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    Evaluating An Enthalpy Change That Cannot Be Measu Essay

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    red Directly. Dr. Watson.

    Introduction. We were told that sodium hydrogencarbonate decomposes on heating to give sodiumcarbonate, water and carbon dioxide as shown in the equation below:-2NaHCO3(s)——–> Na2CO3 (s) + H2O (l) + CO2 (g)= DeltaH1This was given as deltaH1 and we had to calculate as part of the experiment. This however cannot be measured directly, but can be found using the enthalpychanges from two other reactions. These being that of sodium hydrogencarbonateand hydrochloric acid and also sodium carbonate and hydrochloric acid. We were given a list of instructions in how to carry out the experiment, whichare given later.

    List of Apparatus Used. 1 x 500ml Beaker. 1 x Thermometer(-10 to 50oC). 1 x Polystyrene Cup. 1 xWeighing Balance. 1 x Weighing Bottle.

    10 grams of Sodium Hydrogencarbonate. 10grams of Sodium Carbonate. A bottle of 2 molar HCL. Diagram.

    Method. Three grams of sodium hydrogen carbonate was weighted out accurately using aweighting bottle and a balance. Then thirty centimetres cubed of 2 molar HCL wasmeasured using a measuring cylinder. The acid was then placed into thepolystyrene cup and its temperature was taken and recorded using the thermometer. The pre-weighted sodium hydrogencarbonate was then added to the solution, andthe final temperature was recorded.

    The contents of the cup were then emptied out and the cup was washed out withwater and then thoroughly dried. This was done three times for the sodiumhydrogen carbonate so that I could remove any anomalies that were obtained. The experiment was then repeated in exactly the same manner except sodiumcarbonate was used instead of sodium hydrogen carbonate. The results were then tabulated, this table is shown below. Results Table.

    Results Table for Sodium Hydrogencarbonate. Results Table for Sodium Carbonate. Calculations. From these results I had to calculate deltaH2 and deltaH3. DeltaH2 refers to theenthalpy change when sodium hydrogencarbonate reacts with hydrochloric acid, anddeltaH3 is the enthalpy change when the sodium carbonate reacts with the acid.

    Firstly however it is necessary to show the equations for the two reactions:-DeltaH2= 2NaHCO3 (s) + 2HCl (aq)—-> 2NaCl (aq) + 2H2O (l) + 2CO2 (g). DeltaH3= Na2CO3 + 2HCl (aq) —-> 2NaCl (aq) + H2O (l) + CO2 (g)The enthalpy changes of the two reactions can be worked out using the formulashown below :-Energy Exchanged between = Specific Heat Capacity x Mass of the x TemperatureReactants and Surroundings of the SolutionSolutionChange. Therefore the DeltaH2 of the reaction when fitted into the formula is :-Energy Exchanged between = 4. 18 x (84 x 2) x -11. 1 Reactants and Surroundings.

    This gives the enthalpy change for DeltaH2 to be = -7794. 9 Joules per mole. The same formula is used for DeltaH3:-Energy Exchanged Between = 4. 18 x 106 x 21.

    8 Reactants and Surroundings. This gives the Enthalpy change for DeltaH3 to be = 9659. 1 Joules per mole. From these two results we are able to work out what DeltaH1 is likely to be eventhough we have not done the experiment. This is done using the formula :-DeltaH1 = DeltaH3 + DeltaH2 =>DeltaH1 = 9659. 1 + (-7794.

    9) =>DeltaH1 = 1864. 2 Joules per mole. Conclusions. The result obtained will not be a very accurate due to the means by which theexperiment was done. The equipment used was not the most efficient for measuringenthalpy changes, however it does give a rough estimate to work from.

    Someerrors of the equipment would have been heat lost through conduction from thereaction vessel. Also heat may well have been lost through the open top of thecontainer, even though there was a lid this was not very secure some heat willhave escaped through here.In summation the experiment was very difficult to undertake as the enthalpychange for DeltaH1 is hard to determine due to the fact that it thermallydecomposes in the air, causing great problems in calculating its enthalpy changewith its surroundings.Category: Science

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