Enthalpy Changes () and Hess's Law

The heat transfer, q, is a function of the mass of the object (m), the change in temperature undergone by the object (▲T) and the objects specific heat (C).    Temperature change is always defined as T final-T initial.

        Enthalpies of chemical reactions are determined indirectly by measuring temperature changes of water.  In this experiment we used dilute solutions, whose heat capacities are not significantly different from pure water, which has a specific heat of 4.184J/ g°C. If heat is gained (+q) it is an endothermic reaction; if heat is lost (-q) it is an exothermic reaction.

Data:

Calculations

7.  Applying Hess’s Law

        

        ▲H= -54.2 KJ        HCl (aq) + NaOH (aq)          NaCl (aq) + H20 (l)        

        ▲H= +47.0 KJ        NH4Cl         ______       HCl (aq) + NH3 (aq)

        ▲H=?                 NaOH (aq) + NH4Cl (aq)  NaCl (aq) + NH3 (aq) + H20 (l)

                                

                                ▲H= -7.2 KJ

        

        ▲H= -54.2 KJ        HCl (aq) + NaOH (aq)           NaCl (aq) + H20 (l)

        ▲H= +52.4 KJ        NaC2H3O2 (aq) + H20 (l)   HC2H3O2 (aq) + NaOH (aq)

        ▲H=?                           HCl + NaC2H3O2 (aq)      NaCl (aq) + HC2H3O2 (aq)

                                ▲H= -1.8 KJ

Conclusion

The purpose of this lab was to show that Hess’s Law could be used to determine the ▲ H of an overall reaction by adding together the H’s of the reactions that are summed together.  We did this experiment using a technique called calorimetry.  In using this technique we used a calorimeter made of two Styrofoam coffee cups nestled inside one another, aluminum foil, magnetic stir bar and a temperature probe.  In two of the experiments we started with HCl and added NaOH to one and HN3 to the other.  For the third experiment we started with HC2H3O2 and added NaOH.  When all three runs were done we were able to calculate the temperature change by taking the final temperature minus the initial temperature.  For runs one, two and three these are the calculated ▲T respectively: 6.34°C, 5.50 °C, and 6.13 °C.   We then used the q= MC▲T equation with the following variables; mass was equal to 102. grams, C was equal to 4.184 J/g°C, and ▲T for each of the reactions.  The only variable that changed with each calculation was the ▲T.  We then calculated the ▲H by taking the answer we got for q divided it by 0.05 mol.  We did this for run one, two and three.  These are the results for our experiment:  For one run ▲T = 6.34 °C, q = -2710 J, ▲H =-54.2 KJ. For run two ▲T = 5.50 °C, q = -2350 J, ▲H =-47.0KJ.  For run three ▲T = 6.13 °C, q = -2620 J, ▲H =-52.4KJ.  We then used Hess’s Law to determine the ▲H for this reaction HCl + NaOH  NaCl + H20.  In calculating the ▲H we determined it to equal –55.5 KJ.  We also figured ▲H for the reaction of          HCl + NH3 NH4Cl and our results were ▲H = -66.1 KJ.  After all of these calculations were completed we then determined percent error.  The theoretical results for ▲H is –55.5 KJ and our results from this experiment were –54.2 KJ.  We determined our percent error to be 2.34%.  Any error in this experiment that I would site would be the calorimeter that is being used. It was rather crude, along with the aluminum foil that would easily allow heat to escape into the surrounding.  This would be especially true if you made a bigger hole than necessary for the temperature probe.  Once again we used Hess’s Law to determine the ▲H for the two reactions that we did not observe.  We used the ▲H’s that we calculated earlier in the experiment to calculate the ▲H for    NaOH (aq) + NH4Cl (aq)  NaCl (aq) + NH3 (aq) + H20 (l).   We calculated the ▲H for these reactions and found it to be –7.2 KJ.  We also used the same technique for this reaction HCl + NaC2H3O2 (aq)  NaCl (aq) + HC2H3O2 (aq) and found ▲H to equal –1.8 KJ.

        The two biggest ideas I learned in this experiment were the application and utilization of thermodynamics law number one.  I also gained a better understanding of the use and application of Hess’s Law.

References:

Silberberg, Martin S., 2000. Chemistry:  The Molecular Nature of Matter and Change. Third Edition. New York: McGraw-Hill Higher, 2000.

Moore, John W., et al. The Chemical World Concepts and Applications. Orlando: Harcourt Brace & Company 1994.