The value of $K_p$ for the reaction

$$\ce{Br2(l) + Cl2(g) <=> 2BrCl(g)}$$

is $\pu{1 atm}$. At equilibrium in a closed container partial pressure of $\ce{BrCl}$ gas is $\pu{0.1 atm}$ and at this temperature the vapour pressure of $\ce{Br2(l)}$ is also $\pu{0.1 atm}$. Then what minimum moles of $\ce{Br2(l)}$ to be added to $\pu{1 mol}$ of $\ce{Cl2}$ initially to get the equilibrium mixture from above?

In this question should I include the vapour pressure of $\ce{Br2}$ in calculating $K_p$ or not?

  • 2
    $\begingroup$ Yes. How else would you know how much bromine gas is actually in the gas phase. $\endgroup$ Commented Jul 26, 2017 at 10:32
  • $\begingroup$ @Martin-マーチン but why so as the thing that is taking part in the reaction is Br2 liquid and not Br2 vapour which is the cause of vapor pressure. $\endgroup$
    – user161158
    Commented Jul 26, 2017 at 11:45

1 Answer 1



As the reaction is written above, you may incorrectly conclude the bromine is in a liquid state throughout and hence is omitted from the equilbrium constants.

However as the question reminds us bromine is highly volatile and significant amounts of gas will accumulate above the liquid, hence most of the reactions will be between gas molecules of bromine and chlorine rather than gas and liquid.

The vapour pressure tells us the partial pressure of these gas. Liquid bromine will evaporate until the sealed vessel contains bromine vapour with a partial pressure of 0.1 atm.

This means it is actually the equilibrium constant for the all three gases: $$K_p=\frac{[\ce{BrCl}]^2}{[\ce{Br2}][\ce{Cl2}]}$$ where $[\ce{A}]$ is the partial pressure of $\ce{A}$. As the vapour pressure of $\ce{Br2}$ is known to be 0.1 atm, that is the partial pressure of bromine you use in your equilibrium constant.

Hence the minimum amount of bromine to add to establish any equilibrium with the liquid is the point where partial pressure of bromine is 0.1 atm and all the bromine is in the gas phase. Any more bromine added will condense out back into liquid.

  • $\begingroup$ I may have read this question wrong as your equilbrium constant has units suggesting $Kp=\frac{[\ce{BrCl}]^2}{[\ce{Cl2}]}$, when the mine is unitless: (pressure^2 / pressure^2) = 1. If so, I would think there is something wrong with the the chemistry of the question being asked about. $\endgroup$
    – user213305
    Commented Jul 26, 2017 at 23:39

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