Hess’s Law
Chemistry ⇒ Thermochemistry and Energetics
Hess’s Law starts at 11 and continues till grade 12.
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Describe a situation where Hess’s Law would be necessary to determine an enthalpy change.
Describe how Hess’s Law can be used to calculate the enthalpy change for the hydration of copper(II) sulfate.
Describe how you would use Hess’s Law to determine the enthalpy of formation of methane (CH4).
Describe the steps involved in using Hess’s Law to determine the enthalpy change of a reaction.
Given the following data:
C(graphite) + O2(g) → CO2(g) ΔH = –393.5 kJ
CO(g) + ½O2(g) → CO2(g) ΔH = –283.0 kJ
Calculate the enthalpy change for:
C(graphite) + ½O2(g) → CO(g)
Given the following data:
N2(g) + O2(g) → 2NO(g) ΔH = +180.5 kJ
2NO(g) + O2(g) → 2NO2(g) ΔH = –113.1 kJ
Calculate the enthalpy change for:
N2(g) + 2O2(g) → 2NO2(g)
Given the following data:
Na(s) + ½Cl2(g) → NaCl(s) ΔH = –411 kJ
Na(s) + ½O2(g) → Na2O(s) ΔH = –414 kJ
Na2O(s) + 2HCl(g) → 2NaCl(s) + H2O(l) ΔH = –208 kJ
Calculate the enthalpy change for:
H2O(l) + 2Na(s) + Cl2(g) → 2NaCl(s) + H2(g)
Given the following reactions:
2H2(g) + O2(g) → 2H2O(l) ΔH = –571.6 kJ
H2(g) + ½O2(g) → H2O(g) ΔH = –241.8 kJ
Calculate the enthalpy change for:
2H2O(l) → 2H2O(g)
Which law is Hess’s Law based on?
(1) Law of conservation of mass
(2) Law of conservation of energy
(3) Law of definite proportions
(4) Law of multiple proportions
Which of the following best describes Hess’s Law?
(1) The enthalpy change depends on the path taken.
(2) The enthalpy change is independent of the path taken.
(3) The enthalpy change is always positive.
(4) The enthalpy change is always negative.
Which of the following is a correct statement about Hess’s Law?
(1) It is only valid for reactions at standard conditions.
(2) It is valid for any conditions as long as the initial and final states are the same.
(3) It is only valid for reactions involving gases.
(4) It is only valid for reactions in aqueous solution.
Which of the following is a direct consequence of Hess’s Law?
(1) The enthalpy of a reaction can be calculated from enthalpies of formation.
(2) The entropy of a reaction is always positive.
(3) The free energy of a reaction is always negative.
(4) The enthalpy of a reaction depends on the path taken.
Fill in the blank: Hess’s Law allows us to calculate the ________ change for reactions where direct measurement is not possible.
Fill in the blank: Hess’s Law is also known as the law of constant ________ summation.
Fill in the blank: Hess’s Law is applicable only if the initial and final ________ are the same.
Fill in the blank: If a reaction equation is multiplied by a factor, the enthalpy change is multiplied by the ________ factor.
True or False: Hess’s Law can be used to calculate lattice enthalpy.
True or False: Hess’s Law can be used to calculate the enthalpy change for both physical and chemical processes.
True or False: Hess’s Law can be used to determine the enthalpy change for the dissolution of salts.
True or False: Hess’s Law can be used to determine the enthalpy of formation of a compound.
