Classwork Series and Exercises {Chemistry- SS2}: Rate of Reactions

Topic: Rate of Reactions

Introduction

A chemical reaction is a process that transforms one set of chemical substances to another. The substances that take part in chemical reactions are known as reactants and the substances produced by the reaction are known as products.

The reaction rate is a measure of the change in the concentration of reactants or products over time in a chemical reaction. In other words, the rate of chemical change is the number of moles of the reacting substances which disappear or the number of moles of products which appear per unit time for a given quantity of the reacting species. As products are formed reactants are used up.

Measuring Rates

The only way we can find out about the rate of a particular reaction is to carry out an experiment. The balanced equation for the reaction tells us nothing about its rate. So we need to have some way of measuring either:

The rate at which reactants are used up in a reaction, or

The rate at which products are formed in a reaction

 

Reaction Rate =    Change in concentration of reactants/products

Time taken for the change

In real experiments, we find some property of the reacting mixture that changes as the reaction takes place and we measure that. That’s easy in reactions that produce a gas as one of the products – for example, marble chips (containing calcium carbonate) reacting with hydrochloric acid:

In a chemical reaction the reacting particles need to collide. However, not all collisions in a reacting mixture result in a reaction. The particles (molecules or ions) in the mixture will have a whole range of different energies. Some have lots of energy and move about quickly; others have a low energy and move more slowly. The collision must also have enough energy so that the chemical bonds can be broken. Collision without enough energy will not lead to a reaction. An effective reaction is a reaction which does have enough energy and does lead to a reaction.

Chemical reactions occur at different speeds. Some reactions are faster while others are much slower. For example an explosive chemical reaction between two reactants tells us that this is a very fast reaction whereas rusting is a much slower reaction.

Particles need enough kinetic energy to break the bonds and cause a chemical reaction to occur. The minimum amount of kinetic energy that a reaction requires to occur is known as the activation energy. So when the particles collide there must be enough kinetic energy to exceed the activation energy in order for a chemical reaction to occur. Slow reactions such as rusting generally have high activation energies, while explosive chemical reactions generally have low activation energies. The particles with insufficient energy just collide with each other without reacting.

The collision theory states that particles must collide before they can react, and that only collisions with sufficient energy (greater than the activation energy) will result in a reaction.

 

Factors that influence reaction rate are:

1.    Concentration of the reactants

2.    Temperature

3.    Presence of a catalyst

4.    Surface area

5.    Nature of reactant

6.    Intensity of light

Concentration of the Reactants

Rate of a reaction is directly proportional to the concentration of reactants. 

i.e. rate (r) ∝ cⁿ where 

r = kcⁿ 

Where k = specific rate 

c = concentration 

n = order of the reaction 

Raising the concentrations of reactants makes the reaction happen at a faster rate. For a chemical reaction to occur there must be a certain number of molecules with energies equal to or greater than the activation energy. With an increase in concentration the number of molecules with the minimum required energy will increase, and therefore the rate of the reaction will increase. The rate of reaction has doubled by doubling the concentration.

The number of collisions and hence the activated collisions between the reactant molecules increase with increase in concentration. Therefore, according to the collision theory, the rate of a reaction should increase with increase in the concentration since the rate is directly proportional to the collision frequency.

The rate of a reaction decreases exponentially with time as the concentration of reactants is decreasing.

Surface Area

In a reaction between a solid and a liquid, the surface area of the solid will affect how fast the reaction occurs. This is because the two types of molecules can bump into each other only at the liquid-solid interface — that is, on the surface of the solid. Therefore, a larger surface area of the solid allows for a faster reaction. Smaller particles have bigger surface areas than larger particles with the same mass. The total exposed surface area will increase when a larger body is divided into smaller pieces (The surface of a solid can be increased by grinding it to a fine powder). Therefore, if a reaction takes place on the surface of a substance, increasing the surface area should increase the quantity of substance available to react and thus increase the rate of the reaction.

For Example, the reaction between zinc and hydrochloric acid occurs within seconds if the zinc metal is finely powdered. But the reaction will be slower when a zinc wire is used. 

This is also true with the solid catalysts, which are usually employed in finely powdered form, while carrying out a chemical reaction. E.g. finely powdered nickel is used during the hydrogenation of oils.

Pressure

Increasing the pressure of a gas is the same as increasing its concentration. Increase the pressure of a gas by squeezing it into a smaller volume, and if you have the same mass in a smaller volume, the concentration is higher. Changing the pressure for a reaction that involves only solids or liquids has no effect on the reaction rate.

The partial pressure is another way of expressing the concentration for gases. The number of collisions increases with increase in the partial pressures of gases. Hence the rate of reactions involving gaseous reactants increases with increase in partial pressures. However it has no effect on reactions involving reactants in liquid or solid phases.

It is important to keep in mind that the partial pressures of reactants can be increased by increasing the pressure of overall system. However the partial pressures do not increase when an inert gas or a non reacting gas is added to the reaction mixture at constant volume.

Temperature

It has been observed experimentally that a rise of 10°C in temperature usually doubles or triples the speed of a reaction between molecules. The minimum energy needed for a reaction, the activation energy, stays the same with increasing temperature. Thus, an increase in temperature causes a rise in the energy levels of the molecules involved in the reaction. As a result the rate of the reaction increases. Similarly, the rate of reaction will decrease with a decrease in temperature. The higher the temperature of the reaction, the more quickly it will proceed. At higher temperatures, the molecules are moving around more quickly (they have more kinetic energy); this means they will collide with each other with more energy, and it’s more likely that they will overcome the activation energy needed to start the reaction.

The average kinetic energy increases with increase in absolute temperature. As a result, the number of effective collisions between reactant molecules also increases. Therefore, usually it is observed that the rate of reaction increases with increase in temperature.

Catalyst

Catalyst is a substance which alters the rate of a reaction without being consumed or without undergoing any chemical change during the reaction. Catalysts are substances that increase reaction rate by lowering the activation energy needed for the reaction to occur. Biological catalysts are known as enzymes. A catalyst is not destroyed or changed during a reaction, so it can be used again. For example, at ordinary conditions H₂ and O₂ do not combine. But in the presence of a small quantity of platinum, which acts as a catalyst, they do combine, and the reaction occurs rapidly. 

In case of reversible reactions, the catalyst lowers the activation energies of both forward and backward reactions to the same extent and helps in attaining the equilibrium quickly. 

Note: When a catalyst increases the rate of forward reaction, it also increases the rate of backward reaction.  

Some substances may decrease the rate of a reaction. These are generally referred to as negative catalysts or inhibitors. They interfere with the reaction by forming relatively stable complexes, which require more energy to breakup. Thus the speed of the reaction is reduced.

Nature of Reactants

Rate of a reaction depends on the nature of bonding in the reactants. Usually the ionic compounds react faster than covalent compounds

If you mix two gases or two liquids, this represents a homogenous reaction, but if reactants are in different phases, for example, if one is a gas and one is a liquid, then the reaction area is limited to the area where they touch each other, and the larger this area, the faster the reaction will proceed. For example, consider a teaspoon of salt dissolving in water. If you were to dump the salt into the beaker of water and let it float to the bottom without stirring it, it would take much longer for it to dissolve than if you stirred the solution.

The reactions between ionic compounds in water occur very fast as they involve only exchange of ions, which were already separated in aqueous solutions during their dissolution. 

E.g. AgCl is precipitated out immediately when AgNO₃ solution is added to NaCl solution. 

This reaction involves only the exchange of ions and hence occurs very fast. Whereas, the reactions between covalent compounds take place slowly because they require energy for the cleavage of existing bonds. 

E.g. The esterification of acetic acid occurs slowly since the breaking of bonds requires energy.

Intensity of Light

The rate of some photochemical reactions, which occur in presence of light, increases with increase in the intensity of suitable light used. With increase in the intensity, the number of photons in light also increases. Hence more number of reactant molecules get energy by absorbing more number of photons and undergo chemical change. 

E.g. The rate of photosynthesis is more on brighter days. 

However, some photochemical reactions involving the free radicals, generated in a chain process, are not greatly affected by the intensity of the light. Just one photon is sufficient to trigger the formation a free radical. This in turn initiate a chain process in which more free radicals are formed repeatedly in each cycle without the need of extra photons.

Controlling the Rate of Reaction

The rate of the reaction can be controlled by changing the frequency of the collisions or changing the energy of the reactant particle.

Factor	Effect on Rate	Explanation
Concentration	Increasing the concentration of a reactant increases the rate	Increasing the concentration means that the particles are more crowded, so there will be more frequent effective collisions increasing the rate of reaction.
Temperature	Increasing the temperature of a reaction increases the rate	Increasing the temperature increases the kinetic energy that the reactants have, which means that they will collide more often and have more energy available to exceed the activation energy. At lower temperatures the particles have less kinetic energy, are moving much slower and so there are less effective collisions.
Surface area	Increasing the surface area of the solid or cutting the solid into smaller pieces increases the rate of reaction	Increasing the surface area increases the number of particles that are exposed to the reactants which increases the number of collisions and so increases the number of effective reactions. So a powder form of a reactant will have a higher rate of reaction than the same reactant in a block form.
Catalyst	Adding a catalyst to the reaction changes the rate of reaction	A catalyst changes the activation energy in a reaction. It basically speeds up a reaction without being used up itself. It can raise the activation energy in some reactions but it usually lowers it. This means that more particles will have enough kinetic energy to overcome the activation energy level.
Pressure	Increasing the pressure of reactions involving gasses increases the rate of reaction	Once the pressure has been increased the gas particles are in a smaller space meaning that more frequent and effective reactions will occur.

Changing the rate of reaction does not change the volume or mass of product formed, instead it only changes how quickly the product is formed.