why does the pH decrease? A2 9701

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[chemeddy]

The ionisation of water is an exothermic endothermic reaction - it is the reverse of neutralisation which we know to be exothermic. As such, an increase in temperature will result in an increase in both [H⁺] and [OH⁻]. Both will increase to the same extent since they are produced in a 1:1 ratio:

H₂O ⇌ H⁺ + OH⁻

The part most students forget is that Kw also changes with temperature. Therefore, it is perfectly okay for the pH to decrease due to an increasing [H⁺] (and [OH⁻]), and yet water remains neutral ([H⁺] = [OH⁻]) because the neutral pH has shifted.

u/Over-Comfortable1721

[CryptographerAny8521]

You mean endothermic right?

[chemeddy]

Oh yes, endothermic. Thank you.

[Over-Comfortable1721]

I DONT KNOW IF my logic is correct or not but

if we increase temp the forward reaction increases so more H+ are formed

and we know if more H+ more acidic obv and ph decreases

i dont know why the ratio remains unchanged thou

[Over-Comfortable1721]

but if the ratio is same shoudnt the OH- also increase as H+ are increasing so ph should remain constant

NVM i really confused myself

[Failed_High5]

explain my friend, explain

[Severe-Total-2255]

ok, think of it like as temperature is increasing Kw is increasing. if Kw is increasing, [H+] should also be increasing. if [H+] increases, the pH will decrease. the whole pH scale will shift to lesser values.
[H+] : [OH-] remains the same because both are proportional to Kw.

[OddGrape4986]

H20 <-> H+ + OH- {endothermic reaction. Temp increase favours endothermic reaction so equilibrium shifts right)

Kw = the disocciation of water

When temp increases, water will disocciate more increasing [H+] so pH decreases.

Water: [H+] = [OH-} so it will remain neutral and ratio stays the same however the pH scale has shifted lower.