Guided Inquiry

Have your fingers ever been so cold they felt numb? Wouldn’t it be great if you could generate heat to warm your hands up anytime you want to? That’s exactly what a “hand warmer” does. Hand warmers are small packets that people put inside gloves or mittens on cold days to keep their fingers warm. They are very popular with people who work outside in winter or who do winter sports. One type of hand warmer contains water in one section of the packet and a soluble substance in another section. When the packet is squeezed, the water and the soluble substance are mixed, the solid dissolves, and the packet becomes warm. In this experiment, you will learn how a hand warmer works and use chemistry to design an effective, safe, environmentally benign, and inexpensive hand warmer.

Breaking bonds and particulate attractions (“intermolecular/interparticle forces”) absorbs energy from the surroundings, while forming new bonds and particulate attractions releases energy to the surroundings. When an ionic solid dissolves in water, ionic bonds between cations and anions in the ionic solid and hydrogen bonds between water molecules are broken, and new attractions between water molecules and anions and water molecules and cations are formed. The amount of energy required to break these bonds and form new ones depends on the chemical properties of the particular anions and cations. Therefore, when some ionic solids dissolve, more energy is required to break the cation-anion bonds than is released in forming the new water-ion attractions, and the overall process absorbs energy in the form of heat. When other ionic compounds dissolve, the converse is true, and the bond making releases more energy than the bond breaking absorbs, and therefore the process overall releases heat. When heat is absorbed, the enthalpy change, q, is endothermic, and the enthalpy change is positive. When heat is released, the change is exothermic, and the value of q is negative.

Materials provided: Thermometers (or probes), Balance, 100-mL graduated cylinder, 150-mL beaker, Disposable pipet, Distilled water, Sodium chloride, Calcium chloride, Sodium carbonate, styrofoam calorimeter, Magnesium sulfate, Sodium acetate, Lithium nitrate, and Ammonium nitrate,

Challenge:
An ideal hand warmer increases in temperature by 20 °C (but no more) as quickly as possible, has a volume of about 50 mL, costs as little as possible to make, and uses chemicals that are as safe and environmentally friendly as possible. Determine which substances, Sodium chloride, Calcium chloride, Sodium carbonate, Magnesium sulfate, Sodium acetate, Lithium nitrate and Ammonium nitrate, and in what amounts, to use in order to make a hand warmer that meets these criteria.

1. When chromium chloride, CrCl₂, is dissolved in water, the temperature of the water decreases.
(a) Is the heat of solution exothermic or endothermic?
(b) Which is stronger—the attractive forces between water molecules and chromium and chloride ions, or the combined ionic bond strength of CrCl2 and intermolecular forces between water molecules? Explain.

2. A solution was formed by combining 25.0 g of solid A with 60.0 mL of distilled water, with the water initially at 21.4 °C. The final temperature of the solution was 25.3 °C. Calculate the heat released as the solid dissolved, qsoln, assuming no heat loss to the calorimeter.

3. In Question 2 above, the calorimeter was found to have a heat capacity of 8.20 J/°C. If a correction is included to account for the heat absorbed by the calorimeter, what is the heat of solution, qsoln?

4. The solid in Question 2 was aluminum sulfate, Al₂(SO₄)₃. Calculate the molar heat of solution, ΔHsoln, for aluminum sulfate. Hint: The units for molar heat of solution are kilojoules per mole (kJ/mole). First determine the heat released per gram of solid and then convert from grams to mole.

MSDS:
Safety precautions/hazards and disposal for each substances to be tested

Procedure:
Write a detail step by step explanation of what you will do to address the challenge and include labeled drawing to support your explanation. Must include testing of at least three different solids and one of the solids you must test again with a different amount of water.

Substance	2019 Cost per 500 g ($)
NaCl	5.10
CaCl₂	7.75
NaC₂H₃O₂	15.25
Na₂CO₃	10.90
LiNO₃	67.50
NH₄NO₃	18.85
MgSO₄	7.45

Data Table:
Create a data table (or two if necessary) to collect all the necessary data to address the challenge (your group data as well as other classmates data to collaborate.)

4. Are the dissolving processes that were carried out by the class endothermic or exothermic or neither? Justify your response with class data from the investigation.

5. Dissolving ionic compounds involves the separation of the solid ionic compound into cations and anions in water. This process can be represented by an equation showing the solid as a reactant and the aqueous ions as products. The heat of reaction ΔHsoln is written after the products in units of kJ/mol rxn. For example, sodium hydroxide dissolves exothermically, releasing 44.2 kilojoules per mole dissolved. This process is represented as: NaOH (s) --> Na+ (aq) + OH- (aq) ΔHsoln = -44.2 kJ/mol rxn Write equations to similarly represent the dissolving process for each of the four salts that you studied. Include your calculated heat of reaction as in the example.

6. Changes in matter are generally classified as physical or chemical based on whether new substances are formed through the process. Does dissolving represent a physical change, a chemical change, or an intermediate change (combination of the two)? Explain your reasoning, citing evidence from the animation you viewed earlier.

Conclusion:
As usual :) Make sure to discuss the actual precision of your measuring tools and precision of data collection. ( Ex: the scale we used measure to the hundredth of a gram or +/- 0.01)