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40 Terms

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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.

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Enthalpy Change (∆rH°)

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

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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.

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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.

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Exothermic

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

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Endothermic

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

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Standard State

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

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Path Independence

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

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Coefficient

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

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Reverse a Reaction

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

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Multiply Coefficients

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

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Endothermic Reaction Example

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

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Exothermic Reaction Example

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

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Reactants

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

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Products

Substances formed as a result of a chemical reaction.

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Negative ∆fH° Value

Indicates that forming the compound is an exothermic process.

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Positive ∆fH° Value

Indicates that forming the compound is an endothermic process.

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Heat Input

Energy required for an endothermic reaction to proceed.

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Manipulate Reactions

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

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Thermodynamics

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

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Kilojoules per mole (kJ/mol)

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

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Balanced Chemical Equation

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

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Appendix L

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

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Summation Formula for ∆rH°

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

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Heat of Reaction

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

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Energy Changes

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

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Thermal Stability

The ability of a compound to withstand heat without decomposing.

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Chemical Thermodynamics

The study of the interrelationship between chemical reactions and energy.

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System

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

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Surroundings

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

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Heat Capacity

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

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Molar Enthalpy of Formation

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

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Chemical Reaction

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

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Stability of Compounds

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

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Calorimetry

The measurement of heat changes in physical and chemical processes.

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Reaction Mechanism

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

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Thermochemical Equation

A balanced chemical equation that includes the enthalpy change.

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Coefficient Relationship in Reactions

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

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Example of ∆fH°

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

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Stability of Elements

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