SAT Subject test Chemistry, 11th edition (Barron's Sat Subject Test Chemistry) by Wolf Ph.D. Ira K

Author:Wolf Ph.D., Ira K. [Wolf Ph.D., Ira K.]
Language: eng
Format: epub
Tags: book, ebook
Publisher: Barron's Educational Series
Published: 2012-10-31T00:00:00+00:00

Because the quantity of heat absorbed or liberated during a reaction varies with the temperature, scientists have adopted 25°C and 1 atmosphere pressure as the standard state condition for reporting heat data. A superscript zero on ΔH (i.e., ΔH0) indicates that the corresponding process was carried out under standard conditions. The standard enthalpy of formation (ΔH0f) of a compound is defined as the change in enthalpy that accompanies the formation of 1 mole of a compound from its elements with all substances in their standard states at 25°C. This value is called the molar heat of formation.

To calculate the enthalpy of a reaction, it is necessary to write an equation for the reaction. The standard enthalpy change, ΔH, for a given reaction is usually expressed in kilocalories and depends on how the equation is written. For example, the following equations express the reaction of hydrogen with oxygen in two ways:

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