Energy, Thermodynamics and Atomic Structure: Key Chemistry Concepts

Posted on Oct 20, 2026 in Physics

Forms of Energy

3.1

Kinetic: Motion

Thermal: Random motion

Radiant: Sun

Potential: Position

Chemical: Stored within bonds

Nuclear: Energy stored within protons and neutrons (E=mc²)

Electrical, Mass

KE = 1/2mv² | SI unit of energy = Joule (J) = 1 kg·m²/s²

E_electric = kq1q2/r, k = 2.31 x 10^-16

Heat = Transfer of thermal energy

Temperature = measure of thermal energy

Thermodynamics

3.2 Thermodynamics

First law: Energy cannot be created nor destroyed

(System, surroundings, universe)

First law: ΔU_univ = ΔU_sys + ΔU_surr = 0

System energy = ΔU = q + w (q = heat, w = work) (q positive when heat flows into the system; w positive when work is done on the system)

Exothermic: releases energy (energy in products). Vice versa for endothermic.

ΔT > 0 if the substance absorbs heat (vice versa)

Heat capacity (C): amount of heat needed to raise the temperature by 1°

C_m (molar, per mol), C_s (specific, per g), C (per 1°)

Temperature change: ΔT = q/nC_m (q = heat, n = moles)

Heat transferred (molar): q = nC_mΔT

Heat transferred (specific): q = mSΔT

Heat transferred (heat capacity): q = CΔT

q_surroundings = -q_system (keep track of signs for heat flows)

Work = energy used to move an object against an opposing force

W = force x displacement (W_surroundings = -W_system)

W = -PΔV (for gases at constant pressure)

In expansion ΔV > 0, so work < 0

Internal energy = ΔU = q + w

State functions: properties determined by the state of the system regardless of how it was achieved (energy, pressure, volume)

Path functions: properties that depend on how the change occurred (distance)

Energy Changes in Chemical Reactions

3.3 Energy changes in chemical reactions

Thermochemical equations use enthalpy as the energy component

Breaking bonds requires energy; forming bonds releases energy

Bond energy (BE): energy needed to break bonds

ΔH_rxn = ΣBE_{(bonds broken)} – ΣBE_{(bonds formed)}

Calorimetry

3.4 Calorimetry

A device used to measure heat flows

q_reaction = -q_calorimeter   q_calorimeter = C_calΔT

Constant volume calorimeter

Enthalpy

3.5 Enthalpy

H = U + pV

Enthalpy change ΔH_rxn ≈ ΔU_rxn + RTΔn_gas (R = 8.314 J/(mol • K), T = temperature)

Enthalpy of vaporization: ΔH_vap ≈ ΔU_vap + RT_vap

Hess’s Law: The change in enthalpy for a stepwise process is the sum of the enthalpy changes of each of the steps

Standard enthalpies of formation: ΔH_f^o = Σn_productsΔH^o_products – Σn_reactantsΔH^o_reactants

Characteristics of Atoms

4.1 Characteristics of atoms

Atoms possess mass, contain positive nuclei, contain e^–, and occupy volume

Characteristics of Light

4.2 Characteristics of light

Electromagnetic radiation is the best tool for studying the structure of atoms

Light is just a part of the spectrum

c = vλ     c = 2.998 x 10⁸ m/s

E = hv, h = Planck’s constant = 6.626 x 10^-34 J·s

c = vλ, E = hv, E = hc/λ

Photoelectric effect: hv = KE + W

KE = hv – W

If light energy exceeds the energy holding e^– to the metal, electrons are ejected

Any extra energy provided by the light adds to the kinetic energy of the ejected e^–

Absorption and Emission Spectra

4.3 Absorption and emission spectra

A free stationary e^– has 0 energy; thus, bound electrons have negative energies

Ground state: lowest energy state of an atom

Excited state: state reached when an atom absorbs a photon

ΔE = ±hv_photon

Atoms can absorb and emit radiation

Absorption: A source of white light passes through a sample, and the atoms absorb specific frequencies of light

Absorption measures the frequencies of photons that an atom absorbs

Emission: When excited atoms emit photons, the frequencies of the photons are specific

Bohr proposed that electrons can only have specific energy values

E_n = -R_H(1/n²)     n = principal quantum number, R_H = 2.18 x 10^-18 J

ΔE = R_H(1/n_i² – 1/n_f²)

Particle-Wave Duality

4.4 Particle-wave duality

λ_particle = h/mv     m = mass (kg), v = velocity (m/s)

Heisenberg uncertainty principle:

The more accurately we know the position of an electron, the more uncertain we are about its motion, and vice versa

Instead, we have a probable location

Formula: Δx • mΔu ≥ h/4π

Quantum Mechanics

4.5 Quantum mechanics

Orbitals: 3D regions where electrons are likely to be found (maximum probability)

Orbital types: s, p, d, f

n = energy level (size and energy)

l = 0 to n-1, shows the shape of the orbital

m = orientation integer from -l to +l (-1, 0, 1 for p orbitals, extending to d orbitals)

s = spin

Shapes of Orbitals

4.6 Shapes of orbitals

s orbitals are spherical: 1 orbital, 2 electrons

p orbitals are dumbbell-shaped: 3 orbitals, 6 electrons

d orbitals are double-dumbbell-shaped: 5 orbitals, 10 electrons

f orbitals are multi-lobed: 7 orbitals, 14 electrons