# Indirect determination of enthalpy change of decomposition of sodium hydrogen carbonate by thermochemical measurement and Hess's Law.

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Introduction

Chemistry - CH1 Practical Assessment Written Account of Practical Assessment Aim: Indirect determination of enthalpy change of decomposition of sodium hydrogen carbonate by thermochemical measurement and Hess's Law. Background Information: Many standard enthalpy changes of reaction cannot be measured directly, and we therefore have to employ an indirect approach using energy cycles. This method relies on Hess's Law, which states that if a change can be brought about by more than one route, then the overall enthalpy change for each route must be the same, provided that the starting and finishing conditions are the same for each route. We could say that this is a consequence of the first law of thermodynamics, which states that energy can neither be created or destroyed in chemical reactions, and so energy changes for a reaction must be the same, whether it takes place in one step or in a whole series of steps. I will be measuring the enthalpy changes of reaction of sodium hydrogencarbonate and sodium carbonate with dilute hydrochloric acid. By applying Hess's Law to the results, I will be able to calculate the enthalpy change of decomposition of sodium hydrogencarbonate, (?H3), which is impossible to measure directly. The cycle below shows how this is done: The method involves carrying out reactions in separate experiments in insulated calorimeters, calculating the heat absorbed or evolved - allowing for heat losses to the surroundings - and scaling up to molar amounts. Risk Assessment ? Hydrochloric Acid (2M) is a low hazard, but may still cause harm if it comes into contact with eyes or broken skin. If the hydrochloric acid comes into contact with skin, notify supervisor and wash affected area immediately with water. ? Should spillage occur, again notify supervisor and dilute with water before mopping up. ? ...read more.

Middle

I weighed the empty weighing bottle once again, (having emptied the Na2CO3 into the calorimeter), and the weight was 6.136g. I then deducted the result of this from the second weight (bottle + 2.0g Na2CO3): this will give us a more accurate figure for the actual mass of Na2CO3 used, as it accounts for possible error in any material left in the weighing bottle. I found that the actual mass of Na2CO3 I used was 2.125g. After having collected all the results in my table, I used them to plot a graph of temperature against time for this reaction between Na2CO3 and HCl. I then extrapolated the temperature change, ?T, of the reacting solution from the graph. The value of ?T was -4.75?C. I then calculated the heat absorbed by the solution by using the formula c x mass x ?T: Heat absorbed by solution = c x mass x ?T = 4.18J/g x 50g x (-4.75) = = -992.75J I then calculated the mole fraction of Na2CO3 used: n= Mass / Mr : Mr Na2CO3 = 106 Moles Na2CO3 = mass Na2CO3 = 2.125 = 0.020 Mr Na2CO3 106 Finally, I calculated the molar ?H (the heat of solution per mole of solute) by using: heat absorbed / moles. This represents ?H2 with reference to our reaction cycle: Molar ?H = heat absorbed by solution (J) = -992.75= - 49637.5 J/mol-1 = Moles Na2CO3 0.020 = -49.64 kJ/ mol-1 Calculating ?H3: Now that we have the values for ?H1 and ?H2 we can calculate the value of ?H3 from these. Looking at our cycle we can say: ?H1 = ?H3 + ?H2 but we need to find the value of ?H3. ...read more.

Conclusion

I consequently predicted that this reaction would be endothermic. I then estimated the value of ?T for experiment 2 (Na2CO3 and HCl) as being approximately - 4?C, and therefore estimated ?H to be about - 41.8 kJ/mol-1. I consequently predicted that this reaction would be exothermic. Using the reaction cycle and applying Hess's Law, I predicted that the enthalpy change for the reaction 2 NaHCO3 Na2CO3 + H2O + CO2 would be endothermic and would measure about + 64 kJ/mol-1. My actual result was +75.77 kJ/mol-1 so my rough estimate was not too wide of the mark. Calculations & Measurements - at a glance Experiment 1: NaHCO3 and HCl Weight empty bottle = 6.136g (W1) Weight bottle plus 3.0g NaHCO3 = 9.339g (W2) Weight empty bottle after NaHCO3 = 6.136g (W3) Actual mass of NaHCO3 used = W2 - W3 = 3.203g Mr NaHCO3 = 84 Moles NaHCO3 = mass NaHCO3 = 3.203 = 0.038 Mr NaHCO3 84 ?T = +4.75?C (extrapolated from graph - please see separate sheet) Heat absorbed by solution = shc x mass x ?T = 4.18J/g x 50g x 4.75?C = = +992.75 J Molar ?H = heat absorbed by solution = +992.75 = +26125 J/mol-1 = Moles NaHCO3 0.038 = +26.125 kJ/mol-1 Experiment 2: Na2CO3 and HCl Weight empty bottle = 6.136g (W1) Weight bottle plus 2.0g Na2CO3 = 8.261g (W2) Weight empty bottle after Na2CO3 = 6.136g (W3) Actual mass of Na2CO3 used = W2 - W3 = 2.125g Mr Na2CO3 = 106 Moles Na2CO3 = mass Na2CO3 = 2.125 = 0.020 Mr Na2CO3 106 ?T = -4.75?C (extrapolated from graph - please see separate sheet) Heat absorbed by solution = shc x mass x ?T = 4.18J/g x 50g x -4.75?C = = -992.75J Molar ?H = heat absorbed by solution = -992.75 = -49637.5 J/mol-1 = Moles Na2CO3 0.020 = -49.638 kJ/ mol-1 1 ...read more.

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