For class 10 only?
Answer:
at 273 kelvin water has a perfect ip that is the no. of oh- ions are equal to the h3o+ ions and as a reult when ti ionizes it is neutral . thus it has no effect on litmus paper be it blue or red however the ionic product of water is 1*10^-14 and thus its neutral. however at 298 k (25 c) it turns alkaline due to more oh- ions for further doubts clarifiy with isc xii std text book kalyani publishers
298 K = 25 °C
[H3O+ ] = [OH-] = 1.00 x 10^-7 M
also
pH + pOH = 14
and
[H3O+] [ OH-] = 1.00 x 10^-14
The self-ionization of water is the chemical reaction in which two water molecules react to produce a hydronium (H3O+) and a hydroxide ion (OH−):
2H2O(l) ------> H3O+(aq) + OH'(aq)
The reaction is also known as the autoionization or autodissociation of water. It is an example of autoprotolysis, and relies on the amphoteric nature of water.
Water, however pure, is not a simple collection of H2O molecules. Even in "pure" water, sensitive equipment can detect a very slight electrical conductivity of 0.055 µS.cm-1. According to the theories of Svante Arrhenius, this must be due to the presence of ions.
The preceding reaction has a chemical equilibrium constant of Keq = ([H3O+] [OH−]) / [H2O]2 = 3.23 × 10−18. So the acidity constant which is Ka = Keq × [H2O] = ([H3O+] [OH−]) / [H2O] = 1.8 × 10−16. For reactions in water (or diluted aqueous solutions), the molarity (a unit of concentration) of water, [H2O], is practically constant and is omitted from the acidity constant expression by convention. The resulting equilibrium constant is called the ionization constant, dissociation constant, or self-ionization constant, or ion product of water and is symbolized by Kw.
Kw = Ka [H2O] = Keq [H2O]2 = [H3O+] [OH−]
where
[H3O+] = molarity of hydrogen or hydronium ion, and
[OH−] = molarity of hydroxide ion.
At Standard Ambient Temperature and Pressure (SATP), about 25°C (298K), Kw = [H3O+][OH−] = 1.0 × 10−14. Pure water ionizes or dissociates into equal amounts of H3O+ and OH−, so their molarities are equal:
[H3O+] = [OH−].
At SATP, the concentrations of hydroxide and hydronium are both very low at 1.0 × 10−7 and the ions are rarely produced: a randomly selected water molecule will dissociate within approximately 10 hours[2]. Since the concentration of water molecules in water is largely unaffected by dissociation and [H2O] equals approximately 56 mol/l, it follows that for every 5.6 × 108 water molecules, one pair will exist as ions. Any solution in which the H3O+ and OH− concentrations equal each other is considered a neutral solution. Absolutely pure water is neutral, although even trace amounts of impurities could affect these ion concentrations and the water may no longer be neutral. Kw is sensitive to both pressure and temperature; it increases when either increases.
By definition, pKw = −log10 Kw. At SATP, pKw = −log10 (1.0 × 10−14) = 14. pKw also varies with temperature. As temperature increases, pKw decreases; and as temperature decreases, pKw increases (for temperatures up to about 250 °C). This means that ionization of water typically increases with temperature. There is also a (usually small) dependence on pressure (ionization increases with increasing pressure). The dependence of the water ionization on temperature and pressure has been well investigated and a standard formulation exists .
It should be noted that deionized water (also called DI water) is water that has had most impurity ions common in tap water or natural water sources (such as Na+ and Cl−) removed by means of distillation or some other water purification method. Removal of all ions from water is next to impossible, since water self-ionizes quickly to reach equilibrium.
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