A student dissolves 11.0 g of lithium chloride (Lici) in 200. g of water in a well-insulated open cup. Dissolving the substance from the cold pack will cause the temperature to decrease to less than 10 °C (endothermic). It takes energy to break the bonds between the molecules or ions of the solute. 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. The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan. Because the sodium acetate is in solution, you can see the metal disc inside the pack. 1. Note: Comparing the amount of temperature change for different substances by dissolving the same mass of each substance in the same amount of water is fine at the middle school level. 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! The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Many exothermic and endothermic reactions involve toxic chemicals, extreme heat or cold, or messy disposal methods. It is a strong attraction caused by water’s polarity. An example of an easy endothermic reaction is dissolving potassium chloride (sold as a salt substitute) in your hand with water. The enthalpy of solution (ΔHsoln) is the heat released or absorbed when a specified amount of a solute dissolves in a certain quantity of solvent at constant pressure. Download the student activity sheet, and distribute one per student when specified in the activity. But in the case of KCl, the first endothermic step absorbs more heat than it is releasing in the second step. After the hot pack has been agitated, the sodium acetate crystallizes (right) to release heat. Read about our approach to external linking. This results in an increase in temperature. Project the video Temperature Alcohol in Water. Describe the distinction between ΔHsoln and ΔHf. Endothermic reactions take in energy and the temperature of the surroundings decreases. This shows that the reaction is exothermic. Tell students that scientists describe temperature changes that occur when substances interact as either endothermic or exothermic. Students will feel the temperature change that occurs when a cold pack and a hot pack are activated. Tell students that they will explore how some hot and cold packs work. Pour the potassium chloride into the water and swirl the cup. The energy change is negative. Are you loving this? Press the “next” button and explain that this happens in dissolving. 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. Potassium chloride may be purchased at a grocery store under the brand name Nu-Salt Salt Substitute or from Flinn Scientific, product number P0042. The pack can be reused after it is immersed in hot water until the sodium acetate redissolves. Project the animation Energy and Dissolving. Tell students that the liquid inside the fluid-filled bags in both the cold and hot packs is water. Have them shake the packs to get the fluid to spread throughout the bag. The same calorimeter was used for the dissolution of 8.86 g sample of lithium chloride in 100.0 mL of water at 22.2 °C. Watch the thermometer. Because ΔHsoln depends on the concentration of the solute, diluting a solution can produce a change in enthalpy. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Single-use versions of these products are based on the dissolution of either calcium chloride (CaCl 2, ΔH soln = −81.3 kJ/mol) or ammonium nitrate (NH 4 NO 3, ΔH soln = +25.7 kJ/mol). Dissolving the substance from the hot pack will cause the temperature to increase to over 40 °C (exothermic). Most middle school textbooks and curricula associate a change in temperature only with chemical change. [ "article:topic", "enthalpy of solution", "hypothesis:yes", "showtoc:yes" ], An Instant Hot Pack Based on the Crystallization of Sodium. This phenomenon is particularly relevant for strong acids and bases, which are often sold or stored as concentrated aqueous solutions. Bending the metal disk creates tiny scratches, which act as nucleation points where the sodium acetate crystal forms. The urea-based cold pack (Morrison Medical Insta-Cold Compress) is available by the case (24) from Quick Medical Equipment and Supplies (product #6601). Modified by Joshua Halpern (Howard University), Scott Sinex, and Scott Johnson (PGCC). Thanks! Please consider taking a moment to share your feedback with us. Record this initial temperature in the chart on the activity sheet. When lithium iodide (LiI) is dissolved in water, the solution becomes hotter. If 1 mol of each solute is dissolved in 500 mL of water, rank the resulting solutions from warmest to coldest.

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