Carbon monoxide is toxic because it bonds much more strongly...

Carbon monoxide is toxic because it bonds much more strongly to the iron in hemoglobin (Hgb) than does $\mathrm{O}_{2}$. Consider the following reactions and approximate standard free energy changes: $$\begin{array}{clr}\mathrm{Hgb}+\mathrm{O}_{2} \longrightarrow \mathrm{HgbO}_{2} & \Delta G^{\circ}=-70 \mathrm{~kJ} \\ \mathrm{Hgb}+\mathrm{CO} \longrightarrow \mathrm{HgbCO} & \Delta G^{\circ}=-80 \mathrm{~kJ} \end{array}$$ Using these data, estimate the equilibrium constant value at $25^{\circ} \mathrm{C}$ for the following reaction: $$\mathrm{HgbO}_{2}+\mathrm{CO} \rightleftharpoons \mathrm{HgbCO}+\mathrm{O}_{2}$$

Carbon monoxide is toxic because it bonds much more strongly to the iron in hemoglobin (Hgb) than does $\mathrm{O}_{2}$. Consider the following reactions and approximate standard free energy changes:
$$\begin{array}{clr}\mathrm{Hgb}+\mathrm{O}_{2} \longrightarrow \mathrm{HgbO}_{2} & \Delta G^{\circ}=-70 \mathrm{~kJ} \\
\mathrm{Hgb}+\mathrm{CO} \longrightarrow \mathrm{HgbCO} & \Delta G^{\circ}=-80 \mathrm{~kJ}
\end{array}$$
Using these data, estimate the equilibrium constant value at $25^{\circ} \mathrm{C}$ for the following reaction:
$$\mathrm{HgbO}_{2}+\mathrm{CO} \rightleftharpoons \mathrm{HgbCO}+\mathrm{O}_{2}$$

Key Concepts

Standard Free Energy Change

Standard free energy change (?G°) is a thermodynamic quantity that indicates the spontaneity of a reaction under standard conditions (typically 1 M concentration, 1 atm pressure, and a specified temperature, usually 25°C). A negative ?G° means the reaction is thermodynamically favored, while a positive value indicates non-spontaneity. This parameter is crucial because it connects the chemical reaction’s energy profile with its equilibrium behavior.

Equilibrium Constant

The equilibrium constant (K) is a dimensionless value that relates the concentrations or partial pressures of reactants and products at equilibrium. It is directly connected to the standard free energy change by the relation ?G° = -RT ln K, where R is the gas constant and T is the temperature in Kelvin. This relationship allows for the estimation of reaction equilibria from known free energy changes, providing insight into the extent of reactions under specified conditions.

Composite Reaction Free Energy Analysis

Composite reaction free energy analysis involves combining the free energy changes of individual steps to determine the overall free energy change for a net reaction. Since free energy is a state function, the overall ?G° for a reaction that can be broken into steps is the sum of the ?G° for each step. This method is particularly useful in understanding how modifications in binding affinities or reaction pathways can affect the equilibrium constant of a net reaction.

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