Hand warmers are cold weather devices used to provide warmth to cold
hands and fingers. Many modern hand warmers consist of a package consisting of an outer plastic package containing some solid and an inside pouch containing water. The package is activated by squeezing it in one’s hand, causing the solid to dissolve in the water which produces the temperature change.
Solvation occurs because of the polar nature of water and the ionic compound inside the hand warmer. Compounds such as water are polar because one of the atoms in a molecule, which in the case of water is oxygen, has a higher electronegativity and thus attracts the electrons within the electron cloud of the molecule slightly towards it. If every side of a molecule has the same charge as a result of this difference in electronegativity, such as in a molecule like CH4 where all sides of the molecule have a positively charged hydrogen atom, then the molecule is said to be non polar. If there is an imbalance between the distribution of charges, such as for water where the one oxygen atom has a negative charge while the two hydrogen atoms have a positive charge, then the molecule is said to be polar. Because both water molecules and ionic compounds are polar, the positive end of the water molecule is attracted to the negative end of a solute molecule (and vice versa). For ionic solutes such as salt (as shown in the diagram), this attraction causes the salt to split into sodium and chlorine ions which are attracted to the negative and positive ends of the water molecule respectively. All of the compounds being tested in this lab as potential substances for a hand warmer have an ionic component and as such will split into two atoms and/or molecules during solvation.
Along with the polarizability of the solute, other factors that affect solvation include the temperature of the solvent, the surface area of the solute, and the agitation that is used to dissolve the solution. The temperature of the solvation can affect the kinetic energy that is within the molecules of the solution and thus gives more energy for the solute and solvent molecules to move away from like molecules and become attracted to each other. This also creates more openings within the solution for solute ions to squeeze between polar water molecules, thus allowing for the solute to dissolve in the solvent. The surface area of the solute particles affects solvation by affecting how much of the solute is in contact with the water. This can end up making the solvation process either quicker or slower as the more molecules there are which are not touching the water, the more energy is needed to separate and dissolve these particles, thus slowing down the solvation rate. The agitation of the solution also affects the kinetic energy that is within the molecules of the solution and thus gives more energy for solute ions to squeeze between polar water molecules, thus allowing for the solute to dissolve in the solvent at different rate. All three of these factors must be accounted for when conducting the lab by ensuring that the solutes and solvent are kept at a constant temperature for all trials, the solute is ground into a fine powder using a mortar and pestle, and that all of the solutions are stirred at a constant speed in the same manner.
The importance of solvation in the context of a hand warmer is that the solvation of the inner compound with the water is that it allows for heat to be released from the warmer to the hands. Solvation can either be exothermic, where heat energy is released, or endothermic, where heat energy is absorbed. Ideally, the solvation in hand warmers would be exothermic because the released heat energy is what ends up being used to warm the hands. This idea of releasing or absorbing energy follows the first law of thermodynamics which states that energy cannot be created or destroyed. Because the energy used to dissolve a substance must remain the same before and after solvation, heat energy may either be absorbed or released to ensure that the energy on both sides of the process remains the same. The energy or enthalpy change associated with solvation (ΔHsoln) is equal to the sum of the energy required to separate the solute (ΔH1 = +C kJ/mole) and solvent (ΔH2 = +D kJ/mole) molecules minus the energy released when the dipole interaction between the solute and solvent molecules occurs (ΔH3 = −F kJ/mole), as shown in the equation ΔHsoln = ΔH1 + ΔH2 + ΔH3 = (+ C + D −F) kJ/mole. If the sum of the energy required to separate the solute and solvent molecules is greater than the energy released by the dipole interaction, then the solvation is said to be exothermic. If it is the other way around, then the solvation is endothermic.
The amount of heat that is either absorbed or released through the solvation can be measured by utilizing a calorimeter. A calorimeter is a device that measures the heat flow that a substance experiences through processes such as solvation and reactions. Two types of calorimetry which are used to measure change in thermal energy are constant pressure calorimetry, which is a simple process that measures the heat of a reaction/solvation within a solution that involves the use of insulated cups, and constant volume calorimetry, which is a more complex process that used an heavily insulated “bomb” to hold and measure the heat of a combustion reaction. This lab will utilize constant pressure calorimetry as it utilizes 2 polystyrene cups to measure the heat change from solvation.
The amount of heat transfer (q) which occurs during solvation or any reaction can be calculated using the equation q = mcΔT, where m is the total mass of the solution, c is the specific heat constant, which in the case of a hand warmer is assumed to be that of water (4.186 J/g °C), and ΔT is the change in temperature. In an exothermic reaction such as that found in a hand warmer, while this value makes up most of the total heat transfer of the system, some of the heat energy is lost into the calorimeter. In order to calculate this energy, a constant known as the calorimeter constant must be calculated. This constant, referred to as Ccal, is important as it remains the same for all of the trials during the lab due to the same cup being used to dissolve the substances and can be used to find the heat transfer into the calorimeter using the equation q = ΔT * Ccal. The Ccal of the nested polystyrene cups can be found by stirring hot and cold water of known temperatures in the calorimeter for 20 seconds, measuring temperature of the combined water, finding the specific heat of the water using q = mc(Tavg – Tfin) where Tavg is the average of the hot and cold water temperatures and Tfin is the final measured temperature of the combined water, and dividing the negative value of q by the difference between Tfin and the cool water temp. These steps to find the Ccal of the calorimeter are listed out in the part A introductory activity. Finding the heat transfer into the calorimeter will allow for the overall solution heat transfer to be calculated by using the formula q = – (q aq – q cal) where q aq is the heat transfer into the solution and q cal is the heat transfer into the water.
Finally, one factor which must be kept into consideration when selecting a suitable hand warmer compound is the cost efficiency of the compound. Even if a substance provides the greatest temperature increase out of all those tested, if the substance costs significantly more per gram than another substance which is not too far away in terms of temperature increase, then it would be illogical to utilize the better performing substance due to the cost.