Chapter 7: Thermochemistry Flashcards
How are systems classified, regarding thermochemistry.
Systems are classified based on what is or is not exchanged with the surroundings: systems may be isolated, closed, or open, based on the exchange or lack of exchange of energy and matter between the system and its environment.
What is an isolated system? What is a closed system? What is an open system?
Isolated systems exchange neither matter nor energy with the environment such as in a bomb calorimeter.
Closed systems can exchange energy but not matter with the environment such as in a steam radiator.
Open systems can exchange both energy and matter with the environment such as in a pot of boiling water.
What is an isothermal process? Adiabatic process? Isobaric process? Isovolumetric (isochoric) process?
An isothermic process occurs at contact temperature (such as a phase change occurring at a FIXED TEMPERATURE: evaporation of water at a constant temperature) While the temperature stays constant, heat can still be transferred into or out of the system to maintain that constant temperature.
An adiabatic process exchanges no heat with the environment (such as free expansion of a gas into a vacuum as the gas expands against no pressure)
An isobaric process occur at constant pressure (such as water boiling in an open container into constant atmospheric pressure)
An isovolumetric (isochoric) processes occur at constant volume (such as heating a gas in a closed container)
What is a state function?
State functions describe the physical properties of an equilibrium state. They are pathway independent and include P, density, T, V, enthalpy, internal energy, Gibbs free energy, and entropy.
State functions quantitatively describe an equilibrium state of a thermodynamic system, regardless of how the system has arrived in that state.
A state function is a property of a system that only depends on its current state, meaning its value is solely determined by the system’s conditions at a given moment
What are standard conditions? How does this relate to standard state?
Standard conditions are defined at 298 K (approx 76 f and 25 C), 1 atm, and 1 M concentrations.
The standard state of an element is its most prevalent form under standard conditions. Standard enthalpy, standard entropy, and standard free energy are all calculated under standard conditions.
Describe the phase changes at the boundaries between the phases.
Fusion (melting) and freezing (crystallization or solidification) occur at the boundary between solid and liquid phases.
Vaporization (evaporation or boiling) and condensation occur at the boundary between liquid and gas phases.
Sublimation and deposition occur at the boundary between solid and gas phases.
What is a phase diagram?
A phase diagram for a system graphs the phases and phase equilibria as a function of T and P.
Are heat and temperature the same thing? Explain.
Heat and temperature are not the same thing. Temperature is a scaled measure of the average kinetic energy of a substance. Heat is the transfer of energy that results from the differences of temperature between two substances.
The heat content of a system undergoing heating, cooling, or phase changes is the sum of all the respective energy changes.
What is enthalpy?
Enthalpy is a measure of the potential energy of a system found in the intermolecular attractions and chemical bonds.
Enthalpy can also be calculate using heats of formation, heats of combustion, or bond dissociation energies.
H=E+PV
H is enthalpy
E is internal energy
P is pressure
V is volume
What is Hess’s law?
Hess’s law, also known as the law of constant heat summation, 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.
Hess’s law states that the total enthalpy change for a chemical reaction is independent of the reaction’s path or the number of steps involved. It’s an early expression of the law of conservation of energy, stating that the heat released depends on the starting and ending states, not the process itself. Key points of Hess’s law: Enthalpy change is proportional to the amount of reactants and products.
What is entropy?
Entropy is often thought of as disorder. Entropy is a measure of the degree to which energy has been spread throughout a system or between a system and its surroundings.
Entropy is a ratio of heat transferred per mole per unit kelvin.
Entropy is MAXIMIZED at equilibrium. Entropy is maximized at equilibrium because, according to the Second Law of Thermodynamics, in an isolated system, the natural tendency is for disorder (entropy) to increase until it reaches a maximum value, and once at this maximum, the system is considered to be in equilibrium, meaning there will be no further spontaneous changes as any change would result in a decrease in entropy, which is not allowed by the Second Law.
How is Gibbs free energy derived?
Gibbs free energy is derived from both the enthalpy and entropy values for a given system.
What does the change in Gibbs free energy determine?
The change in Gibbs free energy determines whether a process is spontaneous or nonspontaneous.
Gibbs free energy depends on temperature. Temperature dependent processes change between spontaneous and nonspontaneous, depending on the temperature. A simple example being melting of ice: ice melts spontaneously above 0°C (deltaG<0), it does not melt spontaneously below 0°C (deltaG>0)
What is the first law of thermodynamics? What is it telling us?
The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another; essentially, it is the principle of conservation of energy, meaning the total amount of energy in a closed system remains constant.
What is an isothermal process? What does this imply regarding the first law of thermodynamics? Talk about the PV graph of an isothermal process. Provide an example of an isothermal process.
An isothermal process occurs when the systems temperature is constant. (First law of thermodynamics is deltaU=Q-W) Constant temperature implies that the total internal energy of the system (U) is constant throughout the process because T and U are directly proportional. When U is constant, deltaU=0 and therefor Q=W (the heat added to the system equals the work done by the system).
Any phase change occurring at a fixed temperature is an example of an isothermic process. Melting of ice at 0°C, boiling of water at 100°C. To maintain a constant temperature, heat must flow into or out of the system as needed.
What is an adiabatic process? Describe it in terms of the first law of thermodynamics. Describe the PV graph. Give an example of an adiabatic process.
An adiabatic process occurs when no heat is exchanged between the system and the environment. (First law of thermodynamics is deltaU=Q-W) When Q=0, then deltaU= -W (the change in internal energy of the system is equal to work done ON the system (opposite of work done BY the system)). Adiabatic processes appears hyperbolic on a PV graph.
An example of an adiabatic process is compressing air into a tire. As the tire fills up the air inside it rapidly compresses, which causes a rise in temperature. This rise in temperature occurs without any heat being transferred into or out of the system, thereby making it an example of an adiabatic process.
What is an isobaric process? Describe in terms of the first law equation. Describe the PV graph. Provide example of an isobaric process.
An isobaric process occurs when the pressure of a system is constant. (First law of thermodynamics is deltaU=Q-W) Isobaric processes are common because it is usually easy to control temperature and pressure. Isobaric processes do not alter the first law. The PV graph appears as a flat line for isobaric processes.
An example of a isobaric process would be heating a gas in a moving piston where the pressure is allowed to change the volume; or boiling a pot of water in an open container there the volume of water increases and pressure maintains constant (atm).
