Ch 7 - Thermochemistry Flashcards
How are systems classified?
based on what is or is not exchanged with the surroundings
What is the difference between an isolated, closed, and open system?
- isolate: exchange neither matter nor energy with the environment
- closed: can exchange energy but not matter with the environment
- open: can exchange both energy and matter with the environment
How are processes categorized?
based on a single constant property
What is the difference between isothermal, adiabatic, isobaric, and isovolumetric processes?
- isothermal: occur at constant temperature (U = 0, Q = W)
- adiabatic: exchange no heat with the environment (Q = 0, U = -W)
- isobaric: occur at constant pressure (flat line in PV graph)
- isovolumetric (isochoric) occur at constant volume (W = 0, U = Q)
What are state functions?
- describe the physical properties of an equilibrium state
TV HUGS - they are pathway independent and include pressure, density, temperature, volume, enthalpy, internal energy, Gibbs free energy, and entropy
What are standard conditions?
defined as 298 K, 1 atm, and 1 M concentration
What is the standard state and what functions are calculate in this state?
- most prevalent form under standard conditions
- standard enthalpy, entropy, and free energy
Where do phase changes exist?
at characteristic temperatures and pressures
When do fusion and freezing occur?
fusion (meltion) and freeing (crystallization and solidification) occur at the boundary between the solid and liquid phases
When to vaporization and condensation occur?
vaporization (evaporation or boiling) and condensation occur at the boundary between liquid and gas phase
When do sublimation and deposition occur?
at the boundary between solid and gas phase
What happens at temperatures above the critical point?
the liquid and gas phase are indistinguishable and the heat of vaporization is 0
What happens at the triple point?
all 3 phases of matter exist in equilibrium
What does the phase diagram graph?
the phases and phase equilibrium as a function of temperature and pressure
What is the difference between heat and temperature?
- temperature: scaled measure of the average kinetic energy of a substance
- heat: transfer of energy that results from differences of temperature between 2 substances
How can you calculate the heat content of a system undergoing heating, cooling, or phase changes?
sum of all the respective energy changes
What is enthalpy?
a measure of the potential energy of a system found in intermolecular attractions and chemical bonds
Hrxn = Hproducts - Hreactants
What is Hess’s law?
states that the total change in potential energy of a system is equal to the changes of potential energies of the individual steps of the process
How is enthalpy calculated?
using heats of formation, heats of combustion, or bond dissociation energies
What is entropy and how is it calculated?
a measure of the degree to which energy has been spread throughout the system or between a system and its surroundings
change in S = Qrev/T
When is entropy maximized?
at equilibrium
What is entropy a ratio of?
heat transferred per mole per unit kelvin
What is Gibbs free energy derived from?
both enthalpy and entropy values for a given system
change in G = chang in H -TchangeS
What does the change in Gibbs free energy determines?
whether a process is spontaneous or nonspontaneous
- < 0: rxn proceeds forward (spontaneous)
- = 0: rxn in dynamic equilibrium
- > 0: rxn processed in reverse (nonspontaneous)
What does Gibbs free energy depend on?
temperature,
- temperature-dependent processes change between spontaneous and nonspontaneous, depending on temperature
What is the difference between calculating heat transfer when there is or is not a phase change?
- no phase change: q = mcAT
- during phase change: q = mL
What is the first law of thermodynamics equation?
change in U = Q - W
How is standard enthalpy calculated?
Hrxn0 = sum of Hf,products - sum of Hf,reactants
How is bond enthalpy calcualted?
Hrx0 = sum of Hbonds broken - sum of H bonds formed
= total energy absorbed - total energy released
What is the second law of thermodynamics equation?
change Suniverse = change Ssystem + change S surroundings > 0
How is standard Gibbs free energy from equilibrium constant calculated?
change in Grxn0 = -RTlnKeq
How is Gibbs free energy from reaction quotient calcualted?
change in Grxn = change in Grxn0 + RTlnQ = RTln(Q/Keq)
What are process functinons?
path between equilibrium states
- Q (heat), W (work)
What is the difference between specific heat and heat capacity?
- specific heat (c): energy required to raise the temperature of one gram of a substance by 1 C
- heat capacity (mc): product of mass and specific heat and is the energy required to raise any given amount of substance by 1 C
What is the difference between constant-volume and constant-pressure calorimetry?
- constant pressure (coffee cup) is exposed to constant (atmospheric) pressure; as the rxn proceeds, the temperature of the contents is measure to determine the heat of the rxn
- constant volume (bomb): heat of certain rxns (like combustion) can be measured indirectly by assessing temperature change in a water bath around the reaction vessel
What is the difference between endothermic and exothermic process?
- endo: involve an increase in heat content of a system from the surroundings (change in H > 0)
- exo: a release of heat content from a system (change in H < 0)
Which combustion reactions will have the most exothermic standard heat of combustion?
- combustion often involves the reaction of a hydrocarbon with oxygen to produce CO2 and H2O
- longer hydrocarbon chains yield greater amounts of combustion products and release more heat in the process
What relationship is required between Q and Keq in order for a reaction to be spontaneous?
Keq > Q
