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- เผยแพร่เมื่อ 18 ต.ค. 2024
- Using the ionization of water equation as a basis, this explains how changes in temperature affect the hydronium ion concentration, hydroxide ion concentration, pH, pOH, and pKw of pure water.
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0:00here will consider how a change in temperature will affect the pH pH pkw
0:10and neutrality of pure water
0:13we'll just do a quick review of the relationship between hydronium ion
0:17concentration and ph because this is a negative sign the ph and the hydronium
0:22ion concentration have opposite trends
0:25so as we increase the hydronium ion concentration the ph decreases
0:31similarly when we consider hydroxide ion concentration and poh
0:36because this is a negative sign the poh and the 0 h minus concentration have
0:41opposite trends
0:43so as we increase the hydroxide ion concentration the poh decreases
0:48we'll make a note of these up here
0:51now we'll review how the temperature affects the ionization of pure water
0:56remember this equilibrium exists and all samples of water and it's and the
1:00thermic
1:01also recall that increasing the temperature ads heat to this equilibrium
1:05system
1:06according to this Italians principal the equilibrium will counteract the addition
1:11of heat by shifting to the right
1:14as the shift to the right occurs the hydronium and hydroxide ion
1:17concentrations increase water is a liquid so its concentration does not
1:22change
1:23after the shift a new equilibrium is established in which the concentration
1:27of h3o and the concentration of naoh - are both higher than they were before
1:32the shift
1:33it is important to realize that when the temperature increases the concentration
1:37of hydronium and the concentration of hydroxide increase by the same amount
1:43and the concentration of hydronium and the concentration of hydroxide remain
1:47equal to each other
1:49therefore the water remains neutral
1:52so if we increase the temperature and both hydronium and hydroxide ion
1:56concentrations increase
1:59as the concentration of hydronium increases the ph decreases
2:04and as the concentration of hydroxide increases the poh will also decrease
2:10at this point you must be really careful you might mistakenly conclude the
2:15decrease in PH means the solution gets more acidic
2:19a decrease in PH does not mean the solution gets more acidic
2:24when the temperature is increased the concentration of hydronium and the
2:27concentration of hydroxide increase by the same amount
2:31so the water remains neutral
2:34a question we can ask now is what happens to the value of K W when the
2:39temperature is increased
2:41remember KW is defined as the hydronium ion concentration times the hydroxide
2:46ion concentration
2:48so as the temperature is increased and the hydronium and hydroxide ion
2:52concentrations both increase
2:54so does their product aww
2:58also as the temperature is increased the ph and the poh both decrease
3:04so there's some the PK w also decreases
3:09so to summarize as the temperature increases the value of K W also
3:13increases
3:15and as the temperature increases the value of pkw decreases
3:20this makes sense because pkw is the negative log of K W
3:24it's ok w + PK w have opposite trends
3:28and when the temperature increases KW increases and pkw decreases
3:35in this table will summarize what happens to each one of these quantities
3:39as the temperature is increased and decreased
3:43because the ionization is endothermic an increase in temperature causes it to
3:48shift to the right
3:50the shift to the right cause the hydronium ion concentration to increase
3:54and the hydroxide ion concentration to increase
3:58because the concentration of hydronium and the concentration of hydroxide both
4:02increase their product KW also increases
4:07because the concentration of hydronium increases the value of ph decreases
4:12and because the concentration of h minus increases the value of poh also
4:17decreases
4:19because phn poh both decrease
4:22there's some pkw will also decrease
4:26water remains neutral changing temperature alone will never change the
4:30acidity of pure water as long as no acid or base is added water will always
4:36remain neutral
4:38now without looking at the answers for increasing the temperature
4:42pause the video and fill in the column for decreasing the temperature on your
4:46own
4:47be able to justify each of your answered
4:50then resume the video and check your answers
4:54because the ionization is endothermic a decrease in temperature would cause a
4:58shift to the left
5:00a shift to the left would cause the hydronium concentration to decrease
5:05and the hydroxide ion concentration to decrease
5:09because both hydronium and hydroxide decrease their product KW will also
5:15decrease
