A Laboratory Report On Calorimetry
The experiment was conducted to determine the calorimeter constant, change in enthalpy of the neutralization reaction between acetic acid (CH3COOH) and sodium hydroxide (NaOH), and the heat of reaction of magnesium. A coffee cup calorimeter was used to measure the heat energy absorbed or released by the reaction. The results showed that the mean value of calorimeter constant was -39. 8479 J/oC. This negative value of calorimeter constant could be attributed to the greater amount absorbed by cold water (qcold water > qhot water). Moreover, the was found to be exothermic since qreaction is less than zero. The experimental value of agreed to the theoretical value, the calculated percent error was 8. 85789%. The heat of reaction between magnesium and acetic acid was also exothermic because an increased in temperature was observed.
Introduction
The First Law of Thermodynamics states that the heat energy lost by one body is gained by another body. Heat energy can be transferred the heat energy lost by one body is gained by another body. According to Zeroth Law of Thermodynamics, two objects with thermal difference when placed in thermal contact, there will be an exchange of heat. The heat will be lost by the hotter object (TH) and gained by the cooler object (TC), this will continue until both objects will have the same final temperature (TF), so that TH> TF > Tc. This concept governs the calorimetry. Calorimetry is the measurement of energy produced or consumed by a reaction. The basic principles of calorimetry include the mixing of chemicals, measuring of the temperature change, and calculating the heat of reaction. The reaction is identified as an exothermic when the mixture gets hot, this is due to the heat released by the kinetic energy of the chemical reaction. On one hand, a reaction is endothermic when heat is absorbed causing the decrease in temperature. A calorimeter is an instrument used for determining the amount of heat transferred. Also, this provides sufficient insulation from its surroundings. In the case of the experiment, constant-pressure calorimeter was inexpensively constructed using Styrofoam coffee cups. Styrofoam cups were used since they were readily available and economical.
The change in temperature of the calorimeter has direct relationship to the absorbed or released heat energy of the reaction. In this experiment, the was determined graphically. The heat absorbed by the calorimeter is the difference in heat of cold water and hot water. The calorimeter constant is determined using equation 3. (3)(2)The heat of neutralization of acetic acid (CH3COOH) and sodium hydroxide (NaOH) was investigated. The coffee cup calorimeter used in the experiment is constant pressure calorimeter, thus, enthalpy change is equal to the energy supplied as heat in the reaction. Also, in calculating the change in enthalpy of the neutralization it was assumed that the heat capacity and density of the solution were the same as water. Furthermore, magnesium ribbon was reacted to acetic acid. The heat of reaction per mole of magnesium was also computed.
Methodology
General Information
Styrofoam coffee cups were nested to construct the constant-pressure calorimeter. The weighing of solutions and cups were done using analytical balance (Shimadzu ATX224). For the neutralization reaction, 1N NaOH and 1N acetic acid were used.
Determination of Calorimeter Constant
The distilled water (at room temperature) was poured into the coffee cup calorimeter was weighed in analytical balance. On one hand, distilled water (50-mL) was placed in a beaker and weighed. Then, the water in the beaker was heated until 45oC. For 4. 5-minute period, the temperature of distilled water in both containers were recorded. At the five-minute mark, the hot water was poured in to the calorimeter. The mixture was stirred occasionally and the temperature was recorded for ten minutes at half-minute interval.
Neutralization Reaction of Acetic acid and Sodium hydroxide
Acetic acid (50 mL) was pipetted into the calorimeter. The temperature of the acid was measured for a 4. 5-minute period. Then, after 5 minutes the base was added and mixed. The mixture was stirred occasionally and the temperature was recorded for ten minutes.
Heat Reaction of Magnesium
Acetic acid (50 mL) was added into the calorimeter. Approximately 0. 1500g of magnesium ribbon was weighed and cut into pieces. Then, the magnesium was added to the calorimeter. The 10-minute period of recording the temperature was done after the maximum temperature was reached.
Determination of The temperature-time
Data obtained from 2. 3, 2. 4, and 2. 5 were plotted using Microsoft Excel 2016. The initial temperature and final temperature were determined by extrapolating the straight line of the plot before and after mixing, respectively. Polynomial second-order was chosen for most of the graphs in order to have the best-fit-line with reliable value of regression coefficient (R2). Using the equation of the line, the initial and final temperatures were obtained. The 5 minutes was substituted to the x in the equation of the line. The change in temperature ( was difference in the final and initial temperature.
Data Analysis
The data collected were used to calculate the calorimeter constant, , and qreaction of Mg. Mean, standard deviation, and relative standard deviation were computed to analyze the data. Also, percent error was computed to compare the experimental and theoretical values of of acetic acid and sodium hydroxide.
Results and discussion
In this experiment, the direct relationship of of the calorimeter to the absorbed or released heat energy of the reaction was utilized in order to determine the calorimeter constant. Equation 3 was used to calculate the CCal. As shown in the data, the heat absorbed by the cold water is greater than compared to the released heat energy of the hot water. The heat energy assigned to hot water is negative since the heat is released. The heat released by hot water and heat absorbed by cold water did not have same magnitude because some heat is transferred to the Styrofoam cups and to the surroundings. The negative value of the calorimeter constant could be attributed to the fact that qcw > qhw. Accordingly, in the Zeroth Law of Thermodynamics both objects will have the same final temperature (TF), so that TH> TF > Tc, however, in the result of this experiment the final temperature is greater than that of the initial temperature of the cold water. For these reasons, the heat absorbed by the calorimeter is negative.
The reproducibility of the results of this experiment is low because of the value of the relative standard deviation is greater than 10%. This higher value of RSD could be linked to large increased of the change in temperature that occurred in both trials.
In the context of this experiment, it was assumed that the heat capacity and density of the solutions was same as the water. The heat of reaction is calculated using equation 4. The heat of calorimeter was accounted in order to the determine the heat of solution. Since, the coffee cup calorimeter was accounted as constant-pressure calorimeter, thus change in enthalpy is equal to heat. The neutralization involves the reaction between H+ (from acid) and oH- (from base), thus acetic acid must be fully ionized as per reaction. However, acetic acid is ionized to a small extent in solution. The ionization reaction of acetic acid is found to be endothermic reaction. The neutralization of acetic acid and sodium hydroxide not only involves the combination of H+ and OH- but also the dissociation of acetic acid. The dissociation process is accompanied by the absorption of energy. Moreover, the +1. 2 k of heat energy is used up in dissociating acetic acid. The enthalpy change of neutralization in this experiment is -50. 9484 kJ/mol. The change in enthalpy is negative since the reaction is exothermic. To assess the effectiveness of the experiment, percent error was calculated.
Conclusion and recommendations
The results of this experiment indicated that the change of enthalpy in the neutralization reaction between acetic acid and sodium hydroxide is exothermic since q < 0. Also, the experimental value of is in agreement to the theoretical value. This could be attributed that the coffee cup calorimeter is made up of Styrofoam cups which served as insulators. Furthermore, the direct relationship of of the calorimeter to the absorbed or released heat energy of the reaction was utilized in order to determine the calorimeter constant However, the calorimeter constant is negative could be attributed to the greater amount of heat absorbed by cold water and also the final temperature of the mixture is lesser than the initial temperature of hot water. Some of heat produced by the reaction escaped from the calorimeter since the cup used in not insulated. In addition, the stirring was done manually for this reason some of the heat energy transferred to the hands because there is a thermal difference. To increase the effectiveness of the experiment, the experimental set-up must be modified; instead of using plastic cups as cover for the calorimeter the cover must be insulated and the magnetic stirrer must be used in stirring the mixture.
