Hess’s Law
Chemistry ⇒ Thermochemistry and Energetics
Hess’s Law starts at 11 and continues till grade 12.
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See sample questions for grade 12
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.
Explain why enthalpy is considered a state function.
Explain why Hess’s Law is useful in thermochemistry.
Explain why the enthalpy change for a reaction is independent of the reaction pathway.
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)
Given the following reactions:
A → B ΔH = +40 kJ
B → C ΔH = –20 kJ
C → D ΔH = +10 kJ
Calculate the enthalpy change for A → D.
Given the following reactions:
C(s) + 2H2(g) → CH4(g) ΔH = –74.8 kJ
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ΔH = –890.3 kJ
C(s) + O2(g) → CO2(g) ΔH = –393.5 kJ
2H2(g) + O2(g) → 2H2O(l) ΔH = –571.6 kJ
Verify Hess’s Law for the formation of methane.
Given the following reactions:
C(s) + O2(g) → CO2(g) ΔH = –393.5 kJ/mol
CO2(g) + H2(g) → CO(g) + H2O(g) ΔH = +41.2 kJ/mol
H2(g) + ½O2(g) → H2O(g) ΔH = –241.8 kJ/mol
Calculate the enthalpy change for the reaction:
C(s) + ½O2(g) → CO(g)
If the enthalpy change for reaction A → B is +50 kJ and for B → C is –30 kJ, what is the enthalpy change for A → C?
State Hess’s Law.
