INTRODUCTION TO

PHARMACY AND

CHEMISTRY OF

INORGANIC MEDICINALS

WITH QUALITATIVE

CHEMISTRYperiodic table.jpg

What is chemical kinetics?

• is the study of reaction rates and the mechanism or step sequence under which reactions occur.

• The rate can be expressed as:

rate = change in substance/time

for change to occur

Chemical Kinetics

• The rate of a reaction is the amount of chemical change, which takes place in a given interval of time.

• It is measured in terms of products formed per unit time at a given temperature

• It may be measured in terms of disappearance of reactants per unit time at a given temperature.

Chemical Kinetics

• Few chemical reactions occur in a single step. There are usually one or more intermediate products, which form, and the stepwise sequence of reactions is known as the reaction mechanism.

Chemical Kinetics

• Reaction rates are of great importance. The feasibility of a chemical reaction, particularly a commercial chemical process, often depends on the reaction rate. If the process is too slow, it may not be suitable for commercial exploitation.

MOLECULAR COLLISION THEORY

• A chemical change takes place as a result of collision of molecules. The greater the number of collision per unit time, the greater the conversion of initial substances into products per unit time -- that is, the greater the speed of reaction.

• However, not all collisions of molecules necessarily result in chemical change.

Reaction rate tends to increase with concentration phenomenon explained

by collision theory

Two conditions must be met in order to have an effective collision.

• The orientation of the colliding molecules must be favorable for the making and breaking of bonds, and

• Before molecules can react, they must possess a certain minimum energy, termed activation energy, Only molecules with the necessary activation energy will undergo reaction.

FACTORS WHICH INFLUENCE THE SPEED OF CHEMICAL REACTIONS

1. Nature of the Reacting Substances

2. Physical state

3. Concentration

4. Temperature

5. Catalysts

6. Pressure

Nature of the Reacting Substances

• Substances differing activity and hence in the speed 'with which they react with other substances.

• The active metals displace hydrogen vigorously and rapidly from acids, while the less active metals act slowly, if at all. Metals differ in their rates of corrosion because of differences in speed of combination with oxygen and other elements.

• Depending upon what substances are reacting, the reaction rate varies. Acid/base reactions, the formation of salts, and ion exchange are fast reactions.

• When covalent bond formation takes place between the molecules and when large molecules are formed, the reactions tend to be very slow. Nature and strength of bonds in reactant molecules greatly influence the rate of its transformation into products.

Physical state• The physical state (solid, liquid, or gas) of a

reactant is also an important factor of the rate of change.

• This means that the more finely divided a solid or liquid reactant the greater its surface area per unit volume and the more contact it makes with the other reactant, thus the faster the reaction.

Concentration

• Concentration plays a very important role in reactions, because, according to the collision theory of chemical reactions, molecules must collide in order to react together.

• As the concentration of the reactants increases, the frequency of the molecules colliding increases, striking each other more frequently by being in closer contact at any given point in time.

Temperature

Reacting molecules or atoms must collide before they can react.

At a high temperature, the molecules or atoms move faster and therefore should come in contact more often. This is especially true in gaseous reactions. In general, the speed of a chemical change is approximately doubled or tripled for every ten degree rise in temperature regardless of whether the reaction is exothermic or endothermic.

Catalysts

• A catalyst is a substance that alters the rate of a chemical reaction without being used up in the reaction. the catalyst may be recovered unchanged at the conclusion of the process.

Catalysts

• Two types of catalysts:

– accelerators

– inhibitors

Catalysts

• Accelerators or positive catalysts speed up a chemical reaction

• Inhibitors or negative catalysts slow down a reaction and are used to control undesired reactions.

CHEMICAL REACTION

It entails the removal of valence electrons, adding electrons to a partly filled valence shell, or sharing a pair of electrons between two atoms.

Chemical Equilibrium

• Chemical equilibrium applies to reactions that can occur in both directions. In a reaction such as:

CH4(g) + H2O(g) <--> CO(g) + 3H2(g)

• The reaction can happen both ways. So after some of the products are created the products begin to react to form the reactants

• When the net change of the products and reactants is zero the reaction has reached equilibrium.

Equilibrium Constant• To determine the amount of each compound

that will be present at equilibrium you must know the equilibrium constant. Consider the generic equation:

aA + bB <--> cC + dD•

• The upper case letters are the molar concentrations of the reactants and products. The lower case letters are the coefficients that balance the equation.

• Use the following equation to determine the equilibrium constant (Kc).

Law of Mass Action

• The law stating that the rate of any given chemical reaction is proportional to the product of the activities (or concentrations) of the reactants.

• A reversible reaction is a chemical reaction where the reactants form productsthat, in turn, react together to give the reactants back.

Reversible reactions will reach an equilibrium point where the concentrations of the reactants and products will no longer change.

• A reversible reaction is denoted by a double arrow pointing both directions in a chemical equation. For example, a two reagent, two product equation would be written as

A + B ↔ C + D

Le Chatelier’s principle

• If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.

pH and pOH

• The symbol “p” means “the negative of the logarithm of”

• Water undergoes auto- or self-ionization as shown in the following equation

2H2O H3O + OH

An equilibrium is established between the ions produced and the unionized water.

Kw = [H3O ] [OH ]

+ -

+ -

• Kw is used to represent equilibrium constant for the ionization of water. It is a special notation of the general representation Kc.

• The value of Kw when measured at 25˚C has been determined to be 1x10

pH + pOH = pKw

pH + pOH = 14

-14

• pH is a measure of the hydrogen ion concentration, [H+]

• pH is calculated using the following formula:pH = -log10[H+]

• pOH is a measure of the hydroxide ion concentration, [OH-]pOH is calculated using the following formula:pOH = -log10[OH-]

pKa and pKb

pKa = -log (Ka)

Ka is the equilibrium constant for the ionization of acids.

pKb = -log(Kb)

Kb is the equilibrium constant for the ionization of bases.

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