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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.