The University of Adelide Thermochemistry Questions

1. a) Write an equation describing the reaction of HCl with water.

b) Hence, write an equation describing the reaction between an aqueous solution of ammonia and hydrochloric acid. Indicate the acid/base conjugate pairs.

2. Calculate the energy absorbed by the calorimeter alone, qcalorimeter. Refer to Laboratory Manual, Page 2–3.

3. Calculate the energy absorbed by the contents of the calorimeter, qcontents. You may assume that the specific heat and density of the solution equals that of pure water. Refer to Laboratory Manual, Page 2–3.

4. Calculate the total thermal energy, qreaction, of the reaction.

5. Calculate the thermal energy per mole of reactants. Is this energy released or absorbed?

6. Was the reaction exothermic or endothermic?

7. Report the value for the molar enthalpy of neutralisation (ΔHneutralisation) of NH3(aq) with HCl(aq). Be sure to quote the correct units and comply with sign conventions. This value will vary with the concentration of the reactants, so when quoting your ΔHneutralisation you should state the concentration at which it was determined.

8. Calculate the energy required to decrease the temperature of the contents of the calorimeter by the observed ΔT (qcontents). You may assume that the specific heat and density of the ammonium chloride solution are the same as pure water. (Remember, mcontents = the total mass of everything you have added to the calorimeter.)

9. Calculate the energy transfer for the calorimeter alone (qcalorimeter).

10. Did energy flow from the calorimeter to its contents or vice versa?

11. Calculate the enthalpy change when one mole of ammonium chloride, NH4Cl(s), is dissolved in water. Record the concentration of the resulting solution as this will affect the value obtained.

12. a) State Hess’s Law. b) How does Hess’s Law apply in this experiment?

13. In Part One you examined a spontaneous process that was exothermic. In Part Two you examined a spontaneous process that was endothermic. What does this tell you about a spontaneous process?

14. You have considered spontaneous reactions with positive and negative enthalpy changes (ΔHreaction). Is it possible to predict the spontaneity of a process given the enthalpy change of that process alone? If not, what does determine spontaneity and thus what other factors must also be considered? (Use an equation to support your answer.)

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