Thermodynamics Theory

What is the core question thermodynamics answers in reaction engineering?

Whether a reaction is thermodynamically possible and the maximum achievable conversion

What key question does thermodynamics NOT answer?

It does not determine how fast a reaction occurs or how long it takes to reach conversion

What does the enthalpy of reaction ΔH represent?

The heat change associated with a chemical reaction at constant pressure

What does a negative enthalpy -ΔH of reaction indicate?

An exothermic reaction that releases heat to the surroundings

What are the typical reactor consequences of an exothermic reaction?

Reactor temperature tends to increase under adiabatic operation, which can increase reaction rate but reduce equilibrium conversion

What does a positive enthalpy +ΔH of reaction indicate?

An endothermic reaction that absorbs heat from the surroundings

What are the typical reactor consequences of an endothermic reaction?

Temperature tends to decrease unless heat is supplied, and higher temperature increases both reaction rate and equilibrium conversion

What is the key thermodynamic meaning of ΔH in reactors?

It predicts how temperature changes during reaction if no heat is added or removed

What condition defines chemical equilibrium?

The forward and reverse reaction rates are equal

Does a reaction stop at equilibrium?

No, equilibrium is dynamic and molecular reactions continue with no net composition change

Why does equilibrium limit reactor performance?

Equilibrium sets a maximum conversion that cannot be exceeded regardless of reactor size or residence time

Why does increasing residence time near equilibrium give little benefit?

Because the driving force for reaction becomes very small as equilibrium is approached

What does the equilibrium constant represent?

The extent to which products are favoured relative to reactants at equilibrium

What form of equilibrium constant is typically used for liquid-phase reactions?

K_c which is based on species concentrations

What form of equilibrium constant is used for gas-phase reactions?

K_p which is based on partial pressures

What is the relationship between K₍p₎ and K₍c₎?

Kp = Kc (RT)^{\Delta n}

K_p is the equilibrium constant based on partial pressures

K_c is the equilibrium constant based on concentrations
R is the gas constant
T is the absolute temperature
\Delta n is the change in moles of gas (products minus reactants)

What does Δn physically represent in gas-phase equilibrium?

The net change in the number of moles of gas during reaction

On what variables does the equilibrium constant depend?

Temperature only, not reactor type or size

What does the standard Gibbs free energy change indicate?

Whether a reaction is thermodynamically favourable under standard conditions

What is the defining equation for Gibbs free energy change?

\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ

\Delta G^\circ is the standard Gibbs free energy change
\Delta H^\circ is the standard enthalpy change
T is the absolute temperature
\Delta S^\circ is the standard entropy change

What does ΔG° < 0 imply about a reaction?

The reaction is thermodynamically favoured

What does ΔG° = 0 signify?

The reaction is at equilibrium

How is Gibbs free energy linked to the equilibrium constant?

\Delta G^{\circ}=-RT\ln K7

K is the equilibrium constant
R is the gas constant
T is the absolute temperature

What does a large equilibrium constant imply about ΔG°?

ΔG° is large and negative

What does entropy physically represent in reaction engineering?

The degree of molecular disorder or freedom

What changes typically increase entropy?

An increase in the number of gas molecules or gas-phase expansion

Why can entropy make an endothermic reaction favourable?

A large positive entropy change can outweigh enthalpy at high temperature

What equation describes the temperature dependence of equilibrium constants?

\ln\left(\frac{K2}{K1}\right) = -\frac{\Delta H^\circ}{R}\left(\frac{1}{T2} - \frac{1}{T1}\right)

K1, K2 are the equilibrium constants

T1, T2 are the equilibrium temperatures
\Delta H^\circ is the standard enthalpy change
R is the gas constant
T is absolute temperature

How does increasing temperature affect equilibrium for an exothermic reaction?

It decreases the equilibrium constant and lowers equilibrium conversion

How does increasing temperature affect equilibrium for an endothermic reaction?

It increases the equilibrium constant and raises equilibrium conversion

What key trade-off exists between kinetics and thermodynamics?

Higher temperature increases reaction rate but may reduce equilibrium conversion for exothermic reactions

What defines an adiabatic reactor?

A reactor with no heat transfer to or from the surroundings

How does equilibrium behave in an adiabatic exothermic reactor?

Temperature rises during reaction, causing equilibrium conversion to decrease along the reactor

Why can exothermic reactions reach equilibrium early in reactors?

Rising temperature shifts equilibrium toward reactants, reducing the driving force

What reactor design strategy mitigates equilibrium limitations in exothermic systems?

Using multiple reactors with interstage cooling

What does thermodynamics NOT provide in reactor design?

Reaction rates, reactor size, or time required to reach conversion

What does thermodynamics definitively provide?

Reaction direction, maximum conversion, and equilibrium response to temperature and pressure

What is the single most important thermodynamics principle for exams?

Thermodynamics sets the maximum achievable conversion that no reactor can exceed

One-sentence exam statement summarising thermodynamics in reaction engineering

Thermodynamics determines reaction feasibility and maximum achievable conversion, independent of reactor size or residence time