Okay, so we're going to be talking about water equilibrium here. Okay. Now water equilibrium basically what happens to water is if you have a glass of water, what happens is one water molecule collides with another water molecule. Okay.
And what happens is it undergoes ionization. Okay. So one water molecule become the hydronium ion and the other water molecule will become the hydroxide ion. Okay, one becomes the hydronium ion and the other one becomes the hydroxide ion. So this is called self-ion.
ionization all right self-ionization so they they knock into each other and one becomes positive and the other one becomes negative all right so this value this this whole reaction right this whole reaction is actually um this whole reaction is actually endothermic okay let me just redo this so the forward reaction the forward reaction is endothermic. So the forward reaction is endothermic. That means it's a heat-absorbing process.
The forward reaction is a heat-absorbing process. It's endothermic. And at 25 degrees Celsius, at 25 degrees Celsius, which we consider has room temperature, room temperature, it's a heat-absorbing process. 25 degrees Celsius.
The concentration of hydronium ion here is 10 to the power of negative 7 molar and hydroxide is 10 to the power of negative 7 molar. So the concentration of hydroxide and is always always equal to the concentration of hydronium ion okay it's always equal to if you have water okay you cannot have different it must be the same so the concentration of hydroxide ions And the concentration of hydronium ions must always be the same. Cannot be like one higher, the other one lower. And so if we write the K expression for this reaction, it will just look like this.
Hydronium. multiplied with hydroxide. Okay, that's it.
That will be the K expression and we'll put a W here. W here basically means water. So W is for water. Okay, so W is for water. Okay, so this is the x oops this is the expression for kw of water so we don't include water right because what this is liquid this is liquid here we don't include it okay now at 25 degrees celsius I just told you just now the values were this okay these were the values I told you just now so k w k w would be 10 the power of negative 7 times 10 the power of negative 7 which will be 10 the power of negative 14 okay molar square okay because this is molar and this is molar so it becomes molar square so so I can rewrite this whole equation like this this hydronium multiplied with hydroxide okay 10 the power of negative 14 okay so this is an equation which i will manipulate shortly to explain to you okay so um before i before we proceed so i just want to talk to you about other things like for example ph okay ph values you Okay, pH values.
Now, for pH values, right, there is a formula. pH equals to negative log 10, the concentration of hydronium ions. okay it's called it's a formula and we have to memorize this formula and before I go further this this this is called high draw I draw I draw okay I do empire Now how does the hydronium ion come about? You see, normally when you get a hydrogen ion, which we call a proton, it is extremely unstable.
It's very, very unstable in water. So it quickly combines with a water molecule. It quickly combines with a water molecule and it becomes a hydronium ion. So now it becomes aqueous.
That's what hydrodium ions are. So when like for example right when HCl when HCl releases the hydrogen, it breaks up, okay, the hydrogen and the chlorine break up, okay, they break up and they decide to go on their own ways, this hydrogen becomes very unstable, okay, it is unstable, so it quickly combines with water and becomes hydronium, so that's what hydronium is, all right. Now, if pH, if there is something called a pH value, there is something else which is called the pOH values. Thank you. pOH value.
So there is another formula that you have never ever probably come across in your life. It is this formula negative log 10 the concentration of hydroxide ions. This is a formula that you may not have come across.
So there's a formula which is pH equals to negative log 10 hydrogen ions, hydronium ions and pOH which is negative log 10 of hydroxide ions. So now when we look at this equation, again okay let me just rewrite it for you when we look at this equation again right hydronium hydroxide for 10 the power of negative 40 so what i'm going to do next is not important for your syllabus not important for you to know but i'm just going to quickly tell you do this with you if we take negative log 10 okay we take negative log 10 on both sides Okay, both sides. So it becomes negative log 10 hydronium ions, negative log 10 hydroxide ions, and negative log 10, 10 to the power of negative 14, right?
So I just told you negative log 10 hydronium ion is pH. negative log 10 of hydroxide ion is pOH and negative log 10 of 10 to the power of negative 14 is 14. So there is this formula. Now this formula is important. This formula is important. So there are three formulas which are important to us. The first formula, pH equals to negative log 10 concentration of hydronium ion.
So that's the first important formula. Second important formula, pOH equals to negative log 10 concentration of hydroxide ions. Third important formula, pH plus pOH. equals to 14. Okay, pH plus pOH equals to 14. So these are the three important formulas that you need to know or at least memorize them.
Okay, so we started off with water equilibrium. Okay, we started off with water equilibrium and then we have progressed to this. So the next video, what we'll do is we'll be looking at how to calculate pH of solutions.
pH of solutions. All right, so we'll stop here.