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We measured the change of temperature of two reactions to determine the enthalpy change of the third. We first theoretically calculated the enthalpy and then performed experiments.

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Introduction

Shazde Mehkri 11/12/08 Enthalpy change of a chemical reaction Data: Runs Initial temperature +0.1 ?C Final temperature +0.1?C Change in temperature +0.2?C 1 22.74 36.27 13.53 2 21.57 23.36 1.79 3 22.47 35.53 13.06 Final temp. is calculated by ==> Y = mx + b For reaction1: 0.00648(11)+36.2 = 36.27 (See attached graphs) (Values of m,x & b are taken from graph) Sample calculations: Run 1: Change in Temp. = Final Temp. - Initial Temp. 13.53? C = 36.27? C - 22.74??C (Similar calculation done for Run 2 and Run 3) Data processing: Reaction 1:NaOH + HCL ==> NaCl + H2O Net ionic equation: H+ + OH- ==> H2O Theoretical H for reaction 1: -(H Products - H reactants) (-286 - (-230) ...read more.

Middle

Experimental Q = mc t For run 1:reaction between NaOH and HCL NaOH + HCL ==>H2O + NaCl M=D*V D=1.03gml-1 V=100ml M=1.03*100=103 g (Uncertainty for mass is +0.5g) C=4.184 J/?C T=13.53?C Q=5830.78 j =5.830 Kj Uncertainty is +0.014j (Similar calculations done for run 2 & 3) For run 2: reaction between NaOH + NH4CL ==> NH3 + H2O Q=0.771 Kj +0.11j {Q uncertainty is calculated by [uncertainty of temp/ change in temp]} Each solution were 2.0 molar Therefore moles= 0.050 L * 2mol/1 L =. 1 mol To show Hess's law: NaOH + HCL==> H2O NH3 + H2O ==> NaOH + NH4CL {the 2 reactions equation is flipped} In order to get: NH3 + HCL==> NH4CL H for run 1=Q/mol =5.830/0.1=58.3 Kj H ...read more.

Conclusion

We then compared our theoretical values with our experimental. All of the above mentioned steps were done to prove Hess's law. Some errors existed in our experiment. The % error that we calculated was about 2.85%. These possible errors were present because of not placing the temperature probe in its right position. The temperature probe should be placed in such a way that it should be touching the solution. If we placed our probe little lower in the solution then it was we could have eliminated the chance of such error. The mass should be measured using beakers rather than calculating it manually. This is done to increase accuracy. Also, equal time should be given to all reactions in order to generate heat. For this magnetic stirrers should be rotated at constant speed for all reactions. ...read more.

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