Describe three characteristic features of chemical equilibrium.

When $1 \mathrm{mol}$ of ${\mathrm{N}}_2{\mathrm{O}}_4$ gas is allowed to come to equilibrium with ${\mathrm{NO}}_2$ gas under standard conditions, only $20 \%$ of the ${\mathrm{N}}_2{\mathrm{O}}_4$ is converted to ${\mathrm{NO}}_2$.

${\mathrm{N}}_2{\mathrm{O}}_4\rightleftharpoons 2{\mathrm{NO}}_2, ∆{\mathrm{H}}_{\mathrm{r}}=+58 \mathrm{kJ} {\mathrm{mol}}^{-1}$. 

Give the equilibrium expression for this reaction.

When $1 \mathrm{mol}$ of ${\mathrm{N}}_2{\mathrm{O}}_4$ gas is allowed to come to equilibrium with ${\mathrm{NO}}_2$ gas under standard conditions, only $20 \%$ of the ${\mathrm{N}}_2{\mathrm{O}}_4$ is converted to ${\mathrm{NO}}_2$.

${\mathrm{N}}_2{\mathrm{O}}_4\rightleftharpoons 2{\mathrm{NO}}_2, ∆\mathrm{H}=+58 \mathrm{kJ} {\mathrm{mol}}^{-1}$. 

Calculate the value of ${\mathrm{K}}_{\mathrm{c}}$ for the reaction. Assume that the volume of the reaction mixture is $1 {\mathrm{dm}}^3$?

When dissolved in an organic solvent, hydrogen chloride reacts with hydrogen iodide as follows: $\mathrm{HCl}+\mathrm{HI}\rightleftharpoons {\mathrm{H}}_2{\mathrm{Cl}}^{+}+{\mathrm{I}}^{-}$. Use the Bronsted-Lowry theory of acids and bases to explain which reactant is the acid, and which reactant is the base.