Thermodynamics Overview: Spontaneous Changes & Equilibrium

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Questions and Answers

What is the mathematical expression for enthalpy?

H = E - PV

H = P + V + E

H = E + PV (correct)

H = PV + E

What does a positive value of ΔH indicate about a process?

The system is at thermal equilibrium.

The system is losing energy.

The process is exothermic.

The process absorbs heat. (correct)

What is a characteristic of a spontaneous change in a thermodynamic system?

It results in no change in enthalpy.

It occurs without any energy transfer.

It is accompanied by an increase in internal energy.

It is accompanied by a decrease in internal energy or enthalpy. (correct)

According to the Zeroth Law of Thermodynamics, what is true if two systems are in thermal equilibrium with a third system?

They are also in thermal equilibrium with each other. Signup and view all the answers

In the context of the First Law of Thermodynamics, what does ΔE equal?

q - W Signup and view all the answers

What defines a system in thermal equilibrium?

There is no net flow of thermal energy when connected. Signup and view all the answers

What occurs when ΔH is negative?

The process is exothermic. Signup and view all the answers

Which of the following statements is correct regarding thermodynamic equilibrium?

It must be in mechanical, chemical, and thermal equilibrium. Signup and view all the answers

During a reversible expansion of gas, what happens to external pressure?

It equals the internal pressure. Signup and view all the answers

What is the unit for measuring enthalpy change?

Kilojoules (kJ) or kilocalories (kcal) Signup and view all the answers

What does the First Law of Thermodynamics entail about energy in an isolated system?

The total energy remains constant, though it changes form. Signup and view all the answers

What does the change in enthalpy (ΔH) equal when the pressure is constant during a reaction?

ΔH = q Signup and view all the answers

If two objects are in thermal equilibrium, what can be said about their temperatures?

They have the same temperature. Signup and view all the answers

What is the correct expression for the change in enthalpy when considering changes in internal energy and pressure-volume work?

ΔH = (ΔE) + (ΔPV) Signup and view all the answers

What happens to macroscopic properties of a system in thermal equilibrium over time?

They remain constant. Signup and view all the answers

When separated by a diathermic wall, what indicates that two systems are in thermal equilibrium?

There is no net energy transfer. Signup and view all the answers

What happens to the volume of the gas when pressure on the piston, Pext, is increased by dP?

It decreases. Signup and view all the answers

What is the formula for heat capacity when mass is considered as 1 mol?

$C = \frac{(T_2 - T_1)}{1 ext{ mol}}$ Signup and view all the answers

What type of heat capacity is indicated by the change in internal energy at constant volume?

Molar heat capacity at constant volume Signup and view all the answers

Which of the following units is applicable for heat capacity?

Calories per degree kelvin per mole Signup and view all the answers

Which statement about heat and heat capacity is correct?

Neither heat nor heat capacity are state functions. Signup and view all the answers

What type of molar heat capacity is determined at constant pressure?

Molar heat capacity at constant pressure Signup and view all the answers

In the expression $dq = dE + p imes dV$, what does $dq$ represent?

The heat added to the system Signup and view all the answers

What is the mathematical relationship that defines heat capacity?

$c = \frac{Q}{(T_2 - T_1)}$ Signup and view all the answers

What is the expression for the work done by the gas during isothermal reversible expansion?

$W = - nRT ln\frac{V_2}{V_1}$ Signup and view all the answers

What is the relationship between pressure and volume in an ideal gas, as described by the ideal gas equation?

$P imes V = nRT$ Signup and view all the answers

If the external pressure Pext is decreased, what is the effect on the gas?

The gas expands reversibly. Signup and view all the answers

What is the mathematical form of work done by the gas in terms of pressure and volume change?

$dW = P \cdot dV$ Signup and view all the answers

As per the derivation, what does the natural logarithm of the ratio of volumes indicate in the work equation?

The volume change affects the work done. Signup and view all the answers

Which of the following expressions is used to relate pressures in the context of the ideal gas behavior?

$V_1 \cdot P_1 = V_2 \cdot P_2$ Signup and view all the answers

In the formula $W = - nRT ln\frac{P_2}{P_1}$, what does this expression represent?

Isothermal compression work of an ideal gas. Signup and view all the answers

In the work done formula for reversible processes, what does the negative sign indicate?

Work is done by the system. Signup and view all the answers

What do the terms $P_{ext}$ and $P_{gas}$ represent in the context of gas expansion?

They are the same and denote the pressure inside the container. Signup and view all the answers

What is the role of the area (A) and the distance (dL) in calculating work done during gas expansion?

Area affects the piston movement, and distance affects the change in volume. Signup and view all the answers

What is the value of entropy at absolute zero for a pure crystal?

S = 0 Signup and view all the answers

Which of the following describes standard entropy?

The absolute entropy at 25ºC and 1 atm pressure Signup and view all the answers

According to Hess's Law, what can be said about the total heat change in a chemical process?

It is the same regardless of the route taken Signup and view all the answers

What is the standard entropy change ($ riangle S^ heta$) calculated from?

Sum of standard entropies of products minus reactants Signup and view all the answers

The absolute entropy of substances is always:

Positive above 0 K Signup and view all the answers

What is the primary application of Hess's Law?

To derive enthalpies of chemical reactions Signup and view all the answers

What does a positive value of $ riangle S^ heta$ indicate about a reaction?

The reaction tends to be spontaneous Signup and view all the answers

For which of the following is Hess's Law particularly useful?

Determining lattice energies of salts Signup and view all the answers

Study Notes

Spontaneous Changes

A spontaneous change is accompanied by a decrease in internal energy or enthalpy (ΔH)

Zeroth Law of Thermodynamics

If two thermodynamic systems are separately in thermal equilibrium with a third object, they are in equilibrium with each other.
Objects in thermodynamic equilibrium have the same temperature.
A system in thermal equilibrium is a system whose macroscopic properties are not changing in time.
Macroscopic Properties: Macroscopic properties of matter are properties that can be visible to the naked eye, i.e. Pressure, Temperature, Density, Volume, etc.
Two systems are in thermal equilibrium when there is no net flow of thermal energy between them when they are connected by a path permeable to heat.
Two substances/systems are in thermal equilibrium if when separated by a diathermic wall, there is no net energy transfer between them.
A system is in thermodynamic equilibrium if there is no net change in its macroscopic properties with time.
A system is in thermodynamic equilibrium if it is in mechanical, chemical, and thermal equilibrium.
Systems in thermodynamic equilibrium are always in thermal equilibrium, but the converse is not always true.

First Law of Thermodynamics

The total energy of an isolated system remains constant though it may change from one form to another.
Mathematical Expression: H = E + PV
Change in Enthalpy: ΔH = H2 – H1; ΔH = (ΔE) + (ΔPV); ΔH = (ΔE) + (PΔV); ΔH = (ΔE) + W; ΔH = q when change in state occurs at a constant pressure ΔH = qp
Sign Conventions: ΔH is positive if H2>H1 and the process or reaction is endothermic. ΔH is negative if H2<H1 and the process or reaction is exothermic. ΔH = Hproducts – Hreactants = qp
Units: kilocalories (kcal) or kilojoules (kJ)

Reversible Expansion

The reversible expansion of the gas takes place in a finite number of infinitesimally small steps.
Pext = Pgas = P
Work done by the gas is expressed as: dW = P x A x dL; dW = P x dV
The total amount of work done by the isothermal reversible expansion of the ideal gas from V1 to V2 is: W = − ∫ 𝑃 · 𝑑𝑉
W = − 𝑛𝑅𝑇 ln 𝑉2/𝑉1
W = − 𝑛𝑅𝑇 ln 𝑃1/𝑃2
Isothermal compression work of an ideal gas is exactly the same value with the sign changed.

Concept of Heat Capacity

Definition: Heat absorbed by unit mass in raising the temperature by one degree (K or ºC) at a specified temperature.
Mathematical Expression: c = Q / [m × (T2 − T1)]
C = Q / (T2 − T1) = ΔQ / ΔT
C = dQ / dT
Unit: cal K-1 mol-1 or J K-1 mol-1
Heat is not a state function, neither is heat capacity.

Molar Heat Capacity at Constant Volume

dQ = dE + 𝑝 · dV
CV = dQ / dT = dE / dT
Definition: Rate of change of internal energy with temperature at constant volume.
Units: cal K-1 mol-1 or J K-1 mol-1 or eu (entropy units)
A Process accompanied by an increase in entropy tends to be spontaneous.

Third Law of Thermodynamics & Absolute Entropy

Statement: At absolute zero, the entropy of a pure crystal is also zero.
S = 0 at T = 0 K.
Absolute Entropy is the actual amount of entropy that a substance possesses at any temperature above zero K.
Standard Entropy is the absolute entropy of a substance at 25ºC (298 K) and one atmosphere pressure.
Absolute entropy of elements is zero only at 0 K in a perfect crystal.
Standard entropies of all substances at any temperature above 0 K always have positive values.
The standard entropy change ΔSº for chemical reactions can be calculated by: ΔSº = ΣSº(products) – ΣSº(reactants)

Hess’s Law

Statement: If a chemical change can be made to take place in two or more ways whether in one step or two or more steps, the amount of total heat change is the same no matter by which method the change is brought about.
Hess' Law of Constant Heat Summation is useful in the determination of:
- Heats of formation of unstable intermediates like CO(g) and NO(g).
- Heat changes in phase transitions and allotropic transitions.
- Lattice energies of ionic substances by constructing Born-Haber cycles if the electron affinity to form the anion is known.

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Description

Explore the fundamental concepts of thermodynamics focusing on spontaneous changes and the Zeroth Law of Thermodynamics. Understand how thermal equilibrium is defined and the importance of macroscopic properties in thermal systems. This quiz covers key principles essential for mastering thermodynamics.