Kw and Acidity

[MedPR]

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So Kw at 25deg C is 10^-14. At 50deg C it is 6*10^-14.

Why does a larger Kw mean more acidic? Don't both H+ and OH- increase? So shouldn't the pH stay 7?

 

[milski]

In water [H+]=[OH-] since you get one of each once you dissociate one water molecule. Kw=[H+][OH-] by definition, so if Kw increases, both [H+] and [OH-] will be increasing.

If [H+] increases, pH will decrease, it's just -log[H+]. Lower pH -> more acidic.

 

[MedPR]

In water [H+]=[OH-] since you get one of each once you dissociate one water molecule. Kw=[H+][OH-] by definition, so if Kw increases, both [H+] and [OH-] will be increasing.

If [H+] increases, pH will decrease, it's just -log[H+]. Lower pH -> more acidic.

Yea, I think you've explained this to me before. I keep thinking that the increased [OH-] will cancel out the increased [H+].. When in reality increased [OH-] results in a higher pOH and therefore a lower pH, which just adds constructively (not really the right term, but too bad 😉) to the lowered pH induced by increased [H+]


Thanks again.

 

[MrNeuro]

Yea, I think you've explained this to me before. I keep thinking that the increased [OH-] will cancel out the increased [H+].. When in reality increased [OH-] results in a higher pOH and therefore a lower pH, which just adds constructively (not really the right term, but too bad 😉) to the lowered pH induced by increased [H+]


Thanks again.

Also when you increase the temp pH +pOH =/= 14

 

[deleted357625]

Also note temperature is the only thing that changes the Kw value, it will no longer be 14 hence pH will no longer be 7

 

[gettheleadout]

In water [H+]=[OH-] since you get one of each once you dissociate one water molecule. Kw=[H+][OH-] by definition, so if Kw increases, both [H+] and [OH-] will be increasing.

If [H+] increases, pH will decrease, it's just -log[H+]. Lower pH -> more acidic.

Yea, I think you've explained this to me before. I keep thinking that the increased [OH-] will cancel out the increased [H+].. When in reality increased [OH-] results in a higher pOH and therefore a lower pH, which just adds constructively (not really the right term, but too bad 😉) to the lowered pH induced by increased [H+]


Thanks again.

Mind = blown. I've never considered this! (the effect of changing temperature on Kw)

 

[typicalindian]

So increasing Temp causes a decrease in the neutral pH? Then how come neutral pH at body temp (37C) is 7.4? 😕

 

[milski]

What's your definition of neutral? You could say that water is slightly acidic at temps higher than 25 C.

 

[Dasypus]

I think there's a misunderstanding/equivocation in typicalindian's use of 'neutral'. Normally when we say 'neutral' in chemistry, we mean neither acidic nor basic, whereas in biology, 'neutral' means something more like 'homeostatic'. Does that seem right?

 

[typicalindian]

Ok so now I'm just flat out confused...I understand that Kw increases as temperature increases so [H+] increases right? And that means [OH-] increases by the same amount? So why would pH change at all if they both increase?

 

[milski]

Ok so now I'm just flat out confused...I understand that Kw increases as temperature increases so [H+] increases right? And that means [OH-] increases by the same amount? So why would pH change at all if they both increase?

Because pH is concentration of [H+].

A lot of what's acidic/basic/neutral is really a convention. You can actually only compare them between two different solutions/situations. Saying that "A is acidic" is just a shortcut for "A is more acidic than water at standard conditions."

 

[deleted357625]

My view on this topic:

at 25 degrees Celsius [H+][OH-] = 10^-14
pH is 7
pOH is 7

Kw can only be changed by temperature.
If temperature increases and lets say --> [H+][OH-] = 10^-13. Kw value is now 13. Because temp increased and our Kw value is now 13, that means our pH is now 6.5 and pOH 6.5 which is now lower --> neutral condition.

Neutral condition can change if temperature changes - not always pH of 7 (10^-7) - only at 25 Celsius.

 

[typicalindian]

My view on this topic:

at 25 degrees Celsius [H+][OH-] = 10^-14
pH is 7
pOH is 7

Kw can only be changed by temperature.
If temperature increases and lets say --> [H+][OH-] = 10^-13. Kw value is now 13. Because temp increased, that means our pH is now 6.5 and pOH 6.5 which is now lower --> neutral condition.

Neutral condition can change if temperature changes - not always pH of 7 (10^-7) - only at 25 Celsius.

Oh ok right so even though both H+ and OH- concentrations increase and the new pH of 6.5 is a neutral solution, and it's acidic when you compare it to water at 25C?

 

[deleted357625]

Oh ok right so even though both H+ and OH- concentrations increase and the new pH of 6.5 is a neutral solution, and it's acidic when you compare it to water at 25C?

Yes the concentration increases while the pH and pOH decrease obviously.
You have to look at the Kw value. at 25 Celsius the neutral condition for water is 10^-14 where pH = 7 and pOH = 7

If temp increases and I'm picking an arbitrary number the Kw total value is now 10^-13 so that is the neutral condition AT THAT TEMPERATURE. So the pH and pOH are equal to each other and def not 7 since the Kw value is no longer 14, the pH and pOH have lower values pH = 6.5, pOH = 6.5 which equal to Kw value of 13 at that temp

Yes, the pH and pOH are a little more acidic than water at 25C (lower than 7 values) but they are considered neutral conditions don't get that confused - because they both add up equally to the Kw value of 13 at that temperature. At that specific increase of temperature Kw of 13 is considered a neutral condition, at 25 Celsius the Kw of 14 is considered a neutral condition.

Hope that helps

 

[typicalindian]

Yes the concentration increases while the pH and pOH decrease obviously.
You have to look at the Kw value. at 25 Celsius the neutral condition for water is 10^-14 where pH = 7 and pOH = 7

If temp increases and I'm picking an arbitrary number the Kw total value is now 10^-13 so that is the neutral condition AT THAT TEMPERATURE. So the pH and pOH are equal to each other and def not 7 since the Kw value is no longer 14, the pH and pOH have lower values pH = 6.5, pOH = 6.5 which equal to Kw value of 13 at that temp

Yes, the pH and pOH are a little more acidic than water at 25C (lower than 7 values) but they are considered neutral conditions don't get that confused - because they both add up equally to the Kw value of 13 at that temperature. At that specific increase of temperature Kw of 13 is considered a neutral condition, at 25 Celsius the Kw of 14 is considered a neutral condition.

Hope that helps

yes it makes sense now! thanks a lot guys

 

[deleted357625]

yes it makes sense now! thanks a lot guys

No problem. SDN ftw 👍

 

[SaCkO]

Yea, I think you've explained this to me before. I keep thinking that the increased [OH-] will cancel out the increased [H+].. When in reality increased [OH-] results in a higher pOH and therefore a lower pH, which just adds constructively (not really the right term, but too bad 😉) to the lowered pH induced by increased [H+]


Thanks again.

You said that Higher [OH-] results in higher pOH which is false,, Higher [OH-] results in lower pOH and higher pH