CHEM10007 Lecture Notes - Lecture 24: Barium Hydroxide, Conjugate Acid, Nitric Acid
12 Jun 2018
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LECTURE 24
CHAPTER 11 - ACIDS & BASES
11.3 STRONG ACIDS & BASES
•A strong acid is an acid that dissociates
completely in aqueous solution, with
each acid molecule donating a proton to
a water molecule.
•With this definition, for any strong acid
HA, the reaction;
•HA(aq) + H2O(l) = H3)+(aq) + A-
(aq)
•Proceeds to completion.
•Our definition of a strong base says that
OH- is formed quantitatively on
reaction with water, and therefore the
reaction of any strong base B with
water;
•B(aq) + H2O(l) = BH+ (aq) + OH-
(aq)
•Also proceeds to completion.
•The fact that strong acids undergo complete dissociation tells us something about the basicity of
their conjugate bases.
•For example, the equilibrium HCl(aq) + H2O(l) = H3O+(aq) + Cl-(aq) lies almost completely to
the right-hand side, and therefore the reverse reaction occurs to only a negligible extent. This
means that Cl-, the conjugate base of HCl, does not readily accept a proton from H3O+, and
consequently Cl- must be a very weak base.
•Such behaviour is common to all strong acids and we can make the generalisation that the
conjugate base of a strong acid is very weak.
•H3O+ is the strongest acid and OH- is the strongest base that can exist in aqueous solution.
•The strong acid HCl dissociates completely so that every molecule has donated its proton to a
water molecule to form H3O+, and the same is true of all acids stronger than H3O+; they react
completely when dissolved in water to form H3O+ and none of the undissociated acid is present.
•Similarly, any base stronger than OH- will quantitively deprotonate water to give OH-.
pH CALCULATIONS IN SOLUTIONS OF STRONG ACIDS & BASES
•When the solute in an aqueous solution is a strong monoprotic acid (Eg. Hcl, HNO3), we expect
to obtain 1 mole of H30+ for every mole of the acid in the solution. Thus, a 1.0 x 10^-2M
solution of HCl contains 1.0 x 10^-2 mol/L of H3O+ and a 2.0 x 10^-3M solution of HNO3
contains 2.0 x 10^-3 mol/L of H30+.
•To calculate the pH of a solution of a strong monoprotic acid, we use [H3O+] obtained from the
stated molecular concentration of the acid.
•Thus, a 1.0 x 10^-2M HCl solution has [H3O+] = 1.0 x 10^-2M, and therefore the pH = -
log[H3O+] = -log(1.0 x 10^-2) = 2.00.
•For strong bases, calculating the pH from the OH- concentration is similar.
•A 5.0 x 10^-2M solution of NaOH contains 5.0 x 10^-2mol/L of OH- because the base is
fully dissociated and each mole of NaOH releases 1 mole of OH- when it dissociates.
Therefore, pOH = -log(5.0 x 10^-2) = 1.30, and pH = 14.00-1.30 = 12.70 at 25˚C.
• For bases such as Ba(OH)2, we have to recognise that 2 moles of OH- are released by each mole
of the base.
•Ba(OH)2(s) = Ba2+(aq) + 2OH-(aq)
•Therefore, is a solution has 1.0 x 10^-2mol Ba(OH)2 per litre, the concentration of OH-
would be 2.0 x 10^-2M.
•Quickpoll Quiz 2:
•Which of the following is correct?
•B
•H2O and OH- are a conjugate pair and H2O is
the acid and OH- is its conjugate base.
NAMING ACIDS AND THEIR ANIONS
•To name a simple acid take the compound’s name and change the ‘ide’ ending to ‘ic acid’
•For example:
•hydrogen chloride becomes hydrochloric acid.
•The anion is chloride ion.
•In naming oxyacids, the acid name will often end in ‘ic acid’.
•The anion name will end in ‘ate’
•For some oxyacids, the acid name will end in ‘ous acid’.
•These will have fewer O atoms than the ‘ic acid’. The anion name will end in ‘ite’
•The prefixes ‘hypo’ and ‘per’ are also used in some cases along with ‘ous acid’ and ‘ic acid’.
SUMMARY
•An H3O+ ion is a hydrated proton
•Acids are proton donors
•Bases are proton acceptors
•Define amphoteric, diprotic, polyprotic substances
•Write ionisation reactions
•Strong acids fully ionise
•Weak acids partially ionise
•Conjugate pairs differ by one H+
•Naming acids and their anions (conjugate bases)
11.4 WEAK ACIDS & BASES
•Weak acids and weak bases react incompletely with water to give less than stoichiometric
amounts of H3O+ and OH-, respectively.
•The extent to which these reactions occur can be quantified by looking at the values of their
equilibrium constants.
•If we consider a weak monoprotic acid HA, we can write its reaction with water as
•HA(aq) + H2O(l) = H3O+(aq) + A-(aq), and the equilibrium constant expression for this is
therefore: Ka = [H3O+][A-]. !
[HA]
EQUILIBRIUM CONSTANT FOR AN ACID (Ka)
•Ka, Equilibrium Constant for the ionisation of an acid.
•The equilibrium constant for the dissociation of an acid in aqueous
solution is called the acidity constant, and is given the symbol Ka. Ka
tells us how far the reaction has proceeded towards completion when
equilibrium is established.
MEASURING ACID STRENGTH: Ka
•Extent of dissociation of: HA = H+ + A- is described by the
equilibrium constant Ka.
• = acid dissociation constant/ionisation constant/acidity constant.
•For a strong acid the equilibrium lies fully to the right; the value of Ka
is large
•For a weak acid, the equilibrium lies to the left; the value of Ka is
small (Ka for acetic acid = 1.8 x 10^-5).
EQUILIBRIUM CONSTANT FOR A BASE (Kb)
•We can write the equation for the reaction of a weak monoprotic
base B with water as:
•B(aq) + H2O(l) = BH+(aq) + OH-(aq), and the corresponding equilibrium constant expression:
•Kb = [BH+][OH-]!
[B]
•This is called the basicity constant and is given the symbol Kb.
Document Summary
11. 3 strong acids & bases: a strong acid is an acid that dissociates completely in aqueous solution, with each acid molecule donating a proton to a water molecule, with this definition, for any strong acid. Ha, the reaction: ha(aq) + h2o(l) = h3)+(aq) + a- (aq, proceeds to completion, our definition of a strong base says that. Hcl, hno3), we expect to obtain 1 mole of h30+ for every mole of the acid in the solution. The anion name will end in ite": the prefixes hypo" and per" are also used in some cases along with ous acid" and ic acid". Summary: an h3o+ ion is a hydrated proton, acids are proton donors, bases are proton acceptors, define amphoteric, diprotic, polyprotic substances, write ionisation reactions, strong acids fully ionise, weak acids partially ionise, conjugate pairs differ by one h, naming acids and their anions (conjugate bases)
