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Watch this brief video illustrating endothermic and exothermic dissolution processes.
A solution forms when two or more substances combine physically to yield a mixture that is homogeneous at the molecular level. The solvent is the most concentrated component and determines the physical state of the solution. The solutes are the other components typically present at concentrations less than that of the solvent. Solutions may form endothermically or exothermically, depending upon the relative magnitudes of solute and solvent intermolecular attractive forces. Ideal solutions form with no appreciable change in energy.
How do solutions differ from compounds? From other mixtures?
A solution can vary in composition, while a compound cannot vary in composition. Solutions are homogeneous at the molecular level, while other mixtures are heterogeneous.
Which of the principal characteristics of solutions can we see in the solutions of K 2 Cr 2 O 7 shown in [link] ?
When KNO 3 is dissolved in water, the resulting solution is significantly colder than the water was originally.
(a) Is the dissolution of KNO 3 an endothermic or an exothermic process?
(b) What conclusions can you draw about the intermolecular attractions involved in the process?
(c) Is the resulting solution an ideal solution?
(a) The process is endothermic as the solution is consuming heat. (b) Attraction between the K + and ions is stronger than between the ions and water molecules (the ion-ion interactions have a lower, more negative energy). Therefore, the dissolution process increases the energy of the molecular interactions, and it consumes the thermal energy of the solution to make up for the difference. (c) No, an ideal solution is formed with no appreciable heat release or consumption.
Give an example of each of the following types of solutions:
(a) a gas in a liquid
(b) a gas in a gas
(c) a solid in a solid
Indicate the most important types of intermolecular attractions in each of the following solutions:
(a) The solution in [link] .
(b) NO( l ) in CO( l )
(c) Cl 2 ( g ) in Br 2 ( l )
(d) HCl( aq ) in benzene C 6 H 6 ( l )
(e) Methanol CH 3 OH( l ) in H 2 O( l )
(a) ion-dipole forces; (b) dipole-dipole forces; (c) dispersion forces; (d) dispersion forces; (e) hydrogen bonding
Predict whether each of the following substances would be more soluble in water (polar solvent) or in a hydrocarbon such as heptane (C 7 H 16 , nonpolar solvent):
(a) vegetable oil (nonpolar)
(b) isopropyl alcohol (polar)
(c) potassium bromide (ionic)
Heat is released when some solutions form; heat is absorbed when other solutions form. Provide a molecular explanation for the difference between these two types of spontaneous processes.
Heat is released when the total intermolecular forces (IMFs) between the solute and solvent molecules are stronger than the total IMFs in the pure solute and in the pure solvent: Breaking weaker IMFs and forming stronger IMFs releases heat. Heat is absorbed when the total IMFs in the solution are weaker than the total of those in the pure solute and in the pure solvent: Breaking stronger IMFs and forming weaker IMFs absorbs heat.
Solutions of hydrogen in palladium may be formed by exposing Pd metal to H 2 gas. The concentration of hydrogen in the palladium depends on the pressure of H 2 gas applied, but in a more complex fashion than can be described by Henry’s law. Under certain conditions, 0.94 g of hydrogen gas is dissolved in 215 g of palladium metal.
(a) Determine the molarity of this solution (solution density = 1.8 g/cm 3 ).
(b) Determine the molality of this solution (solution density = 1.8 g/cm 3 ).
(c) Determine the percent by mass of hydrogen atoms in this solution (solution density = 1.8 g/cm 3 ).
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