-6.86131kJ/.1moles= -68.61kJ/mol = Delta H rxn Part 3. Verify Hess’s Law 1Q: Write the net ionic equations for the three reactions involved in the experiment. Show how the first two reactions are arranged algebraically to determine the third. 1A: According to Hess's Law, the enthalpy change delta H rxn , for the 3rd reaction equals the sum of the delta H rxn for reactions 1 and 2, after the 2nd reaction is reversed, we also change the sign from negative to positive due to the reverse. Therefore: Delta H of reaction 1: NaOH(aq) + HCl(aq) --> NaCl(aq) + H2O(l) - Delta H of reaction2: NH3(aq) + NaCl(aq) + H2O(l) --> NH4Cl(aq) + NaOH(aq) = Delta H of reaction 3: NH3(aq) + HCl(aq) --> NH4Cl(aq)
The net ionic equations are as follows: (1) HCl (aq) + NaOH (aq) --> NaCl (aq) + H2O (l) H + + Cl - + Na + + (OH) - --> Na + + Cl - + H2O (l) H + + (OH) - --> H2O (l) (2) NH 3(aq) + NaCl (aq) + H2O (l) --> NH4Cl (aq) + NaOH (aq) N - 3 + 3H + + Na + + Cl - + H 2 O (l) -->(NH 4 ) + + Cl - + Na + + (OH) - N - 3 + 3H + + H 2 O (l) --> (NH 4 ) + (OH)- (3) NH 3(aq) + HCl (aq) --> NH 4 Cl (aq) N - 3 + 3H + + H + + C l- --> (NH 4 ) + + Cl - N - 3 + 3H + + H + --> (NH 4 ) + As a result, the net ionic equation for Reaction 1 added to the net ionic equation for Reaction 2 is equal to reaction 3. H + + (OH)- +N - 3 + 3H + + H 2 O (l) --> (NH 4 ) + + (OH) - + H 2 O (l) H + + N - 3 + 3H + --> (NH 4 ) + 2Q: Calculate the value of Delta H for the third reaction from the values of delta H determined for the first two reactions using Hess’s law. 2A:Therefore, -78.6 kJ/mol + 6.2127 kJ/mol = -72.4 kJ/mol So through the method of Hess's law, the estimated (delta H) of the third reaction would be -72.4 kJ/mol. 3Q: Find the percent difference between the calculated and measured values of delta H for the third reaction. 3A: Use percent error formula: %Error= l Theoritical – Experimental l / (Theoritical) x 100 , so it would be l -72.4 - -68.61 l / -72.4 x 100 = 5.23 % error Post-Lab questions 1Q: What is meant by calorimetry? 2Q: How does graphical analysis improve the accuracy of the data? 2A: Graphical analysis does not necessarily make your graph more accurate, for example if you plot the wrong points, it won’t help your accuracy at all. However, with graphical analysis your graph, when done correctly, can accurately display trends going on in the experiment. 3Q: The equation for calculating the heat evolved in each reaction is: q rxn = -[(grams of solution x specific heat of solution x Delta T solution ) + (C cal x Delta T solution )] What is the meaning of the negative sign in front of the brackets.
3A: the negative sign is there to point out that the overall reaction is exothermic. Without the sign the q rxn would be positive, indicating an endothermic reaction. That’s why the negative sign is infront of the brackets.