chemical reaction part 2

COLLISION THEORY AND RATE OF CHEMICAL REACTION

Collision theory: collision theory was propounded by Arrhenius and it was derived from kinetic theory of gases. It explain the conditions that must be achieved before chemical reaction can takes place. It is stated as follows.

Ø  Before chemical reaction can takes place, reactant particles must collide with one another.

Ø  Not all the collision leads to chemical reaction but a very small fraction of it known as effective collision.

Ø  Colliding  particles must possess certain amount of energy before reaction can takes place this minimum amount of energy is known as activation energy. This activation energy is now a barrier that the reactant particles must achieve before chemical reaction can takes place.

RATE OF CHEMICAL REACTION

Rate of reaction is term used to express how far and slow a reaction is with respect to time. It can also be defined as the change in in the concentration of reactant or product with respect to time.

Mathematically R ꞊ ∆ [A].  

                                    t  

where ∆ [A] is the change in concentration of reactant and “t” is the time taken for the reaction to reach completion.

FACTORS AFFECTING RATE OF CHEMICAL REACTION

There are numbers of factors which affect the rate of chemical reactions, the factors are as follow:

1.      Nature of the reactant

2.      Temperature of the reacting system

3.      Concentration or pressure (for gases)

4.      Presence of sunlight

5.      Surface area of the reactants

6.      Presence of catalyst

1.      Nature of the reactant: different chemicals or elements or substance have different characteristics and this we refer to as their nature, in other word, the energy content of different reactants differs from one another, as this affect the way they react with each other. Taken for example someone that does not like beer or alcohol cannot be attracted by it. And this can be established from the reaction between dilute hydrochloric acid and metals like iron, zinc and  gold e.t.c. evolution of hydrogen gas when iron react with HCl will be rapid compare to that of zinc while there will not be a reaction between gold and HCl. Because iron is more electropositive than zinc and gold does not readily react due to its position in the electrochemical series. The same reason why the noble gases does not react because of their stable octet structure.

2.      Temperature: according to kinetic theory, the higher the temperature the higher the kinetic energy of the substance. The same rule apply here the more the temperature, the more the kinetic energy of the reactant particles. Hence, the higher the collision frequency which will result in rapid rate of reaction.

3.      Concentration: imagine the burning sensation you will experience from a soup of two spoons of pure pepper and a soup of 20 spoons of pure pepper! Concentration simply means the amount of solute particles present in a solution. In chemical reaction the higher the concentration the higher the rate of the reaction. For example, the evolution of  carbon (iv)oxide will be slow with dilute HCl acid while it will be rapid with concentrated HCl. This is because concentrated HCl acid contain more solute particles than that of the diluted one.

4.      Presence of sunlight: some reaction like photosynthesis will not takes place without sunlight, this is because sunlight has enough frequency to break water molecules apart. Not only that, it observed that reaction of Chlorine with Hydrogen is negligible in dark room, moderate in day light and violently in bright sunlight. NB: any chemical reaction that needs light to proceed is called photochemical reaction.

5.      Surface area of the reactant: image the time that will take you to completely fill a bucket of water with a tap having diameter of 0.5 meter and a tap having diameter of 5.0! you know one will be faster than other, why? Because one mouth of the tap is bigger than the other and water will rush out of the bigger one than the small one. That is what we refer to as surface area of the reactant. The wider the surface of the reactant particles, the higher the rate of chemical reaction. This is because for solid particles reaction takes place on their surface. Therefore a powered marble chips will evolve carbon (iv) oxide gas more rapidly than that of pellet marble with hydrochloric acid. Because powder particles has a larger surface area.

6.      Presence of catalyst: a catalyst is any chemical substance that alters the rate of chemical reaction but does not take part in the reaction. Catalyst can speed up the rate of chemical reaction by lowering the activation energy of the reactant specie. For example, in the production of oxygen gas using potassium trioxo chlorate (v), manganese (iv) is employed as catalyst which helps to reduce the amount of heat required for the decomposition of KClO₃.

CHARACTERISTICS OF CATALYST

1.      Catalyst are specific in action

2.      Catalyst cannot initiate are action that will not occur on its own.

3.      Catalyst are not used up in the reaction

4.      Little amount of catalyst is required to speedup large amount of chemical reaction.

5.      Catalyst alter the rate of chemical reaction by lowering the activation barrier of reaction.

CHEMICAL EQUILIBRIUM

Have you ever try adding salt to fixed volume of water continuously? Try that and note what will happen at a point. You’ll observe that it will get to a stage that the salt will not dissolve again! Yes, you’ll have your solid salt back in the water. At this point, its not the salt is not dissolving, but as the salt is dissolving its also precipitating back to give you the solid sample. i.e. dissolution and precipitation is occurring at same rate. In order word forward and backward reaction occurs simultaneously. Hence, the system is in equilibrium.

Now what is chemical equilibrium?

This is a stage where there is no observable change in the properties of the chemical system.