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Thermodynamics and Chemical Equilibrium
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Thermodynamics and Chemical Equilibrium
Thermodynamics and Chemical Equilibrium
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1
Question
How do thermodynamic conditions characterize a chemical reaction at dynamic equilibrium?
Answer
Entropy reaches its maximum while Gibbs free energy reaches its minimum. Forward and reverse reaction rates are equal, resulting in constant macroscopic concentrations.
2
Question
Why do stoichiometric coefficients serve as exponents in equilibrium expressions but not automatically in kinetic rate laws?
Answer
Equilibrium constant expressions reflect the overall balanced thermodynamic state (law of mass action). Kinetic rate laws depend strictly on the elementary mechanism and rate-determining step, which must be determined experimentally.
3
Question
How does comparing the reaction quotient \(Q\) to the equilibrium constant \(K_{\text{eq}}\) determine reaction spontaneity?
Answer
When \(Q < K_{\text{eq}}\), \(\Delta G < 0\) and the reaction proceeds forward. When \(Q = K_{\text{eq}}\), \(\Delta G = 0\) at equilibrium. When \(Q > K_{\text{eq}}\), \(\Delta G > 0\) and the reverse reaction is favored.
4
Question
Why are pure liquids and pure solids excluded from the law of mass action expression?
Answer
Equilibrium expressions are technically based on chemical activities rather than concentrations. The activities of pure solids and pure liquids are defined as 1.
5
Question
How does changing temperature affect a system at equilibrium differently from changing concentration or pressure?
Answer
Temperature changes directly alter the value of \(K_{\text{eq}}\) itself. Concentration and pressure changes temporarily alter \(Q\) without changing \(K_{\text{eq}}\).
6
Question
How does the equilibrium constant of a multistep reaction relate to the equilibrium constants of its individual steps?
Answer
The overall equilibrium constant equals the product of the equilibrium constants of the individual steps: \(K_{\text{eq, overall}} = K_1 \times K_2 \times \dots \times K_n\).
7
Question
What mathematical relationship exists between the equilibrium constant of a forward reaction and that of its reverse reaction?
Answer
The equilibrium constant of the reverse reaction is the reciprocal of the forward reaction: \(K_{\text{eq, reverse}} = \frac{1}{K_{\text{eq, forward}}}\).
8
Question
Under what mathematical conditions is the 'x is negligible' approximation valid in equilibrium calculations?
Answer
The approximation is valid when \(K_{\text{eq}}\) has a large negative exponent (at least two orders of magnitude smaller than initial concentrations). It fails if \(K_{\text{eq}}\) is close to 1 or if conversion exceeds a few percent.
9
Question
How does hyperventilation compensate for metabolic acidemia via the bicarbonate buffer system?
Answer
Hyperventilation expels gaseous \(\text{CO}_2\) from the lungs. By Le Châtelier's principle, the equilibrium \(\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-\)$ shifts left, consuming excess \(\text{H}^+\) ions.
10
Question
How does an increase in total pressure affect a gaseous equilibrium system at constant temperature?
Answer
The system shifts toward the side with fewer total moles of gas. This reduces total particle collisions and relieves the applied pressure stress.
11
Question
How does increasing temperature shift the equilibrium position of an exothermic reaction?
Answer
Heat acts as a product in an exothermic reaction (\(\Delta H < 0\)). Increasing temperature shifts the equilibrium to the left, favoring reactants and decreasing \(K_{\text{eq}}\).
12
Question
What distinguishes the kinetic product from the thermodynamic product in terms of activation energy and product stability?
Answer
The kinetic product forms through a lower-energy transition state (lower \(E_a\)) and forms faster at low temperatures. The thermodynamic product has higher \(E_a\) but yields a more stable product with lower overall free energy.
13
Question
Why does the conversion of 2-methylcyclohexanone yield a less-substituted enolate under kinetic control?
Answer
Deprotonation at C-6 is less sterically hindered and has lower activation energy than at C-1. At low temperatures with insufficient thermal energy to overcome C-1 steric hindrance, the less-substituted kinetic enolate forms preferentially.
14
Question
What defines an isolated system in thermodynamics?
Answer
An isolated system cannot exchange either energy (heat and work) or matter with its surroundings. An insulated bomb calorimeter approximates an isolated system.
15
Question
How does a closed system differ from an open system regarding energy and matter exchange?
Answer
A closed system can exchange energy (heat and work) but not matter with its surroundings. An open system exchanges both energy and matter with its surroundings.
16
Question
How does the first law of thermodynamics simplify for an isothermal process in an ideal gas?
Answer
Because temperature is constant, internal energy remains constant (\(\Delta U = 0\)). The first law (\(\Delta U = Q - W\)) simplifies to \(Q = W\).
17
Question
How does an adiabatic process simplify the first law of thermodynamics?
Answer
No heat is exchanged between the system and environment (\(Q = 0\)). The first law simplifies to \(\Delta U = -W\), meaning internal energy change equals work done on the system.
18
Question
Why is work done equal to zero in an isovolumetric (isochoric) process?
Answer
Work is defined as \(W = P\Delta V\). When volume remains constant (\(\Delta V = 0\)), the gas neither expands nor compresses, so no pressure-volume work is performed.
19
Question
How does an isobaric process appear on a pressure–volume (P–V) diagram?
Answer
It appears as a horizontal flat line with a slope of zero. Work corresponds to the rectangular area under the line (\(W = P\Delta V\)).
20
Question
What fundamental distinction separates state functions from process functions?
Answer
State functions describe macroscopic properties of an equilibrium state independent of the path taken. Process functions (such as work and heat) quantify the pathway taken between states.
21
Question
Which eight macroscopic properties constitute the common thermodynamic state functions?
Answer
Pressure (\(P\)), density (\(\rho\)), temperature (\(T\)), volume (\(V\)), enthalpy (\(H\)), internal energy (\(U\)), Gibbs free energy (\(G\)), and entropy (\(S\)).
22
Question
How do standard conditions in thermodynamics differ from standard temperature and pressure (STP) in gas laws?
Answer
Standard conditions are \(25^\circ\text{C}\) (\(298\text{ K}\)), \(1\text{ atm}\), and \(1\text{ M}\) concentrations. STP is \(0^\circ\text{C}\) (\(273\text{ K}\)) and \(1\text{ atm}\).
23
Question
What defines the standard state of a substance?
Answer
The standard state is the most stable physical form of a pure substance under standard conditions (\(298\text{ K}\) and \(1\text{ atm}\)).
24
Question
What occurs at the triple point on a phase diagram?
Answer
All three phases (solid, liquid, and gas) exist simultaneously in thermodynamic equilibrium at a unique temperature and pressure.
25
Question
What occurs physically and thermodynamically at the critical point on a phase diagram?
Answer
The liquid and gas phase boundary terminates where liquid and vapor densities become equal. Above this temperature and pressure, the substance forms a supercritical fluid and the heat of vaporization is zero.
26
Question
Why does water exhibit a solid–liquid equilibrium line with a negative slope on its phase diagram?
Answer
Solid ice is less dense than liquid water due to open hydrogen-bonded crystal lattice spacing. Increasing pressure at constant temperature favors the denser liquid phase, lowering the melting point.
27
Question
How does evaporation differ from boiling in terms of location and temperature constraints?
Answer
Evaporation occurs only at the liquid surface at any temperature when high-energy particles escape. Boiling occurs throughout the entire liquid volume only when vapor pressure equals ambient pressure.
28
Question
Why does evaporation exert a cooling effect on the remaining liquid?
Answer
Evaporation is an endothermic process where molecules with the highest kinetic energy escape. The loss of high-energy molecules decreases the average kinetic energy and temperature of the remaining liquid.
29
Question
What is sublimation and how is its reverse phase transition designated?
Answer
Sublimation is the direct transition from solid to gas without entering the liquid phase. The reverse transition from gas to solid is called deposition.
30
Question
Why does temperature remain constant during a pure substance's phase transition on a heating curve?
Answer
Added thermal energy is utilized entirely to overcome intermolecular forces breaking the lattice or liquid structure rather than increasing particle kinetic energy.