Why is enthalpy of formation negative

Each bond in a molecule has its own bond dissociation energy, so a molecule with four bonds will require more energy to break the bonds than a molecule with one bond. As each successive bond is broken, the bond dissociation energy required for the other bonds changes slightly.

Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:. Skip to main content. Search for:. Standard Enthalpy of Formation and Reaction Standard States and Standard Enthalpy Changes The standard enthalpy of formation refers to the enthalpy change when one mole of a compound is formed from its elements.

Learning Objectives Calculate the standard enthalpy of formation. The standard state for a gas is the hypothetical state at 1 bar; for liquids and solids, the pure substance at 1 bar; for elements, the most stable allotrope of the element; and for a substance in solution solute , concentration at 1 M and 1 bar. The standard enthalpy of formation is the change in enthalpy that accompanies the formation of one mole of the compound from its elements.

The standard enthalpy of reaction occurs in a system when one mole of matter is transformed by a chemical reaction. Standard Enthalpy of Reaction The standard enthalpy of reaction is the enthalpy change that occurs in a system when a chemical reaction transforms one mole of matter under standard conditions.

Learning Objectives Demonstrate how to directly calculate the standard enthalpy of reaction. The enthalpy of reaction is calculated under standard conditions STP. Key Terms standard enthalpy of reaction : The enthalpy change that occurs in a system when one mole of matter is transformed by a chemical reaction under standard conditions. Enthalpy change for a reaction is independent of the number of ways a product can be obtained, if the initial and final conditions are the same.

Heat of Solution Heat of solution refers to the change in enthalpy when a solute is dissolved into a solvent. Learning Objectives Define heat of solution. There are three steps in solvation: the breaking of bonds between solute molecules, the breaking of intermolecular attractions between solvent molecules, and the formation of new solute-solvent attractive bonds. Energy is absorbed during the first two steps, and it is released during the last step. Depending on the relative amounts of energy required to break bonds initially, as well as how much is released upon solute-solvent bond formation, the overall heat of solution can either be endothermic or exothermic.

The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Is the standard enthalpy of formation always non-positive? Ask Question. Asked 5 years, 6 months ago. Active 5 months ago. Note: that the element phosphorus is a unique case. The reference form in phosphorus is not the most stable form, red phosphorus, but the less stable form, white phosphorus.

The enthalpy of formation of carbon dioxide at Write the chemical equation for the formation of CO 2. This equation must be written for one mole of CO 2 g.

In this case, the reference forms of the constituent elements are O 2 g and graphite for carbon. Because O 2 g and C graphite are in their most elementally stable forms, they each have a standard enthalpy of formation equal to Kirchhoff's Law describes the enthalpy of a reaction's variation with temperature changes.