AP Chemistry Unit 6: Deep Dive into Enthalpy

Enthalpy

The total heat content of a system, denoted as H.

Change in Enthalpy ($\Delta H$)

Represents the heat energy exchanged between the system and surroundings at constant pressure.

Exothermic Reaction

A reaction that releases heat to the surroundings, indicated by $\Delta H < 0$.

Endothermic Reaction

A reaction that absorbs heat from the surroundings, indicated by $\Delta H > 0$.

Potential Energy Diagrams

Graphs that show the potential energy of the system as reactants transition to products.

Activation Energy ($E_a$)

The energy barrier that must be overcome for a reaction to occur.

Extensive Property

A property that depends on the amount of substance, such as enthalpy change.

Bond Enthalpy

The energy required to break a bond between two atoms.

Golden Rule of Bond Energy

Breaking bonds is endothermic; forming bonds is exothermic.

Standard Enthalpy of Formation ($\Delta H_f^\circ$)

The enthalpy change when 1 mole of a compound is formed from its elements in standard states.

Zero Enthalpy of Elements

The $\Delta H_f^\circ$ of any element in its standard state is zero.

Hess's Law

The change in enthalpy is independent of the pathway taken between reactants and products.

Reversing a Reaction (Hess's Law)

If the reactants and products are flipped, the sign of $\Delta H$ is also flipped.

Multiplying Coefficients (Hess's Law)

If the reaction coefficients are multiplied by a factor n, $\Delta H$ is also multiplied by n.

Adding Reactions (Hess's Law)

When chemical equations are summed, their $\Delta H$ values are also summed.

Common Mistake: Bond Energy Formula

Applying products minus reactants to Bond Energies; it should be reactants minus products.

Common Mistake: Coefficients

Forgetting to multiply $\Delta H_f^\circ$ by the stoichiometric coefficient.

Common Mistake: Standard States

Overlooking the phase of elements to correctly assess their $\Delta H_f^\circ$.

Common Mistake: Sign Errors

Mistakes in sign when processing negative formation enthalpies.

Common Mistake: States of Water

Confusing liquid water and vapor in combustion reactions; they have different $\Delta H_f^\circ$ values.

Combustion of methane

An example of an exothermic reaction where heat is released.

Decomposition of calcium carbonate

An example of an endothermic reaction where heat is absorbed.

Heat Absorption

In endothermic reactions, the system absorbs heat from the surroundings.

Heat Release

In exothermic reactions, the system releases heat to the surroundings.

Thermodynamic Equation for Enthalpy of Reaction

$\Delta H{rxn}^\circ = \Sigma n \Delta Hf^\circ(\text{products}) - \Sigma n \Delta H_f^\circ(\text{reactants})$.

Definition of State Function

A property of a system that depends only on its current state, not on the path taken.