Chemistry 1Name

The Role of Energy and Entropy in Dissolving

  1. Draw particle-level representations of A) solid sodium chloride and B) aqueous sodium chloride, meaning dissolved in water.

AB

  1. Describe the key characteristics of your drawings in words. Make sure to reference the spacing, organization, and bonding that your drawings illustrate, as well as the motion of the particles that doesn’t show on paper.

In A, Na+ and Cl- ions should be touching, in an alternating crystal lattice pattern and in a 1:1 ratio, with ionic bonds between them. The ions would be vibrating in place. In B, the ions should be separated from each other and spread out in the water, making ion-dipole bonds with the water (Na+ bonded to the O atom in water, Cl- bonded to the H atoms in water). They are no longer organized in a lattice, and their movement includes rotation and translation.

  1. What bonds or forces are broken during the process of NaCl dissolving in water?

Ionic bonds between Na+ and Cl-, some hydrogen bonds between water molecules to make room for ions.

  1. What bonds or forces are formed during this process?

Ion dipole bonds between Na+ and water, and Cl- and water

  1. What bonds or forces are broken during the process of sugar (a polar molecular compound) dissolving in water?

Hydrogen bonds between sugar molecules, and hydrogen bonds between water molecules

  1. What bonds or forces are formed during this process?

Hydrogen bonds from sugar to water

  1. Lead(II) sulfate does not dissolve in water. How could you explain this observation? Claim: The ionic bonds in lead(II) sulfate are stronger overall than the ion-dipole bonds that lead ions and sulfate ions could make with water.

The ion-dipole bonds that Pb2+ and SO42- ions could form with water aren’t strong enough to make up for the strength of the ionic bonds that must be broken. In other words, the dissolving process would be too endothermic, and the entropy increase of the system would not be enough to make up for the large endothermic energy change. Given that the compound does NOT dissolve, this driving force must be overcome by a very unfavorable enthalpy change. In other words, the hypothetical dissolving process would be too endothermic.

  1. Would you expect stearic acid (a solid nonpolar molecular compound) to dissolve in water? Why or why not?

No, because it can only make LDFs. Since it is solid, these LDFs must be pretty strong (i.e. stearic acid is a large molecule). Water is quite bad at making LDFs, since it is a very small molecule with few electrons to form temporary dipoles. Therefore, stearic acid molecules are more stable bonded to each other than bonding to water. Again, the dissolving process would be too endothermic, so it doesn’t occur.

  1. For each pair of substances, circle the one that has higher entropy:

a)Ice cubeORliquid water

Liquids have fewer microstates than solids b/c they lack a crystal lattice

b)Table saltORcurry powder (a mix of spices)

Table salt (NaCl) is a pure substance, curry powder is a mixture with more microstates.

c)CH4(g)ORC6H14(l)

Gas is more spread out with more movement, so it has more microstates than a liquid.

d)DiamondOR silly putty

Diamonds are crystalline, silly putty is not (it is amorphous).

e)2 mol H2OOR5 mol H2O (at the same temperature)

More particles means more microstates.

f)nitrogen gasOR air

Air is a mixture, nitrogen gas is a pure substance.

  1. During each change described, does the entropy of the system increase or decrease?

a)Evaporation of waterincrease

b)Stirring sugar into your coffeeincrease

Increase because the sugar is dissolving and losing its crystal lattice, and spreading out into the coffee.

c)Burning a matchincrease

Increase because the solid molecules in the wood or cardboard are being converted into gases (carbon dioxide and water vapor, mostly).

d)Ice forming on the surface of a lakedecrease

e)Cleaning your roomdecrease

f)Warming up your lunch in the microwaveincrease

Increase because the molecules’ temperature increases so they move more quickly and have access to more microstates.

g)Mixing hot and cold waterincrease

Increase because the energy initially possessed by the hot water spreads out to be shared by all the molecules.

Note: For these last 2 questions, answer them as best you can. If you aren’t sure, make a guess. We will have time in class on Monday for you to debate your answers with your classmates.