This page explains what is intended by the ionic product because that water. The looks at exactly how the ionic product varies v temperature, and also how the determines the pH of pure water at different temperatures.

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Kw and pKw

The crucial equilibrium in water

Water molecule can duty as both acids and bases. One water molecule (acting as a base) deserve to accept a hydrogen ion from a sehunterriverpei.comnd one (acting together an acid). This will be happening almost everywhere there is even a map of water - it doesn\"t have to be pure.

A hydroxonium ion and a hydroxide ion are formed.


However, the hydroxonium ion is a very solid acid, and also the hydroxide ion is a very solid base. As rapid as they are formed, they react to develop water again.

The net impact is the an equilibrium is collection up.


At any kind of one time, there are incredibly small numbers that hydroxonium ions and also hydroxide ion present. Additional down this page, we shall calculate the hunterriverpei.comncentration the hydroxonium ions present in pure water. It turns out to it is in 1.00 x 10-7 mol dm-3 at room temperature.

You might well uncover this equilibrium composed in a simplified form:

This is OK noted you remember the H+(aq) actually describes a hydroxonium ion.

Defining the ionic product for water, Kw

Kw is essentially just an equilibrium hunterriverpei.comnstant because that the reaction shown. Girlfriend may fulfill it in two forms:

Based ~ above the completely written equilibrium . . .


. . . Or on the streamlined equilibrium:


You may discover them written through or there is no the state symbols. Every little thing version friend hunterriverpei.comme across, they every mean precisely the very same thing!

You might wonder why the water isn\"t created on the bottom of this equilibrium hunterriverpei.comnstant expressions. So tiny of the water is ionised at any type of one time, that its hunterriverpei.comncentration continues to be virtually unmodified - a hunterriverpei.comnstant. Kw is identified to protect against making the expression unnecessarily hunterriverpei.commplicated through including an additional hunterriverpei.comnstant in it.

The worth of Kw

Like any type of other equilibrium hunterriverpei.comnstant, the worth of Kw varies v temperature. Its value is usually taken to be 1.00 x 10-14 mol2 dm-6 at room temperature. In fact, this is its worth at a bit less than 25°C.

The systems of Kw: Kw is found by multiplying two hunterriverpei.comncentration terms together. Every of these has actually the systems of mol dm-3.

Multiplying mol dm-3 x mol dm-3 provides you the systems above.


The relationship between Kw and also pKw is precisely the exact same as that between Ka and pKa, or and pH.


The Kw value of 1.00 x 10-14 mol2 dm-6 in ~ room temperature gives you a pKw value of 14. Try it on her calculator! an alert that pKw doesn\"t have any kind of units.

The pH of pure water

Why walk pure water have actually a pH of 7?

That question is in reality misleading! In fact, pure water only has a pH that 7 at a details temperature - the temperature in ~ which the Kw worth is 1.00 x 10-14 mol2 dm-6.

This is just how it hunterriverpei.commes about:

To find the pH friend need an initial to uncover the hydrogen ion hunterriverpei.comncentration (or hydroxonium ion hunterriverpei.comncentration - it\"s the same thing). Climate you hunterriverpei.comnvert it come pH.

In pure water in ~ room temperature the Kw worth tells girlfriend that:

= 1.00 x 10-14

But in pure water, the hydrogen ion (hydroxonium ion) hunterriverpei.comncentration should be equal to the hydroxide ion hunterriverpei.comncentration. For every hydrogen ion formed, there is a hydroxide ion developed as well.

That way that you can replace the term in the Kw expression by an additional .

2 = 1.00 x 10-14

Taking the square root of every side gives:

= 1.00 x 10-7 mol dm-3

hunterriverpei.comnverting that right into pH:

pH = - log10

pH = 7

That\"s where the acquainted value the 7 hunterriverpei.commes from.

The sport of the pH of pure water with temperature

The development of hydrogen ion (hydroxonium ions) and also hydroxide ions from water is an endothermic process. Utilizing the simpler version the the equilibrium:

The front reaction absorbs heat.

Achunterriverpei.comrding come Le Chatelier\"s Principle, if you make a readjust to the hunterriverpei.comnditions of a reaction in dynamic equilibrium, the position of equilibrium moves to hunterriverpei.comunter the readjust you have made.

Note: If you don\"t understand Le Chatelier\"s Principle, you need to follow this link before you go on. Make sure that you know the result of temperature on place of equilibrium.

Use the earlier button on your web browser when girlfriend are prepared to go back to this page.

Achunterriverpei.comrding come Le Chatelier, if you increase the temperature the the water, the equilibrium will relocate to reduced the temperature again. That will carry out that by taking in the extra heat.

That method that the forward reaction will certainly be favoured, and more hydrogen ions and hydroxide ions will certainly be formed. The impact of the is to rise the worth of Kw together temperature increases.

The table listed below shows the result of temperature ~ above Kw. Because that each value of Kw, a brand-new pH has actually been calculated making use of the same method as above. It could be advantageous if you to be to check these pH values yourself.

T (°C)Kw (mol2 dm-6)pH00.114 x 10-147.47100.293 x 10-147.27200.681 x 10-147.08251.008 x 10-147.00301.471 x 10-146.92402.916 x 10-146.77505.476 x 10-146.6310051.3 x 10-146.14

You can see that the pH of pure water falls as the temperature increases.

A indigenous of warning!

Although the pH of pure water transforms with temperature, that is vital to realise the it is tho neutral. In the instance of pure water, there are always going to it is in the same variety of hydrogen ions and also hydroxide ions present. That means that the pure water stays neutral - even if that is pH changes.

The trouble is the we space all so acquainted with 7 gift the pH the pure water, the anything rather feels yes, really strange. Remember that you calculation the neutral worth of pH indigenous Kw. If the changes, climate the neutral worth for pH transforms as well.

At 100°C, the pH that pure water is 6.14. The is the neutral point on the pH scale at this greater temperature. A systems with a pH the 7 at this temperature is slightly alkaline due to the fact that its pH is a bit greater than the neutral value of 6.14.

Similarly, you have the right to argue that a solution with a pH of 7 at 0°C is slightly acidic, since its pH is a little lower 보다 the neutral value of 7.47 at this temperature.

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Questions to test your understanding

If this is the first set of questions you have done, please check out the introduce page prior to you start. Girlfriend will should use the earlier BUTTON on your internet browser to hunterriverpei.comme earlier here afterwards.