Endothermic and Exothermic Processes

System

The part of the world being studied (e.g., the chemicals in a beaker, a salt, or water).

Surroundings

Everything else around the system (e.g., the air, tabletop, thermometer, or beaker walls).

Universe

The combination of the system and the surroundings.

First Law of Thermodynamics (Conservation)

Energy lost by the system is gained by the surroundings, and vice versa (q_sys = -q_surr).

Endothermic Process

A process where heat is absorbed by the system from the surroundings.

Exothermic Process

A process where heat is released by the system to the surroundings.

Sign of q (Heat) for Endothermic

Positive (+q); heat enters the system.

Sign of q (Heat) for Exothermic

Negative (-q); heat leaves the system.

Sign of ΔH (Enthalpy) for Endothermic

Positive (+).

Sign of ΔH (Enthalpy) for Exothermic

Negative (-).

Work (w)

Energy needed to move something against a force (e.g., expanding gases pushing a piston).

Equation for Work

w = -P\ΔV (Work = negative Pressure times change in Volume).

Sign of w (Work) for Expansion

Negative (-w); the system pushes on the surroundings (volume increases).

Sign of w (Work) for Compression

Positive (+w); the surroundings push on the system (volume decreases).

Conversion: L·atm to Joules

1 Latm = 101.325 Joules.

Temperature and Energy Relationship

Temperature changes in a system indicate energy changes.

Potential Energy Diagram: Endothermic

The curve goes "uphill"; products are higher in energy than reactants.

Potential Energy Diagram: Exothermic

The curve goes "downhill"; products are lower in energy than reactants.

Phase Changes: Endothermic

Melting, Boiling, Vaporization, Sublimation (Solid -> Gas).

Phase Changes: Exothermic

Freezing, Condensation, Deposition (Gas -> Solid).

Bond/Interaction Rule: Breaking

Breaking bonds or separating particles requires energy (Endothermic).

Bond/Interaction Rule: Forming

Combining materials or forming bonds releases energy (Exothermic).

Heating Curve slopes vs. plateaus

Sloped lines indicate temperature change (heating); flat lines (plateaus) indicate phase changes where potential energy changes.

Solution Formation: Step 1 (Solvent)

Solvent expands by overcoming intermolecular forces; this is Endothermic.

Solution Formation: Step 2 (Solute)

Solute expands by overcoming intermolecular forces; this is Endothermic.

Solution Formation: Step 3 (Mixing)

Solute and solvent recombine; this is Exothermic.

Temperature Drop in a Solution

Indicates an Endothermic process (the system absorbed heat from the water/thermometer, causing the temp to read lower).

Temperature Rise in a Solution

Indicates an Exothermic process (the system released heat into the water/thermometer).

Freeze Drying (Concept)

Involves Freezing (Exothermic) followed by Sublimation (Endothermic) where ice changes directly to gas.

Born-Haber: Enthalpy of Sublimation

M(s) -> M(g); Endothermic (requires energy to turn solid to gas).

Born-Haber: Ionization Energy

M(g) -> M⁺(g) + e^-; Endothermic (requires energy to remove an electron).

Born-Haber: Enthalpy of Dissociation

Bond breaking; Endothermic.

Born-Haber: Lattice Energy

Formation of solid lattice from gaseous ions; Exothermic (releases energy).

Orange Tree Protection Concept

Farmers spray water on trees before a freeze; as water freezes (Exothermic), it releases heat which is absorbed by the oranges to prevent them from freezing.

Dissolving CaCl₂ (Calcium Chloride)

An exothermic process often used to melt ice on roads (the heat released helps melt the ice).

Gas Expansion Cooling

When a compressed gas expands rapidly (like whipped cream leaving a can), the system does work and absorbs heat, causing the canister to feel cold (Endothermic from the can's perspective).