Enthalpies of Formation and Reaction

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Enthalpy Noun

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[en-thal-pee]

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Enthalpy

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A thermodynamic property of a system representing its total heat content, often measured as the change during a reaction.

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Example: This diagram shows that heat energy, a key part of enthalpy, naturally moves from a hot object to a cold object, representing a change in enthalpy.

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Hess's Law Noun

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[hess-iz law]

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Hess's Law

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A law stating that the total enthalpy change for a chemical reaction is independent of the pathway taken.

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Example: This diagram shows that the total energy change (ΔH) to go from state A to state D is the same whether you take the direct path or a longer, multi-step path (A→B→C→D).

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Hess's Law of Heat Summation Noun

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[hess-iz law uv heet suh-may-shun]

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Hess's Law of Heat Summation

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The principle that the overall enthalpy of a reaction is the sum of the enthalpy changes for its individual steps.

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Example: This diagram shows that the total energy change from a starting point (A) to an end point (B) is the same, whether you go directly or take a longer path (through C).

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Thermochemical Equation Noun

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[thur-mo-kem-i-kul ee-kway-zhun]

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Thermochemical Equation

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A balanced chemical equation that explicitly includes the enthalpy change (ΔH) for the reaction it represents.

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Example: This image shows how chemical reactions either release heat (exothermic, ΔH < 0) or absorb heat (endothermic, ΔH > 0), changing the system's enthalpy.

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Standard Enthalpy of Formation Noun

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[stan-derd en-thal-pee uv for-may-shun]

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Standard Enthalpy of Formation

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The enthalpy change when one mole of a compound is formed from its elements in their standard states.

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Example: This image shows how elements, hydrogen (H₂) and oxygen (O₂), combine to form a new substance, water (H₂O), releasing a specific amount of energy.

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Standard State Noun

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[stan-derd stayt]

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Standard State

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The most stable form of a substance at a pressure of 100 kPa and a specified temperature, usually 25°C.

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Example: This image shows the molecular structure of an element, which could be in its standard state, the most stable form used as a reference for energy changes.

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Standard Enthalpy of Reaction Noun

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[stan-derd en-thal-pee uv ree-ak-shun]

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Standard Enthalpy of Reaction

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The enthalpy change of a reaction carried out at standard conditions, calculated from standard enthalpies of formation.

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Example: This diagram shows that the enthalpy of reaction is the difference in energy between the starting materials (reactants) and the final substances (products).

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Molar Enthalpy of Solution Noun

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[moh-ler en-thal-pee uv suh-loo-shun]

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Molar Enthalpy of Solution

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The heat absorbed or released when one mole of a substance dissolves completely in a large amount of solvent.

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Example: This image shows a solid ionic compound breaking apart into individual ions, which are then surrounded by water molecules, illustrating the process of dissolution.

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Dissolution Noun

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[dis-uh-loo-shun]

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Dissolution

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The process by which a solid, liquid, or gaseous solute dissolves in a solvent to form a solution.

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Example: A solid substance (solute) is added to a liquid (solvent), and it dissolves to form a uniform mixture (solution).

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Endothermic Process Noun

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[en-doh-thur-mik pros-ess]

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Endothermic Process

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A chemical or physical process that absorbs heat from its surroundings, resulting in a positive enthalpy change.

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Example: This diagram shows the inside of an instant cold pack. When the inner bag of water is broken, it mixes with ammonium nitrate, causing a chemical reaction that absorbs heat and makes the pack feel cold.