Writing Equilibrium Expressions

Here are two forms of the compound C₂H₂I₂.

cis-C2H2I2		trans-C2H2I2

The cis form has the iodine atoms on the same side,
and the trans form has them on opposite sides.

The cis and trans forms are stable at room temperature.
However, when heated cis forms some trans and trans gives
some cis. There is an equilibrium between cis and trans forms.