Method 3: Lewis relies on electron pairs in .NET

Drawer EAN / UCC - 13 in .NET Method 3: Lewis relies on electron pairs

In the same year that Br nsted and Lowry proposed their definition of acids and bases, an American chemist named Gilbert Lewis proposed an alternate definition that not only encompassed Br nsted-Lowry theory, but also accounted for acid-base reactions in which a hydrogen ion isn t exchanged Lewis s definition relies on tracking lone pairs of electrons Under his theory, a base is any substance that donates a pair of electrons to form a coordinate covalent bond with another substance, while an acid is a substance that accepts that electron pair in such a reaction As we explain in 5, a coordinate covalent bond is a covalent bond in which both of the bonding electrons are donated by one of the atoms forming the bond All Br nsted-Lowry acids are Lewis acids, but in practice, the term Lewis acid is generally reserved for Lewis acids that don t also fit the Br nsted-Lowry definition The best way to spot a Lewis acid-base pair is to draw a Lewis dot structure of the reacting substances, noting the presence of lone pairs of electrons (We introduce Lewis structures in 5) For example, consider the reaction between ammonia (NH3) and boron trifluoride (BF3): NH3 + BF3 NH3BF3

At first glance, neither the reactants nor the product appear to be acids or bases, but the reactants are revealed as a Lewis acid-base pair when drawn as Lewis dot structures as in Figure 16-1 Ammonia donates its lone pair of electrons to the bond with boron trifluoride, making ammonia the Lewis base and boron trifluoride the Lewis acid

Sometimes you can identify the Lewis acid and base in a compound without drawing the Lewis dot structure You can do this by identifying reactants that are electron rich (bases) or electron poor (acids) Metal cations, for example, are electron poor and tend to act as a Lewis acid in a reaction, accepting a pair of electrons In practice, it s much simpler to use the Arrhenius or Br nsted-Lowry definitions of acid and base, but you ll need to use the Lewis definition when hydrogen ions aren t being exchanged You can pick and choose among the definitions when you re asked to identify the acid and base in a reaction

Identify the acid and base in the following reaction and label their conjugates NH3 + H2O NH4+ + OH

This is a classic Br nsted-Lowry acidbase pair Water loses a proton to ammonia, forming a hydroxide anion (an atom

with a negative charge) This makes water the proton donor, or Br nstedLowry acid, and OH its conjugate base Ammonia accepts the proton from water, forming ammonium (NH4+) This makes ammonia the proton acceptor, or Br nsted-Lowry base, and NH4+ its conjugate acid

Consider the following reaction Label the acid, base, conjugate acid, and conjugate base, and comment on their strengths How can water act as an acid in one reaction and a base in another HCl + H2O H3O+ + Cl

Use the Arrhenius definition to identify the acid or base in each reaction and explain how you know a NaOH(s) + H2O Na+(aq) + OH (aq) + H2O b HF(g) + H2O H+(aq) + F (aq) + H2O

Identify the Lewis acid and base in each reaction below Draw Lewis dot structures for the first two, and determine the Lewis acid and base in the third reaction without a dot structure a 6H2O + Cr3+ Cr(OH2)63+ b 2NH3 + Ag+ Ag(NH3)2+ c 2Cl + HgCl2 HgCl42

A substance s identity as an acid or a base is only one of many things that a chemist may need to know about it Sulfuric acid and water, for example, can both act as acids, but using sulfuric acid to wash your face in the morning would be a grave error indeed Sulfuric acid and water differ greatly in acidity, a measurement of an acid s strength A similar quantity, called basicity, measures a base s strength Acidity and basicity are measured in terms of quantities called pH and pOH, respectively Both are simple scales ranging from 0 to 14, with low numbers on the pH scale representing a higher acidity and therefore a stronger acid On both scales, a measurement of 7 indicates a neutral solution On the pH scale, any number lower than 7 indicates that the solution is acidic, with acidity increasing as pH decreases, while any number higher than 7 indicates a basic solution, with basicity increasing as pH increases In other words, the farther the pH gets away from 7, the more acidic or basic a substance gets pOH shows exactly the same relationship between distance from 7 and acidity or basicity, only this time low numbers indicate very basic solutions, while high numbers indicate very acidic solutions