Determine pH of water at 70 degree celsius

Let's take a look at our discussion question this week!

221219 question

We want to determine which of the above statements about water at 70 degree celsius is correct.

The dissociation in this question is also known as the auto-ionisation of water to give H+ and OH-.

221219 auto ionisation of water

Since 1 water will dissociate to give 1 H+ and 1 OH-, the concentration of H+ and OH- must remain the same.

Hence options C and D are wrong.

We can calculate the pH of water at 25 degree celsius since ionic product of water, Kw, is given.

221219 find pH at 25 deg

pH of water is found to be 7, which is hardly surprising since we associate pH 7 to be neutral.

But we need to remember that pH of water is calculated to be 7 only when Kw is equal to 10-14.

When temperature changes, Kw will change and therefore pH of water must change.

221219 POE at higher temp

At 70 degree celsius, position of equilibrium shifts to the right favouring the endothermic process and absorb excess heat.

221219 pH at higher temp

Concentration of H+ and OH- increase and we can deduce pH of water will be less than 7 at higher temperatures.

Some students might wrongly conclude that water must be acidic at 70 degree celsius since pH is less than 7.

This is false since there is also a proportionate increase in the concentration of OH- and the concentration of H+ and OH- remain the same.

221219 water still neutral

Hence water is still neutral at any temperature, regardless of its calculated pH.

Therefore the answer to this question will be option A.

221219 answer

Topic: Ionic Equilibria, Physical Chemistry, A Level Chemistry, Singapore

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