Hand Warmer Challenge:
The Hand Warmer Challenge:
Between El Nino and the new winter soccer season, cold hands are not an anomaly at San Marin High School. As the STEM Chemistry class, we sought to find a solution. The big question was how could we create a safe, inexpensive, effective, and an environmentally friendly hand warmer. Despite the project finishing in the beginning of spring, the winning hand warmer would be sold in the student store. We would start our journey with a series of labs and notes that would give us background knowledge about energy and its role in reactions.
Calorimetry
Def: a technique in chemistry used to determine enthalpy change of the dissolution.
Our Lab: Using a calorimeter, measure the total enthalpy change when a salt is added to the water.
Results:
Even though the sodium acetate on average lost 0.76 *C per gram, this value was closet to zero compared to all the other salts tested. This gave our group a clue that sodium acetate might have high specific heat, which would mean it could retain heat longer.
Proof:
Sodium Acetate's Specific Heat Capacity: 100.83 J/mol·K
Lithium Chloride's Specific Heat Capacity: 48.03 J/mol·K
What is Energy?
Def: the ability to do work
Enthalpy (q) - the total heat content of a system.
Mass of substance (m) : 80 g
Specific Heat Capacity (c) : 4.184 J/g*C
Temperature Change ( T) : 48*C - 24.5*C = 23.5*C
q = -7,865.92 J = Exothermic Reaction
Exothermic Reaction: a type of reaction where energy is released in the form of light or heat. (Our hand warmer was an exothermic reaction since it released heat.)
Endothermic Reaction: a reaction or process in which a systems absorbs energy from its surroundings.
After our research, we figured out that Lithium Chloride had the largest temperature increase but a low specific heat capacity, meaning the heat wouldn't last long. Not to mention, Lithium Chloride was the most expensive salt by twice the amount. We decided to take an alternative route: a phase change reaction.
Our Phase Change Reaction:
NaC2H3O2 x 3H2O (l) <--> NaC2H3O2 x 3H2O (s) + Heat
The forward reaction which creates the heat in a hand warmer is an exothermic reaction. This is a negative free energy change. This represents the crystallization process.
The reverse reaction which is the melting process is an endothermic reaction with a positive free energy change. This was evident by the need to add heat to, or boil the pouch.
Our Hand Warmer:
A Supersaturated, Supercooled Sodium Acetate Solution
Between El Nino and the new winter soccer season, cold hands are not an anomaly at San Marin High School. As the STEM Chemistry class, we sought to find a solution. The big question was how could we create a safe, inexpensive, effective, and an environmentally friendly hand warmer. Despite the project finishing in the beginning of spring, the winning hand warmer would be sold in the student store. We would start our journey with a series of labs and notes that would give us background knowledge about energy and its role in reactions.
Calorimetry
Def: a technique in chemistry used to determine enthalpy change of the dissolution.
Our Lab: Using a calorimeter, measure the total enthalpy change when a salt is added to the water.
Results:
Even though the sodium acetate on average lost 0.76 *C per gram, this value was closet to zero compared to all the other salts tested. This gave our group a clue that sodium acetate might have high specific heat, which would mean it could retain heat longer.
Proof:
Sodium Acetate's Specific Heat Capacity: 100.83 J/mol·K
Lithium Chloride's Specific Heat Capacity: 48.03 J/mol·K
What is Energy?
Def: the ability to do work
- Enthalpy: the sum of internal energy in a system and the product of pressure and volume.
- Entropy: measure of disorder in a system.
- 1st: the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed.
- 2nd: In any cyclic process the entropy will either increase or remain the same.
Enthalpy (q) - the total heat content of a system.
Mass of substance (m) : 80 g
Specific Heat Capacity (c) : 4.184 J/g*C
Temperature Change ( T) : 48*C - 24.5*C = 23.5*C
q = -7,865.92 J = Exothermic Reaction
Exothermic Reaction: a type of reaction where energy is released in the form of light or heat. (Our hand warmer was an exothermic reaction since it released heat.)
Endothermic Reaction: a reaction or process in which a systems absorbs energy from its surroundings.
After our research, we figured out that Lithium Chloride had the largest temperature increase but a low specific heat capacity, meaning the heat wouldn't last long. Not to mention, Lithium Chloride was the most expensive salt by twice the amount. We decided to take an alternative route: a phase change reaction.
Our Phase Change Reaction:
NaC2H3O2 x 3H2O (l) <--> NaC2H3O2 x 3H2O (s) + Heat
The forward reaction which creates the heat in a hand warmer is an exothermic reaction. This is a negative free energy change. This represents the crystallization process.
The reverse reaction which is the melting process is an endothermic reaction with a positive free energy change. This was evident by the need to add heat to, or boil the pouch.
Our Hand Warmer:
A Supersaturated, Supercooled Sodium Acetate Solution
- Supersaturated and supercooled means that there is more sodium acetate than the saturated solution and the sodium acetate has been cooled below its freezing point of 54*C without crystallization. In a sealed container, the solution can be cooled to -10*C without freezing.
- Start with a supersaturated, supercooled sodium acetate solution and a piece of metal
- The metal circle serves as the nucleation site, or where the reaction occurs. When bent, a single molecule of sodium acetate trihydrate to crystallize and act as a seed crystal that starts the chain reaction.
Reflection:
In our project, there was a series of trial and error required to perfect our product. Even more so with our solution, repetition was what made our solution a success. In the beginning, we definitely didn't grasp the concentration of sodium acetate to water ratio. We started with a 3:16 ratio of sodium acetate to water, however we ended with a ratio of 16:3. Overall, my group worked very well together. Each of us was willing to contribute to the project, which made the work process go smoothly. There was always a ton of ideas that made problems easy to overcome. We were all extremely passionate about our hand warmer which made us more eager to produce a high quality hand warmer. I remember when we finally perfected our hand warmer. Everyone was excited because everyone had a part in making it great. It was and extremely rewarding project.