5.7

Hess’s Law

If a chemical reaction is the sum of two or more other reactions, then the overall enthalpy change (∆rH°) equals the sum of the enthalpy changes of those reactions.

Enthalpy Change (∆rH°)

The difference in enthalpy between the products and reactants of a chemical reaction.

Energy Level Diagram

A visual representation of the energy changes during a chemical reaction showing reactants and products on the y-axis according to their enthalpy.

Standard Enthalpy of Formation (∆fH°)

The enthalpy change when 1 mole of a compound forms from its elements in their standard states at 25°C and 1 bar.

Exothermic

A process that releases heat, resulting in a negative enthalpy change (∆H < 0).

Endothermic

A process that absorbs heat, resulting in a positive enthalpy change (∆H > 0).

Standard State

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

Path Independence

A property of a state function where the value does not depend on the path taken to achieve the end state.

Coefficient

A numerical factor used to multiply the amount of a substance in a balanced chemical equation.

Reverse a Reaction

In Hess's Law, when a reaction is reversed, the sign of its ∆H° must be changed.

Multiply Coefficients

When coefficients in a balanced equation are multiplied, ∆H° must also be multiplied by the same factor.

Endothermic Reaction Example

CaCO₃(s) → CaO(s) + CO₂(g) with ∆rH° = +179.0 kJ/mol.

Exothermic Reaction Example

Na(s) + ½ Cl₂(g) → NaCl(s) with ∆fH° = -411.12 kJ/mol.

Reactants

Substances present at the start of a chemical reaction that undergo change.

Products

Substances formed as a result of a chemical reaction.

Negative ∆fH° Value

Indicates that forming the compound is an exothermic process.

Positive ∆fH° Value

Indicates that forming the compound is an endothermic process.

Heat Input

Energy required for an endothermic reaction to proceed.

Manipulate Reactions

The process of adjusting known reactions to match the target reaction in Hess’s Law.

Thermodynamics

The branch of physical science that deals with the relations between heat and other forms of energy.

Kilojoules per mole (kJ/mol)

Unit for measuring the amount of energy released or absorbed in a chemical reaction.

Balanced Chemical Equation

An equation that has the same number of atoms of each element on both sides of the reaction.

Appendix L

A resource commonly used for reference values of standard enthalpies of formation (∆fH°).

Summation Formula for ∆rH°

ΔrH°=∑nΔfH°(products)−∑nΔfH°(reactants).

Heat of Reaction

The amount of heat absorbed or released during a chemical reaction.

Energy Changes

Variations in energy from reactants to products as a result of chemical reactions.

Thermal Stability

The ability of a compound to withstand heat without decomposing.

Chemical Thermodynamics

The study of the interrelationship between chemical reactions and energy.

System

The part of the universe being studied, which exchanges energy with its surroundings.

Surroundings

Everything in the universe that is not part of the system.

Heat Capacity

The amount of heat energy needed to change a substance's temperature.

Molar Enthalpy of Formation

The heat change associated with the formation of one mole of a substance.

Chemical Reaction

A process that leads to the transformation of one set of chemical substances to another.

Stability of Compounds

The tendency of a chemical compound to remain unchanged under specified conditions.

Calorimetry

The measurement of heat changes in physical and chemical processes.

Reaction Mechanism

The step-by-step sequence of elementary reactions by which overall chemical change occurs.

Thermochemical Equation

A balanced chemical equation that includes the enthalpy change.

Coefficient Relationship in Reactions

The relationship between the number of moles of reactants and products and the total enthalpy change.

Example of ∆fH°

C₂H₅OH(ℓ): 2 C(s) + 3 H₂(g) + ½ O₂(g) → C₂H₅OH(ℓ) with ∆fH° = -277.0 kJ/mol.

Stability of Elements

Elements in their standard state have a ∆fH° of zero.