Not loving this? Have the class watch the thermometer and then ask a student to tell the class the lowest temperature of the solution. After the hot pack has been agitated, the sodium acetate crystallizes (right) to release heat. A student dissolves 11.0 g of lithium chloride (Lici) in 200. g of water in a well-insulated open cup. Hydrogen bromide decomposes to form hydrogen and bromine: The energy change is positive. Bending the metal disk creates tiny scratches, which act as nucleation points where the sodium acetate crystal forms. Sodium carbonate, anhydrous may be purchased from Flinn Scientific, product number S0052. This shows that the reaction is endothermic. Endothermic reactions take in energy and the temperature of the surroundings decreases. Note: This activity deals with a concept that is not often addressed in middle school—that a temperature change occurs during the process of dissolving. The process of dissolving can be endothermic (temperature goes down) or exothermic (temperature goes up). With the thermometer still in the cup, add about 1 teaspoon of the solid substance from the cold pack to the water in one cup. 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. If it takes more energy to separate the particles of the solute than is released when the water molecules bond to the particles, then the temperature goes down (endothermic). Potassium chloride may be purchased at a grocery store under the brand name Nu-Salt Salt Substitute or from Flinn Scientific, product number P0042. Point out that the bags do not feel cold or hot yet. It is a strong attraction caused by water’s polarity. Tell students that the liquid inside the fluid-filled bags in both the cold and hot packs is water. Thus you should never add water to a strong acid or base; a useful way to avoid the danger is to remember: Add water to acid and get blasted! We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. The energy change is negative. When the salt was dissolved, the temperature of the solution was 39.5°C. Dissolving the substance from the cold pack will cause the temperature to decrease to less than 10 °C (endothermic). Have them shake the packs to get the fluid to spread throughout the bag. Show students the contents, but do not handle or allow students to handle the solid substance inside the packs. The process of dissolving is endothermic when less energy is released when water molecules “bond” to the solute than is used to pull the solute apart. The hot packs (Rapid Aid Instant Warm Pack) can be purchased by the case (24) or singly from Lab Safety and Supply (product #144707). But to compare which substance is more or less exothermic or endothermic than another, as students will do in the activity, the solute will be measured in grams. A bond energy is the amount of energy needed to break one mole of a particular covalent bond. Results may vary. The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan. Calcium chloride is used to absorb moisture from the air. Students will be able to correctly classify the process of dissolving as either exothermic or endothermic for each solute. Use this related reading to extend student comprehension after completing the lesson. Download the student activity sheet, and distribute one per student when specified in the activity. This shows that the reaction is exothermic. Then the water molecules pull ions or molecules of the substance apart, which takes energy, as shown by the arrow going in. The cold and hot packs used in this lesson contain urea (cold pack) and magnesium sulfate (hot pack) sealed in a bag with a water-filled bag inside. If the initial dissolution process is exothermic (ΔH < 0), then the dilution process is also exothermic. The process of dissolving is exothermic when more energy is released when water molecules “bond” to the solute than is used to pull the solute apart. Read more about the energy changes in making and breaking bonds in the context of dissolving in the teacher background section. Energy is released when water molecules bond to the solute molecules or ions. Direct the students to activate their packs by following the instructions on the package. Because the sodium acetate is in solution, you can see the metal disc inside the pack. Project the image Endothermic Dissolving. But when atoms, ions, or molecules come together, energy is released. If water is added to a concentrated solution of sulfuric acid (which is 98% H2SO4 and 2% H2O) or sodium hydroxide, the heat released by the large negative ΔH can cause the solution to boil. The energy change in a reaction can be calculated using bond energies. Exothermic reactions transfer energy to the surroundings and the temperature of the surroundings increases. Which solute dissolves the most endothermically and which dissolves the most exothermically in water? For more information contact us at email@example.com or check out our status page at https://status.libretexts.org. Many exothermic and endothermic reactions involve toxic chemicals, extreme heat or cold, or messy disposal methods. 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. Sodium bicarbonate, also known as baking soda, is used in baking, in toothpaste, and numerous other applications. Students will then compare the temperature changes that occur as four different solutes dissolve in water and classify these as either endothermic or exothermic. This phenomenon is particularly relevant for strong acids and bases, which are often sold or stored as concentrated aqueous solutions. Explain that the “bonding” of water molecules to alcohol molecules releases more energy than it takes to separate the alcohol molecules from each other. These hand warmers, called The Heat Solution, are available from Flinn Scientific, product number AP1933, and a variety of other vendors. Sodium bicarbonate is baking soda. Copyright © 2020 American Chemical Society. Record this initial temperature in the chart on the activity sheet. One way to say it is, “It takes energy to break bonds, and energy is released when bonds are formed.”. Legal. Since the purpose of this demonstration is to show whether the temperature simply goes up or down, this type of volume measure is fine. 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. Endothermic reactions take in energy and the temperature of the surroundings decreases. Tell students that they will explore how some hot and cold packs work. Gently swirl the cup to help the substance dissolve. The process of dissolving can be endothermic (temperature goes down) or exothermic (temperature goes up). Is the dissolution of lithium iodide endothermic or exothermic? Tell students that they will compare how much the temperature changes when four household substances dissolve in water. 1. The cold pack quickly becomes cold while the hot pack quickly becomes hot. It takes energy to break the bonds between the molecules or ions of the solute. But in the case of KCl, the first endothermic step absorbs more heat than it is releasing in the second step. As the water molecules and ions bond together in the growing crystal, energy is released. Because ΔHsoln depends on the concentration of the solute, diluting a solution can produce a change in enthalpy.
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