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Validating Hess's law

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Research Question: Is the enthalpy change for a given chemical change the same whether the reaction takes place in a single stage or via several stages, provided the initial and final conditions are the same. Introduction: Hess's Law (1840) states that for a given chemical change the enthalpy change is the same whether the reaction takes place in a single stage or via several stages, provided the initial and final conditions are the same. We will test the validity of this law using the reaction between sodium hydroxide and hydrochloric acid. The reaction between solid sodium hydroxide and dilute hydrochloric acid can be carried out in two ways. Method 1 NaOH(s) + HCl(aq) NaCl(aq) + H2O(l) ?H?1 Method 2 NaOH(s) NaOH(aq) ?H?2 then NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) ?H?3 According to Hess's Law ?H?1 = ?H?2 + ?H?3 In calculating the enthalpy of reaction values in each of the above and following cases, it is assumed that: a. The density of the solutions is 1 gcm-3 b. ...read more.


+ HCl(aq) NaCl(aq) + H2O(l) Initial Temperature = 26.5�C ?0.1�C Maximum Temperature = 48.1�C ?0.1�C ?T = Maximum Temperature - Initial Temperature = 48.1�C - 26.5�C = 21.6�C ?0.2�C % Error in ?T = The enthalpy change ?H?1 is given by multiplying the mass of HCl (m) by its specific heat capacity (Cp) and the change in temperature (?T). Since this reaction takes place in solution, which we assume to be mostly water. Hence the mass of the solution will be 50g and the specific heat capacity is given as 4.2J/g/�C. The calculation for the mass of the solution is as follows: Mass = Density � Volume (Density given as 1g/cm3) Mass = 1g/cm3 � 50cm3 Mass = 50g Hence, ?H?1= m � Cp � ?T ?H?1 = 50g � 4.2J/g/�C � 21.6�C ?H?1 = 4536J ?0.926% ?H?1 = 4.536kJ ?0.042kJ Mass of NaOH Pellets: 2.20g ? 0.01g % Error in mass = 2.20g of NaOH produces 4.536kJ ?0.042kJ 1 mole (40g) will produce: ? ...read more.


Mass = 1g/cm3 � 50cm3 Mass = 50g Hence, ?H?3 = m � Cp � ?T ?H?3 = 50g � 4.2J/g/�C � 9.9�C ?H?3 = 2079J ?2.02% ?H?3 = 2.079kJ ?0.042kJ Volume of NaOH: 0.05dm3 ? 0.0001dm3 % Error in volume = 0.05dm3 of NaOH produces 2.079kJ ?0.042kJ 1 mole (1dm3) will produce: ? 2.22% 1 mole (40g) will produce 41.6kJmol-1 ? 0.924kJmol-1 Conclusion and Evaluation: According to Hess's Law ?H?1 = ?H?2 + ?H?3 So using the results found above we see that: ?H?1 = 82.5kJmol-1 ? 1.14kJmol-1 ?H?2 + ?H?3 = 38.7kJmol-1 + 41.6kJmol-1 = 80.3kJmol-1 ? 1.944kJmol-1 It can be seen that ?H?1 is almost equal to ?H?2 + ?H?3 but due to experimental errors such as heat being lost to the surroundings and not having exact readings they are not equal. Hess's law has therefore been validated but the results would have been much more accurate if there was an insulating capsule around the polystyrene cup so as to prevent any heat from being lost to the environment and by having more accurate apparatus in terms of measuring cylinders and thermometers. ?? ?? ?? ?? ...read more.

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