Week 25 / Thermodynamics 2 Flashcards
Q: What does Hess’s Law state?
A: Hess’s Law states that the standard enthalpy of a reaction is the sum of the standard enthalpies of the reactions into which the overall reaction can be divided.
Q: What is the implication of Hess’s Law?
[what does it depend on]
A: Hess’s Law implies that enthalpy is a state function, meaning the total change in enthalpy depends only on the initial and final states, not on the path taken.
Q: How can Hess’s Law be applied?
A: Hess’s Law can be applied by breaking down a complex reaction into simpler steps and then summing the standard enthalpies of those individual reactions to find the overall enthalpy change.
Q: What does Hess’s Law allow us to calculate?
A: Hess’s Law allows us to calculate the standard enthalpy change of a reaction even if it cannot be measured directly, by using known enthalpy changes of related reactions.
Q: How did Hess’s Law relate to the catalase reaction discussed earlier?
A: The technique used in the catalase reaction can be expanded by applying Hess’s Law, breaking it down into steps and summing the enthalpies to calculate the total enthalpy change.
What does the standard reaction enthalpy (Δ𝐻 ) represent?
The standard reaction enthalpy is the difference between the standard molar enthalpies of the reactants and products, each weighted by the reaction stoichiometry.
Q: What is the formula for calculating the standard reaction enthalpy?
products - reactants
Q: What are the standard states of reactants and products in a reaction?
Reactants and products are in their standard states, which means they are pure and at 1 bar of pressure.
How do we work indirectly to find the standard reaction enthalpy?
We reference each reactant and product to an imaginary reaction where one mole of it is formed from the constituent elements in their most stable states.
Why is the standard reaction enthalpy calculated indirectly?
It is calculated indirectly because each reactant and product is typically referenced to a reaction where it is formed from its constituent elements in the most stable states.
What is a spontaneous process?
A spontaneous process is one that occurs naturally without the need for external work, such as gases expanding to fill empty spaces or hot objects cooling to the same temperature as their surroundings.
Can spontaneous processes be reversed?
Yes, to reverse a spontaneous process, work must be done upon the system of interest.
What is an example of a spontaneous process?
Examples of spontaneous processes include:
Gases expanding to fill empty spaces
Hydrogen and oxygen combining to produce water
Hot objects cooling to match the temperature of their surroundings
Does spontaneity consider the rate of a process?
No, spontaneity considers whether a process can happen but does not consider how quickly it will happen.
What is the significance of spontaneity in thermodynamics?
Spontaneity in thermodynamics helps determine whether a process can occur naturally, but it doesn’t give any information about the speed of the process.
Q: Do spontaneous processes always move in the direction of lower energy?
A: No, spontaneous processes do not necessarily move in the direction of lower energy.
Q: What happens in an isothermal expansion of a perfect gas into a vacuum?
A: In an isothermal expansion of a perfect gas into a vacuum, there is no overall change in energy. The molecules move at the same speed, but the distance between them changes.
Q: Is energy destroyed in a cooling process?
A: No, in a cooling process, the energy lost by the system is transferred to the surroundings, it is not destroyed.
Q: What happens when two metal blocks, one hot and one cold, are placed in contact in a vacuum?
A: Eventually, the two metal blocks will achieve thermal equilibrium, with heat flowing from the hot block to the cold block until both are at the same temperature.
Q: Does spontaneity involve energy loss or transfer?
A: In spontaneous processes, energy is often transferred (e.g., from a hot body to a cold body), but it is not necessarily lost or destroyed.
Q: What is the tendency of energy and matter in spontaneous processes?
A: Energy and matter have a tendency to disperse in spontaneous processes.
Q: Why is it unlikely for gas molecules to move into one corner of a container?
A: The probability of gas molecules moving into one corner of a container is negligible because they move randomly in all directions.
Q: What happens when hot atoms oscillate in a material?
A: When hot atoms oscillate, they collide with neighboring atoms, transferring energy through these collisions.
Q: How does the dispersion of energy relate to spontaneity?
A: The dispersion of energy, such as the random movement of gas molecules or energy transfer between atoms, is a key characteristic of spontaneous processes.
Q: What is the behavior of gas molecules in a container?
A: Gas molecules move randomly, and their movement results in a very low probability of all molecules accumulating in one corner of the container.
Q: What does the Second Law of Thermodynamics state?
A: The Second Law of Thermodynamics states that the entropy of an isolated system tends to increase.
Q: What is entropy a measure of?
A: Entropy is a measure of how dispersed energy and matter are within a system.
Q: What is an isolated system in thermodynamics?
A: An isolated system refers to the reaction vessel and its surroundings, essentially a “mini-universe,” where no energy or matter is exchanged with the surroundings.
Q: How do we typically deal with entropy in thermodynamics?
A: We deal with changes in entropy (ΔS) rather than absolute values, where ΔS = q_rev / T, with q_rev being the heat transferred reversibly and T being the temperature.
Q: What is the significance of entropy as a state function?
A: Entropy is a state function, meaning its value depends only on the current state of the system, not on the path taken to reach that state.
Q: What is entropy in thermodynamics?
A: Entropy is the measure of the tendency of energy and matter to spread out or disperse in a system.
Q: What is the equation for entropy change accompanying heating?
A: The equation for entropy change accompanying heating is ΔS = C ln(Tf/Ti), where C is the heat capacity, Ti is the initial temperature, and Tf is the final temperature.
Q: What happens when Tf > Ti in terms of entropy change?
A: When Tf > Ti, the logarithm is positive, meaning that ΔS > 0, which aligns with the expected increase in entropy as temperature increases.
Q: How does heat capacity affect entropy change?
A: Entropy changes are higher for materials with higher heat capacities because more energy is required to cause a temperature change in these materials.
Q: Are heat capacities constant for all materials?
A: No, for most materials, heat capacities are not constant with respect to temperature, especially for solids at very low temperatures, requiring more complex equations.
Q: What is the relationship between heat passing to/from the surroundings and the heat leaving/entering the system?
A: The amount of heat passing to/from the surroundings is equal and opposite to the heat leaving/entering the system: q_sur = -q.
Q: How is entropy change of the surroundings (ΔS_sur) related to heat transfer?
A: The entropy change of the surroundings is given by ΔS_sur = -q/T, where q is the heat transferred to/from the system, and T is the temperature.
Q: How does the entropy change of the surroundings relate to the system’s energy change?
A: The entropy change of the surroundings can be directly related to the energy change of the system, regardless of whether the process is reversible or not.
Q: What is the relationship between heat transfer at constant pressure and enthalpy?
A: At constant pressure, the heat transferred (q) is equal to the enthalpy change (ΔH), so we can say ΔS_sur = -ΔH/T.
Q: What does the equation ΔS_sur = -ΔH/T indicate?
A: The equation indicates that the change in entropy of the surroundings is tied to the enthalpy change of the system at constant pressure.
Q: What does the Third Law of Thermodynamics state about the absolute entropies of perfectly crystalline substances at absolute zero?
A: The Third Law of Thermodynamics states that the absolute entropies of all perfectly crystalline substances are zero at absolute zero: S° = 0.
Q: What happens to the atoms of a substance at absolute zero?
A: At absolute zero, all atoms would be stationary, at least in a simplistic view.
Q: Why are perfectly crystalline substances considered to be the most ordered possible?
A: Perfectly crystalline substances have a highly ordered arrangement of atoms, with minimal disorder, making them the most ordered state possible.
Q: How can we obtain the standard molar entropy of a substance?
A: We can follow the changes in heat capacity of a given substance with temperature and use this data to obtain its standard molar entropy (S°).
Q: What is the standard reaction entropy (ΔS°)?
A: The standard reaction entropy (ΔS°) is the difference in molar entropy between the products and reactants of a reaction in their standard states: ΔS° = ΣvS°(products) - ΣvS°(reactants).
Q: What does “v” represent in the standard reaction entropy equation?
A: “v” represents the stoichiometric coefficients in the chemical equation.
Q: How is the standard reaction entropy calculated?
A: The standard reaction entropy is calculated by summing the standard molar entropy of the products (weighted by their stoichiometric coefficients) and subtracting the sum of the standard molar entropy of the reactants (also weighted by their stoichiometric coefficients).
Q: What does the 2nd Law of Thermodynamics state?
A: The 2nd Law states that the entropy of an isolated system tends to increase.
Q: What does the 3rd Law of Thermodynamics state about the absolute entropies of perfectly crystalline substances at absolute zero?
A: The 3rd Law states that the absolute entropies of all perfectly crystalline substances are zero at absolute zero.
Q: What determines whether a reaction takes place spontaneously?
A: Reactions take place spontaneously if they allow the overall entropy of the universe to increase.
Q: When do processes occur spontaneously in terms of entropy?
A: Processes occur spontaneously if the overall entropy of the universe is increased.
Q: What do we need to know to determine spontaneity in terms of entropy?
A: To determine spontaneity, we need to know the entropy changes for both the system and the surroundings.
OPEN POWERPOINT FOR GIBBS EQUATIONS
OPEN POWERPOINT FOR GIBBS EQUATIONS
