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Key Concepts
Key Concepts
A physical change alters the physical properties of a substance without changing its composition.
A chemical change, also known as a chemical reaction, involves a change in a substance’s composition.
In a chemical reaction, reactants form products.
The law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction; it is conserved.
Changes in matter
Changes in matter
Physical change
Physical change
Chemical change
Chemical change
Chemical Reactions
Chemical Reactions
Law of Conservation of Mass
Law of Conservation of Mass
cold packs
cold packs
ammonium nitrate
ammonium chloride
hot pack
hot pack
magnesium sulfate
calcium chloride
Hot and Cold Packs: A Thermochemistry Activity
A discussion of chemical hot and cold packs can really warm up a classroom lesson on thermochemistry. In the following activity, students use a coffee cup calorimeter to measure the heat of solution of a chemical salt using 3 different masses. It’s recommended for safety—and for a more green chemistry experience—that students use ammonium chloride for the cold packs and calcium chloride for the hot packs. If chemicals are limited, consider having half the students work with 1 chemical and half with the other. After students have measured the heat of solution for the 3 masses, they graph their data (change in temperature vs. mass) and draw a best fit line. The best fit line can be used to determine what mass of chemical is needed to achieve a specific temperature. Students then design a hot and/or cold pack that utilizes 100 mL of water and can be activated when needed. Ensure that you review student designs before allowing them to perform the activity and that students understand and follow appropriate safety protocols.
Background
Calorimetry
Calorimetry is the science of measuring heat. Many chemical and physical transformations involve energy transfer in the form of heat. The magnitude and direction of heat may be determined using a calorimeter. In reactions that occur in aqueous solutions, the energy is transferred to or taken away from the water. A calorimeter is an apparatus that is insulated and prevents heat from flowing in or out of the system. Since the experiment is run under constant pressure (atmospheric), the change in water temperature that is measured is due to the enthalpy of reaction (heat of reaction). The heat of reaction may be calculated using the values measured for change in the water temperature.The heat transfer, or change in enthalpy in a reaction (qrxn), is related to the mass of the solution (m), the specific heat capacity of the solution (c), and the temperature change (ΔΤ = Τfinal — Τinitial).qrxn= – (m × c × ΔΤ)The specific heat capacity of a substance is the amount of energy required to raise 1 g of the substance 1° C. The specific heat capacity of water is 4.186 J / (° C × g). In experiments conducted in aqueous solution, the specific heat capacity of water is generally used.
Instant Hot and Cold Packs
Many instant hot and cold packs function by dissolving a salt into water. As the salt disassociates, heat is either released in an exothermic reaction or absorbed in an endothermic reaction. Commercial instant cold packs typically use either ammonium nitrate or urea as their salt component; hot packs often use magnesium sulfate or calcium chloride. These reactions happen in a similar manner. When the salt is dissolved in water, the ionic bonds of the salt separate. This process requires energy, which is obtained from the surroundings. The ions then form bonds with the water, a process that releases energy. If more energy is released than taken in, then the process is exothermic, making the solution feel warmer. If more energy is taken in than released, then the process is endothermic, making the solution feel cooler.Commercially, there are 2 other commonly sold types of instant hot packs. One heats up when exposed to air. This hot pack functions as iron reacts with oxygen to form iron (III) oxide, an exothermic reaction. The other type relies on the super cooling of sodium acetate. Upon heating the solution, it can become supersaturated. Without a seed crystal, the sodium acetate will remain in solution as it cools. This type of hot pack typically contains a metal disk that provides a site for crystallization when depressed. As the sodium acetate forms a regular crystal arrangement, heat is released. This hot pack is reusable as it can be regenerated in boiling water to once again form the supersaturated solution.
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