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Hess Law Essay

1570 words - 7 pages


Objective : To determine the heat of reaction by applying Hess’s Law for reactions whose energy cannot be easily measured directly.
An exothermic reaction releases heat into the surroundings and is usually observed as temperature increases in the solvent, container, and other immediate surroundings. The amount of heat released from the reaction can be expressed as
q reaction = q solution + q calorimeter ……..(1)
In this experiment, the calorimeter is assumed to absorb so little heat that q calorimeter is nearly zero and thus the equation simplifies to
q reaction = q solution ...view middle of the document...

Apparatus and materials
Dewar flask, thermometer (0-200 C), 50 ml beaker, spatula 2.00 M NH3(aq), 2.00 M HCl(aq), solid NH4Cl.
Part 1 : The reaction of HCL (aq) and NH3 (aq).
About 25 ml of 2.0 M NH3 was added into a 50 ml beaker. And about 25 ml of 2.0 M of HCL was added directly into Dewar flask. Both solution was kept in room temperature. The temperature of the both solution was recorded. Next, NH3 in the beaker is added into the Dewar flask. The mixture is then stirred slowly and continuously until the maximum temperature is reached and recorded. The experiment is repeated twice and average reading was obtained.

Part 2 : The dissolving of solid ammonium chloride in water
About 25 ml of distilled water was added into the Dewar flask. The solution is then stirred until constant temperature is obtained and the temperature was recorded. About 5 g of NH4CL (s) was weight and by using spatula NH4CL (s) placed into the calorimeter. The solution is then stirred gently with thermometer until the solid is dissolved completely and the maximum temperature was recorded. The experiment was repeated once again and average reading is obtained.
Part 1 : The reaction of HCL (aq) and NH3 (aq)
Number of experiment | 1 | 2 | 3 |
HCL (initial temp) ° C | 25.5 | 26.0 | 25.5 |
NH3 (initial temp) ° C | 24.0 | 24.0 | 24.0 |
HCL + NH3 (max temp) ° C | 27.5 | 36.0 | 36.0 |

Part 2 : The dissolving of solid ammonium chloride in water
Number of experiment | 1 | 2 |
Distilled water (initial temp ) ° C | 25.5 | 26.0 |
NH4CL + Distilled water (max temp) ° C | 16.0Mass : 5.0221 g | 16.0Mass : 5.0041 g |

1. Number of moles of NH4CL formed
n.o.m =MV/ 1000
n.o.m = 2.0 M x 25.00 ml / 1000
n.o.m= 0.05 mol
1 mole of NH3 = 1 mole of NH4CL
Thus n.o.m of NH4CL formed is 0.05 mol
2. Number of moles NH4CL dissolved
Average mass of NH4CL (s) used = (5.0221 g + 5.0041 g)/2
= 5.01 g
n.o.m = mass of NH4CL (s) / molecular mass of NH4CL
n.o.m = 5.01 g / ( 14+4+35.5)
n.o.m = 0.094 mol
3. The ∆H of neutralization between NH3 and HCL.
Q = mc∆T
m = (25.0 ml + 25.0 ml) x 1.00 g/ml
m = 50 g
c = 4.184 J/g °C
∆T = T4 - T3
T4 = (27.5 + 36.0 + 36.0 ) °C/3
T4 = 33.17 °C
T3 = T1 + T2 / 2
T1 = (25.5 + 26.0 + 25.5)/3 T2 = (24.0+ 24.0 + 24.0)/3
= 25.67 °C = 24.0 °C
T3= (25.67 + 24.0)/2
T3= 24.84 °C
∆T= 33.17 - 24.84
∆T = 8.33 °C
Q= (50 g)(4.184 J/ g °C)(8.33°C)
Q= 1742.64 J
∆H= Q/n
∆H = 1742.64 J/0.05 mol
∆H = 34,852.72 J/mol
∆H = 34.85 kJ/mol

4. ∆H of dissolution of NH4CL
m = (5.0221 g + 5.0041 g)/2
m= 5.01 g
c = 4.184 J/g °C
∆T = T2 – T1
T2 = (16.0 + 16.0 )...

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