Enthalpies of Formation (6.4.2) Flashcards
• The standard enthalpy change (∆H°) gives the enthalpy change for a reaction
performed at standard state and a given temperature.
• The standard enthalpy change (∆H°) gives the enthalpy change for a reaction
performed at standard state and a given temperature.
• The standard molar enthalpy of formation describes the standard enthalpy
change for the formation of one mole of a compound from its elements.
• The standard molar enthalpy of formation describes the standard enthalpy
change for the formation of one mole of a compound from its elements.
• Hess’s law can be used to calculate the standard enthalpy change for a reaction
from the standard molar enthalpies of formation of the reactants and products.
• Hess’s law can be used to calculate the standard enthalpy change for a reaction
from the standard molar enthalpies of formation of the reactants and products.
The standard enthalpy change (∆H°) gives the
enthalpy change for a reaction performed at
standard state and is reported for 25 °C, since it
depends on temperature.
At standard state substances are in their most
stable form at 1 atmosphere of pressure at the
given temperature.
When substances have different allotropes such as
carbon (e.g. graphite and diamond), the most stable
form is used (graphite).
The standard molar enthalpy of formation describes
the standard enthalpy change for the formation of
one mole of a compound from its elements.
Standard formation reactions begin with reactants
that are elements in their standard states and
produce one mole of product.
Because they form only one mole of product,
standard formation reactions often contain fractional
coefficients.
The standard molar enthalpy of formation for an
element in standard state is zero.
The standard enthalpy change (∆H°) gives the
enthalpy change for a reaction performed at
standard state and is reported for 25 °C, since it
depends on temperature.
At standard state substances are in their most
stable form at 1 atmosphere of pressure at the
given temperature.
When substances have different allotropes such as
carbon (e.g. graphite and diamond), the most stable
form is used (graphite).
The standard molar enthalpy of formation describes
the standard enthalpy change for the formation of
one mole of a compound from its elements.
Standard formation reactions begin with reactants
that are elements in their standard states and
produce one mole of product.
Because they form only one mole of product,
standard formation reactions often contain fractional
coefficients.
The standard molar enthalpy of formation for an
element in standard state is zero.
The standard enthalpy change for a reaction is
equal to the sum of the standard molar enthalpy of
formations of the products minus the sum of the
standard molar enthalpy of formations of the
reactions. This is an application of Hess’s law,
which states that the enthalpy change for a multistep
process is equal to the sum of the enthalpy
changes for the individual steps.
In the example, the standard molar enthalpy of
formation of each of the reactants and products are
drawn from a table. The coefficients must be taken
into account. The values for elements (aluminum
and iron) are zero.
Using these known values the standard enthalpy
change for the thermite reaction is found to be
highly negative. Therefore, the thermite reaction is
exothermic.
The standard enthalpy change for a reaction is
equal to the sum of the standard molar enthalpy of
formations of the products minus the sum of the
standard molar enthalpy of formations of the
reactions. This is an application of Hess’s law,
which states that the enthalpy change for a multistep
process is equal to the sum of the enthalpy
changes for the individual steps.
In the example, the standard molar enthalpy of
formation of each of the reactants and products are
drawn from a table. The coefficients must be taken
into account. The values for elements (aluminum
and iron) are zero.
Using these known values the standard enthalpy
change for the thermite reaction is found to be
highly negative. Therefore, the thermite reaction is
exothermic.
