The god of science recently told me that and increase in temperature would increase the Kw of water and that hot H(2)O is slightly acidic. I understand the temperature is one of the few things that can affect a rate constant and that increasing it would increase the amount that water splits up into the less stable hydronium and hydroxide ions but I don't understand how that would make water more acidic. When solving for the [H+] and [OH-] at lets say a Kw of 2.0e-14 that would make the [H+] 1.4e-7 making the pH around 6.9, but wouldn't the [OH-] also be 1.4e-7 and the pOH 6.9. From my understanding I would venture to say that we have now redefined what is considered a neutral pH (a pH of 6.9) and pH + pOH would now be 13.8. Wouldn't a pH of 7 now be slightly basic? I'm just confused as to how we can define the hot water as slightly acidic now?
Kw vs. Temp.
- Thread starter Kstar007
- Start date
-
- Tags
- dat gen chem pre dental
- Joined
- May 10, 2015
- Messages
- 301
- Reaction score
- 203
I just watched this video! From what I understand is because you are increasing the amount of water that is being split into [H+] and [OH-] you are adding more [H+] which makes a solution more acidic. I think the number that he used for [H+] and [OH-] was 1.3*10^-13 so if you take the square root of it you would get a pH of around 6.5 that is considered neutral because [H+] and [OH-] are in equilibrium. So if you are comparing a pH of 7 to pH of 6.5 which is considered neutral in this problem pH of 7 would be slightly basic. I am not sure if I explained this well but this is the way I understood it.
Do you mean the number for Kw was 1.3e-13 because if [H+] and [OH-] were 1.3e-13 that would give a Kw of 1.69e-26 which would be a reduction in Kw, not increase. And is your answer that the final solution is considered neutral or acidic?
- Joined
- May 10, 2015
- Messages
- 301
- Reaction score
- 203
Yes Kw in his problem was 1.3*10^-13. pH of 6.5 is considered acidic but since you are changing the temperature it is considered neutral in this problem. So a pH of 7 compared to a neutral pH of 6.5 is where he said a pH of 7 would technically be basic in this problem.
- Joined
- Mar 12, 2005
- Messages
- 4,786
- Reaction score
- 3,485
No.....You kind of missed the point.....As you change temperature, the Kw has changed,,,,,,If the pH falls as temperature increases, this does not mean that water becomes more acidic at higher temperatures. A solution is acidic if there is an excess of hydrogen ions over hydroxide ions (i.e., pH < pOH). In the case of pure water, there are always the same concentration of hydrogen ions and hydroxide ions and hence, the water is still neutral (pH = pOH) - even if its pH changes. The problem is that we are all familiar with 7 being the pH of pure water, that anything else feels really strange. Remember that to calculate the neutral value of pH from Kw . If that changes, then the neutral value for pH changes as well. At 100°C, the pH of pure water is 6.14, which is "neutral" on the pH scale at this higher temperature. A solution with a pH of 7 at this temperature is slightly alkaline because its pH is a bit higher than the neutral value of 6.14. Similarly, you can argue that a solution with a pH of 7 at 0°C is slightly acidic, because its pH is a bit lower than the neutral value of 7.47 at this temperature.
Hope this helps.
Dr. Romano
