Enthalpy of Formation Worksheet
Grade 12 Chemistry worksheet covering standard enthalpy of formation, Hess's Law, and calculations involving enthalpy changes.
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Enthalpy of Formation
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Read each question carefully and answer to the best of your ability. Show all your work for calculation problems.
1. Which of the following statements is true regarding the standard enthalpy of formation (ΔH°f) of an element in its standard state?
It is always positive.
It is always negative.
It is always zero.
It varies depending on the element.
2. Hess's Law states that:
The enthalpy change of a reaction is independent of the pathway.
Energy cannot be created or destroyed.
The total energy of an isolated system is constant.
Heat always flows from hotter to colder objects.
3. The enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states is called the .
4. A reaction with a negative ΔH is an reaction, meaning it releases heat.
5. Calculate the standard enthalpy change (ΔH°rxn) for the combustion of methane (CH₄) given the following standard enthalpies of formation:
ΔH°f [CH₄(g)] = -74.8 kJ/mol
ΔH°f [CO₂(g)] = -393.5 kJ/mol
ΔH°f [H₂O(l)] = -285.8 kJ/mol
The balanced chemical equation is: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
6. The standard enthalpy of formation for O₂(g) is a non-zero value.
True
False
7. Using Hess's Law, calculate the enthalpy change for the reaction: C(s) + 2H₂(g) → CH₄(g)
Given the following reactions:
I. C(s) + O₂(g) → CO₂(g) ΔH = -393.5 kJ
II. H₂(g) + ½O₂(g) → H₂O(l) ΔH = -285.8 kJ
III. CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ΔH = -890.3 kJ