When a chemical reaction occurs, heat is exchanged between the system and the surroundings. This heat exchange is called the heat of reaction, qrxn. In an exothermic reaction, heat is released from the system to the surroundings (qrxn < 0). On the other hand, heat is absorbed by the system from the surroundings (qrxn > 0) in an endothermic reaction. At constant pressure, the heat of reaction of is equal to the enthalpy change, △H, which is measured through calorimetry. This process includes the use of a calorimeter, a device that traps the heat absorbed or released.
In this experiment, a calorimeter was made by inserting a test tube with a cork into a styroball. The temperature was measured by allowing NaOH to react with HCl. After calibrating the calorimeter, the heats of reaction for given reaction systems were then measured. This experiment aims to understand how calorimetry is done by creating a self-made calorimeter.
Answers to Questions:
Calibration of the calorimeter:
1) Give the net ionic thermochemical equation of the reaction used to calibrate the calorimeter NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) OH-(aq) + H+(aq) → H2O(l) a. Is the reaction endothermic or exothermic?
The reaction is endothermic.
b. Which is the limiting reactant?
The limiting reactant is HCl.
c. How much (in moles) limiting reactant was used?
5.00 mL x 1L1000mLx 1M=0.005 mol
d. How much heat was generated (or absorbed) by the reaction? -157 J0.005 mol=-31.4 KJ
2) Relate the sign of the △T to the △H of the reaction used for the calibration. As the temperature increases, the enthalpy also increases. This is due to the fact that enthalpy is proportional to temperature. If △T is positive, then △H is also positive and vice versa. 3) What is the heat capacity of the calorimeter? Relate its sign to the sign of △T. qrxn = -qcalCcal= qcalm△T= 157 J(15 g)(2.50°C) =4.19 J/g°C -157 J = 157 JA positive △T gives a positive heat capacity. △T = 2.50°C
4) In the appendix, show the derivation to obtain the equation used to calculate the heat capacity of the calorimeter. Determination of Heats of Reaction:
5) Give the net ionic reaction for each reaction.
| Reaction System| Net Ionic Equation|
1| 10 mL of 1M NH3 + 5 mL of 1M HCl| NH3(aq) + H+(aq) → NH4+(aq)| 2| 10 mL of 1M NaOH + 5 mL of 1M CH3COOH| OH-(aq) + H+(aq) → H2O(l)| 3| 10 mL of 1M NH3 + 5 mL of 1M CH3COOH| NH3(aq) + H+(aq) → NH4+(aq)| 4| 10 mL of 1M NaoOH + 5 mL of 1M HNO3| OH-(aq) + H+(aq) → H2O(l)| 5| 15 mL of 1M HCl + 0.05 g Mg| Cl-(aq) + Mg(s) → MgCl(aq)| 6| 15 mL of 1M CH3COOH + 0.05 g Mg| CH3COO-(aq) + Mg(s) → MgCH3COO(aq)| 7| 15 mL of 1M CuSO4 + 0.5 g Zn| SO42-(aq) + Zn(s) → ZnSO4(aq)| 8| 10 mL of 0.5M Na2CO3 + 5 mL of 1M CaCl2| Na2CO3(aq) + Cl2(aq) → 2NaCl(aq) + CO32-(aq)| 6) Determine the limiting reactant and the amount of the limiting reactant in each of the reaction performed. Net Ionic Equation| Limiting Reactant| NLR (mol)|
NH3(aq) + H+(aq) → NH4+(aq)| HCl| 5x10-3|
OH-(aq) + H+(aq) → H2O(l)| CH3COOH| 5x10-3|
NH3(aq) + H+(aq) → NH4+(aq)| CH3COOH| 5x10-3|
OH-(aq) + H+(aq) → H2O(l)| HNO3| 5x10-3|
Cl-(aq) + Mg(s) → MgCl(aq)| Mg| 2x10-3|
CH3COO-(aq) + Mg(s) → MgCH3COO(aq)| Mg| 2x10-3|
SO42-(aq) + Zn(s) → ZnSO4(aq)| Zn| 8x10-3|
Na2CO3(aq) + Cl2(aq) → 2NaCl(aq) + CO32-(aq)| CaCl2| 5x10-3| 7) Calculate for the theoretical and experimental enthalpy of each reaction. a. Determine whether the reaction is endothermic or exothermic. b. Give the % error of your experimental values.
| Experimental △H (kJ/mol)| Theoretical △H (kJ/mol)| % Error| Endothermic or Exothermic| 1| 35.2| | | Endothermic|
2| 28.0| | | Endothermic|
3| 38.4| | | Endothermic|
4| 50.2| | | Endothermic|
5| 276| | | Endothermic|
6| 281| | | Endothermic|
7| 8.13| | | Endothermic|