Which salt/compound (ammonium chloride, urea, sodium bicarbonate, potassium chloride and ammonium nitrate) will induce the largest drop in temperature (°C) and what is its optimum mass (5,10,15,20 and 25 g) to be implemented in cooling packs?

Ice packs are immensely significant especially for people who have underwent injury, specifically athletes. They are typically used to combat both swelling and inflammation and thus are considered to be a necessary and essential tool in each athlete’s arsenal. Being a martial arts athlete, I am prone to injury as MMA is a very violent and often dangerous sport which has consequently led me to often resorting to ice packs. Recently we studied the thermal chemistry chapter in the IB chemistry SL course and it made me wonder if there was a correlation between it and the science behind ice packs. Turns out that by breaking an ice pack, it initiates an endothermic reaction between water and a specific compound (R K., 2020). An endothermic chemical reaction refers to any reaction that absorbs heat from the system’s surroundings thus making them cooler. It can thus be deduced that the enthalpy change of endothermic reactions is positive as the energy of the reactants is less than the energy of the products (meaning that the reactants are of higher stability during such reactions). The discovery of this relationship between thermal chemistry and ice packs peaked my interest which consequently led me to further explore this phenomena by investigating which salts / compounds create the highest temperature change and which concentration should they be used in for optimal effectiveness in ice packs (highest decrease in temperature)?

The main salt that is widely used in ice packs is ammonium nitrate as it produces a consistent and reasonable drop in temperature. This exploration will consequently mainly consist of comparing ammonium nitrate to other salts / compounds such as sodium bicarbonate, potassium chloride, urea and ammonium chloride in order to examine which chemical induces the largest drop in temperature and which concentration of this chemical should be utilized in order to obtain optimal effectiveness in producing the “cooling” effect in ice packs. To properly determine the most coherent and efficacious concentration of the chemical, different masses of it will be tested all while keeping a constant amount of water during all trials.

Which salt/compound (ammonium chloride, urea, sodium bicarbonate, potassium chloride and ammonium nitrate) will induce the largest drop in temperature °C and what is its optimum mass (5,10,15,20 and 25 g) to be implemented in cooling packs?

My hypothesis is that ammonium nitrate will produce the largest drop in temperature since it is the most popular substance that is utilized in cooling packs. I also suspect that by increasing the concentration of the substance used, the change in temperature will also become greater. This will occur till a specific concentration is reached and then the change in temperature will become minimal.

Independent

• The different salt or compound used (NaHCO₃, NH₄Cl, NH4NO₃, KCl and CH4N₂O) [ Part 1 of procedure ]
• The mass of salt or compound used, measured in (g) [ Part 2 of procedure ]

Dependent 

• The change in temperature of the newly obtained solution (∆T ), measured in (°C)

Control

• Mass of substance (salt or compound) used, measured in (g) [ Part 1 of procedure only ]
• Initial temperature (room temperature), (23°C)
• Volume of water, (75 mL)
• Time taken before measuring the temperature change (2 minutes 30 seconds)

•  The salts/compounds that will undergo testing (ammonium chloride, urea, sodium bicarbonate, potassium chloride and ammonium nitrate)
•  Stop-Watch (±0.01 s)
• Digital Balance (±0.01 g)
• Beaker
• Thermometer (±0.5°C)
• Thermometer (±0.5°C)
• Distilled Water
• Calorimeter
• Measuring Cylinder (100 mL)
• Mixing Spoons

The procedure is divided into two main parts -

Part one - Finding out which salt/compound will produce the largest decrease in temperature

• Measure 75 mL of distilled water and place it in the calorimeter
• Measure initial temperature of water after waiting exactly 2 minutes and 30 seconds
• Weigh out 15 grams of desired substance and place it in the calorimeter filled with water
• Stir the mixture continuously for 2 minutes and 30 seconds before measuring final temperature
• Calculate the change in temperature
• Repeat all steps 4 times for each of the substances (4 trials each)

Part two - Finding out the concentration to be used for the selected salt/compound

• Measure 75 mL of distilled water and place it in the calorimeter
• Measure initial temperature of water after waiting exactly 2 minutes and 30 seconds
• Weigh out the desired mass (starting from 5 g and up to 25 g; going up each time by increments of 5 [5 g then 10 g then 15 g etc ...])
• Calculate the change in temperature
• Repeat all steps 3 times for the different masses desired
• Calculate the concentration for each of the masses using the formulas: concentration  \(\frac{mol}{vol}\) and mol = \(\frac{mass}{motar mass}\)

Ethical -

This experiment has no ethical implications as there was no animal testing

Analysis -

The results in table 1 showed that Sodium bicarbonate had the lowest change in temperature thus rendering it the least efficient to be utilized in cooling packs. Both ammonium nitrate and potassium chloride were very similar in average changes in temperature as they both had -5°C and -5.5°C respectively. The highest temperature drops corresponded to ammonium chloride and urea; ammonium chloride had an average of -9°C while urea had -9.5°C. This means that the salt “urea” will be tested for the second part of the experimental procedure as it has proven to be the most effective at a mass of 15g. The original hypothesis has thus been proven invalid as Urea induced a larger temperature decrease than Ammonium Nitrate. It can be deduced that both cost and availability are important factors in the incorporation of Ammonium Nitrate in most cold packs since it is cheaper than Urea and also more easily obtained.

Analysis -

The results yielded in the second table showed that as the mass increased the temperature change also increased; meaning that a higher mass, and thus a higher concentration, will always achieve a higher temperature change. This is almost always true untill a specific concentration is reached and then the change in temperature starts to become minimal. It can thus be deduced that the saturation of the solution has approached its all time maximum which consequently means that the change in temperature will evolve at a much slower rate. This is shown here as the change in temperature went from -9°C to -14°C and then from -14°C to -16°C; the change in temperature keeps on becoming lower until the change is negligible / barely noticeable.

One possible extension for this experiment would be to test more salts with the water other then the ones that were already used. This would allow to check if other salts could be a better alternative than Urea or ammonium chloride and if other salts induce a higher temperature drop. The same experimental procedure would be followed. Another possible extension could be the usage of other endothermic mixtures; mixing the salts already present in the experiment together is an option and so is researching other mixtures entirely such as the reaction of thionyl chloride with cobalt(II) sulfate heptahydrate for example. The substances used in each of these mixtures would be used in different concentrations in order to reach and maintain the highest temperature drop. This would allow the potential discovery of more endothermic reactions which could possibly result in a lower temperature change.

In conclusion, the original hypothesis has been proved invalid as Urea has produced the largest temperature drop out of all the other salts and not ammonium nitrate as expected. It can consequently be inferred that the reasons behind the dominance of ammonium nitrate in cooling packs is not due to its actual effectiveness but because of price and/or safety reasons.

- Average ∆T for mass of 5g  -

Average ∆T 

\(=\frac{vatue \,of \,1\,+\,val of\,trial \,2\,+value\,of\,trial\,3}{3}=\frac{(-3)+(-4)+(-4)}{3)} = - 3.666666667 ≈ - 3.7°C\)

Concentration calculation for mass of 5g  - (molar mass of Urea = 60.06)

N.B: the volume of water must be converted from mL to L (75 ml = 0.075 L)

 \(mol=\frac{mass}{molar\, mass}\,=\frac{5}{60.06}\,=\,0.08325008325\)

\(concentration\,=\frac{mol}{vol}\,=\,1.11000111\,mol\,dm^3\)

- Because a thermometer is an analog device, the uncertainty will be half the smallest measurement. The smallest measurement is 1°C consequently meaning that the uncertainty would be ± 0.5°C.

- The uncertainty in mass was deduced as the balance is also an analog device and the smallest value it is able to measure is 0.01.

- The uncertainty in time was deduced as the stop-watch is also an analog device and the smallest value it is able to measure is 0.01.

Best Fit Line Gradient: 0.60

Maximum Fit Line Gradient: 0.65

Minimum Fit Line Gradient: 0.55

N.B: The horizontal error bars are present at a constant value of 0.01 but they’re not visible due to the big difference between the values utilized and the uncertainty value

Analysis of graph -

It can be deduced through the graph that the relationship between mass and temperature change is both linear and proportional as the slope is a straight line. The graph consequently fortifies the endothermic nature of the reaction and also proves that the rate by which the temperature change increases becomes minimal as a certain concentration is reached.

Strengths

• The first part of the experimental procedure was repeated for 4 trials for each of the chemical substances thus limiting random errors and excluding any possible deviations / anomalies
• A multitude of chemical substances were used and there was a wide range in the change of temperature of each one. This provided varied results which allowed an enlarged scope on the effectiveness of each salt in an endothermic setting, specifically in cooling packs.
• The uncertainties in the experiment are low and thus are not necessarily a limitation as they do not majorly affect any of the results obtained.
• The results obtained were consistent throughout the whole experiment thus indicating the validity and reliability of the procedure. The whole experiment was conducted on the same day and the same salt sample was used for each of the trials; this was done to avoid any irregularities in regards to the results
• The experiment procedure is simple and easily to follow

Britannica, L. K. (n.d.). Chemical kinetics. Encyclopedia Britannica. https://www.britannica.com/science/chemical-kinetics

Hwade. (n.d.). Workplace health and safety. The Official Web Site for The State of New Jersey. https://www.nj.gov/health/workplacehealthandsafety/right-to-know/hazardous- substances/

R K. (2020, June 1). The cold pack: A chilly example of an endothermic reaction. Let's Talk Science. https://letstalkscience.ca/educational-resources/stem-in-context/cold-pack-a-chilly- example-endothermic-reaction