Enthalpy Change Design Lab (6/6)How does changing the initial temperature (19C, 25C, 35C, and 45C) of two 40.0 cm3 at 1.00 mol dm-3 solutions of KOH(aq) and HCl(aq) affect the molar enthalpy change of the neutralization reaction

Part of the calculations involved in determining the molar enthalpy change of a reaction is the mass of the reactants. In this investigation, the mass of the reactants will be determined from the volume of the 1.00 mol dm-3 KOH(aq) and HCl(aq) that is measured during the investigation. The volume will be read from two 50.0 cm3 +/- 0.5 graduated cylinders, which will each be assigned to measure either the 40.0 cm3 of 1.00 mol dm-3 HCl(aq) or KOH(aq) so as to also avoid cross-contamination. The mass of the solutions will then be determined through converting cm3 to grams, assuming that the density is that of pure water.

Note: If the mass were to be determined through using an electronic milligram balance, it would have negligible more accuracy since the added density of either KOH(aq) or HCl(aq) is negligible itself – even more so considering the dilute concentration being used in the investigation.

Another component of measuring the molar enthalpy change of a neutralization reaction is the change from the initial temperature of the reactants, to the final temperature of the resulting solution. In this case, the reaction is as follows:

KOH(aq) + HCl(aq) → KCl(aq) + H2O(l)

(Winnipeg School Division, n.d.)

Meaning, the initial temperature will be of the two solutions of the 40.0 cm3 1.00 mol dm3 HCl(aq) and 1.00 mol dm-3 KOH(aq) and the maximum temperature will be that of the combined, 80.0 cm3 of the resulting solution of water and aqueous potassium chloride. The initial temperature will be measured using two Vernier temperature probes, connected to a Vernier LabQuest with DataLogger Pro data collection software. Essentially, it is at either 19°C, 25°C, 35°C, or 45°C, and at the time of the reaction the initial temperature should measure as the same for both 40.0 cm3 solutions of 1.00 mol dm3 HCl(aq) or KOH(aq). After the 1.00 mol dm-3 HCl(aq) is introduced with the 1.00 mol dm-3 KOH(aq), the Vernier LabQuest will be monitored, and once a sustained decrease in the temperature of the resulting solution is measured, the trial can be disposed of, and the LabQuest interface consulted to determine the maximum temperature recorded.

Another aspect of molar enthalpy changes of reactions is the number of moles of the reactants present. What is essential in this neutralization reaction between KOH(aq) and HCl(aq) is that the two be present in equimolar quantities, hence the reason for both being measured in 40.0 cm3 quantities, and both at equal concentrations of 1.00 mol dm-3. With this, the number of moles of both HCl(aq) and KOH(aq) can be determined through calculations to determine the molar enthalpy change of the neutralization reaction of 40.0 cm3  1.00 mol dm-3 of HCl(aq) and 1.00 mol dm-3 KOH(aq).

Control Variables

1. The state of reactants must be kept constant throughout the course of the investigation.

This is because of the fact that at different physical states, substances have different enthalpies because of the nature of the bonds that hold them together. For example, the reaction of solid sodium hydroxide pellets in aqueous hydrochloric acid will result in a much greater molar enthalpy change than that of dissolved sodium hydroxide neutralizing dissolved hydrochloric acid. This is because dissolved sodium hydroxide exists at a lower potential energy than solid sodium hydroxide, and the energy associated with dissolving sodium hydroxide has already been released beforehand when reacting the already aqueous sodium hydroxide and aqueous hydrochloric acid.

This variable will be controlled by ensuring that both reactants – KOH and HCl, are in aqueous states. This will ensure that the molar enthalpies will be exclusively concerning the neutralization of aqueous hydrochloric acid, and aqueous sodium hydroxide.

2. The concentration of both solutions of KOH(aq) and HCl(aq) must be held constant, and in extension, the number of moles of KOH(aq) and HCl(aq) should also be held constant throughout the investigation, across all variations of temperature, and trials.

By doing so, the number of moles of KOH(aq) and HCl(aq) that react can be effectively controlled during the investigation. This is important to control because if the number of moles of HCl(aq) or KOH(aq) was different, the investigation would be void since the neutralization reaction would likely be limited due to the presence of a limiting reactant. This would mean that different quantities are reacting than should be, and a proper molar enthalpy change calculation could not be conducted if concentration was not held constant.

To control this variable, accurate concentrations of HCl(aq) and KOH(aq) will be prepared. Specifically, 1.00 mol dm-3 will be the concentration of both solutions of KOH(aq) and HCl(aq) used in the investigation. This means that they are reacting in equimolar quantities at the same volume, and therefore, there should be no limiting reactant during the investigation (as can also be verified by inspection when looking at the balanced equation for the reaction, displayed earlier).

3. The volume of both 1.00 mol dm-3 HCl(aq) and 1.00 mol dm-3 KOH(aq) solutions must be held constant throughout the course of the investigation, across all variations in temperature, and in all trials.

It is important to control volume in this investigation because it is crucial to determining the molar enthalpy change of the neutralization reaction of 1.00 mol dm-3 HCl(aq) and 1.00 mol dm-3 KOH(aq), since it is being used to determine the mass of the solutions, and therefore, will be incorporated as part of the calculation in determining the heat gained or lost by the surroundings of the neutralization reaction as well as being used to determine the number of moles of reactants present to determine the molar enthalpy change. If the volume is, for one trial, reduced, this adds extra uncertainty to the measurements and therefore limits the accuracy of the dependent variable in pursuit of the research question.

Therefore, to control this variable, 40.0 cm3 of 1.00 mol dm-3 HCl(aq) and 40.0 cm3 of 1.00 mol dm-3 KOH(aq) will be measured out for each variation in temperature (19°C, 25°C, 35°C, and 45°C) and held constant for each of the subsequent trials for each variation. The volume will be measured appropriately using a 50.0 cm3 +/- 0.5 graduated cylinder.

4. The purity of reactants must be controlled in this investigation, namely due to the fact that potassium hydroxide, also while aqueous, is hygroscopic. (Sciencelab, 2010)

This is an important factor to control because the solution of 1.00 mol dm-3 KOH(aq) that is prepared ahead of time, before the investigation is conducted, will absorb moisture from the atmosphere. This means that the volume of the solution will increase and therefore, its concentration decreased the longer it is exposed to the atmosphere. This would mean that less moles of aqueous potassium hydroxide would actually be present if 40.0 cm3 of it was measured after it had exposure to the atmosphere since it will have been diluted. This would therefore render it the limiting reactant in the reaction between it and 1.00 mol dm-3 HCl(aq), and would affect the molar enthalpy change by releasing/gaining less heat from the surroundings than it should ideally.

This variable will be controlled by preparing the 1.00 mol dm-3 KOH(aq) solution as close to the time of the investigation as possible. Specifically, it is recommended that the solution be prepared the morning of the investigation, and the investigation carried out later that day. In addition, the flask containing the 1.00 mol dm-3 KOH(aq) solution should be firmly sealed with a rubber stopper to prevent fresh, moist air entering and further diluting the solution due to the hygroscopic nature of KOH(aq).

5. Heat loss and the time it takes for the reactants to fully react is a variable that must be controlled, or worked towards ensuring is consistent.

In terms of heat loss, it is important to prevent the loss of heat from the neutralization reaction because it means that heat is escaping the calorimeter, and not contributing towards raising the temperature of the resulting solution from the neutralization reaction between 1.00 mol dm-3 HCl(aq) and 1.00 mol dm-3 KOH(aq). A proper analogy is cooking stew on the stove – one wants to insulate the heat inside the pot so that the safe temperature for cooking meat is achieved. The same case exists in the simple calorimeter, in that to achieve as accurate a molar enthalpy change as possible for a trial, it is important to limit the amount of heat lost to the surroundings around the calorimeter so that the most accurate molar enthalpy change for the reaction of 40.0 cm3 of both 1.00 mol dm-3 HCl(aq) and 1.00 mol dm-3 KOH(aq).

On the topic of rate of reaction, it is helpful if both reactants react as ‘instantly’ as possible. This is because the longer it takes for the neutralization reaction of 1.00 mol dm-3 KOH(aq) and 1.00 mol dm-3 HCl(aq) to reach it’s maximum temperature, the greater the opportunity for heat to escape from the calorimeter as time goes on.

To mitigate the effects of heat loss on the temperature change recorded during the investigation, regarding the neutralization reaction of 1.00 mol dm-3 HCl(aq) and 1.00 mol dm-3 KOH(aq), a good insulator should be used as the calorimeter. As such, a doubled polystyrene cup calorimeter is both feasible for a school laboratory setting, and effective at insulating heat. (University of Illinois, n.d.) Therefore, by allowing the neutralization reaction between 40.0 cm3 of 1.00 mol dm-3 HCl(aq) and 40.0 cm3 of 1.00 mol dm-3 KOH(aq) to occur in a doubled polystyrene cup calorimeter, heat will be adequately insulated for the purposes of this investigation.

Note: a plastic lid/cover is not necessary because, with aqueous neutralization reactions, they occur very quickly. Meaning, the temperature change is very sudden since the rate of reaction is quick fast, and so a lid is not necessary since that would only be truly influential in insulating heat for an investigation involving substances that react at slower rates.

To ensure the rate of reaction between the 40.0 cm3 of 1.00 mol dm-3 KOH(aq) and 40.0 cm3 of 1.00 mol dm-3 HCl(aq) is as fast as possible, an electrically powered magnetic stirrer, and magnetic bar will be used for each variation in temperature, and each subsequent trial to agitate the solutions and speed the reaction. The electrically powered magnetic stirrer will be set to the medium speed (5/10 on most dials) for each variation in temperature, and for each subsequent trial, and the same sized magnetic stirring bar is to be used for each variation in temperature, and each subsequent trial throughout the investigation so as to ensure the level of agitation of the solution is consistent.

Materials and Equipment:

• 2 x Clean, dry, polystyrene cups
• 2 x Temperature probe (+/- 0.3°C)
• 2 x Vernier LabQuest interface wih Logger Pro data collection software
• 1 x Electrically powered magnetic stirrer
• 1 x Magnetic stirring bar
• 1 x Electrically powered hot plate
• 2 x 50.0 cm3 graduated cylinders (+/- 0.5 cm3)
• 2 x 150 cm3 clean, dry beakers (+/- 5 cm3)
• 960 cm3 of 1.00 mol dm-3 HCl(aq)
• 960 cm3 of 1.00 mol dm-3 KOH(aq)
• 1 x Ice bath, containing ice cubes and cold water in an insulated, shallow tub
• Pair of heat resistant gloves / oven mitts
• Latex gloves
• Safety Goggles
• Masking tape
• Black marker

Safety Precautions:

• Aqueous potassium hydroxide can be irritant to skin, eyes, and very hazardous in case of ingestion. Safety goggles, and latex gloves should be worn at all times during the investigation. (Sciencelab, 2010)
• Aqueous hydrochloric acid is irritant to skin, eyes and also very hazardous in case of ingestion. Safety goggles and latex gloves should be worn at all times during the investigation. (Sciencelab, 2010)
• A hot plate is being used in this investigation, so when picking up beakers from the hot plate to pour liquids, heat resistant gloves should be worn

Procedure:

Part 1

1. 960cm3 solutions of 1.00 mol dm-3 HCl(aq) and 1.00 mol dm-3 KOH(aq) were prepared earlier in the day, on the day of the investigation and were at room temperature, in appropriately sized flasks and sealed with a rubber stopper.
2. Safety goggles and latex gloves were worn at all times during the investigation. The investigation was conducted beneath a fume hood, and on the same day.
3. A clean, dry, polystyrene cup was placed into another, so that the top one was stably lodged into the lower one, and placed on the electrically powered magnetic stirrer.
4. A magnetic stirring bar was placed inside the doubled polystyrene cup, and the stirring speed was set to medium (usually 5/10).
5. Using a clean, dry 50.0 cm3 graduated cylinder, 40.0 cm3 of 1.00 mol dm-3 KOH(aq) were measured. These 40.0 cm3 of 1.00 mol dm-3 KOH(aq) in the graduated cylinder were then poured into a clean, dry 150 cm3 beaker, and this beaker was labelled ‘KOH(aq)’ using masking tape and a black marker.
6. Using another clean, dry 50.0 cm3 graduated cylinder, 40.0 cm3 of 1.00 mol dm-3 HCl(aq) were measured. These 40.0 cm3 of 1.00 mol dm-3 HCl(aq) in the graduated cylinder were then poured into a clean, dry 150 cm3 beaker, and this beaker was labelled ‘HCl(aq)’ using masking tape and a black marker.
7. The two 150 cm3 beakers labelled ‘HCl(aq)’ and ‘KOH(aq)’ containing 40.0 cm3 of 1.00 mol dm-3 HCl(aq) and 1.00 mol dm-3 KOH(aq) respectively, were placed on an electrically powered hotplate which was set to medium-high heat.
8. Two separate temperature probes, connected to Vernier LabQuest LoggerPro data collection units were inserted into the 150 cm3 beakers, meaning one probe was inserted into the 150 cm3 beaker labelled ‘KOH(aq)’, and the other probe into the 150 cm3 beaker labelled ‘HCl(aq)’ deep enough into the 40.0 cm3 solution of either 1.00 mol dm-3 HCl(aq) or 1.00 mol dm-3 KOH(aq) so that the base of the temperature probe is touching the base of the 150 cm3 beaker.
9. The temperature readings of both Vernier LabQuest LoggerPro data collection units were monitored, and once one of the solutions of either 40.0 cm3 of 1.00 mol dm-3 KOH(aq) or HCl(aq) reached 25.0 °C, it was removed from the hotplate while the other heated up towards 25.0 °C with heat resistant being gloves worn.
10. If the temperature of the 40.0 cm3 solution of either 1.00 mol dm-3 HCl(aq) or KOH(aq) that was removed earlier decreased to below the 25.0 °C target, it was temporarily replaced onto the hotplate to regain the appropriate temperature. The two solutions of 1.00 mol dm-3 HCl(aq) and KOH(aq) were alternated in their respective 150 cm3 beaker on and off the hotplate, until both Vernier LabQuest’s read that the temperature of both solutions was 25.0 °C.
11. Once both Vernier LabQuest’s indicated that the 40.0 cm3 of 1.00 mol dm-3 HCl(aq) and 40.0 cm3 of 1.00 mol dm-3 KOH(aq) were at the 25.0 °C target temperature, the contents of the 150 cm3 beaker labelled ‘HCl(aq)’ were poured into the clean, doubled polystyrene cup, containing the magnetic stirring bar that was already spinning, with the electrically powered stirrer set at a speed of medium.
12. The temperature probe that was being used to monitor the temperature of the 40.0 cm3 solution of 1.00 mol dm-3 HCl(aq) while in the 150 cm3 beaker, was then placed in the doubled polystyrene cup to attain the more accurate initial temperature, and the base of the probe was pressed against the base of the polystyrene cup so as to ensure it was deep enough into the solution. The temperature was recorded.
13.  As quickly and safely as possible, the contents of the 150 cm3 beaker labelled ‘KOH(aq)’ (40.0 cm3 of 1.00 mol dm-3 KOH(aq)) were then emptied into the doubled polystyrene cup, containing 40.0 cm3 of 1.00 mol dm-3 HCl(aq) and the  temperature probe that is connected to the Vernier LabQuest LoggerPro data collector. The Vernier LabQuest LoggerPro was then monitored, until a steady decrease in temperature was observed. The interface was then consulted to determine the maximum measured temperature during the neutralization reaction.
14. The contents of the doubled polystyrene cup were safely disposed of in an appropriate waste container, and the magnetic stirring bar was also retrieved.
15. Steps 3-14 were repeated for a further 2 trials, using the same equipment (except the doubled polystyrene cup and the two 150 cm3 beakers were washed, cleaned, and dried). (Note, the labelling of the beakers in steps 5 and 6 will already be done for subsequent trials, so this part of the procedure is unnecessary after the first trial)
16. Steps 3-15 were repeated for another two target temperatures: 35°C, and 45°C, and each of these variations in temperature were also conducted three times, meaning three trials.

Part 2

1. The same doubled polystyrene cup was used for part 2 of the investigation, and was washed, and dried appropriately, as well as the two 150 cm3 beakers (they were also washed and dried appropriately). The investigation was conducted beneath a fume hood and on the same day.
2. A magnetic stirring bar was placed inside the doubled polystyrene cup, and was placed on the electrically powered magnetic stirrer, which was set to stir at a medium speed.
3. A shallow, insulated tub that was filled with cold tap water, and filled with ice, was obtained.
4. Using a clean, dry 50.0 cm3 graduated cylinder, 40.0 cm3 of 1.00 mol dm-3 KOH(aq) were measured. These 40.0 cm3 of 1.00 mol dm-3 KOH(aq) in the graduated cylinder were then poured into a clean, dry 150 cm3 beaker, and was labelled ‘KOH(aq)’.
5. Using the other clean, dry 50.0 cm3 graduated cylinder, 40.0 cm3 of 1.00 mol dm-3 HCl(aq) were measured out. These 40.0 cm3 of 1.00 mol dm-3 HCl(aq) in the graduated cylinder were then poured into a clean, dry 150 cm3 beaker, and was labelled ‘HCl(aq)’ using masking tape and a black marker.
6. The two 150 cm3 beakers labelled ‘HCl(aq)’ and ‘KOH(aq)’ containing 40.0 cm3 of 1.00 mol dm-3 HCl(aq) and 1.00 mol dm-3 KOH(aq) respectively, were placed in a shallow, insulated tub containing cold water and ice cubes.
7. Two separate temperature probes, connected to Vernier LabQuest LoggerPro data collection units were inserted into the 150 cm3 beakers, meaning one probe was inserted into the 150 cm3 beaker labelled ‘KOH(aq)’, and the other probe into the 150 cm3 beaker labelled ‘HCl(aq)’ deep enough into the 40.0 cm3 solution of either 1.00 mol dm-3 HCl(aq) or 1.00 mol dm-3 KOH(aq) so that the base of the temperature probe is touching the base of the 150 cm3 beaker.
8. The temperature readings of both Vernier LabQuest LoggerPro data collection units were monitored, and once one of the solutions of either 40.0 cm3 of 1.00 mol dm-3 KOH(aq) or HCl(aq) reached 19.0 °C, it was removed from the ice bath while the other also cooled down towards 19.0 °C.
9. If the temperature of the 40.0 cm3 solution of either 1.00 mol dm-3 HCl(aq) or KOH(aq) that was removed earlier rose to above the 19.0 °C target, it was temporarily replaced into the ice bath to regain the appropriate temperature. The two solutions of 1.00 mol dm-3 HCl(aq) and KOH(aq) were alternated in their respective 150 cm3 beaker in and out of the ice bath until both Vernier LabQuest’s read that the temperature of both solutions was 19.0 °C.
10. Once both Vernier LabQuest’s indicated that the 40.0 cm3 of 1.00 mol dm-3 HCl(aq) and 40.0 cm3 of 1.00 mol dm-3 KOH(aq) were at the 19.0 °C target temperature, the contents of the 150 cm3 beaker labelled ‘HCl(aq)’ were poured into the clean doubled polystyrene cup, containing the magnetic stirring bar that was already spinning, with the electrically powered stirrer set at a speed of medium.
11. The temperature probe that was being used to monitor the temperature of the 40.0 cm3 solution of 1.00 mol dm-3 HCl(aq) while in the 150 cm3 beaker, was then placed in the doubled polystyrene cup to attain the more accurate initial temperature, and the base of the probe was pressed against the base of the polystyrene cup so as to ensure it was deep enough into the solution. The temperature was recorded.
12.  As quickly and safely as possible, the contents of the 150 cm3 beaker labelled ‘KOH(aq)’ (40.0 cm3 of 1.00 mol dm-3 KOH(aq)) were then emptied into the doubled polystyrene cup, containing 40.0 cm3 of 1.00 mol dm-3 HCl(aq) and the  temperature probe that is connected to the Vernier LabQuest LoggerPro data collector. The Vernier LabQuest LoggerPro was then monitored, until a steady decrease from the peak temperature was observed. The interface was then consulted to determine the maximum temperature measured during the neutralization reaction.
13. The contents of the doubled polystyrene cup were safely disposed of in an appropriate waste container, and the magnetic stirring bar was also retrieved.
14. Steps 3-14 were repeated for a further 2 trials, using the same equipment (except the doubled polystyrene cup and the two 150 cm3 beakers were washed, cleaned, and dried). (Note, the labelling of the beakers in steps 4 and 5 will already be done for subsequent trials, so this part of the procedure is unnecessary after the first trial)

Works Cited

Neuss, Geoffrey. Chemistry: course companion. Oxford: Oxford University Press, 2007. Print.

Sciencelab. "Material Safety Data Sheet: Hydrochloric Acid." Material Safety Data Sheet. N.p., n.d. Web. 22 Mar. 2012. <www.sciencelab.com/msds.php?msdsId=9924285>.

Sciencelab. "Material Safety Data Sheet: Potassium Hydroxide Solution." Sciencelab Material Safety Data Sheet. N.p., n.d. Web. 21 Mar. 2012. <www.sciencelab.com/msds.php?msdsId=9926720>.

University of Illinois . "Q & A: polystyrene foam insulator | Department of Physics | University of Illinois at Urbana-Champaign." Department of Physics | University of Illinois at Urbana-Champaign. N.p., n.d. Web. 22 Mar. 2012. <http://van.physics.illinois.edu/qa/listing.php?id=1848>.

Winnipeg School Division. "Energetics Review." Winnipeg School Division. N.p.,

n.d.Web.22Mar.2012.<http://www.wsd1.org/kelvin/Departments/science/index/Teacher%20Course%20Pages/Labun_IB_Chem_files/Energetics%20Review1.htm>.