Exam 4 - Enthalpy of formation

Question 1

What is Enthalpy of formation?

Answer 1

It is the enthalpy change associated with constructing a compound from its constituent elements at its most stable state. It is denoted deltaHf.

Question 2

What does the enthalpy of formation depend on?

Answer 2

The state of the reactants and products in the reaction.

Temperature, pressure, and physical state all have an effect on the enthalpy of the system.

Question 3

What is the standard state of thermodynamics?

Answer 3

1 atm and 296 K. It is set in order to validly compare enthalpies of different systems.

Question 4

What are the three rules for calculating the enthalpy of formation?

Answer 4

• For any element, the enthalpy of formation in standard state is 0
• The combination of standard state elements to form a compound is accompanied by the associated deltaHf
• The enthalpy change of a reaction (delta H rxn) can be found from enthalpies of formation of the reactants and products in that reaction (Hess’s law)