Acids and bases. for it cannot be But I am pigeon-liver d and lack gall To make oppression bitter Hamlet. Different concepts Calculations and scales

Transcription

1 Acids and bases for it cannot be But I am pigeon-liver d and lack gall To make oppression bitter Hamlet Different concepts Calculations and scales

2 Learning objectives You will be able to: Identify acids and bases according to Arrhenius, Brønsted and Lewis definitions Identify conjugate acid-base pairs Calculate ph for solutions Write dissociation constant expression K a Calculate dissociation behavior of weak acids and bases Determine ph of weak acid/base solutions Predict ph of salt solutions Identify factors that influence acid strength

3 ACIDS AND BASES The meaning of acid and base has evolved Arrhenius acid is one that generates protons when dissolved in water HA(aq) = H + (aq) + A - (aq) Arrhenius base is one that generates hydroxide ions when dissolved in water MOH(aq) = M + (aq) + OH - (aq)

4 Mineral acids

5 Organic acids

6 Neutralization The mixing of an acid with a base: ACID + BASE = SALT + WATER The reaction of carbonic acid (CO 2 in H 2 O) to give limestone: H 2 CO 3 + Ca(OH) 2 = CaCO 3 + 2H 2 O

7 The essence of neutralization Elimination of the components of acid and base by combination to give H 2 O H 2 O doesn t ionize equilibrium lies to right H + + OH - H 2 O ACID BASE

8 Brønsted and Lowry A broader definition of acids and bases In the reaction NH 3 + HCl = NH 4 Cl has all the elements of acid-base neutralization but no H 2 O as would be required in the Arrhenius definition Brønsted acid donates a proton Brønsted base accepts a proton

9 Brønsted acid HCl + H 2 O = H 3 O + + Cl - DONOR ACCEPTOR

10 Brønsted base H 2 O + NH 3 = NH 4+ + OH - water NH 3 + HCl = NH 4+ Cl - No water DONOR ACCEPTOR

11 The products are themselves acids and bases HA + B BH + + A - DONOR ACCEPTOR DONOR ACCEPTOR

12 Equilibrium: solution contains mixture of all components ACCEPTOR DONOR DONOR ACCEPTOR

13 Conjugate acids and bases: follow the proton The difference is a proton base Conjugate base Conjugate acid acid HA + B A - + HB + Predicting conjugates Conjugate acid-base pair Conjugate acid-base pair

14 An acid solution

15 A basic solution

16 Substances can be both acids and bases depends on environment Note that in one instance H 2 O behaves like a base accepting protons, and in another, behaves like an acid donating protons HCl + H 2 O = H 3 O + + Cl - In presence of an acid H 2 O is a base NH 3 + H 2 O = NH 4+ + OH - In presence of a base H 2 O is an acid

17 It s a competition for protons The substance that is a stronger proton donor becomes the acid HCl + H 2 O = H 3 O + + Cl - The substance that is the stronger proton acceptor becomes the base NH 3 + H 2 O = NH 4+ + OH -

18 Strength and concentration Not all acids completely donate the protons to water molecules in solution HA + H 2 O A - + H 3 O + Degree of ionization is described by strength Total number of moles per unit volume is described by concentration

19 Strong acid: HCl is completely ionized to H 3 O + and Cl -

20 Weak acid: Acetic acid is only partly ionized

21 Strength is measure of dissociation

22 Relative strengths of conjugate pairs As acid gets stronger Loses H + more easily Conjugate base gets weaker Gains H + less easily As base gets stronger Gains H + more easily Conjugate acid gets weaker Loses H + less easily

23 Changing concentration does not change strength Strength refers to degree of ionization: Strong is completely ionized (100 %) Weak is partly ionized (1 % - 1:10 6 ) Concentration refers to number of moles per unit volume An acid (or base) can be strong and concentrated, weak and concentrated, strong and dilute, weak and dilute

24 Hydronium ion is the active ingredient of an acid in aqueous solution Protons do not exist in solution CH 3 CO 2 H + H 2 O = H 3 O + + CH 3 CO 2 - Vinegar in water produces hydronium ions

25 Hydroxide ion is the active ingredient of a base in aqueous solution NH 3 + H 2 O = NH 4+ + OH - Ammonia, a base, dissolves in water and produces hydroxide ions

26 Amphotericity A substance that behaves as an acid and a base is amphoteric (amphiprotic). Water is a good example Equilibrium lies far to left Acid Base

27 Ionization of water Even in pure water, a tiny fraction of the molecules are ionized and concentrations of OH - and H 3 O + are equal H 2 O + H 2 O H 3 O + + OH - [H 3 O + ] = [OH - ] Why is H 2 O not an electrolyte if it contains ions?

28 In all aqueous solutions, product of concentrations is a constant [H 3 O + ][OH - ] = K w At 25 C, [H 3 O + ] = 1 x 10-7 M Increasing [H 3 O + ] decreases [OH - ] (acidic conditions) Increasing [OH - ] decreases [H 3 O + ] (basic conditions) Playing with K w

29 Three types of solution

30 Calculating [OH - ] from [H 3 O + ] [H 3 O + ][OH - ] = K w [OH - ] = K w /[H 3 O + ] Example: if [H 3 O + ] = 1 x 10-3 M Then [OH - ] = /10-3 M = M

31 The ph scale reduces large range of numbers to small In water [H 3 O + ][OH - ] = Range of [H 3 O + ]: ph = - log 10 [H 3 O + ] 10 M (conc acid) M (conc base) Range of ph: -1 (conc acid) to +15 (conc base) Low ph = acid; high ph = basic ph = 7 = neutral

32 Relating ph to [H 3 O + ] For ph, take exponent of [H 3 O + ], change sign Acid HCl(aq): [H 3 O + ] = 1 x 10-1 M, ph = 1 Pure H 2 O: [H 3 O + ] = 1 x 10-7 M, ph = 7 Base: NH 3 (aq): [H 3 O + ] = 1 x M, ph = 11 Note: change of 1 unit in ph is factor of ten in [H 3 O + ] When [H 3 O + ] is 1 x 10 x, ph is whole number

33 Estimating ph Stranded on a desert island without a calculator you need to estimate the ph

34 Deconstructing ph ph = - log 10 [H 3 O + ] = -log 10 (a x 10 -b ) = -log 10 a + b Exponent ph = 3.00 [H 3 O + ] = 1.0 x 10-3 M Prefix 2 S.F. ph = Exponent [H 3 O + ] = 2.55 x 10-3 M Prefix 3 S.F.

35 Alternative pathways [OH - ] can be obtained from [H + ] using [OH - ] = K w /[H 3 O + ] Alternatively define poh = -log 10 [OH - ] ph poh 14

36 ph scale and common substances

37 Indicating ph All indicators involve a conjugate acid-base pair which have different colours HIn(aq) + H 2 O = H 3 O + (aq) + In - (aq) Colour A Colour B

38 Strong acids and bases and ph Monoprotic acids are completely ionized HCl + H 2 O H 3 O + (aq) + Cl - (aq) Polyprotic acids are not completely ionized even if strong (stay tuned) Strong bases are completely dissociated NaOH + H 2 O Na + (aq) + OH - (aq) CaO + H 2 O Ca 2+ (aq) + 2OH - (aq)