AP Chemistry Unit 6: Thermodynamics Study Notes

Thermodynamics

The study of energy and its interconversions.

First Law of Thermodynamics

States that energy cannot be created or destroyed, only transferred or transformed.

Temperature

A measure of the average kinetic energy of the particles in a substance.

Kinetic Energy

The energy associated with the motion of particles.

Thermal Energy

The total kinetic energy associated with the random motion of atoms and molecules.

Heat ($q$)

The transfer of energy between two substances due to a temperature difference.

Thermal Equilibrium

The point at which two substances reach equal temperatures.

System

The specific part of the universe being studied.

Surroundings

Everything else in the universe outside the system.

Internal Energy ($$)

The total energy contained within a system.

Work ($w$)

Energy transfer due to pressure-volume changes.

Exothermic Process

A process that releases energy from the system to the surroundings.

Endothermic Process

A process that absorbs energy from the surroundings into the system.

Activation Energy ($E_a$)

The minimum energy required to initiate a chemical reaction.

Enthalpy Change ($_H$)

The difference in potential energy between products and reactants.

Catalyst

A substance that speeds up a reaction without being consumed.

Specific Heat Capacity ($c$)

The amount of heat required to raise the temperature of 1 gram of substance by 1°C.

Calorimetry

The experimental measurement of heat flow.

Heat Equation

The formula to calculate heat absorbed or released: $q = mc T$.

Conservation of Energy

In a perfectly insulated system, heat lost equals heat gained.

Phase Change

A transition between different states of matter that involves changes in potential energy.

Enthalpy of Fusion ($_{fus}$)

The enthalpy change when a substance melts or freezes.

Enthalpy of Vaporization ($_{vap}$)

The enthalpy change when a substance boils or condenses.

Heating Curve

A graph of temperature vs. heat added, showing phase changes and temperature increases.

Bond Enthalpy

The energy needed to break a bond; it's endothermic for bond breaking and exothermic for bond forming.

Standard Enthalpy of Formation ($_f^$)

The enthalpy change when one mole of a compound is formed from its elements in their standard states.

Hess's Law

States that the enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps.

Enthalpy of Reaction ($_{rxn}$)

The change in enthalpy during a chemical reaction.

Bond Breaking

An endothermic process that requires energy to separate atoms.

Bond Forming

An exothermic process that releases energy when atoms are combined.

Melting/Freezing

A phase change that involves enthalpy of fusion.

Boiling/Condensing

A phase change that involves enthalpy of vaporization.

Heat of Dissolution

The overall heat change when a solute dissolves in a solvent.

Sign Convention of $$

Negative for exothermic reactions and positive for endothermic reactions.

Common Mistakes in Thermodynamics

Including sign errors, unit mismatches, and confusion between bond energies and enthalpies of formation.

Total Energy in Reaction

Includes both kinetic and potential energy involved in breaking and forming bonds.

Demo of $_f^$ of Elements

Diatomic elements in their standard states have a $_f^$ of zero.

Potential Energy Diagram

Visual representation of energy changes during a reaction.

Sloped Line on Heating Curves

Indicates a single phase where kinetic energy increases.

Flat Plateaus on Heating Curves

Indicates a phase change where potential energy increases.

Phase Change Temperature

Remains constant during a phase transition, reflecting energy absorbed without a temperature rise.

Discrepancy in $$ and $_{rxn}$

The discrepancy arises from incorrect application of energy changes.

Measurement Units for Heat

Commonly measured in Joules (J) or kilojoules (kJ).