Thermochemical Changes: Theories of Energy, Chemical Changes, Activation Energy, and Catalysts

Comprehensive vocabulary flashcards covering thermochemical changes, Hess's Law, enthalpy calculations, energy efficiency, fossil fuels, collision theory, activation energy, and industrial and biological catalysts.

Hess's Law

The principle stating that the enthalpy change of a physical or chemical process depends only on the initial and final states, so that the overall enthalpy change is the sum of the enthalpy changes of its individual steps.

Calorimetry Limitations

Reactions unsuitable for a calorimeter include those that occur extremely slowly or those that release vast amounts of energy, such as explosive detonations of nitroglycerin.

Standard Molar Enthalpy of Formation ($\Delta_f H^\circ$)

The enthalpy change that occurs when one mole of a compound is synthesized directly from its elements in their most stable states under standard ambient temperature and pressure (SATP).

Reference Point for Elements

The standard molar enthalpy of formation assigned to pure elements in their most stable state at SATP, defined as $0\,kJ\,mol^{-1}$.

Thermal Stability

The ability of a chemical substance to resist decomposition when heated; substances with higher positive enthalpies of decomposition require greater energy inputs to break down and are more stable.

Hess's Law Summation Formula

The mathematical formula used to predict reaction enthalpy using formation enthalpies: $\Delta_r H^\circ = \sum (n\,\Delta_f H^\circ_{\text{products}}) - \sum (n\,\Delta_f H^\circ_{\text{reactants}})$.

Reversing an Equation (Hess's Law)

A manipulation method where reactants and products are exchanged so the reaction proceeds in reverse, requiring the sign of $\Delta H^\circ$ to be multiplied by $-1$.

Multiplying Coefficients (Hess's Law)

A manipulation method where all stoichiometric coefficients in an equation are multiplied by an integer or fraction factor, requiring $\Delta H^\circ$ to be multiplied by that same factor.

Efficiency

The percentage ratio of useful energy delivered to total energy supplied: $\text{Efficiency} = \frac{\text{Energy output}}{\text{Energy input}} \times 100\%$.

Energy Output

Energy delivered to the consumer in a usable form or the actual work done, calculated via formulas such as $Q = mc\Delta T$ and $\Delta H = n\,\Delta_x H$.

Energy Input

The ideal energy content of a fuel, solar energy invested in fuel creation, or the energy consumed in extracting fuel, transporting it, and maintaining power plants.

Renewable Energy Sources

Energy sources that are continually and infinitely available, such as solar, wind, water, geothermal, tidal, and biomass.

Non-renewable Energy Sources

Energy sources that exist in limited supply and are irreplaceable once consumed, such as nuclear fuels and fossil fuels.

Clean Fuels

Fuels that emit comparatively lower amounts of carbon dioxide ($\text{CO}_2$) into the atmosphere during combustion.

Heat Content of a Fuel

The quantity of energy released per kilogram of a fuel, used in combination with environmental pollution and economic costs to evaluate energy options.

Collision Theory

The chemical theory stating that reacting particles must collide with correct geometric orientation and sufficient collision energy equal to or exceeding activation energy to react.

Activation Energy ($E_a$)

The minimum kinetic energy barrier that colliding reactant particles must possess for a successful chemical reaction to take place.

Kinetic Energy Distribution Curve

A curve showing that increasing temperature from $T_1$ to $T_2$ broadens the distribution and increases the fraction of particles colliding with energy equal to or greater than $E_a$, speeding up reaction rates.

Potential Energy Diagram

A graphical representation that illustrates the reaction pathway, showing the enthalpy change ($\Delta H$) alongside the activation energy barrier ($E_a$) as a hill.

Exothermic Reaction Potential Energy Diagram

A potential energy curve where products end at lower potential energy than reactants, featuring a reverse activation energy related by $E_{a(\text{rev})} = E_{a(\text{fwd})} + \Delta H$.

Endothermic Reaction Potential Energy Diagram

A potential energy curve where products end at higher potential energy than reactants, featuring a reverse activation energy related by $E_{a(\text{rev})} = E_{a(\text{fwd})} - \Delta H$.

Activated Complex (Transition State)

The unstable, high-energy arrangement of atoms located at the maximum potential energy peak along the reaction pathway.

Catalyst

A substance that accelerates a chemical reaction without being consumed by providing an alternative pathway with a lower activation energy.

Catalyzed Reaction Pathway

An alternate reaction pathway characterized by a lower activation energy barrier ($E_a$) while leaving reactants, products, and overall enthalpy change ($\Delta H$) identical.

Vanadium(V) Pentoxide ($V_2O_{5(s)}$)

The solid chemical catalyst used industrially to accelerate the slow oxidation of $SO_{2(g)}$ and $O_{2(g)}$ into $SO_{3(g)}$ during the production of sulfuric acid.

Platinum-Rhodium ($\text{Pt-Rh}$) Mesh Gauze

The catalyst used at a high temperature of $800\,^\circ\text{C}$ in the industrial manufacture of nitric acid, a starting material for ammonium nitrate fertilizer ($NH_4NO_{3(s)}$).

Enzyme

A biological protein catalyst in the human body that allows vital chemical reactions to proceed rapidly at body temperature ($37\,^\circ\text{C}$).

Active Site

The unique structural region of an enzyme that binds specifically to a substrate molecule to create an enzyme-substrate complex.

Catalase

A biological enzyme that protects human cells by rapidly breaking down toxic hydrogen peroxide ($H_2O_{2(aq)}$) into harmless water and gaseous oxygen.