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Solutes and Solubility

Investigation 2 – Concept Day

 

 

 

 

 

 

 

Solutes and Solubility: Investigation 2

Concept Day

SLIDE SOL-2-1

Note: This is a very simple Investigation. What we want to accentuate that solubility requires the molecular interaction between solute and solvent molecules. In the experiments that you will perform in this lab, baking soda (sodium bicarbonate), salt (NaCl), or sugar (C12H22O11, sucrose) will be dissolved in 100ml of water at room temperature.

As a means of increasing molecular collisions, stirring will or will not be applied to samples of solutes in the solvent water.

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SLIDE SOL-2-2

  • This first slide was used in the previous Investigation, Investigation 1. It is included here simply to review to potential outcomes of adding an identical amount of solute to an identical amount of solvent.
  • As shown on the right of the slide, three potential results are possible. Either the solvent is completely insoluble (top) or soluble (middle) in the solvent, or it is partially soluble (bottom beaker) and saturates the solution.
  • The extent to which the solubility reaction proceeds is dependent in part on the amount of kinetic energy applied to the reaction.
  • In this Investigation, we will explore the effect of kinetic energy in the form of physical mixing on the solubility of solutes in solvents (water).

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SLIDE SOL-2-3

  • In this slide, we are focusing on one particular solute, salt (NaCl). As shown in this illustration, solid NaCl forms a crystal in which the positively charged sodium molecules interact with negatively charged chloride molecules in the NaCl geometric conformation.

Note: You may well be able to recall that table salt forms regular geometric shapes, crystals. Thus, the macroscopic samples that we can observe in everyday life are the result of the underlying molecular arrangements of the molecules in table salt (NaCl).

Note: This is one of the solid solutes that you will add to water in your experiments in Investigation 2 Lab.

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SLIDE SOL-2-4

  • This slide depicts the arrangement of water molecules in the liquid state. The molecules move freely and interact with each other loosely. They form and break “hydrogen bonds” between the hydrogen atom of one water molecule and the oxygen atom of an adjacent water molecule.

  • A solid sample of sodium chloride salt (NaCl) is then added to the water solution.
  • The question must now be posed: what will happen on a molecular level in order for the atoms in the salt molecule to interact with the atoms in the solvent molecule.
  • The answer to this question is shown on the following slide.

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SLIDE SOL-2-5

  • In order for the salt molecules (NaCl) to become soluble in the solvent (H2O) solution, molecular collisions must occur that break the salt molecules apart into positively charged sodium ions (Na+) and negatively charged chloride ions (Cl).
  • Once this dissociation occurs, the positively charged sodium ions (Na+) will associate with the negatively charged oxygen atoms of the water molecule, and the negatively charged chloride ions of the solute will interact with the positively charged hydrogen atoms of the solvent.
  • These ionic, charged-mediated interactions act to pull the solid sodium chloride salt (NaCl) apart, solubilizing it in the water solvent.
  • Notice how each of the ions from the solute NaCl ( Na+ and Cl) are surrounded by their opposite charges from the water molecules (the solvent).

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SLIDE SOL-2-6

  • This slide shows a soluble solution of NaCl in water. Each Na+ ion from the solute is surrounded by negatively charged oxygen atoms of the solvent, water. Also, each negatively charged chloride ion from the solute NaCl is surrounded by positively charged hydrogen atoms of the solvent molecule, water.
  • The following slide will show what happens if more solid salt (NaCl) is added than the amount of solvent water molecules can surround and solubilize.

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SLIDE SOL-2-7

  • This slide shows a saturated NaCl solution. All of the water molecules present in the solution are engaged in positive-negative or negative-positive ionic interactions with the solvent atoms.
  • There are not enough free water molecules to interact with additional solute NaCl molecules to aid in their solubilization.
  • As a result, some solid, insoluble NaCl molecules are found to sink to the bottom of the beaker or flask and remain solid and non-soluble in a saturated NaCl/water solution.

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SLIDE SOL-2-8

  • This slide simply emphasizes the importance of molecular collisions in the solubilization reaction.
  • In order to increase the number of molecular collisions, one must increase the movement of molecules in the solution so that the proper hydrogen-chloride ion and oxygen-sodium ion interactions are optimized.
  • As suggested, increasing the amount of kinetic energy, the energy of movement and motion, will increase the chances of the proper molecular interactions that favor solubilization.

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SLIDE SOL-2-9

  • The point of this slide is that mixing a solution adds kinetic energy, which increases the chances of molecular and atomic interactions, which leads to solubilization of the solute molecules in the solution of solvent molecules.

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SLIDE SOL-2-10

  • This final slide simply explains where the kinetic energy of mixing comes from. It comes from the arm muscles, in this case, of the scientist that is swilling this flask.

Note: Where does the energy in the scientist’s muscles came from. The answer would be from the food the scientist consumed. Where did this energy come from? It came from vegetable matter that consumed energy from the Sun through photosynthesis and produced the carbohydrates that the scientist ate.

  • Energy must be applied to the solute/solvent system to increase the solubility reaction. In this Investigation (Investigation 2) this energy is supplied by the kinetic energy of the physical movement of stirring. In Investigation 3, we will see that we can increase kinetic energy and solubility by an entirely different mechanism.