General Chemistry Study Notes: Thermochemistry, Energy, and Heat Capacity

Thermochemistry and Energy

Introduction to Thermochemistry

Thermochemistry is the study of matter and energy associated with chemical reactions or physical changes. It focuses on how energy is transferred and transformed during chemical processes.

• Energy: The capacity to do work or produce heat.

Classification of Energy

Energy can be classified based on its source and effect on matter:

• Energy of Position of Atoms: Related to the arrangement of atoms.

• Energy of Motion of Atoms: Related to the movement of atoms (kinetic energy).

• Energy of Chemical Bonds of Atoms: Stored in chemical bonds (potential energy).

• Energy of Temperature of Atoms and Molecules: Related to thermal energy.

Energy Conversion Factors

Energy can be measured in different units. The SI unit for energy is the joule (J), named after James Joule.

Unit	Conversion
1 calorie (cal)	4.184 J
1 Calorie (Cal)	4184 J
1 kilowatt-hour (kWh)	3.60 × 106 J

Mechanical Energy

Kinetic and Potential Energy

Mechanical energy is the energy an object possesses due to its motion (kinetic energy) or position (potential energy).

• Kinetic Energy (K.E.): Energy due to motion.

• Potential Energy (P.E.): Energy due to position or configuration.

Kinetic Energy Formula

The kinetic energy of an object is given by:

$K.E. = \frac{1}{2}mv^2$

• m: Mass of the object (kg)

• v: Velocity of the object (m/s)

Potential Energy Formula

The potential energy due to gravity is given by:

$P.E. = mgh$

• m: Mass (kg)

• g: Acceleration due to gravity (9.8 m/s2)

• h: Height above ground (m)

Mechanical Energy Conversions

Kinetic and potential energy can be converted between each other:

$K.E. \leftrightarrow P.E.$

First Law of Thermodynamics

Energy Conservation

The First Law of Thermodynamics states that energy cannot be created nor destroyed, only transferred between a system and its surroundings.

• System: The part of the universe being studied.

• Surroundings: Everything outside the system.

Heat and Work

Definitions

• Heat (q): Transfer of thermal energy from a higher temperature object to a lower temperature object.

• Work (w): Movement of reacting molecules against gravity or an opposing force.

Heat and Work Applications

• Heat can be released (exothermic) or absorbed (endothermic).

• Work can be done by the system on the surroundings or by the surroundings on the system.

Internal Energy

Definition and Calculation

Internal energy (E) is the total energy from all forms of kinetic and potential energy of a system.

Internal energy can be calculated as:

$\Delta E = q + w$

• q: Heat

• w: Work

Work-Pressure Volume Formula

When work is done by changing the volume at constant pressure:

$w = -P\Delta V$

• P: Pressure

• ΔV: Change in volume

Enthalpy (ΔH)

Enthalpy is the amount of heat released or absorbed during a chemical reaction at constant pressure.

Endothermic and Exothermic Reactions

Endothermic Reactions

Endothermic reactions absorb thermal energy from the surroundings, breaking chemical bonds and increasing molecular motion.

• Feels cold to the touch.

• Examples: Water boiling, melting ice.

Exothermic Reactions

Exothermic reactions release thermal energy to the surroundings, forming chemical bonds and decreasing molecular motion.

• Feels warm to the touch.

• Examples: Water freezing, combustion.

Thermochemical Equations and Stoichiometry

Thermochemical Equations

Thermochemical equations show the enthalpy change (ΔH) associated with chemical reactions.

• Example: $2 \text{Mg} (s) + \text{O}_2 (g) \rightarrow 2 \text{MgO} (s) \quad \Delta H = -1204 \text{ kJ}$

Stoichiometric Calculations

Stoichiometry uses the balanced chemical equation to relate quantities of reactants and products.

• Convert grams of given substance to moles.

• Use mole-to-mole comparison based on coefficients.

• Convert moles of unknown to grams or other units as needed.

Heat–Temperature Relationship

Proportionality

Heat added to an object is directly proportional to its temperature change:

$q \propto \Delta T$

Molar and Specific Heat Capacity

Definitions

• Heat Capacity (C): Amount of heat required to change the temperature of a weighted substance by 1 K.

• Specific Heat Capacity (c): Amount of heat required to change the temperature of 1 g of substance by 1 K.

• Molar Heat Capacity (Cm): Amount of heat required to change the temperature of 1 mole of substance by 1 K.

Heat Capacity Formulas

Type	Formula
Molar Heat Capacity	$q = nC_m\Delta T$
Specific Heat Capacity	$q = mc\Delta T$

• q: Heat (J)

• n: Moles

• m: Mass (g)

• c: Specific heat capacity (J/g·°C)

• Cm: Molar heat capacity (J/mol·°C)

• ΔT: Temperature change (°C or K)

Rearranged Specific Heat Formula

To solve for heat or temperature change:

$q = mc\Delta T$

Specific Heat Capacities Table

Substance	Specific Heat Capacity (J/g·°C)
Aluminum, Al	0.900
Copper, Cu	0.385
Compounds (H2O)	4.184
Ethanol, C2H5OH	2.450
Wood	1.760

Applications and Practice Problems

• Calculate energy conversions between kinetic and potential energy.

• Determine heat released or absorbed in chemical reactions using thermochemical equations.

• Use specific heat capacity to find temperature changes in substances.

• Apply stoichiometry to relate reactant and product quantities in energy-related reactions.

Additional info: These notes expand on the provided study prep slides, filling in missing definitions, formulas, and context for clarity and completeness.