Thermochemistry - Energy of Chemical Reactions

Vocabulary flashcards covering core concepts of thermochemistry, including heat, work, enthalpy, First Law of Thermodynamics, Hess's Law, standard enthalpies of formation, and bond dissociation energies.

Thermochemistry

The science of heat (energy) flow in physical and chemical processes.

Exothermic Reaction

A chemical reaction in which more energy is released in forming bonds than is used in breaking bonds, releasing energy as heat, light, sound, or work, characterized by $\Delta H < 0$ ($H_{\text{final}} < H_{\text{initial}}$).

Endothermic Reaction

A chemical reaction in which more energy is used in breaking bonds than is released in forming bonds, requiring energy input from light, work, or cooling of surroundings, characterized by $\Delta H > 0$ ($H_{\text{final}} > H_{\text{initial}}$).

Specific Heat Capacity

The heat energy required to change the temperature of a given mass of substance, defined as $\text{Specific heat capacity} = \frac{\text{heat lost or gained by substance (J)}}{(\text{mass, g}) \times (\text{T change, K})}$

Heat of Fusion (Water)

The energy change involved when ice transitions to water at $0\,^{\circ}\text{C}$, equal to $333\,\text{J/g}$.

Heat of Vaporization (Water)

The energy change involved when water transitions to vapor at $100\,^{\circ}\text{C}$, equal to $2260\,\text{J/g}$.

Heating/Cooling Curve for Water

A diagram illustrating the stages of energy transfer and phase changes as heat is added or liberated, including melting ice, heating water, and evaporating water.

Energy

The capacity to do work or transfer heat.

Heat ($q$)

The form of energy that flows between two samples because of a difference in temperature.

Work ($w$)

The form of energy that results in a macroscopic displacement of matter, such as gas expansion or motion of an object ($\text{force} \times \text{distance}$).

Work of Expansion ($w_{\text{exp}}$)

The work performed by an expanding ideal gas when enclosed, expressed by the equation $w_{\text{exp}} = -P\Delta V$.

First Law of Thermodynamics

The principle stating that energy is conserved, expressed by the equation $\Delta E = q + w$, where $q$ and $w$ are positive when transferred from surroundings to the system.

Enthalpy ($H$)

A thermodynamic function defined as $H = E + PV$, where heat transferred at constant pressure is $q_p = \Delta H = H_{\text{final}} - H_{\text{initial}}$.

State Function

A property that depends only on the current state of the system and not on how it got there, such as $\Delta H$, volume ($V$), temperature ($T$), pressure ($P$), and energy.

Hess's Law

The principle stating that if a reaction is the sum of two or more other reactions, the net enthalpy change $\Delta H$ is the sum of the $\Delta H$ values of those constituent reactions.

Hess's Law Reaction Path Diagram

A diagram demonstrating that the total enthalpy change along a direct reaction path equals the sum of enthalpy changes along an alternate multi-step reaction path ($\sum \Delta H \text{ along one path} = \sum \Delta H \text{ along another path}$).

Standard Conditions (Thermochemistry)

Refers to conditions where pressure $P = 1\,\text{atm}$ ($= 760\,\text{torr} = 101.3\,\text{kPa}$), concentration $= 1\,\text{mol/L}$, temperature $T = \text{usually } 25\,^{\circ}\text{C}$, with all species in standard states.

Standard Molar Enthalpy of Formation ($\Delta H^{\circ}_f$)

The enthalpy change when $1\,\text{mol}$ of a compound is formed from its constituent elements under standard conditions, where $\Delta H^{\circ}_f = 0\,\text{kJ/mol}$ for elements in their standard states.

Standard Enthalpy of Reaction Equation

The general relationship used to calculate overall reaction enthalpy when standard enthalpies of formation are known: $\Delta H^{\circ}_{\text{rxn}} = \sum \Delta H^{\circ}_f(\text{products}) - \sum \Delta H^{\circ}_f(\text{reactants})$

Bond Dissociation Energy ($D$)

The energy required to break a specific chemical bond in a gas-phase molecule under standard conditions.

Bond Order and Bond Energy Relationship

The relationship stating that the greater the number of shared electron bonds (bond order), the higher the bond dissociation energy ($D$).

Enthalpy of Reaction from Bond Energies Equation

The equation used to estimate reaction energy: $\Delta H_{\text{rxn}} = \sum D(\text{bonds broken}) - \sum D(\text{bonds made})$

Reaction Energy Pathways via Gaseous Atoms

A visual model illustrating that endothermic reactions cost more energy in bond breaking to gaseous atoms than gained by bond making, while exothermic reactions release more energy in bond making from gaseous atoms than required for bond breaking.