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The aim of this experiment is to find the enthalpy change for the decomposition of sodium hydrogen carbonate.

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Introduction

Assessed Practical: Planning Introduction: The aim of this experiment is to find the enthalpy change for the decomposition of sodium hydrogen carbonate. 2NaHCO = Na2CO3 + CO2 + H2O Using the enthalpy change of the following reactions. Sodium Hydrogen Carbonate: NaHCO3 + HCl = NaCl + CO2 + H2O Sodium carbonate: Na2CO3 + 2HCl = 2NaCl + CO2 + H2O Apparatus Sodium hydrogen carbonate Sodium carbonate Polystyrene Cup x 2 Measuring cylinder 50cm� x 2 Weighing scale Weighing boats Thermometer degrees Spatula HCl acid 2M Prediction Background Information Hess' Law states that the enthalpy change for a reaction is the same whether the reaction occurs directly or in steps. This is a direct consequence of the fact that enthalpy, is a state function. One of the applications of Hess' Law is to determine the enthalpy change for a reaction by combining other reactions to get the desired reaction, then combining the enthalpy changes for the reactions to get delta H for the reaction under consideration. An exothermic enthalpy change is always given a negative value, as the energy is lost from the system to the surroundings. An endothermic enthalpy change is always given a positive value as the energy is gained by the system from the surroundings. ...read more.

Middle

The Mr is 84. I will now use the following formula to find the mass of NaHCO3. Mass = Moles x Mr 0.1 X 84 Mass = 8.4g The mss of Sodium hydrogen carbonate I need is 8.4g. I will now find the amount of Sodium Carbonate. In this reaction I am using twice as much HCl acid so I will need to divide it by two top ensure I have the same no of moles in each reaction 100 x 2 = 0.2 moles 1000 0.2/ 2 = 0.1 moles. The Mr of Na2CO3 is 106. I will need to divide this by 2, because the stoichoimetry is 1:2. So if I have divided HCl acid by 2 I will also need to divide the mass of sodium carbonate by 2. 106 x 0.1 = 10.6 10.6 = 5.3g 2 The mass of Sodium carbonate I need is 5.3g. Method. Measure out approx 8.4g of sodium hydrogen carbonate into a weighing boat. Then fill the 50cm� measuring cylinder with HCl acid to the mark. Then pour the HCl acid into the polystyrene cup. Measure the temperature for a couple of minutes so you will get a steady temperature and then pour in some of the sodium hydrogen carbonate, a couple of seconds later add the rest of the sodium hydrogen carbonate. ...read more.

Conclusion

I could have also repeated the experiment twice so I could see that if the experiment was right, by comparing the end points. Results NaHCO3 Initial temperature 21 Final Temperature 11 change in temperature -10 Mass of HCl -50g Specific heat capacity- 4.2Jg�� K�� ?T- 10?C ?H= 4.2g��?C�� x 50g x 10?C = 2100J We then divide this by the no of moles used = 2100 = 21000J 0.1 21000 = 21KJ mol �� 1000 ?H= - 21KJ mol �� Na2CO3 Initial temperature 21 Final Temperature 28 Change in temperature 7 Mass of HCl used - 50g Specific heat capacity - 4.2g��?C�� ?T- 7?C ?H= 4.2g��?C�� x 50g x 7?C = 1470J We then divide by the no of moles used 1470 = 29400 0.05 29400 = 29.4KJ mol�� 1000 ?H= - 29.4 KJ mol�� It is a minus as it is a endothermic reaction. 2NaHCO3 = Na2CO3 + CO2 + H2O In order to get the enthalpy change for the above equation I would need to multiply the sodium hydrogen carbonate reaction by 2 and multiply the sodium carbonate reaction by -1. and add the two numbers together. Sodium hydrogen carbonate ?H= 21 x 2 = 42KJmol�� Sodium carbonate ?H= - 29.4 x -1 = 29.4KJ mol�� 42 + 29.4 = 71.4 KJ mol�� The enthalpy change for the decomposition of sodium hydrogen carbonate is 71.4 KJ mol��. ...read more.

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