An Investigation Of Hess' Law

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Aim: To verify Hess law Research Question: How can Hess’ Law be used to find the enthalpy of solution of sodium hydroxide using the neutralization reaction between sodium hydroxide and hydrochloric acid. Theory: Following the law of conservation of energy, Hess’s Law states that the total enthalpy change for a reaction is equal to the energy lost or gained by the system. This law remains true no matter what the reaction or the steps and ways in which a particular reaction occurs. Theoretically, energy is never ‘lost’ from a system. It is simply converted to another set of products. In this experiment, NaOH(s) + HCl(aq) NaCl(aq) + H2O(l) [H1] NaOH(s) NaOH(aq) [H2] NaOH(aq) + HCl(aq) NaCl(aq) + H2O [H3] According to Hess’s Law [H1] = [H2] + [H3] Therefore [H2] = [H1] - [H3] Variables: Independent Variable: The state of NaOH used in the three experiments. These different states enable the data that will be used to verify Hess’s law to be collected. Dependent Variable: The enthalpy changes of the three reactions that will vary with different states of NaOH used. Controlled Variables: The room temperature and pressure. The temperature of all the solutions used kept constant at room temperature. Apparatus: * 50 cm3 measuring cylinder (± 0.5 cm3) * Polystyrene cup with lid * Cotton * Thermometer (± 0.5 °C) * 100 cm3 of 1M HCl * 50 cm3 of 1M NaOH * 50 cm3 of distilled water * 2 grams of solid NaOH * Stopwatch * Pan balance (± 0.001 g) Procedure: Measurement of [H1] 1. Measure 50 cm3 of HCl (1M) and pour it into the polystyrene cup. 2. Close the lid of the cup and insulate it with cotton. 3. Weigh out approximately 2 grams of solid NaOH pellets. 4. Measure the temperature of the HCl and start the stopwatch. 5. Measure the temperature once

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