Heat of Solution for Various Salts - Lab Report Example

LAB REPORT - HEAT OF SOLUTION FOR VARIOUS SALTS Aim The intention of this experiment is to determine the heats of solution associated with various salts. Background Ionic substances are characterized by different charges in their structure. A salt can either be neutral, basic, or acidic depending on the ions used in forming the salt. When a salt dissolves in water, it dissociates to its constituent ions. This process is accompanied with a change in energy of the system. A system in this case refers to a set of conditions under observation. The system in this kind of experiment consists of the water, the salt to be dissolved, and the containers used in the experiment. These three factors will determine whether the system is open, closed, or isolated. These factors will all play a role in influencing energy change, in the process of an experiment. Energy changes when a salt dissolves in water are indicated by temperature change of the system, usually change in water temperature. Since the environment surrounding the system under observation can influence the temperature of the system, it is necessary to use a set-up that will shield this effect. For this to be possible, a calorimeter is used. This experiment may be undertaken in the laboratory. The results obtained, though will be inclusive of external interference thus not accurate. For this laboratory, an open system will be used. The variables in this experiment include the quantities to be used, and the temperature changes. The different salts to be used constitute the independent variables in this experiment. Temperature change is the dependent variable, since it keeps changing throughout the experiment. The amounts of the salt, and water as measured all through the experiment. Hypothesis: If the dissociation of the ions of the salt is accompanied by energy change, this will be transferred to the water. The temperature change of the solution is directly proportional to the energy change of the system. Materials Measuring scale Filter paper 50 mL beakers Distilled water Potassium Nitrate salt Ammonium Sulphate salt Potassium Hydroxide salt Sodium Nitrate salt Sodium Hydroxide salt Stirring rod Thermometer Procedure 1. To 50 mL beaker, 25 Ml of distilled water was added. 2. The temperature of the water at room temperature was measured using a thermometer and recorded. 3. Using the measuring scale, 1.0 g of potassium nitrate salt was measured on a filter paper. 4. The salt was poured into the water in the beaker, with stirring. 5. The thermometer was put into the solution to measure the temperature changes as they occur. 6. The final temperature that which stabilizes is noted and recorded. 7. The procedure (steps 1 – 6) was repeated for the second and third time using potassium nitrate. These three trials were all recorded. 8. Steps one to seven were repeated for the other four salts that were used in this experiment. Each time, the experiment would be done in three trials and the temperature changes noted and recorded. 9. Using the results obtained, a detailed study of heat of solution for the various salts was carried out. Results Salt Trial 1 Trial 2 Trial 3 Average Initial Temp (0C) Average Final Temp (0C) Temperature Change (0C) Initial Temp. (0C) Final Temp (0C) Initial Temp (0C) Final Temp (0C) Initial Temp (0C) Initial Temp (0C) KNO­3 25.1 23 25 22.6 25.1 22 25.1 22.5 -2.6 (NH­­4­­)­2­SO4 25.5 23.9 25.1 25 25.4 25.1 25.3 24.7 -0.6 KOH 25.5 31.2 25.5 33.2 25.2 31.4 25.4 31.9 6.5 NaNO3 26.0 23.6 26.2 23.9 25.3 24 25.8 23.8 -2 NaOH 25.0 35.1 25.2 35 25.1 35.1 25.1 35 9.9 Note: Calculations involving temperature changes are usually done in Kelvin. Analysis of the results in this case, however, the results remain in degrees Celsius. This is because when heat changes consider temperature differences. Whether, in Kelvin or not, the resultant heat change retains the same number of units. Temperature may be in Kelvin or degrees Celsius; but a change of two units while in Kelvin is the same in 0C. Calculations KNO­3 Heat of Solution, Q =m c ΔT Q= 25g × 4.2J g-1 K-1­ × -2.6 K Q = -273 J Change in Molar Heat, ΔH = Q/n n=25g/101.1032g/mol = 0.25 moles ΔH = -273J/0.25 moles = 272.75 J/mol (NH­­4­­)­2­SO4 Heat of solution, Q = m c ΔT Q = 25g × 4.2J g-1 K-1­ × -0.6 K Q = 36 J Change in Molar Heat, ΔH = Q/n n = 25g/132.1402g/mol = 0.19moles ΔH = 36J/0.19moles = 189.47 J/mole KOH Heat of solution, Q = m c ΔT Q = 25g × 4.2J g-1 K-1­ × 6.5 K Q = 682.5 J Change in Molar Heat, ΔH = Q/n n=25g/56.1057g/mol = 0.45moles ΔH = 682.5J/0.45moles = 1533.7 J/mol NaNO3 Heat of solution, Q = m c ΔT Q = 25g × 4.2J g-1 K-1­ × -2 K Q = -210.4 J Change in Molar Heat, ΔH = Q/n n=25g/84.9948g/mole = 0.29moles ΔH = -210J/0.29moles = -724.13 J/mol NaOH Heat of solution, Q = m c ΔT Q = 25g × 4.2J g-1 K-1­ × 9.9 Q = 1039.5 J Change in Molar Heat, ΔH = Q/n n = 25g/39.9971g/mole = 0.625moles ΔH = 1039.5J/0.625moles = 1663.2J/mol Discussion The salts used in this experiment are all categorized as ionic salts. The bonds in the structure are due to electrostatic attraction between different chemical charges, also referred to as ionic bond. An ion is a chemical species with a characteristic charge. For each of the salts in this experiment, two species can be identified within one salt. The positive charge is the cation, and the negative charge the anion. For Ammonium Sulphate for instance, the ammonium ion (NH4+) is the cation, while the sulphate ion (SO42-) is the anion. When a salt goes into solution, the bond between different ion species in the salt are broken down. The different ions that were held together in the crystal lattice go into solution as different species (Kotz, Treichel and Weaver 623). This dissociation of bonds is accompanied by energy change. The results obtained in this experiment vary from negative, to positive temperature changes. Negative temperature change indicates that heat was absorbed as the salt went into solution. This means the breaking down of ionic bonds in the salt required input of energy (Kotz, Treichel and Weaver ). As the process absorbs energy from the surrounding, the temperature of the water falls. Potassium Nitrate, Ammonium Sulphate, and Sodium Nitrate illustrated this property. The solvation process in these cases is endothermic since energy is absorbed from the surrounding. The energy absorbed is from the water. This is why the temperature of water falls in these cases. For Potassium Hydroxide and Sodium Hydroxide, a different trend in temperature change is noticed as the salts dissolve. The process goes together with increase in the temperature of the solution. The temperature of the water rises because there has been a release of the same as the ionic bonds are broken down. This is known as an exothermic reaction since it gives off energy to the surrounding. In the calculation of heats of solution, enthalpy changes are calculated. The assumption is made that the amount of heat either absorbed or given out is obtained from or absorbed by the water respectively. That is to say that the amount of heat given out as a salt goes into solution is absorbed by the water causing a similar increase in temperature. When the process is endothermic, the energy that will be used for breaking of the ionic bonds comes from the water, thus will be shown by the fall in water temperature (Reger, Goode and Ball 482). By making this assumption, it is possible to quantify heat change for the salt dissolving. Conclusion Measuring of heat changes using the above methods is not very accurate. To improve the accuracy of such procedures, a calorimeter is used. A calorimeter consists of a container in which the reaction is carried out. The heat produced is channeled to a known amount of water. Alternatively, a known material may be used instead of the water. This ensures that the heat capacity of the surrounding material is known. The heat capacity of water is 4.2J g-1 K-1. Knowing this, heat change may be calculated using the formula Q = m c ΔT. Q is the heat m is the mass of solution c is the specific heat capacity of the surrounding material ΔT is the energy change observed during the experiment, it may be positive or negative The errors that were associated with this experiment were largely due to human error. Errors may have arisen during the process of measuring quantities of the salt, reading of the temperature, or inconsistency with following the results. Errors of judgment may also have played in influencing the direction of the results. Instrumental errors, such as when measuring quantity of salt may have occurred leading to discrepancies between the obtained, and expected results. This experiment was successful since the heat changes as the salt went into solution were obtained. The results obtained may not be in line with literature values due to the errors discussed above. The elimination of these errors, as stated above, may e possible with the use a calorimeter. For instance, the molar heat of solution of KNO3 is 35KJ/mol, for KOH is -56KJ/mol, for NaNO3 is 20.4KJ/mol, and for NaOH is -44.3KJ/mol. Works Cited Kotz, John C., et al. Chemistry and Chemical Reactivity. Stamford: Cengage Learning, 2009. Print. Reger, Daniel L., Scott R. Goode and David W. Ball. Chemistry: Principles and Practice. Stamford: Cengage Learning, 2009. Print. Read More