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Hesss Law Lab, use Hesss law to find the enthalpy change of combustion of magnesium which is a highly exothermic reaction.

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Introduction

Hess's Law Lab Hess's Law Hess's Law states that that the value of ?H for a reaction is the same whether it occurs directly or as a series of steps. Purpose To use the Hess's law to find the enthalpy change of combustion of magnesium which is a highly exothermic reaction. Determining the enthalpy change for the reactions of Mg in HCl(aq) and MgO(s) HCl(aq) experimentally and alongside that use the enthalpy change for hydrogen gas (Table value) when it is combusted , to find the enthalpy change for the combustion of magnesium. HYPOTHESIS The heat of combustion for magnesium can be determined by determining the heat of reaction for the following reactions: 1) Mg(s) + 2HCl (aq) � MgCl2(aq) + H2(g) 2) MgO(s) + 2HCl(aq) � MgCl2(aq) + H2O(l) 3) H2(g) + 1/2 O2(g) � H2O(l) Once these values have been obtained, I can add or subtract these values in order to achieve the heat of combustion for magnesium. Apparatus � Goggles � Lab apron � Electric balance � 100 mL graduated cylinder � 2 plastic foam cup � Thermometer � Beaker ( transportation of HCl) Procedure I began this experiment by gathering all the required lab equipment and the necessary chemicals. I found a lab station and started my investigation. I decided to start with hydrochloric acid and magnesium first. I measured 50 cm3 1.00 mol/dm3 amount of HCl solution using a graduated cylinder and put it in the foam-cup and took the initial temperature reading. I cleaned approx. 10 cm of the magnesium ribbon using emery paper and weigh off approx. 0.25 g (cut into small pieces) using a weighing boat and a scale. I measured the temperature of the of the HCl solution every 30 seconds for 3 minutes. After exactly 3 minutes, I added the magnesium from the weighing boat. I Stirred using the thermometer and read the temperature every 10 seconds for 2 minutes and every 30 seconds for another 3 minutes.I recorded all the changes I observed during the course of the reaction. ...read more.

Middle

+ 1/2 O2(g) � H2O(l) - 2.86*102 kJ/mol (given) We have to find the heat of combustion of magnesium from the above 3 reactions. Mg +1/2O2 � MgO ? To find the ?H of this reaction mathematical way, we should have MgO on the product side in ?H 2, so we reverse ?H 2 and thus change the sign to positive for the Heat of reaction which should be 1.27 * 102 kJ/mol . So ?H combustion (Mg)= ?H 1 +?H 2 + ?H 3 = (-4.44* 102 kJ/mol) + 1.69 * 102 kJ/mol + (- 2.86*102 kJ/mol) ?H combustion (Mg) = - 5.61*102 kJ/mol + 76 kJ/mol Heat of reaction= heat of formation of reactants - heat of formation of products. Calculation of absolute uncertainty for the first trial First of all we have to find the percentage error for both of the experiments. Mg As Q= mc?T So AU(?T) = +/-(0.5 +0.5)oC = � 1 oC RU(?T) = (AU(?T)/ ?T)*100% So RU(?T)= +/-(1/22 *100) = +/- 4.5 % AU(V(HCl)) = +/- 1 cm3 RU(V(HCl)) = (AU(V(HCl))/ V(HCl))*100% RU(V(HCl)) = +/- 1/50 * 100 = +/- 2% As mass = volume * density By Setting AU of density to zero because it has been as that of water which is a table value Therefore RU(m(HCl))= RU(V(HCl)) RU( C) = 0 ( table value ) So percentage uncertainty for RU(Q)=RU(m)+ RU(?T) RU(Q) = 2+ 4.5 = +/- 6.5 % So percentage uncertainty for AU(m(Mg))=+/- 0.01 g Then RU(m(Mg))= (AU(m(Mg))/ m(Mg))*100% RU(m(Mg)) = (0.01/0.25)*100 = 1/0.25 = � 4 % As AU(M(Mg))=0 because this value is given and not calculated by us so, RU(n) =RU(m) So RU(n) = � 4 % RU(?H1)=RU(Q) + RU(n) RU(?H1) = 4% + 6.5% = 10.5 % Then AU(?H1)=( ?H1* RU(?H1)/ 100) AU(?H1 )= (10.5/100)* 444KJ/mol = �47 kJ/mol MgO As Q= mc?T So AU(?T) = +/-(0.5 +0.5)oC = � 1 oC RU(?T) = (AU(?T)/ ?T)*100% So RU(?T)= +/-(1/8 *100) = +/- 12.5 % AU(V(HCl)) = +/- 1 cm3 RU(V(HCl)) ...read more.

Conclusion

The easiest we could have done that was by using a proper lid and double walls with air between them. Although our results were not far shot from the Table values, but by using proper equipment we could have achieved a more accurate result. � Thermometer should not be used as a stirrer, as the chances of getting inaccurate results are very high. A magnetic stirrer provided that we take it's specific heat capacity into consideration. � To ensure the even distribution of magnesium, magnesium powder should have been used instead of the magnesium strip, which would allow for a better reaction due to the larger surface area. � During calculations, we could have measured the mass instead of the volume, because the scale has a smaller RU, and by doing that, we did not need to know the density. � Through the benefit of more trials, the heat of combustion could have been calculated more accurately by reducing random errors. � 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 a small error that we must take account of though. We could have determined the specific heat capacity for this solution in an experiment. Discussion After collection and organization the data was interpreted. Through the uses of Hess's Law of heat summation, we were capable of deriving the change in enthalpy of a reaction from alternative reactions. Our final answers were fairly precise accurate. These errors may be accounted for by inaccurate measurements, mathematical mistakes, incomplete reactions, poor heat collection, incorrect recording of data, and poorly calibrated tools. To avoid such error one should label all materials, check each tool before use, take extra care in reading and recording of measurements, double check all calculations, and most of all be patient, labs take time and a rushed procedure leads to inaccurate data and incorrect analysis. ?? ?? ?? ?? Haroon Sahibi 1. februar 2011 ...read more.

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