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Rate of Chemical Reactions

Everyday experience tells us that chemical reactions take place in widely varying rates. Infact rates of various reactions vary from very slow to very fast. Some reactions are so rapid that they occur as soon as the reactants are mixed. For example, the reactions involving ionic species (known as ionic solutions) are very fast.

Common examples of such type are:

i) Acid-Base Neutralisation

ii) Precipitation from aqueous solution of NaCl and AgNO₃

iii) Ionic Reactions

These reactions occur in about 10^-¹⁴ to 10^-¹⁶ seconds and are very fast. These reactions involve only the ions and no bonds are broken. Therefore their rates are very fast.

On the other hand there are certain reactions which occur at an extremely slow speed.

Examples of very slow reactions are:

Rusting of iron

Conversion of monoclinic sulphur to rhombic sulphur etc.

In between very slow and very fast reactions, we have reactions which proceed at a moderate speed which can easily be measured.

Examples of moderate speed reactions are:

i) The reaction between nitrogen peroxide and carbon monoxide

ii) Decomposition of hydrogen peroxide

iii) Decomposition of dinitrogen pentoxide

Rate of a Reaction

The rate of a reaction is broadly defined as the speed with which the reactants are converted into products. This can be determined both quantitatively and qualitatively.

Qualitatively, (concerned with or depending on constitiuents) an idea about the rate of a reaction can be obtained by observing either the speed of disappearance of the reactants or the speed of appearance of the products. For example when a piece of magnesium is put into a beaker containing dilute hydrochloric acid, hydrogen gas is rapidly evolved and magnesium disappears at once. On the other hand, when a piece of iron is placed in the same acid, hydrogen gas evolves at a slower rate and iron also disappears very slowly. This means that the reaction of magnesium and hydrochloric acid is faster than reaction of iron and hydrochloric acid.

Quantitatively, (measured or measurable by quantity) the rate of a reaction may be expressed as change in concentration of any one of the reactants or products per unit time.

Units of rate of reaction

Units of concentration (in aqueous solutions) is moles per litre

Unit of time is seconds, minutes etc.

For e.g., Consider a hypothetical reaction

The rate of a reaction may be expressed in either of the following two ways:

1) The rate of disappearance or decrease in concentration of A (reactants)

2) The rate of appearance or increase in concentration of B (products)

Equation A

B shows that one mole of A produces one mole of B and thus the rate at which the concentration of A decreases will be the same as the rate at which the concentration of B increases.

The decrease or increase in the concentration of the reactants or products may also be expressed in terms of change in their concentration during time interval Dt as

Where D[A] gives the decrease in concentration of A and D[B] represents the increase in concentration of B. The square brackets around the substances are used to express the concentration (in terms of mol/litre). It may be noted that in the case of concentration of reactants minus sign is used. This implies that the concentration of the reactants is decreasing with time.

For e.g., consider the reaction:

The rate of the reaction can be expressed either by decrease of concentration of any of the reactants (NO or O₃) or by increase in concentration of any of the products (NO₂ or O₂). Thus,

However for the reaction

We observe from the stoichiometry of the reaction, that one mole of H₂ reacts with one mole of I₂ to form two moles by hydrogen iodide. This means that the rate of disappearance of H₂ and I₂ is the same but the rate of appearance of HI must be twice the rate of disappearance of H₂ and I₂_.

Thus we may write,

But for having unique value of the reaction rate (independent of the concentration term chosen), we divide the rate of the reaction, defined with respect to any of the reactants or products by the number of moles involved in the reaction.

Thus, for the above reaction, rate may be expressed as (by dividing by 2).

Consider another example,

When acidified hydrogen peroxide (H₂O₂) is added to a solution of potassium iodide (KI), iodine is liberated.

Concentration of iodine is zero initially. With the passage of time, the concentration of iodine increases and the reaction solution becomes brownish.

Concentration of iodine can be measured at different intervals of time by titrating against sodium thiosulphate solution.

If the concentration of iodine rises from 0 to 10^-⁵ mol L^-¹ in seconds,

[Here symbol D represents a change and I₂ represents the molar concentration of iodine].

This change in concentration of the product (iodine) takes place in ten seconds. Thus the rate can be called the average reaction rate.

The change in concentration over a shorter time interval will give a more accurate estimate of reaction at any moment.

Rate of reaction depends on the concentration of the reactants. With lapse of time, concentration of reactant/(s) decreases and therefore, rate of reaction also decreases.

In order to determine the rate of a particular instant, one should make the time interval almost zero.

d[I₂] denotes a very small change in concentration of I₂, and dt denotes very small change in time.

This rate is instantaneous rate.

The concentration of H₂O₂ decreases with time and thus rate in terms of H₂O₂ and KI is expressed as

The negative sign indicates that the concentration of H₂O₂ and KI decreases with time.

In this above reaction, rate of consumption of KI is twice the rate of consumption of H₂O₂.

Thus to make the two rates equal we divide by two and write

hope u get a gud idea..................

source :tutorvista.com

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