Energy of Reactions – Thermochemistry Vocabulary

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15 vocabulary flashcards covering enthalpy, ΔH, system and surroundings, endothermic/exothermic reactions, and energy change representations.

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15 Terms

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Enthalpy

The total energy contained in a system (sum of chemical potential energy and kinetic energy), symbolized as H.

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ΔH (enthalpy change)

The change in enthalpy during a reaction, ΔH = H(products) − H(reactants); indicates heat absorbed or released at constant pressure.

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System

The part of the universe being studied (the reaction mixture and its components) in a chemical change.

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Surroundings

Everything outside the system, such as the container, air, and the external environment.

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Exothermic reaction

A reaction that releases heat to the surroundings; ΔH is negative.

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Endothermic reaction

A reaction that absorbs heat from the surroundings; ΔH is positive.

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Chemical potential energy

Energy stored in the chemical bonds of molecules; part of the enthalpy of a system.

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Kinetic energy

Energy of motion of particles; increases with temperature and contributes to enthalpy.

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Energy profile diagram

A graph showing how energy (enthalpy) changes from reactants to products, illustrating ΔH.

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Bond breaking

The process of breaking chemical bonds, which requires energy input (an endothermic step).

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Bond forming

The formation of new chemical bonds, which releases energy (contributes to an exothermic change).

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H(reactants)

The enthalpy of the reactants—the total energy of the starting substances.

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H(products)

The enthalpy of the products—the total energy of the substances formed.

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Heat energy

Energy transferred due to a temperature difference between the system and surroundings.

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Combustion (exothermic example)

A chemical reaction, such as burning a fuel, that typically releases heat energy (exothermic).