Introduction
The aim of the investigation is to experimentally determine how much iron (Fe) is in each heat pack and analyse the results obtained, relating to principles of thermochemistry.
Energy and Enthalpy
Energy transformation is the change of energy from one form to another. Energy gives the capacity to do work and may exist as potential, kinetic, thermal, electrical, chemical and other various forms. Energy and energy transformations are essential for day-to-day life and are happening all around us. Many chemical reactions involve energy transfer in the form of heat, which include heat packs with need this process to occur to produce its effects.
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Enthalpy (signified as H) is the total energy of a system at a constant pressure. “Enthalpy is the amount of heat (energy) absorbed by the system to cause a change in the system or the amount of heat expelled by the system as a result of the change in the system”, (Michel van Biezen). Enthalpy is the heat that is being absorbed or expelled if the enthalpy is positive, energy is absorbed and if the energy is negative, the energy is expelled.
If = endothermic
(Exothermic reactions are spontaneous)
= Change in enthalpy
= Change in internal energy
= Work done by the system
When heat is added to the system (enthalpy) it increases the internal energy and expands the system against the pressure of the atmosphere, the equation shows the addition of these two factors.
Exothermic reactions
[image: ]Reactions that release heat are described as exothermic reaction which occurs when the energy used to break the bonds between the reactants is less than the energy released when new bonds are made in the products. This reaction releases a by-product of light or heat.
This graph shows that energy has been released and the energy change is negative. The reactants have more energy than the end products. It also indicates that the enthalpy change is negative as heat is lost to the surroundings resulting in a temperature increase.
Whereas, endothermic is a reaction that absorbs heat from the environment. In endothermic reactions, the products have more enthalpy than the reactants. Thus, an endothermic reaction is said to have a positive enthalpy of reaction. This means that the energy required to break the bonds in the reactants is more than the energy released when new bonds form in the products; in other words, the reaction requires energy to proceed.
How does the heat pack produce heat?
Air-activated hand warmers are long-lasting chemical reactions that begin to release heat as soon as exposed to oxygen, when the air-tight wrapper is opened. When exposed to oxygen, aid diffuses through the pouch, initiating an exothermic oxidation reaction between the Fe and the oxygen producing iron oxide (Fe2O3), otherwise known as rust and releases a by-product of heat:
4Fe(s) + 3O2(g) -> 2Fe2O3(s)
Iron (Fe) as a metal holds more energy in this form than it does as iron oxide (Fe2O3), therefore when the oxidation reaction process occurs it releases energy in the form of heat. The warmer is a mixture of Iron, water, cellulose, vermiculite, activated carbon and salt. Iron powder when exposed to oxygen and water, oxidises and produces heat. The added contents are used to help aid the reaction process by maintaining heat or quickening the oxidation process. The chloride ions in salt acts as a catalyst, to further accelerate the oxidation process and produce a more porous form of rust (β-FeOOH). Due to the electrochemical nature of the reaction, dissolved electrolytes in water aid the reaction, rust occurs more quickly in saltwater than in pure. The carbon disperses the heat throughout the warmer and helps bring the oxygen to the iron particles through gas adsorption. The vermiculite is an inert light- weight mineral acts as an insulator, keeping the heat from dissipating too rapidly by maintaining an optimal moisture level for rusting to occur and cellulose is an added filler to assist with the oxidation process by exposing more iron particles to be oxidised. These ingredients are all surrounded by a polypropylene bag, which allows air to permeate the ingredients while holding moisture. The performance of such a hand warmer depends largely on its weight of ingredients, size and air circulation which is due to the breathability and type of fabric of the package.
Surface area in reactions.
The temperature attained by a warming device is largely dependent on the rate of the oxidative process. The rate is controlled by the amount of iron and oxygen available to react. Solid blocks of iron do oxidize but do so slowly and release oxidative heat slowly. The small surface area of a solid limits how fast the iron is consumed by oxidation. Rust on the surface of the solid acts as a barrier to oxygen diffusion further inhibiting the reaction rate. The iron inside the pack is milled extremely fine to cause a rapid oxidation process, with the addition of salt as a catalyst. By subdividing the solid block to a powder, the surface area has drastically increased, making more iron molecules available to react and the reaction process proceeds at an accelerated rate. Both, the solid block and the powdered iron will be completely consumed by the oxidative process and ultimately release the same amount of heat, however the powered iron will give off more heat and attain a higher temperature as it is consumed by the heat producing exothermic process in a few hours as compared to years for the solid block.
Discussion
The investigation aimed to experimentally determine the amount of iron present inside iron heat packs. The iron pack was situated inside a bomb calorimeter made from two Styrofoam cups fused by sticky tape to create a stable internal environment. A thermometer was inserted through a small hole through the top of the cup and pushed down until reaching the heat pack. Once the heat pack is opened from its casing and exposed to oxygen, it is placed in the bomb calorimeter for the temperature to be recorded every 5 minutes for an hour.
The results obtained, shown in graph 1, produced a steady exponential growth with respect to time in proportion to temperature. The heat packs steadily increased over the 55 minutes and it took approximately 50 minutes for the first 2 trialled heat packs to reach the expected temperature of 57oC, further the third only reached 55oC. After the 55 minutes, it is predicted that the heat released by the heat pack will plateau off at an approximate temperature of 57oC.
Using the initial and final two temperature points, 23.0oC and 56.3oC, the amount of Fe present inside the average heat pack is calculated. The average of the three heat packs is 24.271g and following calculations in appendix 1, approximately 0.32g of Fe, 1.3%4.2. is present inside the heat pack.
These calculations in appendix 1, are all based on assumptions gathered by literature:
- 1 gram of Fe can release 1.7 kilocalories of heat. http://sciencefocus.ust.hk/the-chemical-magic-of-hand-warmers
- The hand warmer releases 10 hours of heat.
- The heat pack remained at approximately 56.3oC for 10 hours.
- The iron stopped oxidizing after 10 hours.
These assumptions listed above will affect all results obtained however when used in calculations the results will still be proportional to one another. It was assumed that iron releases 1.7kcal of heat, so therefore if that theoretical value was to be incorrect the results will still be proportional as a fair test was conducted and all the results obtained will use this value to calculate results, therefore, all calculations and results concluded will be proportional to one another.
It was assumed that the heat pack will provide 10 hours of heat at approximately 57oC, which cannot be guaranteed, however, more testing can be done to ensure that this will provide accurate results, though for the calculations the packet data is used and trusted due to the engineering of the heat packs.
Problems/complications
During the experiment four heat packs were trialled; however, it was observed that the temperature released by the one heat pack was considerably smaller than expected and recorded temperatures around 20oC lower than the other trialled heat packs. It was decided to refuse this data as it was producing major outliers and would interrupt the other data. It was predicted that the heat pack had already been the oxidation process as it was partially exposed to oxygen and had already started the oxidation process before the experiment began.
Another factor that could have either affected this ‘faulty’ pack or possibly the data provided, was that the packs were not shaken up as sometimes suggested. The pack states that it isn’t essential to shake the pack up before use however it could help speed up the oxidation process as the particles could have agglomerated and reduced the surface area available for oxidation, therefore, reducing the rate of reaction.
Conclusion
Risk vs reward
The investigation aimed to determine the quantity of Fe in a disposable iron heat pack and analyse the results obtained from the experiment. It was calculated that the heat pack contains approximately 0.32g per pack which covers 1.3% of the 24.3g heat pack to maintain an average temperature of 56.3oC for 10 hours consecutively. This value expresses that 1.3/100 of the pack is Fe and considering that the pack is called an “iron heat pack”, costumers may feel as if they are being ‘ripped off’ as the product contains more filler ingredients such as vermiculite and cellulose.
It is possible to get a burn from direct contact between the hand warmer and skin, so the packaging warns users to put the product on the outside of a sock or glove, to give some protection from the skin. Iron packs are manufactured specifically to ensure that a sensible amount of heat is released for use. The packaging for the iron heat pack states that the heat pack can reach up to 57oC and should not exceed use if too hot or uncomfortable. Calculations in the appendix show that approximately 0.32g of iron is in the heat pack. This amount ensures that the reaction should not exceed 57oC and burn any users. The iron pack on average has a mass of 24.3g, so referring to appendix the iron powder only accounts for 1.3% of the packet. Burn Centre Care, suggests that temperature as low as 44oC can burn the skin if held on the area for too long and that temperatures around 80oC can produce more serious burns in short periods. Customers may feel as they are being “ripped off” however the heat packs have been engineered specifically to ensure that the temperature released is suitable for the use on skin. The greater the amount of iron results in a greater oxidation process which evidentially produces higher temperatures.
Iron heat packs are single use and produce up to 10 hours of heat. This can be an issue when purchasing however pros and cons can be discussed. The heat packs are only single use as they aren’t reversible reactions as once the Fe has oxidised and turned into rust, it cannot be reversed however the reaction can be stopped and resumed. This can occur if the heat pack is sealed air-tight again to stop the oxidation process for the moment until needed again as Fe can only oxidise and produce heat by oxygen and if that is removed then the process ultimately stops until oxygen is provided again.
A single unit heat pack costs approximately $1, so in cases such as skiing or extremely cold weather, they can be provided with 10 hours of heat for their hands or feet for minimal pricing. However, an issue occurs with the word ‘disposable’. If using these heat packs frequently, the packing for each individual unit adds up and can create an environmental issue. So other options such as sodium acetate heat packs could be useful as they are reusable and just as effective as single use. The only downside is that to reverse the reversible reaction, they must be boiled for a certain period until useable again and the cost is much higher at approximately $10 per unit however this reduces the cost and packaging use in the long run.
To increase validity, changes to the experiment can be made to ensure that highly accurate results are obtained. Such as ensuring that the heat packs have not been exposed to oxygen prior and to ensure all bomb calorimeters are extremely like one another to each other stable environment for all heat packs tested. It is also suggested to do multiple tests to ensure for accurate and fair data.
Overall, the aim of the investigation was to experimentally determine how much iron (Fe) is in each heat pack and analyse the results obtained, relating to principles of thermochemistry.
References
- Anon, (2019). [online] Available at: https://www.uhms.org/images/SafetyArticles/air_activated_warmers_raleig.pdf [Accessed 1 Nov. 2019].
- Anon, (2019). [online] Available at: https://www.uhms.org/images/SafetyArticles/air_activated_warmers_raleig.pdf [Accessed 1 Nov. 2019].
- En.wikipedia.org. (2019). Exothermic reaction. [online] Available at: https://en.wikipedia.org/wiki/Exothermic_reaction [Accessed 1 Nov. 2019].
- Science made simple. (2019). Exothermic and Endothermic reactions -. [online] Available at: http://www.sciencemadesimple.co.uk/curriculumblogs/chemistryblogs/exothermic-and-endothermic-reactions [Accessed 1 Nov. 2019].
- Sciencefocus.ust.hk. (2019). [online] Available at: http://sciencefocus.ust.hk/the-chemical-magic-of-hand-warmers [Accessed 1 Nov. 2019].
- YouTube. (2019). Physics - Thermodynamics 2: Ch 32.7 Thermo Potential (4 of 25) What is Enthalpy?. [online] Available at: https://www.youtube.com/watch?v=QoRzoHW800E [Accessed 1 Nov. 2019].
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