Thermochemical Equations

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Thermochemical Equation Noun

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[thur-moh-kem-i-kuhl ee-kway-zhuhn]

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Thermochemical Equation

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A balanced chemical equation that includes the physical states of all substances and the associated change in enthalpy (ΔH).

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Example: This image shows a thermochemical equation in two ways: with heat as a product, and with a negative enthalpy change (ΔH), indicating an exothermic reaction.

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Enthalpy (ΔH) Noun

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[en-thal-pee]

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Enthalpy (ΔH)

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A quantity representing the total heat content of a system, where its change measures energy absorbed or released in a reaction.

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Example: Adding heat to water increases its temperature and can cause it to boil, changing from a liquid to a gas. This change in heat content is called enthalpy change (ΔH).

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Exothermic Process Noun

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[ek-soh-thur-mik pros-es]

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Exothermic Process

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A process or reaction that releases energy, usually as heat, into its surroundings and has a negative ΔH value.

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Example: This diagram shows burning wood, a combustion reaction. The equation shows reactants (wood, oxygen) becoming products (smoke, water) and releasing energy (ΔH), which is an exothermic process.

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Endothermic Process Noun

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

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Endothermic Process

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A process or reaction that absorbs energy, usually as heat, from its surroundings and has a positive ΔH value.

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Example: The Earth absorbs energy from the sun, which is an endothermic process where a system (Earth) takes in heat from its surroundings (the sun).

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Combustion Noun

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[kuhm-buhs-chuhn]

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Combustion

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A chemical reaction between a fuel and an oxidizing agent, such as oxygen, that produces heat and light energy.

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Example: This diagram shows a combustion reaction where a hydrocarbon fuel (like wood or charcoal) reacts with oxygen to produce heat, light, water, and carbon dioxide.

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Enthalpy of Combustion (ΔHcomb) Noun

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[en-thal-pee uhv kuhm-buhs-chuhn]

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Enthalpy of Combustion (ΔHcomb)

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The specific enthalpy change that occurs during the complete burning of one mole of a substance under standard conditions.

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Example: A hydrocarbon (like charcoal) reacts with oxygen to produce carbon dioxide, water, and energy in the form of heat and light.

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Standard Enthalpy Change (ΔH°) Noun

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[stan-derd en-thal-pee cheynj]

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Standard Enthalpy Change (ΔH°)

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The enthalpy change of a reaction when all reactants and products are in their defined standard states at standard conditions.

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Example: This diagram shows a simple calorimeter, a device used to measure the heat released or absorbed during a chemical reaction, which determines the enthalpy change.

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Standard Conditions Noun

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[stan-derd kuhn-dish-uhns]

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Standard Conditions

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A defined set of conditions for measurements, specified as 1 atmosphere of pressure and a temperature of 298 K (25 °C).

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Example: This image shows that the '°' symbol in thermochemistry represents 'Standard Conditions,' which are a specific pressure (100 kPa) and temperature (298 K).

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Vaporization Noun

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[vey-per-uh-zey-shuhn]

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Vaporization

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The endothermic phase transition process in which a substance changes from a liquid state to a gaseous state or vapor.

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Example: This image compares evaporation and boiling, two types of vaporization. Evaporation is a slow surface process, while boiling is a rapid process that occurs throughout the liquid when heated.

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Fusion Noun

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[fyoo-zhuhn]

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Fusion

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The endothermic phase transition process, also known as melting, where a substance changes from a solid to a liquid state.

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Example: Two small atomic nuclei (Deuterium and Tritium) combine in a process called fusion, forming a larger nucleus (Helium) and releasing a large amount of energy.