5.5

Enthalpy change (ΔH)

The heat transferred at constant pressure during a chemical reaction.

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).

Standard state

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

Positive ΔrH°

Indicates an endothermic reaction that absorbs heat from surroundings.

Negative ΔrH°

Indicates an exothermic reaction that releases heat to surroundings.

Reversing a chemical equation

Reverses the sign of ΔrH°.

Scaling the reaction

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

Different physical states

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

ΔH calculation step 1

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

ΔH calculation step 2

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

Given reaction

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

454 g of butane

Example mass used for energy change calculations in the reaction.

Moles of C₄H₁₀ from grams

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

Stoichiometry for ΔH

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

Result of ΔH calculation

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

Check physical states

Important for accurate enthalpy calculation and interpretation.

Coefficients in reactions

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

Unit conversion importance

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

ΔrH° applications

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

Exothermic reaction

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

Endothermic reaction

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

Heat Transfer

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

Thermodynamic standard conditions

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

Chemical Reaction Sign Convention

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

Condensation of water vapor

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

Reversible reactions

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

Physical state impact

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

Reactants in standard states

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

Molar mass definition

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

Energy per mole concept

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

Units of enthalpy change

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

ΔH significance

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

Stoichiometric coefficients

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

Balanced chemical equation

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

Heat of formation

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

Calorimetry

The measurement of heat transfer in chemical reactions.

Enthalpy of vaporization

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

Endothermic vs exothermic

Endothermic absorbs heat, exothermic releases heat during reactions.

Molecular interpretation of ΔH

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

Importance of ΔrH° in thermodynamics

Critical for predicting the direction and spontaneity of chemical reactions.

Chemical energy,

Potential energy stored in the bonds of chemical compounds.