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Test Hess' law by converting NaOH to NaCl by using two different routes and measuring the enthalpy change for each one.

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Introduction

Chemistry lab report Aim- to test Hess' law by converting NaOH to NaCl by using two different routes and measuring the enthalpy change for each one Theory- the Hess' law states that the total enthalpy change on converting a given set of reactants to a particular set of products is always constant irrespective of the pathway it takes. Method- Route 1- Reaction1- NaOH(s) + HCL(aq) NaCl + H2O(l) 1.take 50 cm3 of HCl in an insulated calorimeter using a pipette. The initial temperature should be recorded. 2. 1 gram of NaOH was grinded in a crucible and added to it 3. The apparatus was stirred. The final temperature was found using a cooling curve. 4. The heat given out by the reaction was recorded using the formula Heat given out by the reaction=heat absorbed by water 5. The enthalpy change is calculated using the formula H1=(Heat absorbed by water/no. of moles of the limiting reactant Route2- Reaction2-NaOH(s) + H2O(l) NaOH(aq) 1.25- cm3 cube of water was taken in an insulated calorimeter using a pipette. ...read more.

Middle

No.of Moles of NaOH Vol.of HCl (+/-0.04 cm3 ) Initial temp. (+/-0.5 oC) 1.04 0.025 50 28.0 Heat evolved by the reaction = heat absorbed by water Heat absorbed by water- mass of water x sp. heat capacity of water x change in temperature Time (seconds) Temperature (+/- 0.5 degree Celsius) 30 28.0 60 28.0 90 28.0 120 28.0 (add NaOH(s) ) 150 29.0 180 30.0 210 31.5 240 32.0 270 32.5 300 32.5 330 33.0 360 34.0 390 35.5 420 35.0 450 35.0 480 34.5 510 34.5 540 34.5 570 34.5 600 34.0 630 34.0 the maximum temperature reached as seen in the graph= 36oC therefore heat absorbed by water=mass x sp. heat capacity x change in temperature = 50 x 4.18 x (36-28) = 1672 joules = 1.672 KJ no .of moles of NaOH= 0.025 therefore the enthalpy change per mole = 1.672/0.025 =66.88 KJ/mol reaction 2- Mass of NaOH(+/- 0.01 g) No.of moles of NaOH Initial temperature(+/-0.5oC) Volume of water(+/-0.04 cm3) ...read more.

Conclusion

Conclusion and evaluation- the following factors would have caused a shift from the actual values- 1. All the heat would not be transferred to the water. Some heat would be lost to the surroundings, which would cause a decrease in the value. 2. Also it is assumed in reaction 1 and 3 that HCL(aq) would have the same sp. Heat capacity as that of water which is not true. 3. The stirring was not adequate. A cooling curve was plotted when the reaction was slow. This would give us the value of the final temperature which would be obtained had the reaction been fast. The difference between both the enthalpies is significantly large. This could be due to the above stated reasons. It was expected that enthalpy value reaction 1 would be equal to that of reaction 2 + reaction3 (since Hess' law states that the total enthalpy change on converting a given set of reactants to a particular set of products is always constant irrespective of the pathway it takes). ...read more.

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