DF - Hess's law Flashcards
What are the 3 types of standard enthalpy change?
Standard enthalpy change of reaction.
Standard enthalpy change of formation.
Standard enthalpy change of combustion.
What is standard enthalpy change of reaction?
The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions.
What is standard enthalpy change of formation?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions.
What is the standard enthalpy change of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions.
What does Hess’s law say?
The total enthalpy change of a reaction is always the same, no matter which route is taken.
When is Hess’s law handy?
For working out enthalpy changes that you can’t find directly by doing an experiment.
What diagrams does Hess’s law use?
Enthalpy cycles also known as Hess’s cycles.
What are enthalpy changes of formation useful for calculating?
Enthalpy changes that you can’t find directly.
What do you need to know to calculate enthalpy changes by using the enthalpy change of formation?
You need to know the enthalpy change of formation for all of the reactants and products that are compounds - the enthalpy change of formation is always 0 for elements.
What can you use to calculate reaction enthalpies?
Bond enthalpies, enthalpies of combustion and enthalpies of formation.
What can you use bond enthalpies to calculate?
Reaction enthalpies.
What can you use enthalpies of combustion to calculate?
Enthalpy changes of reaction.
