Chapter 17.docx

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Department
Chemistry & Biochemistry
Course
CHM 142
Professor
Dr.James D.Garrity
Semester
Summer

Description
Acid  Arrhenius definition—molecules that dissociate in water to yield H O+ ions  Bronsted-Lowry Definition—Proton donors (H+) Base  Arrhenius definition—molecules which dissociate in water to yield OH- ions  Bronsted-Lowry definition—proton acceptor (H+) Differences in definitions  All Bronsted-Lowry acids are Arrhenius acids o With Arrhenius, proton acceptor must be water  Bronsted-Lowry expands definition of bases beyond metal hydroxides (ex. NH 3) Strong acid—completely ionized in water (technically, never true. Kinfinity) Weak acid—only partially ionizes in water Equilibrium constant (K) is called K, the acid dissociation constant  For weak acids, K a<1 Any molecule with a donateable H+ is a potential acid; any molecule with a lone pair is a potential base Conjugate acid-base pairs—differ from each other only by the presence of the H+ ion  The stronger an acid is, the weaker its conjugate base is Strong base—hydroxide or oxide of Group I or II metals Weak base-molecule which can accept a proton To be a base:  Must have a lone pair  Most commonly, have N pH= -log([H 3O+]) Amphiprotic/amphoteric—capable of acting as a bronsted-lowry acid or base. (Can donate or accept a proton). (ex. Water) Kc=[H2O+][OH-]/([H2O][H2P]) Kw =[H3O+][OH-]= 1*10^-14 In the absence of other acids or bases at 298K: [H3O+]=[OH-]=1*10^-7 M pH value will appear to have one more significant digit than the concentration because technically the first (7 in 7.00) isn’t significant because it is from the exponent, it’s a placeholder pH = -log[H3O+] For any aqueous solution at 25C  1*10^-14 = {H3O+][OH-]  14= pH + pOH px = -log[x] Factors affecting Acid strength  The stability of the conjugate base (anion=negative ion) formed when acid donates the proton o The more stable the conjugate base, the stronger the acid o The electronegativity of the element that the H is bonded to  The greater the electronegativity, the more stable the anion, the stonger the acid  Periodic trend: left  right o Radius of anion  Greater size of electron cloud, more stable anion, stronger acid  Group trend, top  bottom  Oxyanions o The greater the number of oxygen atoms, the stronger the acid o Electronegativity of element to which OH is bonded For very weak aci
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