and boiling/melting point temperatures.

The conservation of atoms in chemical reactions leads to the principle of conservation of matter and the ability to calculate the mass of products and reactants. As a basis for understanding this concept:

a.

  Students know how to describe chemical reactions by writing balanced equations.

b.

  Students know the quantity one mole is set by defining one mole of carbon 12 atoms to have a mass of exactly 12 grams.

c.

  Students know one mole equals 6.02x1023 particles (atoms or molecules).

d.

  Students know how to determine the molar mass of a molecule from its chemical formula and a table of atomic masses and how to convert the mass of a molecular substance to moles, number of particles, or volume of gas at standard temperature and pressure.

e.

  Students know how to calculate the masses of reactants and products in a chemical reaction from the mass of one of the reactants or products and the relevant atomic masses.

f.

* Students know how to calculate percent yield in a chemical reaction.

g.

* Students know how to identify reactions that involve oxidation and reduction and how to balance oxidation-reduction reactions.

Procedure

        Pour 200 milliliters of alcohol into a graduated cylinder.  Then, pour 200 milliliters of water
          into the same graduated cylinder.  Read the level of liquid in the graduated cylinder at its lowest point in the meniscus.
          Identify the liquids as alcohol and water. Observe traces of liquid inside the contains first holding the liquids.
          Recognize that the traces of liquid could not account for the loss in volume.
 
When I actually performed this demo in front of my colleagues I added food coloring to both the water and lacohol to show them mixing more clearly and I used 95% ethanol and their was know appreciable loss of volume.  It was suggested to use rubbing alcohol (isopropyl alcohol) instead at the highest concentration you can find.  Perhaps the food coloring molecules interferred with the water molecules from moving into the interstitial spaces between the larger alcohol chains but then again ethanol does not react the expected way as mentioned in the sources I found.  Sorry all for failing to make the magic happen.

Questioning Script

Prior knowledge & experience:

Does water and alcohol mix and are volumes of liquids always conserved?

Root question:

What will happen to the sum volume when you the alcohol and water are mixed together in the same container? Similar phenomenon is shown when sugar and water are added and mixed together.

Target response:

Observers will predict that volumes when added to create the sum expected in simple math. For example, if 50 milliliters are added to 50 milliliters of water then one would predict that 100 milliliters will be measured. 

Explanation: When 50 milliliters of water are added to 50 milliliters of alcohol, the volume of the two mixed together is only about 96 or 97 milliliters.  When mixed together, the combined molecules fit together better than when they are alone, so they take up less space. Water and ethanol mix to form a solution. The solution formation is exothermic, and a substantial amount of heat is released.  The reason is that the sizes of the individual molecules are different enough that the smaller molecules can slip into the spaces between the big molecules. More specifically, the reason that it is less is due to molecular bonding - specifically, the creation of Hydrogen bonds. The OH- component of alcohol interacts with the H+ of the water molecules. These bonds attract each other to the point of making "hydrogen bonds". These bonds result in a tighter molecular formation, thereby reducing the volume of the combined liquids.

Common Misconceptions:

The alcohol and water react and one evaporates away as gas (at least not immediately true).
They react and form another smaller molecule.
There is always the evoking of incomprehensible magic phenomenon or trickery.

Photographs and Movies and Explanation

Bill Nye Science Guy showing the discrepant event of water and alcohol.

In the case of ethanol (alcohol) and water the volume of some concentrations is less than the sum of the components. Liquid water has a somewhat "open" structure that is broken up by the addition of ethanol so the mixture "collapses".

The emission spectra obtained in the study of methanol and water reveal that the water and alcohol molecules in solution form complex hydrogen-bonded networks and mix very little at the microscopic level. The results illustrate the technique's potential to provide new and valuable information about the microscopic origins of the properties of liquids and solutions.

The results show that the structure of liquid methanol at room temperature is a combination of rings and chains, each made up of either 6 or 8 methanol molecules. When water is added, the methanol chains interact with varying numbers of water molecules. These "bridging" water molecules bend the chains into open-ring structures that are stable because their glue-like hydrogen bonds are saturated. This means that the mixing of alcohol and water on the microscopic level is incomplete no matter how long you wait.

chains and rings
At the molecular level, very little mixing of alcohol and water occurs in solution. Instead, chains of methanol molecules react with clusters of water molecules to form stable open-ring structures, which lower the solution's entropy.
 In general there is no good way of predicting volumes of mixing of either liquids or of liquids and solids.

References

Methanol and Water analysis.

Hydrogen Bonding--biological Importance --a video/animation

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