or
Chemicals: Spatula 2.00 M , 2.00 M HCl, solid
Glasswares: Dewar flask, thermometer (0 - 200)
Procedure:
Part 1 : The reaction of hydrochloric acid solution and ammonia aqueous solution
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Measured 25.0ml of 2.00 M using a graduated cylinder into a 50 ml beaker and recorded the initial temperature.
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Measured 25.0ml of 2.00 M of directly in to Dewar flask and recorded the initial temperature.
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Added theto the in the calorimeter. Stirred the mixture slowly and continuously with thermometer and recorded the maximum temperature reached by the mixture. Repeat twice.
Part 2 : The dissolving of solid ammonium chloride in water
- Placed 25.0 ml of distilled water into the Dewar flask. Stirred carefully with a thermometer until a constant temperature was reached and recorded down.
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Weight 5g of solid and grinded. Re-weighted the grinded.
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Using a spatula, placed the solid into the water in the calorimeter. Stirred gently until the solid completely dissolved and recorded the maximum temperature reached. Repeat twice.
Result and calculation:
Part 1:
Part 2:
Part 1 : The reaction of hydrochloric acid solution and ammonia aqueous solution
The average initial temperature of and NH3 =
=26.5 °C
The average final temperature of and NH3 =
= 38.8 °C
After adding 25.0 ml of 2.00 M NH3 to 25.0 ml of 2.00 M in calorimeter,
Total volume of solution in calorimeter = 50.0 ml
Mass of solution (assuming density of solution is 1.00 g/ml), = 50.0 g
Final temperature of mixture = 39.0 °C
Change in temperature after reaction, = 38.8 °C – 26.5 °C
= 12.3 °C
Calculation of in Joules from
= C = 4.184 J/g °C
= 50.0 g x 4.184 J/g °C x 12.3 °C
= 2573.16 J
HCl and NH3 have equal number of moles,
n = MV/1000
= (25.0 x 2.00)/1000
= 0.050 mol
0.05 mol of HCl react with NH3, 2573.16 J of heat is released.
Since this is an exothermic reaction, the ΔH has a negative sign.
ΔH1 = -
= - 51463.20 J/ mol
Part 2: The dissolving of solid ammonium chloride in water
Average initial temperature of water =
= 26.2 °C
Average final temperature of water =
= 14.7 °C
Average weight of NH4Cl =
= 5.0067 g
Total volume of water in calorimeter = 25.0 ml
Mass of water (assuming density of water is 1.00 g/ml), = 25.0 g
Initial temperature of water in calorimeter = 26.2 °C
After adding NH4Cl, the total mass in calorimeter = 30.0067 g
Final temperature of mixture = 14.7 °C
Change in temperature, = 14.7 °C – 26.2 °C
= -11.5 °C
Calculation of in Joules from
= m x C x ΔT C = 4.184 J/g °C
= 30.0067 g x 4.184 J/g °C x (-11.5 °C)
= -1443.80 J
Conversion of the total heat of reaction to a molar enthalpy:
Molar mass of NH4Cl = 53.5
Number of moles in 5.0067 g of NH4Cl =
=
0.0936 mol of NH4Cl dissolve in water, 1443.8 J of heat is absorbed.
Since this is an endothermic reaction, the ΔH has a positive sign.
ΔH2 = -
= + 15425.21 J/ mol
Enthalpy cycle:
By using Hess’s law,
ΔH3 + ΔH4+ ΔH5 +ΔH1 = ΔH2
ΔH3 = ΔH2 - ΔH4 - ΔH5 - ΔH1
Given that, ΔH4 = - 36640 J/mol
ΔH5 = -75140 J/mol
From experiment,
ΔH1 = -51463.20J/mol,
ΔH2 = +15425.21 J/mol,
∴ ΔH3 = +15425.21 – (-34640) – (-75140) – (-51463.20)
= + 176668.41 J/mol (endothermic)
The enthalpy change of is 176668.41 J/mol.
Discussion:
From the experiment, when the ammonia solution was added to the hydrochloric acid solution in the Dewar flask the temperature of the solution increased. Thus the reaction is in exothermic reaction, the change of the temperature, is a positive value while the enthalpy change of the reaction is a negative value due to the heat will released to the surrounding when the system is under closed or opened system. Besides, when the solid ammonium chloride was added to the water in the Dewar flask the temperature of the solution decreased. This reaction known as an endothermic reaction, the change of the temperature, is a negative value while the change of the enthalpy of the reaction is then having a positive value.
The heat of reaction of ammonium chloride dissolved can be taken place in several steps. Thus according to the Hess’s law the standard reaction enthalpy of the ammonium chloride can be calculated from the standard enthalpies of the intermediate reactions. Thus from the reaction, the value discovery is 176668.14 Joules while the theoretical value is 175910 joules. This experiment can conduct as a perfect experiment. Although is a perfect experiment, the value still have a litter inaccuracy. This is due to the impurities of the compound or during the running of the experiment the Dewar flask was always opened caused the exchange of the energy between solutions and surrounding.
Precaution:
- Measure the temperature every 15 minutes to get the minimum or maximum temperature.
- Stir the mixture by using the thermometer for prevent the losses of the energy.
Conclusion:
The experiment show the value of enthalpy change of the ammonium chloride form hydrochloric acid gas and ammonia gas is 176668.14 Joules per mole which is very nearer to the theoretical value. Thus from the calculation, the change of the temperature is directly proportional to the enthalpy change of the reaction. While the change of volume of solution is not affect the enthalpies change of reaction.
References:
- http://www.shodor.org/unchem/advanced/thermo/
- http://www.mindspring.com/~drwolfe/WWWolfe_dat_enthalpies.htm
- http://www.chemguide.co.uk/physical/energetics/sums.html#top
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Thomas, E., Gary, D., & Philip, R. (2008). Physical Chemistry for the Life Scicnce. Prson Education, Inc.
Question:
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a) When the recorded temperature of the solution is lower in part 1, means that the change of the temperature, ΔT is smaller.
=
According to the above equations, heat of solution, is directly proportional to the change of the temperature. Thus the result of part 1, increased because the reaction is exothermic reaction.
b) ΔH3 = ΔH2 - ΔH4 - ΔH5 - ΔH1
Because of the value of ΔH1 increased while other remains same. Thus the final value of the enthalpy change of ammonium chloride, ΔH3 become smaller
- a) In Part 1, the volume of the hydrochloric acid solution and ammonia solution is 75.0ml. Thus the total volume of the mixture becomes 150ml.
= 150.0 g x 4.184 J/g °C x 12.3 °C
= 7719.48 J
Heat produces by 25ml each solution,
= = 3
The heat discovery of using 75ml of each solution is 3 times than using 25ml of each solution.
The number of moles of 75ml HCl and 75ml of are equal, thus
=
= 0.150mol (3 times larger than using 25 ml of both solution)
While,
The enthalpy change,
Since using the 75ml of each solution of both the heat of reaction,and number of moles is 3 times larger than using the 25ml of each solution which is the change of volume not affect the enthalpy change of reaction. Therefore the enthalpy change of the reaction is remains same
In Part 2 the volume of distilled water is 75.0ml, thus the total mass in calorimeter is 80.0067g because the mass of solid is same which is 5.0067g. (Assuming that the densities of all solution are 1.00g/ml)
Molar mass of = 53.5mol/g
Number of moles in 5.0067g of solid = 5.0067/53.5
= 0.0936mol
The numbers of moles of solid remain unchanged, thus
b) From the calculation in (ii) (a),
From equation,
ΔH3 = ΔH2 - ΔH4 - ΔH5 - ΔH1
= +41128.21 – (-34640) – (-75140) – (-51463.20)
= +202371.41 J/mol
The final calculation value for the enthalpy change of ammonium chloride is increased due to the increased of ΔH2.