Christine Musich Science 660 04 April 2013 Misconceptions: Is Dissolving the Same as Melting?

Christine MusichScience 66004 April 2013

Misconceptions: Is Dissolving the Same as

Melting?

•NO, it is not!•Dissolution is the process by which a solute is

dissolved by a solvent and goes into solution-It is related to strength of intermolecular forces

between the solute and solvent •Melting (or fusion) is the phase change from a

solid to a liquid-Intermolecular forces between the particles of a

substance are important

Is Dissolving the Same as Melting?

• In both melting and dissolving, a solid appears to turn into a liquid•The use of hot water speeds up dissolving, just as heat speeds up melting.

Sources of the Misconception

Regents Chemistry: Dissolution of Sodium Chloride (NaCl)

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/molvie1.swf

• Sucrose. In this space filling model, red represents oxygen, light gray represents hydrogen, and dark gray represents carbon.• http://www.inquiryinaction.org/f/chapter_1/water_dissolves_sugar.mov

Regents Chemistry: Dissolution of Sucrose

Regents Chemistry: Dissolution• Involves mixtures (solutions)• Increasing the temperature is not a requirement, although it does increase the solubility of a solute and the rate at which the solute dissolves.

Table G:Solubility Curvesat StandardPressure

Regents Chemistry: Melting of Ice

• As ice is heated (energy is added), the motion of molecules increases. • Eventually, the motion overcomes the attraction between molecules, and the ice melts and becomes a liquid.• Entropy increases

Regents Chemistry: Melting of Ice

Regents Chemistry: Particle Diagrams

Regents Chemistry: Heating Curves

Regents Chemistry: Heating Curves

AP Chemistry: Solutions at Equilibrium Learning Objectives:•The student can predict the solubility of a salt, or rank the solubility of salts, given the relevant Ksp values.

•The student can interpret data regarding solubility of salts to determine or rank, the relevant Ksp values.

•The student can interpret data regarding the relative solubility of salts in terms of factor (common ions, pH) that influence the solubility.•The student can analyze the enthalpic and entropic changes associated with the dissolution of a salt, using particulate level interactions and representations.

AP Chemistry: Solutions at Equilibrium

•The dissolution of a substance in a solvent is a reversible reaction, and so has an associated equilibrium constant. For dissolution of a salt, the reaction quotient, Q, is referred to as the solubility product, and the equilibrium constant for this reaction is denoted as Ksp, the solubility-product constant.•The solubility of a substance can be calculated from the Ksp, for the dissolution reaction. This relation can also be used to reason qualitatively about the relative solubility of different substances.

Ag2CO3 2Ag+ + CO32-

Ksp = [Ag+]2 [CO32-]

= [2 *(1.2 x 10-4)]2 [1.2 x 10-

4] = 6.9 x 10-12

Calculate Ksp of Silver Carbonate if it’s solubility is 1.2 x 10-4 M.

AP Chemistry: Solutions at Equilibrium

AP Chemistry: Solutions at Equilibrium

DG = DH - TDS

•The free energy change (DG°) for dissolution of a substance reflects both the breaking of the forces that hold the solid together and the interaction of the dissolved species with the solvent. In addition, entropic effects must be considered. Qualitative reasoning regarding solubility requires consideration of all of these contributions to the free energy.

College Level: The Thermodynamics of Dissolution: EnthalpyDivide the process into 3 steps to determine the enthalpy change:

1. Separation of solute particles.2. Separation of solvent

particles.3. Formation of new interactions

between solute and solvent.

College Level: Enthalpy Change in forming Solution

Start

End

EndStart

DHsoln = DH1 + DH2 + DH3

AP Chemistry: Phase Changes

•The student is able to use calculations or estimation to relate energy changes associated with a phase transition to the enthalpy of fusion/ vaporization, relate energy changes associated with a chemical reaction to the enthalpy of the reaction, and relate energy changes to PDV work.