Heat of Formation Flashcards
Define heat of formation.
The heat of formation of a compound is the best change that takes place when one mole of a compound in it’s standard state is formed from it’s elements in their standard states
What is the standard state of an element or compound?
It’s normal form at 25 degrees Celsius and 101 kPa pressure (normal atmospheric pressure)
What is normal atmospheric pressure?
101 kPa pressure
What is the standard state of oxygen?
The gaseous state
What is the standard state of water?
Liquid state
What is the standard state of magnesium?
The solid state
Do the heats of formation of most compounds have positive or negative values?
Most have negative values
Why do most compounds have negative values?
Because compounds are generally more stable than the elements from which they are formed.
What is the heat of formation of any element in it’s standard state?
Zero
What did Hess study?
He studied the heat changes involved in a wide variety of different chemical reactions
What did Hess observe?
He found that the quantity of heat produced in going from one substance to another was the same no matter what route was taken.
Define Hess’ Law
Hess’ Law states that if a chemical reaction takes place in a number of stages, the sum of the heat changes in the separate stages is equal to the heat change if the reaction is carried out in one stage.
What can Hess’s law be used to find?
To indirectly find the value of the heat of reaction of a chemical reaction that may be difficult to measure directly in the laboratory.
What is Hess’s Law said to be a re-statement of?
The Law of Conservation of Energy
Define the Law of Conservation of Energy
The Law of Conservation of Energy states that energy cannot be created or destroyed but can only be converted from one form of energy into another
