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41 Terms

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Enthalpy change (ΔH)

The heat transferred at constant pressure during a chemical reaction.

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Standard enthalpy change of reaction (ΔrH°)

The enthalpy change when reactants in their standard states form products in their standard states at 1 bar pressure and 25°C (298 K).

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Standard state

The most stable form of a substance at 1 bar and a specified temperature (usually 25°C).

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Positive ΔrH°

Indicates an endothermic reaction that absorbs heat from surroundings.

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Negative ΔrH°

Indicates an exothermic reaction that releases heat to surroundings.

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Reversing a chemical equation

Reverses the sign of ΔrH°.

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Scaling the reaction

If you multiply the coefficients in a reaction, multiply ΔrH° by the same factor.

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Different physical states

ΔrH° values differ for different physical states of products/reactants.

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ΔH calculation step 1

Convert grams to moles using mol=mass (g)/molar mass (g/mol).

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ΔH calculation step 2

Use ΔrH° to calculate ΔH for actual amount as ΔH=(mol of substance)×(ΔrH°/mol-rxn).

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Given reaction

2C₄H₁₀(g) + 13O₂(g) → 8CO₂(g) + 10H₂O(l), ΔrH° = -5755 kJ/mol-rxn.

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454 g of butane

Example mass used for energy change calculations in the reaction.

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Moles of C₄H₁₀ from grams

454 g of C₄H₁₀ converts to 7.811 mol using 58.12 g/mol as molar mass.

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Stoichiometry for ΔH

ΔH = moles × (ΔrH°/mol-rxn) for calculating energy changes.

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Result of ΔH calculation

For 7.811 mol of C₄H₁₀, ΔH equals -22,500 kJ using given reaction data.

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Check physical states

Important for accurate enthalpy calculation and interpretation.

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Coefficients in reactions

Determine how ΔrH° scales and should be accounted for in calculations.

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Unit conversion importance

Carefully convert grams to moles and then to kJ in calculations.

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ΔrH° applications

Represents energy per mole of the overall reaction, not per mole of a single substance.

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Exothermic reaction

A reaction characterized by a negative ΔrH°, indicating heat release.

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Endothermic reaction

A reaction characterized by a positive ΔrH°, indicating heat absorption.

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Heat Transfer

The exchange of thermal energy during chemical reactions, quantified by ΔH.

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Thermodynamic standard conditions

Refers to the 1 bar pressure and 25°C (298 K) under which standard enthalpy changes are measured.

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Chemical Reaction Sign Convention

The sign of ΔrH° signifies the heat flow direction in a reaction.

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Condensation of water vapor

A specific reaction with ΔrH° of -44.0 kJ/mol.

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Reversible reactions

A reaction whose ΔrH° values will change sign when the direction is reversed.

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Physical state impact

The physical state (gas, liquid, solid) can significantly affect ΔrH° values.

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Reactants in standard states

The form of reactants at 1 bar and 25°C before a reaction occurs.

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Molar mass definition

The mass of one mole of a substance, typically in grams per mole (g/mol).

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Energy per mole concept

ΔrH° is calculated as energy change per mole of the entire reaction.

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Units of enthalpy change

Typically measured in kJ/mol, indicating energy per mole.

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ΔH significance

Indicates the energy change associated with chemical reactions under specified conditions.

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Stoichiometric coefficients

Represent the proportional amounts of reactants and products in a reaction.

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Balanced chemical equation

An equation that shows equal numbers of atoms for both reactants and products.

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Heat of formation

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

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Calorimetry

The measurement of heat transfer in chemical reactions.

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Enthalpy of vaporization

The heat required to convert a substance from a liquid to a gas at constant temperature and pressure.

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Endothermic vs exothermic

Endothermic absorbs heat, exothermic releases heat during reactions.

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Molecular interpretation of ΔH

Reflects the energy absorbed or released due to bond breaking and forming in reactions.

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Importance of ΔrH° in thermodynamics

Critical for predicting the direction and spontaneity of chemical reactions.

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Chemical energy,

Potential energy stored in the bonds of chemical compounds.