Sodium hydroxide proton acceptance

In one type of titration, a solution of a strong base such as sodium hydroxide is added slowly to a solution that contains an unknown amount of an acid. Each hydroxide ion added to the acid solution accepts one proton from a molecule of acid. As the titration proceeds, fewer and fewer acid molecules remain in the acid solution, but the solution is still acidic. At the stoichiometric point, just enough hydroxide ions have been added to react with every acidic proton present in the acid solution before the titration was started. The hydroxide ions in the next drop of titrant do not react because acid molecules are no longer present in the solution. Before the stoichiometric point, the solution contains excess acid. After the stoichiometric point, the solution contains excess OH". Figure 4-11 shows a titration setup and molecular views illustrating titration of a strong acid by a strong base. 

Thus, sodium hydride, NaH, reacts with water to give a solution of sodium hydroxide. In terms of acid/base theory, the hydride ion is a stronger base than OH and readily accepts a proton from water. 

But what is a base When a base is dissolved in water it liberates negatively charged hydroxyl ions (OH-). When a base is neutralized, these ions take on — or accept — positively charged protons from another substance. A base is a substance that will accept and combine with protons from another substance. Bases are "proton acceptors. The most important bases are sodium hydroxide ( lye, NaOH), ammonium hydroxide ( ammonia, NIL OH), and calcium hydroxide ("slaked lime, Ca(OH),). 

It follows that sodium hydroxide, because it supplies OH- ions, which accept protons, is a Bransted base. Ammonia is a base because, as we see from Eq. 1, it accepts protons from water and forms NH4+ ions. 

Hydroxides, such as sodium hydroxide and calcium hydroxide, are also strong bases in water. However, there is an interesting point here if an OH" ion accepts a proton from a water molecule, the water molecule donating the proton becomes a hydroxide ion, so there is no net change ... 

There are also substances which, once dissolved, accept a proton from a water molecule to bind it to a particle of their own. In this way the so-called hydroxide ions OH are formed which are characteristic for an alkaline solution. Three kinds of compounds are capable of this a) certain salts, e.g. sodium cyanide (NaCN), b) hydroxides, e.g. sodium hydroxide (NaOH) and c) molecular bases, e.g. ammonia (NH3). 

Not all acid-base reactions occur in a 1 1 combining ratio (as hydrochloric acid and sodium hydroxide in the previous example). Acid-base reactions with other than 1 1 combining ratios occur between what are termed polyprotic substances. Polyprotic substances donate (as acids) or accept (as bases) more than one proton per formula unit. 

Here is a tiny technical point that I really do need to introduce. Chemists now refer to a proton-accepting molecule and ion as a base . Thus, OH is a base. They keep the term alkali for bases dissolved in water. So, for instance, sodium hydroxide, NaOH, dissolves in water, separating into Na ions and OH ions. It is therefore a source of the base OH and the solution is an alkali. I shall use the term base from now on, because it is more general than alkali (a molecule or ion doesn t need to be present in water to be a base). 

The Arrhenius definitions and those of Brpnsted and Lowry are essentially equivalent for aqueous solutions, although their points of view are different. For instance, sodium hydroxide and anunonia are bases in the Arrhenius view because they increase the percentage of OH ion in the aqueous solution. They are bases in the Brpnsted-Lowry view because they provide species (OH from the strong electrolyte sodium hydroxide and NH3 from ammonia) that can accept protons. ... 

Equivalent weights of acids and bases. One equivalent of an acid is the quantity that transfers (gives up) 1 mole ofH (1 mole of protons). Correspondingly, 1 eq of a base is the quantity that accepts 1 mole of H. Sodium hydroxide, NaOH, can react with phosphoric acid, H3PO4, for example, in any one of three ways ... 

Strong bases are those compounds that totally dissociate in water, yielding some cation and the hydroxide ion. It is the hydroxide ion that we normally refer to as the base, because it is what accepts the proton. Calculating the hydroxide ion concentration is really stredghtforward. Suppose that you have a 1.5 M (1.5 mol/L) NaOH solution. The sodium hydroxide, a salt, completely dissociates (breaks apart) into ions ... 

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

Base is a substance that accepts protons, that is, indicate the alkalinity of water. A strong base ionizes in solution and yields OH ions. Sodium and potassium hydroxides are examples of strong inorganic bases ... 

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