Thermochemistry Flashcards
First Law of Thermodynamics
ΔU = Q - W
ΔU: Change in internal energy of the system.
Q: Heat added to the system.
W: Work done by the system.
Key point:
Heat increases energy (𝑄>0, and doing work decreases energy (W>0).
Mnemonic: “Energy is like a bank account: Deposits (heat) add energy, and withdrawals (work) reduce it.”
Specific Heat Formula: Formula for the heat transferred to or from a substance during a given change in temperature.
q = heat transferred in J
m = mass in kg
c = specific heat in J kg-1 K-1 or J kg-1 ℃-1
𝛥T = change in temperature in Kelvin or ℃
Key point:
Larger mass or higher specific heat means more heat is needed to change the temperature.
Mnemonic: “Heat loves mass, specific heat, and big temperature changes.”
Standard Heat of Reaction: The enthalpy change of a reaction is the difference between the total enthalpy of the products and the reactants.
𝛥H°f = enthalpy of formation in kJ/mol
Key point:
ΔHrxn<0: Exothermic (releases heat).
ΔHrxn>0: Endothermic (absorbs heat).
Mnemonic: “Products minus reactants equals reaction energy.”
Gibbs Free Energy: Gibbs free energy (Δ𝐺) determines whether a reaction is spontaneous.
𝛥G = Gibbs free energy change in kJ
𝛥H = enthalpy in kJ
T = temperature in Kelvin
𝛥S = entropy change in kJ/K
Key point:
ΔG<0: Reaction is spontaneous.
ΔG>0: Reaction is non-spontaneous.
Standard Heat of Reaction: Formula for the enthalpy of a reaction given the bond-dissociation energies of the bonds broken and bonds formed in the reaction.
𝛥H°bond = bond-dissociation energy in kJ/mol
Formula for the entropy change of a reversible, isothermal process
𝛥S = entropy change in J/K
Qrev = heat transfer in J
T = temperature in Kelvin
Work (W) is done when a gas expands or compresses.
W = -PΔV
P: Pressure of the gas.
ΔV: Change in volume.
Key point:
If ΔV>0 (gas expands), work is negative (system loses energy).
If ΔV<0 (gas compresses), work is positive (system gains energy).
Mnemonic: “Expanding gases do negative work.”
This equation relates the internal energy (U) of an ideal monatomic gas to temperature.
U = 3/2nRT
n: Number of moles of gas
R: Ideal gas constant (8.31J/mol)
T: Absolute temperature (in Kelvin)
Key point: Internal energy is directly proportional to temperature for ideal gases.
Mnemonic: “More moles and more heat mean more internal energy.”
Heat during a phase change (e.g., melting or boiling).
Q = mHL
Q: Heat.
m: Mass.
HL: Latent heat (heat needed for phase change per unit mass).
Key point: There is no temperature change during a phase change; all heat goes into changing the phase.
Mnemonic: “Latent heat loves phase changes.”
Thermo equations often involve
heat (Q), work (W), or energy (U).
