## Test #2

$\ln K = -\frac{\Delta H^{\circ}}{RT} + \frac{\Delta S^{\circ}}{R}$

Emily Ng_4C
Posts: 65
Joined: Fri Sep 28, 2018 12:17 am

### Test #2

the ionic dissociation of water is given by the following reaction: the deltaH for the reaction is 58 kJ/mol. The Kw for the reaction at 25 degree Celsius is 10 ^-14. Is the pH of 7 acidic or basic at 10 degree Celsius?
2H20 --> H3O+ + OH-

Lily Benitez 2G
Posts: 63
Joined: Fri Sep 28, 2018 12:29 am

### Re: Test #2

the ionic dissociation of water is given by the following reaction: the deltaH for the reaction is 58 kJ/mol. The Kw for the reaction at 25 degree Celsius is 10 ^-14. Is the pH of 7 acidic or basic at 10 degree Celsius?
2H20 --> H3O+ + OH-

You have to start by using the Van't Hoff equation which is Ln(k2/k1)=-(deltaH/R)(1/T1 - 1/T2)
You substitute everything in and find your K2. Since H3O and OH are equal concentrations you get the equation x^2 = k2. Once you find X you can take the -log and you find your ph. Then you compare 7 to the new ph value. If 7 is higher then the calculated ph it is basic, if it is lower it is acidic.

BenJohnson1H
Posts: 68
Joined: Fri Sep 28, 2018 12:17 am

### Re: Test #2

I still don't conceptually understand how the pH would change if H+ and OH- are at equivalent concentrations

Joonsoo Kim 4L
Posts: 61
Joined: Fri Sep 28, 2018 12:29 am

### Re: Test #2

pH refers to the concentration of [H+] in solution, not necessarily the acidity of the solution. When the temperature is lowered, the Kw is also lowered (due to the van't hoff equation) so the [H+] of a neutral solution of water would also be lowered, which raises the neutral pH. If the neutral pH is raised, then a pH of 7 would be considered acidic at this new temperature.

Daisylookinland
Posts: 46
Joined: Fri Sep 28, 2018 12:28 am

### Re: Test #2

for my K2 value I got 2.78*10^-14

On my test I took the negative log of this value but got the answer wrong, is that the wrong approach?

Roberto Gonzalez 1L
Posts: 39
Joined: Fri Sep 28, 2018 12:17 am

### Re: Test #2

The key here is to use the Van't Hoff Equation to find K2 at the temperature 30 celsius. I got 1.471^-14. Then take the square root of Kw to solve for your hydronium concentration and take the -log of the value. This will result in your new neutral pH. Comparing to see wether 7 is larger (meaning basic) or smaller (meaning acidic) compared to your neutral pH is how you solve this.