thermodynamics

Thermodynamics

Thermodynamics deals with heat and temperature and their relation to energy, work, radiation, and properties of matter.

Energy

Energy is the capacity to do work.

Thermal energy

Thermal energy is the energy associated with the random motion of atoms and molecules (example: heating water increases thermal energy).

Chemical energy

Chemical energy is the energy stored within the bonds of chemical substances (example: energy released in combustion).

Nuclear energy

Nuclear energy is the energy stored within the collection of neutrons and protons in the atom (example: nuclear fission).

Electrical energy

Electrical energy is the energy associated with the flow of electrons (example: current in a wire).

Potential energy

Potential energy is the energy available by virtue of an object's position (example: water stored behind a dam).

Law of conservation of energy

In interactions between a system and its surroundings the total energy remains constant; energy is neither created nor destroyed.

Heat (q)

Heat is a form of energy transferred between system and surroundings due to a temperature difference.

System

A system is the part of the universe chosen for study.

Surroundings

Surroundings are the portion of the universe with which a system interacts.

State parameters

State parameters are measurable macroscopic properties such as pressure, volume, temperature, and number of moles.

State functions

State functions are mathematical relationships between state parameters and depend only on the state of the system, not the path.

Temperature

Temperature is a measure of hotness or coldness indicating the direction of spontaneous heat flow (example: heat flows from hot to cold).

Heat capacity

Heat capacity is the quantity of heat required to raise the temperature of a substance by 1°C.

Specific heat (s)

Specific heat is the amount of heat required to raise the temperature of one gram of a substance by 1°C.

Heat capacity (C)

Heat capacity is the amount of heat required to raise the temperature of a given quantity of a substance by 1°C.

Heat capacity formula

C = m × s (example: doubling mass doubles heat capacity).

Heat equation

Q = m s Δt (example: heating 50 g of water by 10°C).

Temperature change

Δt = t_final − t_initial.

Isolated system

An isolated system exchanges neither matter nor energy with its surroundings.

Open system

An open system freely exchanges both matter and energy with its surroundings.

Closed system

A closed system exchanges energy but not matter with its surroundings.

Adiabatic system

An adiabatic system exchanges no heat but work can be done on the system.

Exothermic reaction

A chemical reaction in which heat is liberated and ΔH is negative (example: combustion).

Endothermic reaction

A chemical reaction in which heat is absorbed and ΔH is positive (example: melting ice).

Internal energy (E)

Internal energy is all the energy of a system and depends on the state and composition of the system.

Change in internal energy

ΔE = E_final − E_initial.

First Law of Thermodynamics

The change in internal energy of a system equals the heat transferred plus the work done on the system.

First law equation

ΔE = Q + W.

Positive heat flow

Heat absorbed by the system is positive (endothermic, +q).

Work (W)

Work is energy transfer that occurs when a force is applied over a distance (example: gas expansion).

Pressure-volume work

Pressure-volume work occurs when volume changes against an external pressure.

PV work equation

w = −P_ext ΔV.

Sign of work (expansion)

If ΔV > 0 work is negative and done by the system (example: gas expansion).

Sign of work (compression)

If ΔV < 0 work is positive and done on the system.

Enthalpy (H)

Enthalpy is the heat content of a system at constant pressure.

Enthalpy definition

H = E + PV.

Enthalpy change

ΔH equals heat exchanged at constant pressure.

Enthalpy change of reaction

ΔH = ΣH_products − ΣH_reactants.

Exothermic enthalpy change

For an exothermic reaction ΔH < 0.

Endothermic enthalpy change

For an endothermic reaction ΔH > 0.

Thermochemical equation

A balanced chemical equation that includes the enthalpy change.

Reversing reactions

Reversing a reaction changes the sign of ΔH.

Scaling reactions

Multiplying a reaction by a factor n multiplies ΔH by n.

Standard enthalpy of formation (ΔHf°)

The heat change when one mole of a compound forms from its elements in their standard states at 1 atm.

Standard enthalpy of formation of elements

The ΔHf° of an element in its most stable form is zero.

Standard enthalpy of reaction (ΔH°rxn)

The enthalpy change for a reaction carried out at standard conditions.

Standard reaction enthalpy equation

ΔH°rxn = ΣnΔHf°(products) − ΣmΔHf°(reactants).

Hess's Law

The enthalpy change of a reaction is independent of the reaction path and depends only on initial and final states.

Spontaneous process

A spontaneous process occurs without outside intervention.

Entropy (S)

Entropy is a measure of the randomness or disorder of a system.

Entropy as state function

Entropy is a state function: ΔS = S_final − S_initial.

Second Law of Thermodynamics

The total entropy of an isolated system never decreases over time.

Entropy and spontaneity

All spontaneous processes increase the entropy of the universe.

Third Law of Thermodynamics

The entropy of a pure crystalline substance at absolute zero is zero.

Standard entropy (S°)

Standard entropy is the molar entropy of a substance in its standard state.

Entropy and complexity

Larger and more complex molecules generally have higher entropy.

Entropy change of reaction

ΔS°rxn = ΣS°products − ΣS°reactants.

Gibbs free energy (G)

Gibbs free energy combines enthalpy and entropy to predict spontaneity.

Gibbs free energy equation

ΔG° = ΔH° − TΔS°.

Spontaneity and ΔG

If ΔG < 0 the reaction is spontaneous.

Equilibrium and ΔG

If ΔG = 0 the system is at equilibrium.

Nonspontaneous process

If ΔG > 0 the reaction is spontaneous in the reverse direction.

Temperature dependence of ΔG

Temperature affects spontaneity through the TΔS term.

Free energy of formation (ΔGf°)

The standard free energy change for forming one mole of a compound from its elements.

Free energy of elements

ΔGf° for an element in its standard state is zero.

Free energy and equilibrium constant

ΔG° is directly related to the equilibrium constant Keq.

Reaction quotient (Q)

Q describes the ratio of product to reactant concentrations at non-equilibrium conditions.