Timeline for What are the units of Kc and Kp?
Current License: CC BY-SA 4.0
6 events
| when toggle format | what | by | license | comment | |
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| Jan 4, 2019 at 9:09 | comment | added | porphyrin | @voffch, you are correct, I was careless and I have changed my answer to be clearer, I was thinking of $K_x$ when I used 'coefficients'. | |
| Jan 4, 2019 at 9:06 | history | edited | porphyrin | CC BY-SA 4.0 |
typo corrected
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| Jan 3, 2019 at 19:27 | comment | added | voffch | The answer is OK-ish but there is one obvious error, which is now repeated in the comments. Actually, thermodynamic equilibrium constant is defined in terms of activities (fugacities) and not just activity coefficients! Fugacity, for example, is $f=\gamma \frac{p}{p^○}$ where $\gamma$ is an activity coefficient and $p^○$ is a standard pressure. Mind also that the latter is tricky: it was 1 atm in the past, but now IUPAC recommends it to be $10^5$ Pa, or 1 bar in non-SI units. So, you see that it is the activity (fugacity) which is dimensionless. Ask Atkins if you don't believe me. | |
| Jan 3, 2019 at 15:06 | comment | added | andselisk♦ | @AnuragB. porhyrin is correct. There is a trap: $K_\mathrm{p}$ is indeed dimensionless as it deals not really with the pressures, but fugacity $f$, which, in turn is expressed as $p_i/p°$, where $p_i$ is partial pressure and $p°$ is standard state of pressure so that the units of pressure are cancelled out. Also, no sarcasm this time:) | |
| Jan 3, 2019 at 14:24 | comment | added | Anurag Baundwal | >The equilibrium constant Kp is dimensionless as it is defined, ultimately, in terms of the ratio of activity coefficients, which are themselves dimensionless. Excuse me? Won't $K_p$ have dimensions of pressure raised to some power? And what about $K_c$? I'm guessing $[L^{-3}]^a$ ... a is any number ... I think mol is dimensionless so I'm only considering the dimensions of litres. | |
| Jan 3, 2019 at 13:57 | history | answered | porphyrin | CC BY-SA 4.0 |