Chapter 9 Thermochemistry – Key Vocabulary

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A set of core vocabulary flashcards covering enthalpy, energy transfer, standard states, and Hess’s law from Chapter 9 Thermochemistry notes.

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24 Terms

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Thermochemistry

Branch of chemistry that studies the relationships among heat, work, and other forms of energy during chemical and physical processes.

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Internal Energy (U or E)

The total of all kinetic and potential energies of the particles in a system; a state function.

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Heat (q)

Energy transferred between a system and its surroundings because of a temperature difference; positive when absorbed by the system, negative when released.

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Work (w)

Energy transfer resulting from a force acting through a distance; positive when done on the system, negative when done by the system.

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First Law of Thermodynamics

Energy cannot be created or destroyed; for a system, ΔU = q + w and ΔEsys = −ΔEsurr.

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Pressure–Volume (Expansion) Work

Work associated with volume change against an external pressure (PΔV); w = −PΔV.

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State Function

A property that depends only on the current state of the system, not on the path taken (e.g., U, H).

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Enthalpy (H)

A thermodynamic quantity defined as H = U + PV; a state function useful for constant-pressure processes.

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Enthalpy Change (ΔH)

Difference in enthalpy between final and initial states; equals qp (heat at constant pressure) when only P-V work occurs.

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Heat of Reaction at Constant Pressure (qp)

The heat exchanged during a reaction carried out at constant pressure; numerically equal to ΔH.

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Exothermic Reaction

Process with ΔH < 0 that releases heat to the surroundings.

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Endothermic Reaction

Process with ΔH > 0 that absorbs heat from the surroundings.

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Thermochemical Equation

A balanced chemical equation accompanied by a ΔH value specifying the heat change for the stated stoichiometric amounts and physical states.

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Extensive Property

Property that depends on the quantity of matter present (e.g., ΔH, mass, volume).

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Intensive Property

Property independent of the amount of substance (e.g., temperature, ΔH per mole).

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Standard State

Reference condition of 1 bar pressure for gases, 1 M concentration for solutions, and pure substances at 1 bar and 298 K unless otherwise noted.

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Standard Enthalpy Change (ΔH°)

Enthalpy change for a process when all reactants and products are in their standard states.

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Standard Enthalpy of Formation (ΔHf°)

Enthalpy change when exactly 1 mol of a compound forms from its elements in their most stable standard states.

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Standard Enthalpy of Combustion (ΔHc°)

Enthalpy change when 1 mol of a substance burns completely in oxygen under standard-state conditions.

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Hess’s Law

If a reaction is carried out in several steps, the overall ΔH equals the sum of the ΔH values for each step because enthalpy is a state function.

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Zero ΔHf° of Elements

The standard enthalpy of formation of any element in its most stable form (e.g., O2(g), C(s, graphite)) is defined as zero.

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Energy Equation ΔU = q + w

Fundamental relation linking changes in internal energy with heat and work transfers for a system.

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Stoichiometric Coefficient (n) in ΔH Calculations

The multiplier used with ΔHf° values when applying Σ nΔHf°(products) − Σ nΔHf°(reactants) to find a reaction’s standard enthalpy change.

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Hess’s Law Using Formation Enthalpies

ΔH°reaction = Σ nΔHf°(products) − Σ nΔHf°(reactants), enabling calculation of reaction enthalpy from tabulated ΔHf° values.