Project the animation Breaking and Making Bonds. Tell students that there is an important rule in chemistry: Energy is required to pull apart atoms, ions, or molecules that are attracted to each other. But when atoms, ions, or molecules come together, energy is released. Project the animation Energy and Dissolving. When water molecules are attracted to and bond to the molecules or ions of a substance, some energy is released as shown by the arrow going out.
Then the water molecules pull ions or molecules of the substance apart, which takes energy, as shown by the arrow going in. Project the image Exothermic Dissolving. Because more energy is released than is used, the molecules of the solution move faster, making the temperature increase. Project the image Endothermic Dissolving. Because less energy is released than is used, the molecules of the solution move more slowly, making the temperature decrease.
In the hand warmer, the water molecules and the ions of the solute come together to form a crystal. Bending the metal disk creates tiny scratches, which act as nucleation points where the sodium acetate crystal forms.
As the water molecules and ions bond together in the growing crystal, energy is released. This results in an increase in temperature. Project the video Temperature Alcohol in Water. The American Chemical Society is dedicated to improving lives through Chemistry. Skip Navigation. Lesson 5. Engage Allow students to feel the temperature change in an activated cold pack and an activated hot pack.
Materials for the Demonstration 1 disposable cold pack 1 disposable hot pack Procedure Select two student volunteers—one to activate one cold pack and another to activate one hot pack.
Have each student feel each bag and guess what is inside each. They should notice a dry pellet-like solid and a fluid-filled bag. Point out that the bags do not feel cold or hot yet. Pass the cold and hot packs around the room. Expected Results The cold pack quickly becomes cold while the hot pack quickly becomes hot. Do a demonstration to show how cold and hot packs work. Materials for the Demonstration 1 disposable cold pack 1 disposable hot pack Graduated cylinder, 50 mL or smaller Water room temperature 2 clear plastic cups 2 thermometers 1 teaspoon Procedure Carefully cut open one cold pack and one hot pack.
Show students the contents, but do not handle or allow students to handle the solid substance inside the packs.
Tell students that the liquid inside the fluid-filled bags in both the cold and hot packs is water. Pour about 10 mL of room-temperature water in two separate clear plastic cups. Place a thermometer in each cup and select two student volunteers to tell the class the starting temperature of the water in each cup.
With the thermometer still in the cup, place about 1 teaspoon of the substance from the hot pack in the water in the other cup. Have the class watch the thermometer and then ask a student to tell the class the highest temperature of the solution. Introduce the terms endothermic and exothermic.
Explore Introduce the dissolving activity students will do and help students identify the variables. Introduce the crystals students will dissolve: Potassium chloride is a common salt substitute.
The formation of this solution clearly involves an increase in disorder, since the helium and argon atoms occupy a volume twice as large as that which each occupied before mixing.
Ideal solutions may also form when structurally similar liquids are mixed. Placing methanol and ethanol, or pentane and hexane, in the bulbs shown in Figure 2 will result in the same diffusion and subsequent mixing of these liquids as is observed for the He and Ar gases although at a much slower rate , yielding solutions with no significant change in energy.
Unlike a mixture of gases, however, the components of these liquid-liquid solutions do, indeed, experience intermolecular attractive forces. But since the molecules of the two substances being mixed are structurally very similar, the intermolecular attractive forces between like and unlike molecules are essentially the same, and the dissolution process, therefore, does not entail any appreciable increase or decrease in energy.
These examples illustrate how diffusion alone can provide the driving force required to cause the spontaneous formation of a solution. In some cases, however, the relative magnitudes of intermolecular forces of attraction between solute and solvent species may prevent dissolution. Three types of intermolecular attractive forces are relevant to the dissolution process: solute-solute, solvent-solvent, and solute-solvent.
As illustrated in Figure 3 , the formation of a solution may be viewed as a stepwise process in which energy is consumed to overcome solute-solute and solvent-solvent attractions endothermic processes and released when solute-solvent attractions are established an exothermic process referred to as solvation. The relative magnitudes of the energy changes associated with these stepwise processes determine whether the dissolution process overall will release or absorb energy.
In some cases, solutions do not form because the energy required to separate solute and solvent species is so much greater than the energy released by solvation. For example, cooking oils and water will not mix to any appreciable extent to yield solutions Figure 4. Hydrogen bonding is the dominant intermolecular attractive force present in liquid water; the nonpolar hydrocarbon molecules of cooking oils are not capable of hydrogen bonding, instead being held together by dispersion forces.
Forming an oil-water solution would require overcoming the very strong hydrogen bonding in water, as well as the significantly strong dispersion forces between the relatively large oil molecules.
And, since the polar water molecules and nonpolar oil molecules would not experience very strong intermolecular attraction, very little energy would be released by solvation. On the other hand, a mixture of ethanol and water will mix in any proportions to yield a solution. In this case, both substances are capable of hydrogen bonding, and so the solvation process is sufficiently exothermic to compensate for the endothermic separations of solute and solvent molecules.
As noted at the beginning of this module, spontaneous solution formation is favored, but not guaranteed, by exothermic dissolution processes. While many soluble compounds do, indeed, dissolve with the release of heat, some dissolve endothermically.
Ammonium nitrate NH 4 NO 3 is one such example and is used to make instant cold packs for treating injuries like the one pictured in Figure 5. A thin-walled plastic bag of water is sealed inside a larger bag with solid NH 4 NO 3. When the smaller bag is broken, a solution of NH 4 NO 3 forms, absorbing heat from the surroundings the injured area to which the pack is applied and providing a cold compress that decreases swelling.
Endothermic dissolutions such as this one require a greater energy input to separate the solute species than is recovered when the solutes are solvated, but they are spontaneous nonetheless due to the increase in disorder that accompanies formation of the solution. 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. Breaking solvent-solvent attractions endothermic , for instance that of hydrogen bonding Forming solvent-solute attractions exothermic , in solvation. Improve this answer. F'x F'x Lots of reactions are endothermic. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown.
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