PCl5 (g) <=> PCl3 (g) +Cl2
what is the property that might be observed in order to determine when equlibrium has been reached?
The question is a little vague. Is the property visual or some other property? I don't think vision (color) will work for PCl5 is a yellowish color and chlorine gas is ayellow green. But I wonder about the possibility of noting that 1 mole of PCl5 produces 2 mols of gas; therefore, if the PCl5 is placed in a sealed container (at a constant temperature) that had a pressusre guage attached, I wonder if equilibrium for the reaction would be noted by the pressure not changing? Check my thinking.
Your thinking is correct! One property that can be observed to determine when equilibrium has been reached in the reaction PCl5 (g) <=> PCl3 (g) + Cl2 (g) is the pressure.
Since the reaction involves a change in the number of moles of gas, monitoring the pressure can be a good indicator of equilibrium. At the beginning of the reaction, when only PCl5 is present, the pressure will be high. As the reaction proceeds and PCl5 is converted into PCl3 and Cl2, the total number of moles of gas decreases, resulting in a decrease in pressure. Eventually, when equilibrium is reached, the pressure will stop decreasing, indicating that the forward and backward reactions are occurring at the same rate and the system has reached a dynamic equilibrium.
To observe this property, you can set up the reaction in a sealed container with a pressure gauge attached. As the reaction progresses, monitor the pressure. Initially, the pressure will decrease rapidly, but eventually, it will stabilize and remain constant once equilibrium is reached.