For example, let's say we have the equation $$\ce{PCl_3(g) + Cl_2(g) -> PCl_5(g)},$$ and the temperature is held constant.

Would the pressure in the container when equilibrium is reached be greater than, less than, or equal to the pressure when the $\ce{PCl3}$ and the $\ce{Cl2}$ are initially mixed?

I have a few ideas, but they all come up with different conclusions.

  1. The pressure increases. This is because the volume is decreasing because of less molecules when equilibrium is established and by Boyle's law, the pressure increases. However, container has a constant volume, so I don't think this is the correct answer.
  2. The pressure decreases. This is because less molecules present implies less molecules hitting off the walls of the container.
  3. The pressure remains the same. This is because even though less molecules implies less pressure, they will hit the walls of the container with more force. I'm speculating that more force will cancel out the fewer molecules hitting the walls of the container.
  • $\begingroup$ Boyle's law is not applicable here. Boyle's law imply a constant amount of substances, and no chemical reaction. $\endgroup$
    – Maurice
    Commented Jun 24, 2023 at 11:30
  • 1
    $\begingroup$ Try $\Delta (pV) = \Delta (nRT)$ $\endgroup$
    – Poutnik
    Commented Jun 24, 2023 at 12:37

1 Answer 1


Consider $\pu{1 mol}$ of $\ce{PCl3}$ and $\pu{1 mol}$ of $\ce{Cl2}$ at the beginning for the above reaction. Let at equilibrium $x \text{ mol} $ of each reactants get converted to products. Then $\ce{PCl5}$ is $x \text{ mol}$ and $\ce{PCl3}$ is $1-x\text{ mol}$ and $\ce{Cl2}$ is $1-x\text{ mol}$.

$$\begin{array}{ccccc} \ce{PCl3} & + & \ce{Cl2} & \ce{->} & \ce{PCl5} \\ 1 & & 1 & & 0 \\ 1-x & & 1-x & & x \\ \end{array}$$

This means at equilibrium, total number of moles $= 1-x+1-x+x=2-x\text{ mol}$. Initially 2 moles were there and now 2-x. Practically pressure must decrease as all the reactants and products are gaseous.

  • $\begingroup$ No, initially it is only PCl3 and Cl2 in the container, no PCl5 is present. Assuming there is 1 mole of Pcl3 and cl2, at equilibrium there would be 2-x moles present. ((1-x) + (1-x) + x). Note that 2-x is less than 2. $\endgroup$
    – photon
    Commented Jun 24, 2023 at 11:58
  • 1
    $\begingroup$ I have edited the answer $\endgroup$ Commented Jun 24, 2023 at 12:12
  • $\begingroup$ Consider \ce{} for chemical formulas and reactions. $\ce{PCl3}$ for $\ce{PCl3}$, $\ce{PCl3 + Cl2 -> PCl5}$ for $\ce{PCl3 + Cl2 -> PCl5}$. $\endgroup$
    – Poutnik
    Commented Jun 24, 2023 at 16:27
  • $\begingroup$ @Poutnik, thanks for the suggestion $\endgroup$ Commented Jun 24, 2023 at 16:58
  • $\begingroup$ I have also created the array structure for the tabelated content. // SE Markdown supports tables too: SE meta - tables $\endgroup$
    – Poutnik
    Commented Jun 24, 2023 at 16:58

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