Chemistry of LifeLecture 3

Prepared by IJn

Acids, bases, pH and buffers

H+

Acids

A hydrogen atom consists of one electron and one proton.

Loss of electron Proton – a hydrogen ion, H+

An acid is a substance which can act as a proton donor.

or,

An acid is defined as a substance which ionizes in water to give H+ ions as the cation (positive ion).

A strong acid Hydrochloric acid (undergoes almost complete dissociation)

HCl + Cl-

A weak acid Acetic acid (a small proportion of the acid dissociates to give hydrogen ions)

CH3COOH CH3COO- + H+

Acids

• Strong Acid = pH 1-3

• High in H+ ions

• Lower number of OH-ions

Bases

• Substance that can accept protons, so they can raise thepH of fluids and make them basic, or alkaline.

Examples: NaOH, KOH etc

• Strong Base = pH 11 – 14

• High in OH-ions

• Lower in number of H+ ions

Salts

A salt is a substance in which replaceable hydrogen of anacid has been partly or completely replaced by a metal.

For example, Sodium Chloride (NaCl) where the hydrogenatom of hydrochloric acid has been replaced by an atom ofsodium.

HCl + NaOH NaCl + H2O

(Salt)

Acids, Bases and Salts

a) In water, Hydrochloric acid (HCl) dissociates into H+ and Cl-.

b) Sodium hydroxide (NaOH) a base, dissociates into OH- and Na+ in water.

c) In water, Table salt (NaCl) dissociates into positive ion (Na+) and negative ion(Cl-), neither of which are H+ or OH-.

pH

The pH is defined as the negative logarithm to the base 10 of thehydrogen ion concentration; determined in moles per liter [H+].

Pure water contains 1x107 moles of hydrogen ions per liter. ThepH of water is therefore,

pH = -log1010-7 = 7

• pH is a measure of the concentration of hydrogen ions in a solution.

pH = 7 : A neutral solution (H+ and OH- are equal)

pH < 7 : an acidic solution (H+ concentration> OH-

concentration)

pH > 7 : an alkaline solution (OH- concentration > H+

concentration)• The pH scale ranges from 0 - 14

• A change of one whole number represents a tenfold change from the previous concentration.

• A solution of pH 1 has 10 times more H+ than a solution of pH 2 and 100 times more H+ ions than a solution of pH 3.

pH scale

The pH of few common substances

Approximate pH Common Examples

Strong Acids 0-2Stomach acid (HCl),

battery acid (H2SO4)

Weak Acids 3-6 Lemon juice, vinegar

Neutral 7 Pure water

Weak Bases 8-11 Bicarbonate solution

Strong Bases 12-14 Solutions of NaOH, KOH

Human blood pH is 7.4 – Mild BasicWater pH is 7.0 -- Neutral

Gastric juice pH is 2.0 --- Strong acid

Buffers

Living organisms Takes up

nutrientsExcretes waste

Chemical reactions

Balance of acids and bases change

Change of pH

Living organisms overcome this adverse effect by means of pH buffers

A buffer solution is a solution containing a mixture of a weak acid and its solublesalt. It acts to resist changes in pH. Such changes can be brought about by dilutionor addition of acid or alkali.

Increased acidity More H+

Free anion (negative ion)

from salt

Removal of H+ from

solution

Drop in pH

At increased alkalinity

Decrease in acidity Tendency to release hydrogen ions

Thus buffer solution tends to maintain a constant, balanced hydrogen ion concentration

Example,

NaHCO3 Na+ + HCO3 -

Sodium hydrogen carbonate

Sodium ion Hydrogen carbonate ion

HCO3 -

Hydrogen carbonate ion

+ H+ H2CO3

Carbonic acidHydrogen Ion(removal of hydrogen ions from the solution)

Lowering solution’s acidity

HCO3 -

Hydrogen carbonate ion

+ OH-CO3

2- + H2O

(removal of Hydroxyl ions from the solution)

Lowering solution’s alkalinity

Diffusion

• The difference in concentration of a substance between two areas is called aconcentration gradient.

• Particles move down a concentration gradient by diffusion, until they arespread evenly.

• Diffusion is a passive process: it requires no input of energy.

Examples of diffusion across concentration gradients in organism

Place Particles move From To

Gut Digested Gut cavity Blood in capillary of villusFood products

Lungs Oxygen Alveolar air space Blood circulating around the lungs

Diffusion in water

What affects the rate of diffusion?

• Concentration gradient: The greater the difference in the concentration of a substance in two areas, the faster the rate of diffusion

The rate of diffusion is

directly proportional to

the concentration

gradient

The larger the surface area the higher the rate of diffusion (e.g. in gases diffusing into/out of leaves)

• Osmosis is the diffusionof water.

• It is the net movementof solvent (water)molecules from aregion of their higherconcentration to aregion of their lowerconcentration, througha partially permeablemembrane.

Osmosis