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.

(-286 – (-230) = 56

Reaction 2:NaOH + NH4OH ➔ NH3 + H2O

Net ionic equation:

NH4+ + OH- ➔ NH3 + H2O

Theoretical   H for reaction 2: -366 – (-362) = -4

Reaction 3:HCL + NH3 ➔ NH4Cl

Net ionic equation:

NH3  + H+ + Cl- ➔ NH4+ + Cl-

Theoretical   H for reaction 3: -247-(-299)=52

 

Therefore according to Hess’s law 2 reactions are added to get the  H of third

R3= R1 + R2

52= 56 + -4

        

Further calculations:

 

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 for run 2=0.771/0.1=-7.71 Kj                {the sign is –ve because the equation was flipped}

 

Therefore according to Hess’s law R1+R2=R3

                                         58.3+-7.71=50.59Kj + 0.015j

Percent Error:

        Is calculated by

                        (Theoretical-experimental/theoretical)*100

        Hence, in this case:                 

(52-50.59/52)*100 = 2.85%

        

  Conclusion and evaluation:

        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. 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.