Thermochemistry (Chapter 5) Study Notes

Chapter 5: Thermochemistry

Introduction to Thermochemistry

Thermochemistry studies how energy relates to matter and change in matter, focusing on energy changes during chemical reactions. The primary form of energy considered is heat, but other forms exist. The First Law of Thermodynamics states that energy cannot be created or destroyed, only converted.

• Key Terms: Energy, Heat, Work, System, Surroundings, Universe

Systems vs Surroundings

In thermochemistry, the system is the part of the universe chosen for study, while the surroundings are everything else. The universe is the sum of system and surroundings.

• Equation: $\text{Universe} = \text{System} + \text{Surroundings}$

Types of Systems

• Open System: Can exchange both heat and mass with surroundings.

• Closed System: Can exchange heat but not mass.

• Isolated System: Cannot exchange heat or mass.

First Law of Thermodynamics

Energy can be converted from one form to another, but it is neither created nor destroyed.

• Example: Chemical energy in food is converted to heat and work in the body.

Nature of Chemical Energy

Chemical energy is primarily stored as potential energy in charged particles. The most important form is electrostatic potential energy:

• Equation: $E_{el} = \frac{kQ_1Q_2}{d}$

• $Q_1$ and $Q_2$ are charges, $d$ is distance, $k$ is a constant.

• Unit: Joule ($1\,J = 1\,kg\,m^2/s^2$)

Attraction Between Ions – Energy in Bonds

Electrostatic attraction between oppositely charged ions releases energy when bonds form ($E_{el} < 0$) and consumes energy when bonds break ($E_{el} > 0$).

• This contributes to the internal energy of chemicals.

Internal Energy

The internal energy ($U$) of a system is the total energy (potential + kinetic). We are interested in changes in internal energy ($\Delta E$), which indicate energy flow into or out of a system.

• Equation: $\Delta E = E_{final} - E_{initial}$

• $\Delta E$ is a state function (depends only on initial and final states).

Thermodynamic Quantities: Number, Unit, Sign

• Every thermodynamic quantity has three components: a number, a unit, and a sign.

• Positive $\Delta E$: system gains energy from surroundings.

• Negative $\Delta E$: system loses energy to surroundings.

Internal Energy Change: Release and Absorption

• If $\Delta E < 0$, system releases energy.

• If $\Delta E > 0$, system absorbs energy.

Relationship of $\Delta E$ to Heat and Work

Energy exchanged between system and surroundings is either heat ($q$) or work ($w$).

• Equation: $\Delta E = q + w$

Heat ($q$)

• Heat is random molecular motion.

• Transferred as energy from high to low temperature.

• Can cause temperature or phase changes.

Work ($w$)

• Work is organized molecular motion.

• "PV work" is common in chemistry: $w = -P\Delta V$ (pressure-volume work).

Signs of $\Delta E$, $q$, and $w$

Quantity	Positive (+)	Negative (-)
q	System gains heat	System loses heat
w	Work done on system	Work done by system
$\Delta E$	Net gain of energy by system	Net loss of energy by system

Exchange of Heat: Endothermic and Exothermic Processes

• Endothermic: System absorbs heat from surroundings; temperature drops.

• Exothermic: System releases heat to surroundings; temperature rises.