Chemical Reactions 2: Equilibrium & Oxidation-Reduction

Acid-Base Equilibrium

Acids

Fruit juices, soft drinks, gastric juicesSour tasteRed and blue litmus paper turn RedArrhenius: acids release H+ when dissolved in Water (aqueous solution)

HF (l) + H2O H+(aq) + F-

(aq)

H2SO4 (l) + H2O 2H+(aq) + SO4

2-(aq)

HI (l) + H2O H+(aq) + I-

(aq)

Acid-Base Equilibrium

Acids

Strong Acids dissociate completelyWeak Acids DO NOT dissociate completelyWeak Acids establish a dissociation equilibrium

HA(aq) H+(aq) + A-

(aq)

*In the case of strong acids, such equilibrium is almost entirely displaced toward formation of ions

Acid-Base Equilibrium

Dissolution Constant of Acids

aHA(aq) bH+(aq) + cA-

(aq)

_Ka value is a measure of acid’s strength

_Weak acids have low values of Ka

Acid-Base Equilibrium

Acid-Base Equilibrium

Bases

Cleaning products, some heartburn medicationBitter tasteFeel slippery (react with oil in skin forming a kind of soap)Red and blue litmus paper turn BlueArrhenius: bases release OH- when dissolved in Water (aqueous solution)

NaOH (s) + H2O Na+(aq) + OH-

(aq)

Mg(OH)2 (s) + H2O Mg2+(aq) + 2OH-

(aq)

NH4OH (l) + H2O NH4+

(aq) + OH-(aq)

Acid-Base Equilibrium

Bases

Strong Bases dissociate completelyWeak Bases DO NOT dissociate completelyWeak Bases establish a dissociation equilibrium

BOH(aq) B+(aq) + OH-

(aq)

*In the case of strong bases, such equilibrium is almost entirely displaced toward formation of ions

Acid-Base Equilibrium

Dissolution Constant of Acids

aBOH(aq) bB+(aq) + cOH-

(aq)

_Kb value is a measure of base’s strength

_Weak bases have low values of Kb

Acid-Base Equilibrium

Acid-Base Equilibrium

pH

Property that distinguishes acidic, basic and neutral solutions

Measures the concentration of H+ ions in solution

Can be determined by indicator, pH paper, pH meter, etc.

Acid-Base Equilibrium

pH

pH scale: 0 – 14 pH < 7 (Acidic solution)

_0 < pH < 3 (STRONGLY Acidic)

_4 < pH < 6 (Slightly Acidic ) pH = 7 (NEUTRAL solution) pH > 7 (Basic solution)

_8 < pH < 11 (Slightly Basic)

_12 < pH < 14 (STRONGLY Basic )

Acid-Base Equilibrium

pH

pH = - log c(H+)

Log makes differences of one unit, to be ten times different

Example: If A (pH = 3), B (pH = 1), C (pH = 9)

Then B is 100 times more acidic than A

Acid-Base Equilibrium

c(H+) pH = - log c(H+) Solution

10-1 mol/L 1 Acid (H+)

10-2 mol/L 2 Acid (H+)

10-3 mol/L 3 Acid (H+)

10-4 mol/L 4 Acid (H+)

10-5 mol/L 5 Acid (H+)

10-6 mol/L 6 Acid (H+)

10-7 mol/L 7 Neutral (H2O)

10-8 mol/L 8 Basic (OH-)

10-9 mol/L 9 Basic (OH-)

10-10 mol/L 10 Basic (OH-)

10-14 mol/L 14 Basic (OH-)

[H+] = 10-pH

Acid-Base Equilibrium

Indicate True or False:__B (c(H+)=10-2 M) has pH = 3

__A (c(H+)=10-4 M) is 100 times more basic than B

__C (neutral pH) is 1000 times more basic than D (c(H+)=10-5 M)

__D (c(H+)=10-5 M) is a million times more acidic than E (pH = 10)

Order these substances from more acidic to most basic:

Order these substances from highest pH to lowest:

F

T

F

F

B – A – D – C – E

E – C – D – A – B

Acid-Base Equilibrium

pH & Environment (Water bodies) BEFORE

High Pollution High Toxic levels At mercy of private companies disposal policies No environmental regulation

Acid-Base Equilibrium

pH & Environment (Water bodies) AFTER

Lower levels of Pollution Lower levels of Toxicity Constant monitoring of pH levels Government environmental regulation

Acid-Base Equilibrium

Neutralization Reaction

Involves an acid (pH < 7) and a base (pH > 7) Produces salt and water (pH = 7)

Acid-Base Equilibrium

Titration (CaVa = CbVb)

Experimental technique to determine the concentration of an acid or a base

Uses known volumes of acid and base Uses a known value of concentration With all three variables (Va, Vb, Ca or Cb) finds Cb or Ca

using the Dilution formula (CaVa = CbVb)

Titration needs an indicator to determine the final point

Acid-Base Equilibrium

What is an indicator?!

Acid-Base Equilibrium

Indicator

Usually weak acids or weak bases Indicator’s colour changes according to the level of

pH of the solution that it is added to Colour change usually in a range (e.g. pH = 4.5 – 5.5)

Acid-Base Equilibrium

Titration technique

Acid-Base Equilibrium

Example: Determine the concentration of a solution of HCl, if 15ml of this acid are titrated with 25ml of NaOH. The concentration of the latter is 0.45M. Determine the pH of the HCl solution.

CaVa = CbVb

Ca = CbVb / Va

Ca = (0.45M) (25ml) / (15ml)Ca = 0.75 M

pH = - log c(H+)pH = - log (0.75)

pH = - (-0.12)pH = 0.12