Dissociation and Solvation

Scribe: Juliette Ovadia

Tuesday, October 29th, 2013

Dissociation and Solvation

Agenda

First, we checked in the Dissociation homework from last night and picked up handouts (the power point notes on solutions and ChemThink LabSim Questions). Then we checked the homework and went over questions. Finally, we watched a sugar and salt simulation and went over the notes. 

Dissociation Homework

When going over this homework, Mrs. Friedmann reminded us that dissociations are "breaking apart" reactions. When the reactant dissolves in water, it breaks apart. She reminded us that the products of dissociations are always aqueous because the solute is, in this case, always dissolved in water and breaks apart in water. When making compounds, one should balance the charges, but when separating compounds, one should balance equations. 

For example, in problem #6, there was one molecule, a crystal, of Rb₂SO₄, which had two Rb atoms and one SO_4. This needs to also be shown in the products, so you add a coefficient of 2 in front of the Rb+. A trick is to take the subscript and turn it into a coefficient. We can imagine the Rb₂SO₄  as a molecule breaking apart into different pieces even as we understand that ionic compounds do not actually form these types of molecules.

So, Rb₂SO_{4 (s)}  à 2 Rb⁺ _(aq)  + SO₄^-2_(aq)

Simulation

We wanted to understand what was going on when we dissolved a solute in water, which this simulation (an atomic level picture) demonstrated to us.  The simulation showed a salt shaker filled with NaCl, and we “shook” the salt into the water. We all know that if we shake NaCL in water it dissolves (NaCl is a crystal of an Na ion and a Cl ion, but when mixed with water the Na ions and Cl ions separate). We also know that of things that mix with water, some break apart, and some don’t.

The simulation showed lots of H₂O molecules, and on the molecules were +s and –s and a greek lower case delta. The delta indicates “partial” charge, and the presence of the delta indicates that O₂ has a partial – charge and H₂ has a partial + charge.

The simulation then showed us that H₂O is a polar molecule, meaning hydrogen is positive and oxygen is negative. The simulation showed that when we dissolve NaCl in water, the +s from the hydrogens wrap around the Cl- and the –s from the oxygens wrap around the Na+. Water surrounds the Na+ and Cl- and pulls them apart and dissociates them, because the ability of the H₂O to break the NaCl apart is stronger than the ability of the NaCl to stick together. Other ionic compounds haves +s and –s that have the ability to stick together that is stronger than the ability of H₂O to break them apart. H₂O with salt conducts electricity because the +'s and –'s solvated and conduct electricity.

The process outlined above is called Solvation.

The second part of the simulation showed sugar dissolved in water. Sugar molecules shake apart from each other when dissolved in water but do not dissociate because they do not break into the small pieces that make them up; rather,  only their molecules break apart. Water solvates molecules but not the molecular bonds because the molecular bonds are stronger than the ability of water to break them apart, which is why sugar in water does not conduct electricity.

Answers to questions during the simulation:

•  Each granule of sugar is a huge clump of sugar molecules. Moisture results in the crystals sticking together and crystallizing into a lump of sugar, for instance.
• If one evaporates off the water, a crust of salt would remain because dissociation is physical.
• You can only tell if the ionic compound is stronger than water’s ability to break it apart by testing, and there is a set of rules about what dissolves called the “Solubility Rules” that one must memorize.
• Only ionic compounds are strong electrolytes. Electrolyte: a compound that conducts electricity
• Electrolytes are potassium sodium, calcium and magnesium, and all are ions that perform important jobs in the body. For example, the action potential is when potassium is pushed to one side and chlorine to another to set up a gradient. Electrolytes are necessary to set up gradients in the body!
• Dissolve: to mix at the molecular level (sugar and water dissolve but do not dissociate, while salt dissolves and dissociates.)
•   Remember: If dissociates, must dissolve.

Example: Iced Tea

You cannot sweeten iced tea by simply pouring in sugar because the sugar just settles to the bottom. However, if you heat up the tea, the sugar will dissolve and the tea will taste sweet. Then you would cool the tea back to its former cold temperature. The sugar in the cold iced tea is an example of Slight Solubility, and if you heat a slightly soluble solution like iced tea, the heat will get the sugar to completely dissolve.

Slight Solubility: when you shake something into water and it partially dissolves but some does not dissolve, which was also shown in our simulation.

The sugar in iced tea example uses Colligative Properties that explain the way adding a solute changes the properties of the solvent itself.

Sugar in the iced tea that is heated and dissolves and then cooled results in a Supersaturated Solution, which is delicate and can come out of solution very quickly.

This video shows sodium acetate ("Hot Ice"), a supersaturated solution, crystallizing. 

Example: Rock candy is made with supersaturated sugar water

Homework

ChemThink Lab Simulation

The assignment is on the right side, under the heading Labs, under the heading Chemical Reactions, under the heading Precipitate Lab.

Complete the lab and the ChemThink LabSim worksheet that was handed out.

The next scribe post author is Suvd D.