Exothermic reaction Essay

The object of this investigation is to determine the enthalpy change for the reaction CaCO3 (s) i CaO (s) + CO2 (g) by an indirect method based on Hess’ Law. Hess’s law states that the enthalpy change for any chemical reaction is independent of the route taken provided that the initial and final conditions are identical. So the temperature change during these reactions below can be measured and the enthalpy changes ? H1 and ? H2 calculated. For Example: Using Hess’ law with the calculated values for ? H1 and ? H2 it is possible to calculate a value for ? H3. Results Table. Temperature change during reaction   The results for the temperatures are to the nearest whole number as it is unrealistic to measure to a point of a ? C with this type of thermometer and the masses rounded up to 2 decimal places for greater accuracy. Calculations It’s possible to use the formula E = mc ? T, where E = energy transferred, m = mass of HCl, c= specific heat capacity of HCl and ? T = temperature change. This formula can be used for calculating the energy transferred in the following reactions ? ?H1, CaCO3 (s) + HCl and ? H2, CaO (s) + HCl. Seeing as the molar mass of CaCO3 = 100. 00 ?H1 = 420 x (1 x 0. 0250) = – 16. 80 kJmol-1 I will not include the last result in my average for ? H1, which is – 16. 80 kJmol-1. This is because it’s way off the other results and would significantly affect my average results, it’s an anomaly. AVERAGE for the ? H1 for the reaction between CaCO3 + HCl: (- 25. 09 kJmol-1) + (- 24. 90 kJmol-1) 2 ?H1 = – 25. 00 kJmol-1 This value for ? H1 is negative because heat is lost to the surroundings. It’s an exothermic reaction. Calculations for ? H2 for the reactions between CaO (s) + HCl: 1. I will not include the – 102. 86 kJmol-1 result in my average for ? H2. This is because it’s way off the other results and would significantly affect my average results, it’s an anomaly. AVERAGE for the ? H2 for the reaction between CaO + HCl: (-128. 05 kJmol-1) + (- 111. 43 kJmol-1) 2 ?H2 = – 119. 74 kJmol-1 This value for ? H1 is negative because heat is lost to the surroundings. It’s an exothermic reaction. Using Hess’ cycle I will use the values that I have calculated for ? H1 and ? H2 to work out the value for ? H3. ?H3= ? H1 ? ?H2 = (- 25. 00 kJmol-1) – (- 111. 43 kJmol-1)= 86. 43 kJmol-1 This value is positive because heat is absorbed from the surroundings. It’s an endothermic reaction. I have been told the actual value for ? H3, which is 178. 00. So I will calculate the percentage by which my value is out by the actual value. 178. 00 ? 86. 43 = 91. 57 (91. 57 ? 178. 00) x 100 = 51% Evaluation Errors in procedure: When the CaO and CaCO3 were put into the cup there was a delay before the lid was put on. This could have caused heat to escape out of the cup and the temperature change would not have been as great compared to if there was no delay.