Experiment 3: Determine the Enthalpy of Dissolution of NH4Cl in Water 1. Took a

Question

Experiment 3: Determine the Enthalpy of Dissolution of NH4Cl in Water

1. Took a new calorimeter from the Containers shelf and place it on the workbench.
2. Took water from the Materials shelf and added 25.0 mL to the calorimeter.
3. Took a thermometer from the Instruments shelf and attached it to the calorimeter. Record the temperature inside the calorimeter in your Lab Notes.

- Thermometer attached to the calorimeter(water) : 21.5 C

(4)Took ammonium chloride (NH4Cl) from the Materials shelf and add 5.000 g to the calorimeter.
5. Observed the temperature change in the calorimeter as the reaction proceeds. Record the new temperature your Lab Notes.

- Thermometer(water) from (3) at starting : 21.5 C
- Thermometer(water + NH4Cl) from (4) : 9.1 C

6. Cleared your station by dragging all containers and instruments to the recycling bin. Remember to press Save Notes

QUESTIONS

1. What was the heat of solution (qsoln) for the dissolution of ammonium chloride in water? Choose the closest answer.

A. 1560 J

B. 1740 J

C. -1560 J

D. -1740 J

2. What was the value of qcal for the dissolution of ammonium chloride in water? Choose the closest answer.

A. -1740 J

B. 181 J

C. -181 J

D. 14.6 J

3. What was the change of temperature (deltaT) for the dissolution of ammonium chloride in water? Choose the closest answer.

A. 9.1 oC

B. 21.5 oC

C. -12.4 oC

D. 12.4 oC

4. What does Hess's Law state?

A. If a reaction takes place in several steps then its enthalpy is the difference between the enthalpies of the intermediate reactions.

B. The enthalpy change of an overall process cannot be determined from the enthalpy of its individual steps.

C. The enthalpy change of the overall process is the sum of the enthalpy changes of its individual steps and does not depend on the route taken.

D. If the heat energy of a reaction increases the enthalpy of the reaction decreases proportionally.

5. Based on the molar enthalpies calculated for experiment 2 and 3, use Hess's Law to write a series of reactions that will sum up to the reaction of the decomposition of ammonium chloride. In addition to the molar enthalpies from experiment 2 and 3, you will also need to include the following reactions which account for the enthalpy change when HCl and NH3 change state.

NH3(g) -> NH3(aq) (deltaH = -34,640 J/mol)

HCl(g) -> HCl(aq) (deltaH = -75,140 J/mol)

Choose the equation that shows the correct direction and sign for the enthalpy value for the reaction between NH3 and HCl in your series of reactions.

A. NH4Cl(s) -> HCl(g) + NH3(aq) -deltaH

B. NH4Cl(aq) -> HCl(aq) + NH3(aq) +deltaH

C. HCl(g) + NH3(g) -> NH4Cl(aq) +deltaH

D. HCl(aq) + NH3(aq) -> NH4Cl(s) -deltaH

Explanation / Answer

1. What was the heat of solution (qsoln) for the dissolution of ammonium chloride in water? Choose the closest answer.

mass of ssytem = mass of water

C = 4.184 J7gC for water

Qsolution = m*C*(Tf-Ti) = 25*4.184*(9.1-21.5) =

Qsolution = -1297.04 J

2. What was the value of qcal for the dissolution of ammonium chloride in water? Choose the closest answer.

Qcal = -Qsystem

Q = - - 1297.04 = 1297.04 J

3. What was the change of temperature (deltaT) for the dissolution of ammonium chloride in water? Choose the closest answer.

dT = Tf-Ti = (9.1-21.5) = -12.4°C

Total heat:

HRxn = -Q/n

n = mass/MW = 5/53.491 = 0.09347

HRxn = 1297.04 /0.09347 = 13876.537 J/mol = 13.87 kJ/mol

Hdissolution = 13.87 kJ/mol

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