As you can see from the very small value of K W, water is not very dissociated at room temperature and our assumption that the concentration of water being constant is a good one.įor pure water, the values of and are equal and thus their values are both 1.0 x 10 -7. The value of this constant at 25 oC is 1.0 x 10 -14.
The resulting equation is given a special designation K W called the ionization constant of water: If we now write this reaction into an equilibrium expression:īut remember one of our rules regarding the writing of equilibrium expressions states that we do not include solvents (liquids) as their concentration stays pretty constant during a reaction so the water portion of the equation is essentially equal to 1 and thus "disappears". This reaction does fit the Brønsted-Lowry definition since one water molecule is acting as an acid "donating" a proton and the other is acting as a base "accepting" the proton: Let's start by looking at the Ionization of Water: And we will now tie the concept of acids and bases into equilibrium: According to this theory, an acid is a "proton donor" and a base is a "proton acceptor."įor now we will stick with the Brønsted-Lowry definition. Water reacts with itself, for example, by transferring an H + ion from one molecule to another to form an H 3O + ion and an OH - ion. The Brønsted-Lowry definition: Brønsted argued that all acid-base reactions involve the transfer of an H + ion, or proton.
The Lewis definition: A Lewis acid is one that can accept a pair of electrons and form a coordinate covalent bond. The Arrhenius definition: By the 1884 definition of Svante Arrhenius (Sweden), an acid is a material that can release a proton or hydrogen ion (H +). There are several definitions of what constitutes an acid:
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