Question

Write the chemical reaction for the auto-ionizatooj for water and the Kw expression in terms of the symbolic concentration of the reaction species. Label each species physical state

Answer 1

water undergo autcionization to form H₂ot and onions the equilibrium constant for the actoionization of water kw is 10th cet as c At neubel satution 0+1= [OH J → Acidic Solution [H2017 > Com'] 5 Basic solution com]X[40+] → for agus solution at 25°c the following relationship always towe Kw = [heut] [or ] =10-14 pH + pOH = 14 y the contribution of autoionization of water [hzet ] and [OH-] becomes significant for extormly dilute cuid and base solution Reaction - HOW 4H2O(l) 2 Hot tot 2H20 3 430+ + OH Kw=[H₃O+] [OH-] = 1810-14 If we knows the concentration of either of hychonium ions or Hydroxide ion in water solution the other ion concentration can be determined [on 1=kw ] = 14 lut or [H3Of] = kwa eg what is (oht ] concentration in water solution that is a oso M NaOH ?

NAOH → strong base so yo dissociated into its compononis ions therlore the concentration of love ion will be 0.050M So we can calculade (not) concentration (H207 ] = [1 X 10"") - kW = d.ox 163M Com] 0.050 Behaviour species inwater is - Solute added to water aneutral, acidic, basic Solm will arises . pH depends upon rxn beth solute and H₂O To When aid added to water there is Boonsted Lowery auid-Base reaution where ceid behave as an cuid and watu acts as base. When buve geins a prolon it becomes aid called conjugated cuid HA + B + A + BHT acid - Buse conjugated conjugeted baje cud Acc to Bronsted - Lowery concept Reaction between I aid and base occur form new acid and new bay aid & base exist in pair When and added to water B-L acid base oxn when and behave as auid and wateract by buse Coilled Acid ionization reaction НА + Hzoa) < HATCan) Hot (9) cuid B. L acid &xn where A base added to water buse act as base 6 water ad as cid Base ionization Reaution

Be able to identify or provide the conjugate acids and conjugate bases of any acid or base.

Remember, a conjugate

base will have one less hydrogen than the acid and the conjugate acid will have one more hydrogen.

Keep your

charges straight. Remember the hydrogen is removed as a proton.

Categories of behavior of solutes in water –

what are the major species in water

1) If a solute has strong covalent bonds there is no reaction between solute and water. The pH will be

unchanged when the solute dissolves. These are molecules with no H in the formula or contain an H that will

not come off easily such as that found in methanol (CH3OH), ethanol (CH3CH2OH) or alkane chains such as

butane (CH3CH2CH2CH3)

2) The solute is covalent and undergoes a reaction with water to produce a proton.

a) There is a complete reaction with water when the solute is a strong acid (HClO4, H2SO4, HNO3, HI,

HBr, HCl0) There will be no undissociated acid molecules left. The major species in solution after this

reaction will be the conjugate base, H3O

+

, the anion, and H2O

HNO3(aq) + H2O(l) -> NO3-(aq) + H3O+(aq)

b) There is a partial reaction with water when the solute is a weak acid. Any acid that is not a strong acid is considered to be a weak acid. There will be an equilibrium and the major species in solution will be the weak acid and water. There will be small amounts of the conjugate base and the hydronium ion.

HF(aq) + H2O(l) -> F-(aq) + H3O+(aq) (acid ionization)

3) The solute is ionic. It will dissociate into ions and the component ions may undergo reaction with water.

a) Strong bases will dissociate to form OH-. Strong bases are the hydroxides of Group I and Group II

metals. (NaOH and KOH are common strong bases. LiOH, RbOH, and CsOH are less common

strong bases. Ca(OH)2, Ba(OH)2 and Sr(OH)2 will produce two moles OH-

per mole of solid). These strong bases will completely dissociate in water. The major species in water will be the metal ion, the hydroxide ion and the water

The reaction of interest is:

OH-(aq) + H2O(l) -> H2O(l) + OH-(aq). Since there is no net change

in the molecules present after the reaction, it is sufficient to know that the base dissociates in water.

b) anion example: NaCN(aq) -> Na+(aq) + CN-(aq)

Na+ + H2O -> no reaction

CN- + H2O -> HCN + OH- (base ionization)

c) cation example CH3NH3Cl(aq) -> CH3NH3 (aq) + Cl- (aq)

Cl-(aq) + H2O(l) -> no reaction

CH3NH3+(aq) + H2O(l) -> CH3NH2-(aq) + H3O+(aq) acid ionization

Be able to write the chemical equations that show the acid base reaction of solutes in water.

☆Acid Ionization, Ka and pKa ->

The acid ionization reaction is the transfer of a proton from the acid to water.

HA(aq) + H2O(l) -> A-(aq) + H3O+(aq)

The equilibrium expression for this reaction is

Keq = [A-][H3O+]/[HA] = Ka

For the acid ionization, Keq is written Ka and is called the acid dissociation constant.

Ka values allow one to compare the strength of acids. Since it is an equilibrium constant, the larger the Ka, the more products there are, which means there will have been more dissociation of the acid and more protons formed. Thus, the larger the Ka, the stronger the acid. For strong acids, the reaction lies so far to the right that [HA] is very small and it is difficult to determine the Ka accurately. However, comparing the values of weak acids is very useful.

The p notation can be employed, such that pKa = -log(Ka).

Note that the larger the Ka, the smaller the pKa. Thus the stronger the acid, the smaller the pKa.

Although we will discuss bases more later, a comment can be made now on the relative strength of bases. The relative strength of an acid is based on the extent to which it produces protons. The relative strength of a base depends on its affinity for a proton. The greater the affinity, the stronger the base.

In general, the stronger acid will produce a weaker conjugate base.

Consider:

HF(aq) + H2O(l) -> F-(aq) + H3O+

(aq) Ka = 7.2 x 10-4

CH3COOH(aq) + H2O(l) -> CH3COO- + H3O+

Ka = 1.85 x 10-5

In the first reaction there is a competition between H2O and F-for the proton.

In the second reaction there is a competition between H2O and CH3COO-for the proton.

The first reaction lies further to the right than the second reaction. Thus water can steal the proton from F-easier than from CH3COO-. Thus, F- is the weaker base and HF is the stronger acid.

That is – the stronger the acid, the weaker the conjugate base or the weaker the base, the

stronger the acid