Abstract
In this experiment, two calorimeters were designed and assembled. One was made using Styrofoam cups and another one using glass and foil. The calorimeters were studied for their performance in keeping the heat of the system. They were studied by measuring the heat loss of hot water over time. The heat lost in each calorimeter over 30 minutes was calculated. The calorimeter made of glass worked better and less heat was lost in it. The assembled calorimeters were used to find out the heat of reaction between NaOH and HCl per one mole of water. This value was found and comprised 62,760 J/mol.
Introduction
The objective of this experiment was to design the two apparatus and determine which of them is better as a calorimeter and to find experimentally the molar heat of a reaction.
Typically, when chemical processes occur the energy of the system changes. If in the process of reaction heat is given off to surroundings, then the reaction is called exothermic. If, in contrary, the heat is absorbed during reaction then it is called endothermic. The system is usually called the substances reacting, as well as the products of the reaction and a solvent if the reaction is taking place in a solution, all else opposing the system is called the surroundings. Under stable atmospheric conditions, the energy change is usually described by the change in enthalpy (ΔH). While it cannot be directly measured, temperature change in the system can be easily detected in the course of chemical reaction. If the temperature change during reaction, mass of the system (reactants, products, and solvent), and its specific heat, or heat capacity are known, then the enthalpy of energy change in the reaction can be found. In practice, such measurements are carried out in an apparatus called calorimeter. It is an insulated cup which ensures little energy exchange between system and outside world, so that the temperature change can be accurately and precisely measured. The heat change of a system can be found by using equation:
Here C is heat capacity, ΔT is the temperature change, c – specific heat, m – mass.
The two designs of the calorimeter apparatus were the following – two Styrofoam cups, one inserted into other and a glass wrapped with aluminum foil. These two apparatuses were tested for their efficiency as calorimeters.
Experimental details
The experimental part consisted of two parts. In the first one, the calorimeters are tested for their efficiency using hot water by finding their heat loss over time. In the second, the molar heat of reaction between NaOH and HCl was found.
Materials used in the experiment:
distilled water;
2.0 M NaOH solution;
2.0 M HCl solution;
1 glass beaker;
aluminum foil;
4 styrofoam cups;
plastic lids;
graduated cylinder;
microwave oven.
For the first part of the experiment, the calorimeters were assembled and 75 mL of water measured into the Styrofoam cup. The cup was put in microwave oven for 5 minutes. When the water was heated, it was carefully transferred to the calorimeter. The calorimeter was covered with a lid and the thermometer was put inside it. The same procedure was repeated for the second calorimeter – 75 mL of water measured, heated and transferred into it. The initial temperature of water was recorded and over the next 30 minutes, every 5 minutes the temperature was recorded to measure heat loss over time. Overall, there were 7 temperature readings for each calorimeter. The experiment was repeated 2 more times resulting in 3 trials for each calorimeter.
The second part of the experiment was carried out the following way. Into the calorimeter, 50.0 mL of 2.0 M HCl were measured using a graduated cylinder. The graduated cylinder was thoroughly rinsed with distilled water and used to measure out 50.0 mL of 2.0 M NaOH. The temperature of the solutions was measured using a thermometer which was washed well with distilled water after each measurement. When the solutions were at the same temperature ± 0.5 °C, the NaOH solution was poured into the calorimeter, the thermometer was placed in the solution and used to stir it. Maximum temperature of the solution was recorded.
Results
The experimental results for the first part are presented in the table below.
The experimental data for the second part of the experiment is presented in Tab.3.
Discussion
Figure 1. Temperature vs. Time for Both Calorimeters
It is evident from the graph that in the wrapped glass calorimeter the temperature difference between the initial and final measuring is lower. Therefore, it is better than the double Styrofoam cup one because its insulating properties are stronger.
Quantitatively, the difference between them can be found by calculating the amount of heat lost in each calorimeter during the measured time period. In this experiment, the hot water was releasing energy, while surroundings were absorbing it. The heat change can be found using the following equation:
q=C×∆T=c×m×∆T
In this case, the calculation using specific heat will be used. For both calorimeters the mass of water was 75 g, specific heat of water is 4.184 J/g °C, the temperature difference is individual for each calorimeter.
For double Styrofoam cup calorimeter: q1=4.184 Jg ℃×75 g×85.0 ℃-41.7 ℃=13587.54 J
For wrapped glass calorimeter: q2=4.184 Jg ℃×75 g×85.0 ℃-45.3 ℃=12457.86 J
In the wrapped glass calorimeter, the amount of heat lost during 30 minutes was 12,457.86 J, while in the double cup calorimeter, it was 13,587.54 J. Judging by this data, the wrapped glass calorimeter performed better.
The part 2 was intended to find the molar heat of reaction between HCl and NaOH and see if this value is different when determined using different calorimeters. The equation of the reaction is
HCl + NaOH → NaCl + H2O
As a result of reaction, the temperature of the solution has risen, this means that reaction has given off energy and that it is exothermic. The surroundings were absorbing heat from the reaction. The heat released during reaction can be found:
qr=4.184 Jg ℃×(50 mL+50 mL)×1.0gmL×40 ℃-25 ℃=6276 J
moles HCl=2.0 M×50 mL×1 L1000 mL=0.1 mol
moles NaOH=2.0 M×50 mL×1 L1000 mL=0.1 mol
The reactants react in a 1:1 ratio, and since their amounts are equal, there is no limiting reagent. If one of the quantity of one reagent was lower than the other then, the reaction would go until one of the components would end, and thus, the quantity of the products of reaction would be calculated according to this substance. The quantity of substance of water is equal to HCl and NaOH and is 0.1 mol.
Accordingly, the molar heat of reaction is:
∆H=6276 J0.1 mol=62,760Jmol
Conclusion
The purposes of the experiment were fulfilled and the experiment can be considered successful. The glass wrapped with aluminum foil performed the best of the two calorimeter experimental designs and showed lower heat loss during the studied time. However, the difference between the two designs is not significant. The molar heat of neutralization reaction of HCl and NaOH was determined and was 62,760 J/mol H2O.
