Standard Heats of Formation Worksheet
This worksheet covers standard heats of formation, Hess's Law, and their application in calculating enthalpy changes for chemical reactions at a Grade 12 Chemistry level.
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Standards
Standard Heats of Formation
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Read each question carefully and provide your answers in the space provided. Show all your work for calculations.
1. The standard enthalpy of formation (ΔH°f) for an element in its standard state is defined as .
2. Hess's Law states that if a reaction can be expressed as the sum of two or more steps, the enthalpy change for the overall reaction is the of the enthalpy changes for the individual steps.
3. A reaction with a negative ΔH° is considered , meaning it releases heat.
1. Which of the following represents the standard state of oxygen?
O(g)
O2(g)
O3(g)
O(l)
2. The enthalpy change for a reaction is independent of the pathway between the initial and final states. This statement is known as:
Law of Conservation of Energy
First Law of Thermodynamics
Hess's Law
Le Chatelier's Principle
1. Define standard enthalpy of formation (ΔH°f).
2. Explain why the standard enthalpy of formation for a pure element in its most stable form is zero.
Use the following standard enthalpy of formation values to answer the questions below:
ΔH°f [CO2(g)] = -393.5 kJ/mol
ΔH°f [H2O(l)] = -285.8 kJ/mol
ΔH°f [C2H5OH(l)] = -277.7 kJ/mol
ΔH°f [O2(g)] = 0 kJ/mol
1. Calculate the standard enthalpy change (ΔH°rxn) for the combustion of ethanol (C2H5OH) using the given standard heats of formation.
C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)
2. Is the combustion of ethanol an exothermic or endothermic reaction? Justify your answer.
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