Calculating Enthalpy Change

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Enthalpy Change (ΔH) Noun

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[en-thal-pee cheynj]

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Enthalpy Change (ΔH)

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The amount of heat energy absorbed or released by a chemical system during a process at constant pressure.

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Example: This diagram shows a simple calorimeter setup. By measuring the temperature change of the reaction mixture with the thermometer, we can calculate the enthalpy change (ΔH).

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Hess's Law Noun

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[hes-iz law]

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Hess's Law

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A law stating the total enthalpy change for 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 (ΔH) to go from reactants to products is the same, even if the reaction takes a different path.

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Allotrope Noun

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[al-uh-trohp]

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Allotrope

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One of two or more different physical forms in which an element can exist in the same physical state.

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Example: The image shows that diatomic oxygen (O2) and ozone (O3) are allotropes: different structural forms of the same element (oxygen) in the same physical state (gas).

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Standard State Noun

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[stan-derd steyt]

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Standard State

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The defined set of conditions, 1 atm pressure and 298 K, used as a reference point for thermodynamic measurements.

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Example: This image shows the molecular structure of white phosphorus, the form the element phosphorus naturally takes under standard conditions used for scientific measurements.

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Standard Enthalpy of Formation (ΔH°f) Noun

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[stan-derd en-thal-pee ov fawr-mey-shuhn]

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Standard Enthalpy of Formation (ΔH°f)

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

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Example: This diagram shows hydrogen and oxygen atoms combining to form a water molecule, which releases a specific amount of energy called the enthalpy of formation.

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Standard Enthalpy of Reaction (ΔH°rxn) Noun

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[stan-derd en-thal-pee ov ree-ak-shuhn]

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Standard Enthalpy of Reaction (ΔH°rxn)

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The enthalpy change for a chemical reaction where all reactants and products are maintained in their standard states.

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Example: This diagram compares exothermic and endothermic reactions, showing how enthalpy (heat energy) changes. Exothermic reactions release heat, so the final enthalpy is lower (negative ΔH).

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Endothermic Reaction Noun

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[en-doh-thur-mik ree-ak-shuhn]

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Endothermic Reaction

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A chemical reaction that absorbs heat energy from its surroundings, resulting in a positive enthalpy change (ΔH) value.

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Example: This icon shows a chemical reaction in a test tube absorbing energy (represented by arrows pointing inward) from its surroundings, which is an endothermic process.

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Exothermic Reaction Noun

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[ek-soh-thur-mik ree-ak-shuhn]

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Exothermic Reaction

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A chemical reaction that releases heat energy into its surroundings, resulting in a negative enthalpy change (ΔH) value.

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Example: Burning wood (a hydrocarbon) combines with oxygen in an exothermic reaction, releasing energy as heat and light, and producing carbon dioxide and water.

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Calorimeter Noun

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[kal-uh-rim-i-ter]

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Calorimeter

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An insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process.

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Example: This diagram shows a coffee-cup calorimeter, a simple device used to measure heat change by recording the temperature change (ΔT) of a reaction in water.

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Decomposition Noun

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[dee-kom-puh-zish-uhn]

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Decomposition

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A chemical reaction in which a single compound breaks down into two or more simpler elements or compounds.

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Example: This diagram shows a large abstract 'problem' being broken down into smaller, more manageable parts, illustrating the general concept of decomposition.