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# Determining energy change by indirect methods

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Introduction

TO DETERMINE AN ENERGY CHANGE BY AN INDIRECT METHOD Aim: To determine the energy change in the reaction Mg(s) + H2O(l) -->MgO(s) + H2(g) Through an indirect method Apparatus: * Polystyrene cup with a lid * 100�C Thermometer (+0.5�C) * Distilled water. * 100 ml of 2 molar HCl * 0.24g of Mg ribbon * 0.1g of MgO powder * Beaker (100ml +0.1ml) * 100 ml measuring cylinder * Balance (+0.0005g) * Small beaker for holding MgO and Mg. * Sand paper * Stirrer Method: 1. Wash all the equipment with distilled water. 2. Clean the Mg ribbon with sandpaper to ensure that all the MgO that formed on it is removed. 3. Measure out 0.24g of Mg on the weighing scale. 4. Now measure 50 ml of 2 molar HCl in the 100 ml measuring cylinder. 5. Now fix a thermometer in the hole of the lid of the polystyrene cup. 6. Now in the same polystyrene cup add the 50ml of HCl and then add the 0.24g of Mg that you measured out earlier and immediately cover with the lid of the polystyrene cup(which already has the thermometer fixed in it). ...read more.

Middle

==>mass of water= mass of acid (ASSUMING) ==>specific heat capacity of water= 4.2kJ/kgK ==>change in temperature= 9.5 K = 0.05kg x 4200 J/kgK x 9.5 K = 1995J/mol = 1.995 kJ/mol This is the amount of energy which is given out for 0.24g. If we convert this to mol, we will get = (mass/molar mass) = (0.24g/24gmol-1) = 0.01mol. Therefore, for one mol the energy given out would be: 1.995 kJ/mol x 100 = 199.5 kJ/mol. For MgO= Energy change= mass x specific heat capacity x change in temperature. ==>mass of water= mass of acid (ASSUMING) ==>specific heat capacity of water= 4.2kJ/kgK ==>change in temperature= 15.1 K = 0.025kg x 4200 J/kgK x 15.1 K = 1585.5 J/mol = 1.5855 kJ/mol This is the amount of energy which is given out for 0.1g of MgO. If we convert this to mol, we'll get = (mass/molar mass) = (0.1g/ 40.3gmol-1) = 0.00248mol. Therefore, for one mol the energy given out would be: 0.00248 kJ/mol x 100 = 0.248 kJ/mol. ...read more.

Conclusion

occurred are: * Reusing the polystyrene cup can lead us to inaccurate results as the second reaction could be contaminated if the cup was not washed properly with distilled water. * The lid on the polystyrene cup was placed on it only after both the substances were put into the cup and the reaction started occurring. This would give us inaccurate results as the temperature was not measure justly as a lot of initial heat given off in the reaction was not taken into account. * The thermometers could have been influenced by the temperature outside of the cup as the experiment was being carried out next to a window. * During a couple of the trials, the lid was not able to be fixed properly on the cup. * One of the major errors was the fact that we assumed that the hydrochloric acid was equal to the amount of water. We can't really change this considering that 100% pure HCl is in gaseous state. It is an error that we must take account of though. ************ ...read more.

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