Thermodynamics Fundamentals

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Questions and Answers

A scientist is studying a chemical reaction in a sealed, insulated container. Can the system exchange energy or matter with its surroundings? What type of system is this?

No, it cannot exchange energy or matter. This is an isolated system.

A researcher measures the temperature and pressure of a gas in a closed container. Are these properties considered extensive or intensive? Briefly explain why.

These are intensive properties because they do not depend on the amount of gas present in the container.

Consider two beakers, one containing 50 mL of water and the other containing 100 mL of water, both at the same temperature. Which property, volume or temperature, is an extensive property in this scenario? Explain.

Volume is an extensive property because it changes with the amount of water. The beaker with 100 mL of water has a larger volume.

A student is conducting an experiment where heat is released during a chemical reaction in a calorimeter. Is heat a state function or a path function? Explain your answer.

Heat is a path function because the amount of heat exchanged depends on the specific process or path taken during the reaction. Signup and view all the answers

Imagine a scenario where you have a balloon that can expand or contract. If you heat the balloon, causing it to expand, which macroscopic properties are most likely to change? Name two.

Volume and potentially temperature. The volume will increase as the balloon expands, and the temperature may increase depending on the amount of heat. Signup and view all the answers

A chemist investigates a reaction in a system that allows the transfer of energy but not matter. What type of system is the chemist working with, and what is an example of such a system?

This is a closed system. An example would be a sealed container with a fixed amount of gas that can be heated or cooled. Signup and view all the answers

Two different pathways are used to convert reactant A to product B. If the change in internal energy is measured to be the same for both pathways, what does this indicate about internal energy?

This indicates that internal energy is a state function, as the change in its value is independent of the path taken. Signup and view all the answers

How does increasing the amount of a substance in a system affect its extensive properties, and why is this distinction important in thermodynamic studies?

Increasing the amount of a substance increases its extensive properties because these properties depend on the amount of matter. This distinction is important because it helps in scaling up or down processes and understanding the effects of quantity on system behavior. Signup and view all the answers

Under what specific condition is the change in internal energy ($\Delta U$) of a system equal to the heat absorbed or released at constant volume ($q_v$)?

When the volume is constant, meaning (\Delta V = 0). Signup and view all the answers

How does the first law of thermodynamics relate the change in internal energy ($\Delta U$) to heat (q) and work (w)?

The first law of thermodynamics states that (\Delta U = q + w), where q is the heat absorbed by the system and w is the work done on the system. Signup and view all the answers

For an isothermal reversible process, what is the relationship between heat (q) and work (w) in terms of volume change?

For an isothermal reversible process, (q = -w = 2.303nRT \log \frac{V_2}{V_1}), where (V_2) and (V_1) are the final and initial volumes, respectively. Signup and view all the answers

What is the change in internal energy for a free expansion process and why?

The change in internal energy is zero because no work is done since the external pressure is zero. Signup and view all the answers

Differentiate between internal energy and enthalpy.

Internal energy (U) comprises all energies within a system, while enthalpy (H) sums internal energy and pressure-volume energy, i.e., (H = U + PV). Signup and view all the answers

In an adiabatic process, what condition applies to the heat transfer (q), and how does this simplify the first law of thermodynamics?

In an adiabatic process, (q = 0). Therefore, the first law simplifies to (\Delta U = w), meaning the change in internal energy is solely due to work done. Signup and view all the answers

What happens to the internal energy of system containing only solid/liquids? Explain why.

The volume is constant, (\Delta V = 0 ). Therefore, (\Delta U = q). Signup and view all the answers

What is free expansion? Is work done during free expansion?

Free expansion is the expansion of gas into vacuum where external pressure is 0. No work is done. Signup and view all the answers

Explain how Hess's Law allows for the calculation of enthalpy changes for reactions that are difficult or impossible to measure directly.

Hess's Law states that the total enthalpy change for a reaction is the same whether it occurs in one step or multiple steps. This allows us to calculate the enthalpy change of a reaction by summing the enthalpy changes of a series of reactions that add up to the overall reaction. Signup and view all the answers

How does the enthalpy of fusion relate to the intermolecular forces within a solid?

The enthalpy of fusion is the energy required to overcome the intermolecular forces holding the solid together, allowing it to transition to the liquid phase. Stronger intermolecular forces result in a higher enthalpy of fusion. Signup and view all the answers

Why is the standard enthalpy of combustion always a negative value?

The standard enthalpy of combustion is always negative because combustion reactions are exothermic, meaning they release heat. The release of heat corresponds to a decrease in the enthalpy of the system. Signup and view all the answers

What is a thermochemical equation, and what information does it convey beyond a standard chemical equation?

A thermochemical equation is a balanced chemical equation that includes the enthalpy change ($\Delta rH0$) for the reaction. It specifies the physical states of all reactants and products and indicates whether heat is absorbed or released. Signup and view all the answers

Describe how the enthalpy of vaporization differs from the enthalpy of sublimation at a molecular level.

Enthalpy of vaporization involves a phase change from liquid to gas, requiring energy to overcome intermolecular forces in the liquid. Enthalpy of sublimation involves a direct phase change from solid to gas, overcoming stronger intermolecular forces in the solid state. Signup and view all the answers

Explain why the physical states of reactants and products are important to specify in a thermochemical equation.

The enthalpy change of a reaction is dependent on the physical states of the reactants and products (solid, liquid, gas) because the energy required to change phases (e.g., vaporization, fusion) contributes to the overall enthalpy change. Different physical states have different internal energies. Signup and view all the answers

If a reaction occurs in two steps where $\Delta H_1$ is +50 kJ/mol and $\Delta H_2$ is -30 kJ/mol, what is the overall enthalpy change for the reaction, and is the reaction endothermic or exothermic?

The overall enthalpy change is +20 kJ/mol (+50 kJ/mol - 30 kJ/mol). Since the overall $\Delta H$ is positive, the reaction is endothermic, meaning it absorbs heat from the surroundings. Signup and view all the answers

How can you determine the enthalpy change for the reverse of a given reaction if you know the enthalpy change for the forward reaction?

The enthalpy change for the reverse reaction is equal in magnitude but opposite in sign to the enthalpy change for the forward reaction. If the forward reaction has an enthalpy change of $\Delta H$, the reverse reaction will have an enthalpy change of -$\Delta H$. Signup and view all the answers

Explain why a decrease in energy is not the sole criterion for spontaneity, providing an example to support your explanation.

A decrease in energy cannot be the only criteria because some spontaneous processes, such as the melting of ice, involve the absorption of heat, which increases the energy of the system. These processes occur due to an increase in entropy that overcomes the energy increase. Signup and view all the answers

How does the second law of thermodynamics define the spontaneity of a process in terms of entropy?

The second law of thermodynamics states that for a spontaneous process, the total entropy of the universe (system plus surroundings) must increase. Signup and view all the answers

Describe the relationship between entropy change and equilibrium in a system.

At equilibrium, the entropy of the system is at its maximum, and the total entropy change (S Total) for the system and surroundings is zero. Signup and view all the answers

Differentiate between a spontaneous and a non-spontaneous process, giving an example of each.

A spontaneous process occurs without external intervention (e.g., the rusting of iron), while a non-spontaneous process requires continuous external energy input to proceed (e.g., electrolysis of water). Signup and view all the answers

A system absorbs 500 J of heat reversibly at a temperature of 300 K. Calculate the change in entropy (S) for this process.

$\Delta S = \frac{q_{rev}}{T} = \frac{500 \text{ J}}{300 \text{ K}} = 1.67 \text{ J/K}$ Signup and view all the answers

Explain how entropy generally changes during phase transitions (solid to liquid, liquid to gas) and why.

Entropy increases during phase transitions from solid to liquid and liquid to gas because molecules gain more freedom of movement and occupy a larger volume, increasing the disorder or randomness of the system. Signup and view all the answers

Is the mixing of gases considered a spontaneous process? Briefly justify your answer in terms of entropy change.

Yes, the mixing of gases is a spontaneous process because it results in an increase in entropy as different gas molecules occupy a greater volume and become more disordered. Signup and view all the answers

For the reaction $N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$ with $_rH^0 = -46.1 \text{ kJ/mol}$, is the reaction spontaneous at all temperatures? Explain your reasoning.

It cannot be determined if the reaction is spontaneous at all temperatures based on only enthalpy. To determine the spontaneity, you would need to know the entropy change. If entropy is positive, it is spontaneous at high temperatures. If entropy is negative, it may be spontaneous at low temperatures. If the entropy change is zero, then temperature does not affect spontaneity. Signup and view all the answers

Explain why $C_p$ is always greater than $C_v$ for an ideal gas. Relate your answer to the molecular level.

At constant volume, all heat added goes into increasing the internal energy (kinetic energy) of the molecules, thus raising the temperature. At constant pressure, some of the added heat is used to do work against the surroundings as the gas expands, so less heat is available to raise the temperature. Signup and view all the answers

A certain reaction has a positive $\Delta rH$. Explain what this indicates about the energy required for the reaction to occur and the stability of the products relative to the reactants.

A positive $\Delta rH$ indicates that the reaction is endothermic, meaning it requires energy input to proceed. The products have higher enthalpy (energy) than the reactants, implying that the reactants are more stable than the products. Signup and view all the answers

Consider a reaction where ΔH is negative. How does this information help you predict the effect of decreasing the temperature on the equilibrium constant (K) of the reaction?

A negative ΔH means the reaction is exothermic. Decreasing the temperature will shift the equilibrium towards the products, increasing the equilibrium constant K. This is because the system will try to counteract the decrease in temperature by favoring the heat-releasing (exothermic) direction. Signup and view all the answers

Explain how the third law of thermodynamics allows for the determination of absolute entropy values.

The third law states that the entropy of a perfectly crystalline substance is zero at absolute zero. This provides a reference point, allowing for the calculation of absolute entropy by measuring entropy changes relative to this zero point using thermodynamical data. Signup and view all the answers

Suppose the standard enthalpy of formation ($\Delta_fH^0$) for a particular element in a non-standard state is non-zero. Explain what this implies about the stability of the element in that non-standard state compared to its reference state.

If the standard enthalpy of formation ($\Delta_fH^0$) of an element in a non-standard state is non-zero, it implies that the element in its reference state is more stable than in the non-standard state. A non-zero value suggests that energy is either required (positive) or released (negative) when converting from the reference state to the non-standard state. Signup and view all the answers

Define Gibbs energy (G) and explain its significance in determining the spontaneity of a reaction.

Gibbs energy (G) represents the maximum amount of energy available from a system to do useful work at constant temperature and pressure. It is defined as G = H - TS. A negative change in Gibbs energy (∆G < 0) indicates a spontaneous process, while a positive change (∆G > 0) indicates a non-spontaneous process. Signup and view all the answers

Derive the relationship between the change in Gibbs energy (∆G), change in enthalpy (∆H), and change in entropy (∆S) at constant temperature.

Starting with G = H - TS, at constant temperature, the change in Gibbs energy is derived as ∆G = ∆H - T∆S, where ∆H is the change in enthalpy, T is the temperature, and ∆S is the change in entropy. Signup and view all the answers

For the reaction: $N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$, what information is needed to calculate the standard reaction enthalpy using standard enthalpies of formation?

To calculate the standard reaction enthalpy, we need the standard enthalpies of formation ($\Delta_fH^0$) of all reactants and products. Specifically, we need the $\Delta_fH^0$ for $N_2(g)$, $H_2(g)$, and $NH_3(g)$. Signup and view all the answers

A bomb calorimeter measures heat at constant volume. How is the heat measured in a bomb calorimeter related to the change in internal energy ($\Delta U$) of the system?

In a bomb calorimeter, the heat measured at constant volume ($q_v$) is equal to the change in internal energy ($\Delta U$) of the system. This is because, at constant volume, no work is done ($w = 0$), so all the heat goes into changing the internal energy. Signup and view all the answers

Describe the conditions under which a reaction is always spontaneous in terms of ∆H and ∆S.

A reaction is always spontaneous when the change in enthalpy (∆H) is negative and the change in entropy (∆S) is positive. This ensures that ∆G is always negative, regardless of temperature. Signup and view all the answers

Explain how temperature influences the spontaneity of a reaction when both ∆H and ∆S are positive.

When both ∆H and ∆S are positive, the spontaneity of a reaction depends on temperature. The reaction will be spontaneous at high temperatures, where the T∆S term is greater than ∆H, resulting in a negative ∆G. Signup and view all the answers

Consider two reactions: (1) $A \rightarrow B$ with $\Delta rH = -50 kJ/mol$ and (2) $X \rightarrow Y$ with $\Delta rH = +25 kJ/mol$. Which reaction is more favorable thermodynamically at standard conditions, and why?

Reaction (1) $A \rightarrow B$ is more favorable thermodynamically. A negative $\Delta rH$ indicates that the reaction is exothermic, meaning it releases heat and leads to a lower energy state for the products compared to the reactants, making it more spontaneous than the endothermic Reaction (2). Signup and view all the answers

Explain the significance of the reference state in the context of standard enthalpy of formation. Why is it important to define a reference state.

The reference state provides a common baseline for comparing the relative stability of different compounds. Defining a reference state (the most stable form of an element at standard conditions) allows for a consistent assignment of zero enthalpy to elements in their standard states. This makes it possible to calculate and compare the enthalpy changes of various reactions. Signup and view all the answers

For a reaction at equilibrium, what is the value of ∆G and how does it relate to the total entropy change of the system and surroundings?

At equilibrium, the change in Gibbs energy (∆G) is zero. This corresponds to a state where the total entropy change (∆S_total) of the system and surroundings is also zero, indicating no net tendency for the reaction to proceed in either direction. Signup and view all the answers

Contrast the behavior of spontaneous vs. non-spontaneous processes in terms of both $\Delta G_{syst}$ and $\Delta S_{total}$.

For a spontaneous process, $\Delta G_{syst}$ is negative and $\Delta S_{total}$ is positive. Conversely, for a non-spontaneous process, $\Delta G_{syst}$ is positve and $\Delta S_{total}$ is negative. Signup and view all the answers

Flashcards

Thermodynamics

Deals with the relationship between heat and work.

System (Thermodynamics)

The part of the universe under observation.