This section introduces the basic terms and concepts necessary to discuss thermodynamics in chemistry.
- The system is the part of the universe we are studying, and the surroundings are the rest of the universe.
- State variables describe a system's thermodynamic state.
- Intensive state variables do not scale with the size of the system.
- Extensive state variables scale with the size of the system.
- Equations of state describe the relationships between state variables at equilibrium.
- The ideal gas equation, PV=nRT, is an equation of state that describes the relationship between the state variables that describe an ideal gas.
- Path variables, such as heat (q) and work (w), change depending on the path taken between two states.
- Heat (q) and work (w) are positive when they result in an increase in energy of the system and negative when they result in a decrease in energy of the system.
- The First Law of Thermodynamics states that the energy of the universe is conserved.
- Calorimetry is a measurement of heat flow during a process.
- For a process carried out at constant volume, the energy change for a system is equal to the heat flow.
This section introduces the state variable enthalpy, H.
- The enthalpy of a system is defined as H≡E+PV
- For a process carried out at constant pressure, the change in enthalpy of the system is equal to the heat flow for the process.
- Hess' Law states that if a reaction/process can be written via multiple steps, then ΔH for the overall reaction/process is equal to the sum of the ΔH for each individual step.
- Hess' Law can be extended to apply to any state variable.
- In a formation reaction, the product is the chemical of interest and the reactants are the elements (in their standard states) necessary to form that product.
- Standard state is a standardized reference state, in which the pressure is 1 bar, all chemicals are pure, and solutions have a concentration of 1 M.
- A chemical species' enthalpy of formation is the enthalpy change associated with its formation reaction.
- The enthalpy of formation for a pure element in its standard state is 0 kJ/mol.
- Enthalpy of formation tables can be used to efficiently calculate the enthalpy change of many reactions.
This section introduces the state variable entropy.
- A microstate is one possible way to arrange the particles and distribute the energy in a system.
- Two microstates are part of the same macrostate if they result in the same macroscopic properties.
- When more microstates are part of the same macrostate, this macrostate is more likely to occur (higher probability) and is associated with a higher entropy.
- The entropy change for a process is qrev/T, where qrev is the heat flow when the process is carried out reversibly.
- The 2nd Law of Thermodynamics states that for all process, ΔSuniverse≥0.
- ΔSuniverse>0 for all spontaneous processes.
- When ΔSuniverse=0 the system is at equlibrium.
- The 3rd Law of Thermodyanmics states that a perfect crystal at 0 K has an entropy of 0 J/molK.
This section explores how we can use state variables for the system, only, to measure ΔSuniverse.
- Gibbs free energy, G, is defined as G≡H-TS
- At constant temperature and pressure, ΔG=-TΔSuniverse
- The standard Gibbs free energy change, ΔG°, is the Gibbs free energy change associated with going from 100% reactants at standard state to 100% products at standard state.
- ΔG° relates to the equilibrium constant, K, by ΔG°=-RTlnK.
- A van't Hoff plot is a plot of lnK vs 1/T for a reaction.
- A van't Hoff plot can be used to measure a reaction's ΔH° and ΔS°.
This section reviews acids, bases, and buffers, and their associated calculations.
- An Bronsted-Lowry acid is a chemical species that donates H+.
- An Bronsted-Lowry base is a chemical species that accepts H+.
- Strong acids and bases dissociate completely in water, while weak acids and bases only partially dissociate.
- The pH of a solution is a measure of the [H3O+], pH=-log[H3O+].
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- The pH of a strong acid or strong base aqueous solution is calculated by assuming the acid or base reacts completely with water.
- The pH of a weak acid or weak base in water can be estimated by assuming that its dissociation in water is minimal.
- Strong acids/bases react complete with weak bases/acids.
- The pH of a salt solution is determined by considering how each ion would react with water.
- A buffer, which consists of similar quantities of a weak acid and its conjugate base, resists change to pH upon addition of strong acid or base.
- The Henderson-Hasselbalch equation, pH=pKa + log(Cb/Ca), can be used to calculate the pH of a buffer solution.