Enthalpy change and Hess’s law. Essay

Custom Student Mr. Teacher ENG 1001-04 16 November 2017

Enthalpy change and Hess’s law.

Introduction: Enthalpy is the total energy of a system, some of which is restored as chemical potential energy in the chemical bonds. During reactions, bonds are broken and formed. As a result, all reactions are accompanied by a change in the potential energy of the bonds, and hence an enthalpy change. This enthalpy change of reaction can be measured and is given the symbol H. The temperature of a system is a measure of the average kinetic energy of the particles present. Therefore, during a reaction, as bonds are broken and formed, there is a change in this average kinetic energy and this will result in a change in the temperature of the system. On the other hand, heat is the measure of the total energy of the substance. When the temperature of a substance increases or decreases, heat energy is absorbed or released from the environment. In order to measure this change in temperature, the following formula can be used:-

Heat energy = m.C.T ……where, m= mass of the substance

C= specific heat capacity

T= change in temperature

Aim of experiment: – This particular experiment focuses on Hess’s law. Hess’s law states that the total enthalpy change on converting a given set of reactants to a particular set of products is constant, irrespective of the way in which the change is carried out. For this experiment, the conversion of NaOH to NaCl by two different routes was used and the change in enthalpy for each path was measured to test Hess’s law.

Route one: – NaOH + 2M HCl 2M NaCl

2M NaCl + H2O 1M NaCl

Route two: – NaOH + H2O 2M NaCl

2M NaOH + 2M HCl 1M NaCl

Materials: – beakers, NaOH, 2M HCl, stopwatch, thermometer, pipettes, measuring cylinder

Methods: – for the first route, 4g of NaOH was weighed. Then using a measuring cylinder 50 ml of 2M of HCl was measured and poured into a can. A thermometer was used to record the temperature of the hydrochloric acid in the can. Afterwards NaOH was added into the can. Using stopwatch and a thermometer the change in temperature was measured as 2M of NaCl was being formed. Once the temperature of the 2M NaCl was stable, 50 ml of water was added to change the sodium chloride into 1M of NaCl. Using stopwatch and thermometer the change in temperature was recorded.

For the second route, 4g of NaOH was weighed. Then to form 2M of NaOH, 50 ml of water was added. A thermometer and stopwatch was used to monitor the change in temperature. Then 50ml of HCl was measured. Then its temperature was measured. These two solutions were then mixed together in a can. The temperature change was measured using a thermometer and stopwatch.

Results for route one.

In table1.1, the temperatures of NaOH and 2M HCl before the reaction began were 22.5�0.1�c. In table1.2, the temperature of 2M NaCl was 57.5�0.1�c before 50ml of water was added to it.

Graph 1.2 temperature changes when 50 ml of water was added to 2M of NaCl to form 1M of

NaCl.

Results for route two.

After 50ml of water was added to 4g of NaOH to form 2M of NaOH, it had a temperature of 32.4�0.1�c. On the other hand, 2M of HCl had a temperature of 22.5�0.1 c�. For table 1.3 the beginning temperature of these two reactants is the mean value of their temperature that is 27.4�0.1�c. This was done because there was not enough time to wait until both reactants reach the same temperature.

Table 1.4 temperature changes during the addition of

50ml of water to 4 g of NaOH to form 2M

of NaOH.

Table 1.3 temperature changes during the reaction of 2M of NaOH

with 2M of HCl to form 1M of NaCl.

Graph 1.3 temperature changes during the reaction of 2M of NaOH with 2M of HCl to form 1M of NaCl.

Graph 1.4 temperature changes during the addition of 50ml of water to 4 g of NaOH to form 2M of NaOH.

Discussion

To test Hess’s law the temperature change in each of the routes during the reaction can be used. The formula for heat change, which is found in the introduction section of this report, is also useful. For this particular experiment, water that is the byproduct in the formation of sodium chloride is used as a way of finding the enthalpy change. The density of water is taken to be 1g/ml and is used to determine the mass of the solutions. The specific heat capacity of water is 4.18 (g-1k-1).

Route one: – To form 2M of NaCl the change in temperature () was 71.6 – 22.5= 49.1�c. The mass of the solution of NaOH and 2M of HCl was found to be 50g by using the density of water as 1g/ml and the volume of the solution being 50ml. change in enthalpy is:-

Heat change= m. c.

=

= 10.2 KJ

The change in enthalpy in changing 2M of NaCl to 1M of NaCl can be calculated in the same way where the mass of the solution becomes 100 g when 50 ml of water is added. The change in temperature was -17.6 �c.

Heat change= m. c.

=

= -7.3 KJ

The sum of the two changes in enthalpy will give a value of 2.9 KJ.

Route two: – the change in temperature in forming 2M of NaOH from NaOH was 9.9�c. To do this 50 ml of water was added and the mass of the solution was 50 g. using the formula the change in enthalpy can be calculated as follows:-

Heat change= m. c.

=

= 2.0 KJ

Once 2M of NaOH was formed, 50ml of 2M of HCl was added to form 1M of NaCl, which is the final product. The change in temperature was 8.5�c. The mass was 100g and the change in enthalpy was

Heat change= m. c.

=

= 3.5 KJ

The sum of the two changes of enthalpy is 5.5 KJ.

Conclusion: – The change in enthalpy for the formation of 1M of NaCl using route one was 2.9 KJ. Using route two it was 5.5 KJ. There is a significant difference of 2.6 KJ. This shows that the experiment done did not prove the Hess’s law effectively. This may be due to certain factors that will be discussed below.

Limitation: – There could be lose of heat to the surrounding environment that can have vital effect on the results of the experiment. Human error and mistake matter in this kind of experiment. Instead of using a simple thermometer appropriate can be used.

Improvements:- the use of data pro logger or calorimeter can minimize the human and device error in measuring temperature and can be better than simple thermometer. Using insulator such as Styrofoam cups to prevent heat from escaping to the surrounding.

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