Rate of Reaction - Definition, Factors, Formula, FAQs

Arguably the single simplest concept in all chemistry, the reaction rate is the rate at which reactants change to products. Chemical reactions can occur, and their rates can span a very wide range, from explosively fast to essentially zero masses to even taking years to complete. For almost all practical applications, therefore, understanding factors that will affect the reaction rate assumes a lot of importance, be it in processes in industry, biochemical engineering, or environmental sciences. Hence, one would expect the terms 'reaction rate', 'factors which affect reaction', and the importance of the study to chemical kinetics only at the beginning.. Consider a rate of reactions ranging from combusting a wooden log to rusting of iron; the broad variations in the rates are simply incredible. In this very process itself, the students get to understand in varied ways the conditions and variables that dependently relate to a change in the rate of occurrence of chemical reactions and hence can apply this understanding in an academic area as well as in real life.

What is the Reaction Rate?

The rate at which products are formed in the course of a reaction. In chemistry, a reaction rate is the speed at which chemistry products are forming from reactants. It is a universal fact that there exists a wide extent of variation in rates in all chemical reactions. Of the chemical reactions, some take place almost instantaneously, while others, in general, take some time to reach the final equilibrium.

This paper is just going to forward to the students what the rate of reaction is concerning a given chemical compound.

According to the general definition, the rate of a reaction defines the speed at which a given reaction takes place.

For example, the burning of wood has a reaction that takes place very fast and quickly and hence can be said to have a high reaction rate while the rusting of iron has a reaction rate being less due to the process being slow

Factors Affecting the Rate of Reaction

The following part enlists a few of the factors that can ultimately affect the chemical reaction rate.

• Nature of the Reaction

The rate of reaction depends greatly on the very nature of the reaction. Very few are naturally very fast, while only a very few are such that naturally nothing is so slow.

Other myriad factors that also highly affect the rate include the number of reactants, the physical state of reactants, and the complexity of the reaction.

The rate of reaction is generally low in liquid as compared to gases and slow in solid as compared to liquid. The size of the reactant also matters a lot. The smaller the size of the reactant, the faster the reaction.

• Effect of Concentration on Reaction Rate

Concentration refers to the amount of matter in a solution. From, when the concentration and the reactants are increased, then the rate at which all the reaction is increased also.

With an increase in concentration, the rate of chemical reaction varies directly according to the action of mass law.

In a broader sense, this would mean that the rate of chemical reaction will

also increase with an increment of concentration, and conversely, it would decrease when the concentration of reactants is decreased.

Time is said to be the most crucial factor. It alters the reactant and product concentration from time to time. So, time, too can be said perceived as a highly crucial factor governing the rate of reaction.

• Pressure factor

At higher pressure concentrations, the gases are more; therefore, the rate of reaction is higher. In this fashion, at less number of gaseous molecules, the rate of reaction is high, and a versa it's low.

It is, therefore, very intuitive in a realistic view to see that when pressure is directly proportionate to the concentration, it will influence the rate at which the reaction occurs.

How does temperature affect the reaction rate?

From the collision theory, it happens for instance, in a chemical reaction that is done at a high temperature, the produced energy is considered very much in instances when a low temperature is applied.

This is because the more collision particles bearing the needed activation energy are at a high temperature and more-than-expected collisions of reactant beings take place.

Some reactions are not temperature-dependent. Chemical reactions that are not affected by temperature do not have an activation barrier.

Solvent

The reaction rate is also a function of the solvent nature. Both solvent character and ionic strength are significant in this case

Order

Reaction order is the factor that relates to the pressure or concentration of reactant concerning reaction rate.

Electromagnetic Radiation

E.M. waves forms of energy and an appearance at the reaction of the chemicals could accelerate the rate of reaction since it provides to the reactant`s particles more energy.

Intensity of light

Even the rate of reaction is influenced by the intensity of light. The rate of generation of excited particles increases with the increase in the intensity of light.

Presence of Catalyst

A catalyst is a substance that has increased the speed of the reaction without getting itself using a chemical reaction. From the definition itself how catalyst can affect the chemical reaction can be produced.

An increase in the surface area increases the rate of reaction, i.e., fine particles will react more quickly than large-size particles. The reaction speed of a chemical reaction of a heterogeneous type with fine particles sticks to a high rate constant. A catalyst is present that increases the speed of the reaction, both of the forward and reverse reactions. This is provided by the alternative path having low activation energy.

Surface Area of the Reactants

Thus, the nature of the reactants, part of the surface area of the reactants influences the rate of reaction. If the size of a particle is small, the surface area will be larger and this influences the velocity of the heterogeneous chemical reactions.

Average Rate of Reaction:

It is defined as "The rate of change of concentration of a reactant or a product per unit time"

Rate of reaction $(r)=\frac{C_2-C_1}{t_2-t_1}$

s rate of reaction varies greatly with time, so generally, average reaction rate and instantaneous reaction rates are used.

For a reaction ${A} \rightarrow \mathrm{P}$
Rate of disappearance of $\mathrm{A}=-\frac{\Delta[\mathrm{A}]}{\Delta \mathrm{T}}$
Rate of appearance of $\mathrm{P}=\frac{\Delta[\mathrm{P}]}{\Delta \mathrm{T}}$

It is to be noted that the rate of reaction is always a positive quantity and hence, there is a negative sign that has to be included in the expression for rate.

$Unit of average velocity =\frac{ Unit of concentration }{ Unit of time }=\frac{ mole }{ litre second }= mole litre ^{-1} second ^{-1}$

However, depending on the data given in the question, different units may be used.

Instantaneous Rate of Reaction

As the average reaction rate fails to predict the rate at a particular moment so we use the instantaneous rate which is equal to a small change in concentration (dx) during a small interval of time (dt). It is given as dx dt.

$\lim _{\Delta \mathrm{t} \rightarrow 0} \frac{\Delta \mathrm{c}}{\Delta \mathrm{t}}=\frac{\mathrm{dc}}{\mathrm{dt}}$

$Rate of reaction = slope of curve =\frac{\mathrm{dx}}{\mathrm{dt}}$

It can be written for any of the reactants or the product in terms of stoichiometric coefficients Vj which is negative for reactants and positive for products as follows: $\frac{\mathrm{dt}}{\mathrm{Vt}}=\frac{1 \mathrm{~d}(\mathrm{~J})}{\mathrm{dt}}$
For example, if we have the reaction

$\mathrm{aA}+\mathrm{bB} \rightarrow \mathrm{cC}+\mathrm{dD}$

$Rate w.r.t. [\mathrm{A}]=-\frac{\mathrm{d}[\mathrm{A}]}{\mathrm{dt}} \times \frac{1}{\mathrm{a}}$
$Rate w.r.t. [\mathrm{B}]=-\frac{\mathrm{d}[\mathrm{B}]}{\mathrm{dt}} \times \frac{1}{\mathrm{~b}}$
$Rate w.r.t. [\mathrm{C}]=-\frac{\mathrm{d}[\mathrm{C}]}{\mathrm{dt}} \times \frac{1}{\mathrm{c}}$
$Rate w.r.t. [\mathrm{D}]=-\frac{\mathrm{d}[\mathrm{D}]}{\mathrm{dt}} \times \frac{1}{\mathrm{~d}}$
For the reactants, the negative sign indicates a decrease in concentration, and for products positive sign indicates an increase in concentration.
For a reversible reaction at dynamic equilibrium, the net reaction rate is always zero as: $\left(\frac{\mathrm{dx}}{\mathrm{dt}}\right)_{\text {forward }}=\left(\frac{\mathrm{dx}}{\mathrm{dt}}\right)_{\text {\backward }}$

There are various factors on which the rate of reaction depends:

Nature of reactant and product:

For ionic reactants reaction rate is fast as activation energy is zero for them. For example:BaCl₂+H₂SO₄ $\rightarrow $BaSO₄+2 HCl

Molecules have slow reaction rates due to the need for more activation energy. For example:2CO+O₂ $\rightarrow$ 2CO₂

The physical state of reactants: Rate also changes with the physical state.
Gaseous states > Liquid states > Solid states

Pressure: For gaseous reactants rate varies with pressure just like concentration.frac{mathrm{dx}}{mathrm{n}. mathrm{dt}} propto Pressure ( as mathrm{P} propto mathrm{C})

Surface Area: The Greater the surface area, the faster is the rate of reaction due to more number of active sites.Rate (mathrm{dx} / mathrm{dt}) propto Surface area

There are various factors on which the rate of reaction depends:

Temperature: The rate of reaction increases with the increase of temperature as it increases the number of effective collisions. It is observed that for every 10^oC rise in temperature, -dx/dt or rates become nearly double. Temp. Coefficient (mu)=frac{mathrm{K} text { at } mathrm{t}^{circ} mathrm{C}+10^{circ} mathrm{C}}{mathrm{K} text { at } mathrm{t}^{circ} mathrm{C}}
The value of the temperature coefficient lies in between 2-3. In case we increase the temperature by more than 10 ^oC the above relation can be given as:

frac{mathrm{K}_{mathrm{T}_2}}{mathrm{~K}_{mathrm{T}_1}}=(mu)^{Delta mathrm{T} 10}

left[right. Here left.Delta mathrm{T}=mathrm{T}_2-mathrm{T}_1right]

begin{aligned}
& log _{10} frac{mathrm{K}_{mathrm{T}_2}}{mathrm{~K}_{mathrm{T}_1}}=frac{Delta mathrm{T}}{10} log _{10} mu
& frac{mathrm{K}_{mathrm{T}_2}}{mathrm{~K}_{mathrm{T}_1}}=text { Antilog }left[frac{Delta mathrm{T}}{10} log _{10} muright]
end{aligned}

Catalyst: It increases the rate of a reaction by decreasing the activation energy by accepting a new alternative smaller path for the reaction. It is reversed in the case of negative catalyst to that of positive catalyst. Catalysts are more effective in 'Solid powdered form' due to larger surface area, that is, more active site

.

The intensity of light: The rate of photochemical reactions depends upon the intensity of light radiations.frac{mathrm{dx}}{mathrm{ns} . mathrm{dt}} propto Intensity of radiation

Concentration of reactants: The rate increases with the increase of concentration as due to more number of reactants there are more collisions.Rate of reaction (mathrm{dx} / mathrm{dt}) propto Concentration

Recommended topic video on (Rate of Reaction )

Some Solved Examples

Example 1

For the reaction (2A + B right arrow C), the rate equation is found to be: rate = (K[A][B]). The correct statement about this reaction is:

1. Unit of (K) must be s^-1
2. (+r₂) is a constant
3. The rate of formation of (C) is twice the rate of disappearance of (A)
4. The value of (K) is independent of the initial concentration of (A) and (B).

Solution

The rate law equation for a reaction can be given as:
Rate = K[A^pB^q]
where (p) and (q) are the orders of the reaction concerning reactants (A) and (B), respectively.

In this case, the rate equation is:
Rate ={K}[A][B]

The value of (K), the rate constant, is determined experimentally and does not depend on the initial concentrations of (A) and (B). Thus, the correct statement is that the value of (K) is independent of the initial concentration of (A) and (B).

Example 2

Chemical Kinetics tells about the process or a reaction.

1. Feasibility of a reaction
2. Direction of a reaction
3. Rate of a reaction
4. All of above

Solution

Chemical Kinetics is the branch of chemistry that deals with the rate and mechanism of chemical reactions. It studies the influence of factors such as temperature, pressure, concentration, and catalysts on the rate of a chemical reaction.

Thermodynamics, on the other hand, tells about the feasibility and direction of a reaction. Therefore, Chemical Kinetics specifically focuses on the rate of a reaction.

Hence, the answer is option (3).

Example 3

For the reaction (2A + B right arrow 3C), what is the rate of the reaction concerning (A)?

1. (-frac{d[A]}{dt})
2. (-frac{d[A]}{dt} + frac{d[C]}{dt} - frac{d[B]}{dt})
3. (-frac{d[B]}{dt})
4. (-frac{1}{2}frac{d[A]}{dt}) (Correct)

Solution

The rate of reaction is defined as the rate of change of concentration of a reactant or product per unit of time. For the reaction (2A + B right arrow 3C):

The rate of disappearance of (A) can be written as:
[{Rate} = -frac{1}{2}frac{d[A]}{dt} ]

This accounts for the stoichiometric coefficient of 2 in front of (A). Therefore, the rate of the reaction for (A) is:
[ -frac{1}{2}frac{d[A]}{dt} ]

Hence, the answer is option (4).

Conclusion

The rate of reaction is one of the core quantities of chemical kinetics, which mainly explains how the reactants convert into products concerning the time taken. Gases in general and solutions are the usual cases where this difference in rate is large, due to the change of species characteristics of reactants, change of physical form, change of concentration or pressure, change in solution temperature and its solvent, or induced catalysts. Awareness of these factors arms scientists and engineers with the capability to manipulate conditions of a reaction effectively toward the realization of set goals for product formation in an efficient way and as safely as it can be. The concept of average and instantaneous rate of reaction discloses much in the dynamics of the chemical process to help advance the chemical engineering field much better. Moreover, it has helped in the conservation of the environment, biochemistry, and very many other critical technological developments in the fields of biotechnology and pharmaceutics. Finally, with a proper understanding of the working of the rate reactions and what influences these, there ought to be knowledge put into place to advance it further by taking the practical applications afield in various fields. This will advance continually ameliorating the phenomena and immediate progression according to principles which will be divulged both in view and to be exercised on academic knowledge and industrial applications.