19.1 Chemical equilibrium and reaction coordinates
3 min read•july 23, 2024
is a balancing act in reactions. It's when forward and reverse reactions happen at the same speed, keeping concentrations steady. The change hits zero at this point, showing the system's found its sweet spot.
map out a reaction's energy journey. They show how Gibbs free energy changes as the reaction progresses, helping us predict if a reaction will happen on its own and how far it'll go.
Chemical Equilibrium and Reaction Coordinates
Concept of chemical equilibrium
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Dynamic state forward and reverse reactions occur at equal rates resulting in no net change in concentrations of reactants and products
At equilibrium Gibbs free energy change (ΔG) equal to zero represents minimum Gibbs free energy for system at constant and
Relationship between Gibbs free energy change and (K) given by equation ΔG∘=−RTlnK
ΔG∘ standard Gibbs free energy change
R universal gas constant (8.314 J/mol·K)
T absolute temperature (K)
Interpretation of reaction coordinate diagrams
Plot Gibbs free energy of system as function of reaction progress
Reactants represented on left side products on right side
highest point represents energy barrier that must be overcome for reaction to occur
Direction of chemical reaction determined by comparing Gibbs free energy of reactants and products
Products lower than reactants reaction spontaneous in forward direction (exothermic)
Reactants lower than products reaction spontaneous in reverse direction (endothermic)
Extent of chemical reaction determined by difference in Gibbs free energy between reactants and products
Larger difference results in greater extent of reaction (higher yield)
Calculations with law of mass action
equilibrium constant (K) equal to product of concentrations of products raised to stoichiometric coefficients divided by product of concentrations of reactants raised to stoichiometric coefficients
General reaction aA+bB⇌cC+dD, equilibrium constant K=[A]a[B]b[C]c[D]d
Square brackets denote concentrations of respective species at equilibrium (mol/L or M)
Equilibrium concentrations calculated using equilibrium constant and initial concentrations of reactants and products
Set up using law of mass action
Substitute initial concentrations into equilibrium constant expression
Solve resulting equation for equilibrium concentrations (quadratic formula or ICE table)
Effects on equilibrium state
system at equilibrium subjected to change in temperature pressure or will shift equilibrium position to counteract change and re-establish equilibrium
Effect of temperature depends on whether reaction is endothermic or exothermic
Endothermic reaction increasing temperature shifts equilibrium towards products (favors forward reaction)
Exothermic reaction increasing temperature shifts equilibrium towards reactants (favors reverse reaction)
Effect of pressure depends on stoichiometry of reaction
Increasing pressure shifts equilibrium towards side with fewer moles of gas (favors reaction that decreases number of gas molecules)
Decreasing pressure shifts equilibrium towards side with more moles of gas (favors reaction that increases number of gas molecules)
Effect of concentration predicted using equilibrium constant expression
Increasing concentration of reactant shifts equilibrium towards products (favors forward reaction)
Increasing concentration of product shifts equilibrium towards reactants (favors reverse reaction)
Adding inert gas at constant volume has no effect on equilibrium (does not change partial pressures)