Here donates the proton to water, so in the forward reaction it acts as the acid, and water is still the base. ’s conjugate base is NH3, and water’s conjugate acid is again the hydronium ion, H3O+.
Relative Strengths of Acids and Bases Certain acids are stronger than other acids, and some bases are stronger than others. What this means is that some acids are better at donating a proton, and some bases are better proton acceptors. A strong acid or base dissociates or ionizes completely in aqueous solution. A weak acid or base does not completely ionize. Strong Acids There are six strong acids that you’ll need to memorize for the SAT II Chemistry test:Hydrohalic acids: HCl, HBr, HI Nitric acid: HNO3Sulfuric acid: H2SO4Perchloric acid: HClO4 Let’s take a closer look at how acids differ in strength by focusing on perchloric acid. In general, the 美国GREater the number of oxygen atoms in a polyatomic ion, the stronger the acid.
So HClO4 is stronger than HClO3, which is stronger than HClO2, which is stronger than HClO. (Perchloric acid is the strongest among the six, but all the other oxyacids of chlorine are not considered strong acids.) Now think about why, as you take away oxygens, the strength of the acid decreases. The hydrogen (proton) to be removed is bonded to an oxygen atom. The oxygens are highly electronegative and are pulling the bonded pair of electrons away from the site where the hydrogen is bonded, thus making it easier to remove the H+. As the number of oxygen decreases, the molecule becomes less polar, and the H+ is harder to remove. Strong Bases There are fewer strong bases to memorize for the exam. These are hydroxides (—OH), oxides of 1A and 2A metals (except Mg and Be), H-, and . Remember that the stronger the acid, the weaker its conjugate base, and the converse is also true. The figure below illustrates the relative strengths of some common conjugate acid-base pairs.
The pH Scale As you know, water can act as either a proton donor (in the form of the hydronium ion, H3O+) or a proton acceptor (as OH-). In solution, a water molecule can even donate a proton to or accept a proton from another water molecule, and this process is called autoionization:
2H2OH3O+ + OH-
Since this reaction takes place in equilibrium, we can write an equilibrium expression, Keq, for it:
Keq = [H3O+][OH-]
And since this expression refers specifically to the ionization of water, we can write the equilibrium expression as Kw. At 25oC, the value of Kw, which is known as the ion-product constant, is 110-14. This means that the [H3O+] = [OH-] and each is equal to 110-7. When the concentrations of H+ and OH- are equal in a solution, the solution is said to be neutral. In acidic solutions, the concentration of H+ is higher than that of OH-, and in basic solutions, the concentration of OH- is 美国GREater than that of H+. The pH of a solution is calculated as the negative logarithm in base 10 of the hydronium ion concentration—it is an expression of the molar concentration of H+ ions in solution:英语作文
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