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# Determining the enthalpy change for different chemical reactions.

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Introduction

Topic: Determining the enthalpy change for different chemical reactions. I familiarised myself with the Material Safety Data Sheets of toxic substances. PLANNING (A) Enthalpy (H)1 - The sum of the internal energy of the system plus the product of the pressure of the gas in the system and its volume: Esys is the amount of internal energy, while P and V are respectively pressure and volume of the system. However, to make it simpler, this definition can be shortened. Enthalpy (H) is a measure of heat in the system. To measure the enthalpy we have to first figure out the mass of a substance under a constant pressure and determine the internal energy of the system. The enthalpy change (H)2 is the amount of heat released or absorbed when a chemical reaction occurs at constant pressure. Standard conditions3 are used in order to allow experiments that are taken at different locations to come out with the same results. Standard pressure is 1 atmosphere or 1.0135 x 105 pascals. Standard temperature is 25o C. Standard state is the physical state at which an element or a compound exists at standard conditions. ...read more.

Middle

� Na2CO3 (s) + H2O (l) + CO2 (g) This may be also shown in the form of an enthalpy cycle: 2HCl (aq) + 2NaHCO3 (s) 2NaCl (aq) + CO2 (g) + H2O (l) Na2CO3 (s) + H2O (l) + CO2 (g) + 2HCl (aq) The enthalpy change for the decomposition of sodium hydrogencarbonate may be obtained by determining the enthalpy change of reaction between sodium carbonate and hydrochloric acid and that between sodium hydrogencarbonate and hydrochloric acid. ?H = H(products) - H(reactants) ?H = 0.92 kJ - 26.54 kJ = - 25.62 kJ Experiment II Part I PLANNING (B) Requirements: - 1 beaker [250 cm3] - 2 test tubes - thermometer - 60 cm -3 of 2 mol dm-3 hydrochloric acid - solid calcium oxide (CaO) [3 g] - balance Procedure: We were provided with 2 mol dm-3 hydrochloric acid, solid calcium carbonate and solid calcium oxide. 1. One person in each pair measured 30 cm3 of approximately of 2 mol dm-3 hydrochloric acid into the beaker. 2. We took the temperature of the acid and recorded it in table 3. 3. We weighted a test tube empty and than again when it contained 3.00 g of solid calcium oxide. ...read more.

Conclusion

If the enthalpy has a negative sign, like in the Experiment I, then the reaction is exothermic. Heat energy is evolved, so the beaker becomes hotter4. If the sign of enthalpy is positive, then similarly the reaction is endothermic, like in the Experiment II. Heat energy is absorbed and the beaker becomes colder5. The physical properties of reactions (different temperatures of beakers) can be easily distinguished in the real life, even without using any instruments. To evaluate this lab I would suggest using the calorimeter to make the records more reliable than by using thermometer. Room temperature might have had an influence on our results and this was probable the most important source of uncertainty. Masses of substances were measured accurately, although some minute amounts might have been lost while pouring. The pressure remained the same, however little changes may have appeared. We also should pay attention to the amount of gas (CO2) that might have escaped during the experiment. It ought to have been gathered and stored to make the results reliable. 1 The definition comes from http://www.chem.tamu.edu/class/majors/tutorialnotefiles/enthalpy.htm 2 The definition comes from http://www.ausetute.com.au/enthchan.html 3 The definition comes from http://library.thinkquest.org/17869/chemweb/book/thermo.htm 4 The graph comes from http://www.sasked.gov.sk.ca/docs/chemistry/mission2mars/contents/glossary/images/exo_h1.gif 5 The graph comes from http://www.sasked.gov.sk.ca/docs/chemistry/mission2mars/contents/glossary/images/endo_h1.gif DETERMINING THE ENTHALPY CHANGE FOR DIFFERENT CHEMICAL REACTIONS 1 ...read more.

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