Reaction Kinetics — Short Questions

Successful collision A successful collision is a collision between reactant particles that actually leads to the formation of products.
Conditions for successful collision: A collision is successful only when.

• The colliding particles have energy equal to or greater than activation energy.
• Secondly the particles collide with the proper orientation.

The increase in temperature increase the reaction rate because as the temperature increases, the kinetic energy of the particles also increases. As a result, the particles start moving fast leading to more vigorous motion. The fast moving particles will have more chances to collide with each other Effective collisions are increased in this way. Hence rate of reaction is also increased.

Activation energy According to collision theory, the colliding particles will lead to effective collisions only when they possess sufficient energy to break the bonds present in the reactant molecules. This minimum energy required to start a chemical reaction is called its activation energy.

Burning of sulphur in pure oxygen: It has been observed that burning of sulphur proceed slower in air than in pure oxygen.
S(s) + O₂(g) → SO₂(g)
In the presence of air it slows down because air is a mixture of different gases. It has 21% of oxygen the rest is mostly nitrogen which does not support combustion. In pure oxygen, more frequent collisions between sulphur and oxygen molecules result in more successful collisions and increase in rate of reaction.

A catalyst is mostly used in finely divided form because it increases the rate of reaction due to:
Large surface area – more active sites
Finely divided catalysts (powdered, porous solids) react rapidly than do large pieces of the same substances.
Example: Aluminium foil reacts slowly with hot sodium Hydroxide but in finely divided state it reacts rapidly with even cold aqueous solution of sodium hydroxide evolving hydrogen gas.
2A ℓ(s) + 2NaOH(aq) + 6H₂O(ℓ) → 2NaA ℓ(OH)₄(aq) + 3H₂(g)

Rate of reaction at the beginning of reaction: The rate of reaction is very fast at the beginning because.
Maximum concentration of reactants: At the start, the concentrations of reactants are highest so more frequent collisions results in increase the rate of reaction.

Magnesium requires high energy to react with oxygen, which it does not get at room temperature. At room temperature, oxygen molecules in air do not have enough energy to overcome the activation energy.
At higher temperature
Magnesium atom gains enough kinetic energy at higher temperature and the collision between magnesium and oxygen become more effective which results in fast reaction releasing lot of energy and giving intense white light.
2Mg(s) + O₂(g) ---heat---> 2MgO(g)

When reactant particles possessing activation energy climbs the energy hill, after this stage the reactants are converted into stable products and potential energy of the system falls down.

A catalyst lowers the activation energy by providing an alternative reaction pathway that requires less energy for the reactants to be converted into products. When the activation energy is lowered, more and more particles of the reactants start forming the high energy state and the rate of reaction will increase.

Features of the catalytic action of enzymes
The reactions catalysed by enzymes have following characteristics.
(i) Specific in nature: Each enzyme acts on a specific type of reaction.
(ii) Lowers activation energy: Enzymes provide an alternative pathway for the reaction by lowering activation energy.
(iii) Not consumed in reaction: Enzymes increase the rate of reaction without being consumed chemically in the reaction.

Reaction kinetics
The branch of chemistry that tells the rates of chemical reactions and factors affecting the rates of chemical reactions is called reaction kinetics.

Collision Theory According to collision theory, a reaction take place only when the participating particles (atoms, molecules or ions) collide with one another. A very small fraction of molecule lead to products because they have collisions which are effective collisions:

Frequency of collisions The number of collisions per second is called frequency of collisions.
A greater number of collision per second will also result in the increase in the number of effective collision. The frequency of collision increases at higher temperatures.

Importance of activation energy

• The information provided by the energy of activation is very important in understanding the mechanism of the reaction.
• It tells that the reaction is either feasible or not.

The larger the number of particles per unit volume, larger is the possibility of effective collisions. In other words, higher concentration of the reactants, more the number of molecules and hence more will be the number of collisions between them.

Some reactions appears to proceed with a change in mass because a reactant or a product is a gas in such reactions. So gas can leave or enter the system, in this way change in mass can occur.

If a reaction is carried out in an open container where gasses can escape or enter, the apparent mass change can occur, if a gas is either evolved or absorbed.
Example: If a gas is evolved, we shall observe a decrease in mass and if a gas is absorbed, the apparent mass will increase.

The graph of time against concentration is called concentration-time graph. When a reaction moves ahead the concentrations of the reactants decreases until all the reactants are used up. At the same time, concentration of the products increase and reach the maximum value at the end of reaction. This is shown in graph.

Exothermic reactions In these reactions heat energy is released which is absorbed by the surrounding causing its temperature to increase.
Example: When quick lime (CaO) reacts with water, slaked lime Ca(OH)₂ is formed and a lot of heat energy is released.
CaO(s) + H₂O(ℓ) → Ca(OH)₂(s)

Endothermic reactions In these reactions, heat energy is absorbed causing surrounding to cool down.
Example: When ammonium chloride is dissolved in water, heat energy is absorbed causing container to cool down.

An increase or decrease in concentration of reactants will result in increase or decrease in the rate of reaction respectively.
Example: Combustion of coal in air (21% oxygen) proceeds relatively slowly as compared to the reaction in pure oxygen.

When a graph is drawn between the time in seconds or minutes again the volume of gas evolved as shown in figure. The graph obtained is in the form of curve.

The graph of volume of gas against time is obtained in the form of curve. The rate of reaction is measured during a time interval by drawing a tangent on this curve.

If reactants in a reaction are present in gas phase, their concentration can be changed by changing pressure.
Example: The rate of reaction between hydrogen and chlorine is doubled if the pressure of chlorine gas is doubled.

Finely divided solids reacts more rapidly because of greater surface area available than do large pieces of the same substances.
Example: Finely divided aluminium reacts rapidly with sodium hydroxide as compared to aluminium foil.

In many cases, the rate of reaction is nearly doubled when the temperature is increased by 10°C.

According to collision theory, the rate of reaction depends upon the frequency of successful collisions between reactants. As the temperature is increased, the kinetic energy of the reacting particles also increases. As a result, successful collisions increases, enhancing the reaction rate.

The role of catalyst is to alter the way the reaction takes place by substantially lowering its activation energy. When the activation energy is lowered, the rate of reaction will increase.

Enzymes are biological catalyst. These are basically proteins which help to speed up specific chemical reactions taking place in living organisms.

The active sites present in enzymes, binds with the reactant molecules. They convert the reactants into products, then free to bind other reactants. Active sites are highly specific such that a particular enzyme can catalyse a specific reaction only.

The rate of chemical reactions which are involved during food ripening and food spoilage are studied for different types of fruits and vegetables. The information this obtained is used to minimize the losses due to spoilage.

Fruits and vegetables ripen owing the reactions which involve enzymes. These reactions convert starches into sugars, soften tissues and develop characteristics flavour and colours. With the help of rates of these enzymatic reactions, farmers can predict when the product will reach at its peak quality and ready for harvesting.

We can increase the frequency of collision by different ways:
(i) By increasing the temperature. Kinetic energy of molecules is increased which results is increase of frequency of collision to overcome activation energy.
(ii) We can also increase the frequency of collision by increasing concentration of reactant.
(iii) It can be increased by increasing the pressure if reactants are present in gaseous state.
(iv) Frequency of collision can also be increased by increasing the surface area of a solid.

Example of reaction with increase in mass: If a ribbon of magnesium is burnt in an open crucible, the mass of the product (MgO) formed will be greater than the reactant (Mg). This increase in mass has occurred due to the reaction of magnesium with oxygen present in air
However, if the two reactants (Mg and O₂) are allowed to react in a closed container, the total mass will remain the same before and after the reaction.

Higher energy state (Transition state of molecules): Molecules get the energy to attain a higher energy state from their surroundings usually in the form of:

• Heat
• Light
• Chemical energy (released by bond formation) after this state the reactants are converted into stable products and the potential energy of the system falls down.

2SO₂(g) + O₂(g) → 2SO₃(g)

Vanadium pentoxide as a catalyst: V₂O₅ (vanadium pentoxide) acts as a catalyst in this reaction and lowers the activation energy by providing an alternative pathway in which SO₂ is oxidized to SO₃ in intermediate steps and requires less energy than un-catalyzed reaction.

Under the conditions 450 °C and 2 – 3 atm pressure, 98% SO₂ gas is converted to SO₃, which is regarded a major role of a catalyst.

See Q.12 from theory.

See Q.9 from theory.

See Q.2 from theory.

See Q.21 from theory.
(b) Effect of temperature at which a reaction is carried out.
Ans. (a) See Q.7 from theory.
(b) See Q.10 from theory.

See Q.13 from theory.

See Q.12 from theory.