Rate of a Reaction

Chemical kinetics is the branch of physical chemistry that studies how chemical reaction rates are measured and predicted, as well as how reaction-rate data can be used to deduce likely reaction mechanisms. Chemical kinetics concepts are used in many disciplines, including chemical engineering, enzymology, and environmental engineering. In science, reaction rate refers to the rate at which a chemical reaction occurs. It is frequently expressed in terms of either the concentration (amount per unit volume) of a product formed in a unit of time or the concentration that is the amount per unit volume of the reactant consumed in a unit of time. It can also be defined in terms of the number of reactants consumed or products formed in a given unit of time.

Overview

Chemical reactions occur at varying rates depending on the nature of the reacting substances, the type of chemical transformation, the temperature, and other factors. In general, reactions involving atoms or ions (electrically charged particles) occur very quickly, whereas reactions involving covalent bonds (bonds in which atoms share electrons) are much slower. The rate of a given reaction will vary depending on the temperature, pressure, and amount of reactants present. Reactions typically slow down over time due to the depletion of the reactants. In some cases, the addition of a substance that is not a reactant, known as a catalyst, speeds up a reaction.

The rate constant, also known as the specific rate constant, is the proportionality constant in the equation that expresses the relationship between a chemical reaction’s rate and the concentrations of the reacting substances. Chemical kinetics is a branch of chemistry that deals with the measurement and interpretation of reactions.

Rate of a reaction

In general, the reaction rate, also known as the rate of reaction is the rate at which a chemical reaction occurs, defined as proportional to the increase in product concentration in terms of time unit and the decrease in reactant concentration per unit time. The rates of reaction can vary greatly. We can say that the oxidative rusting of iron in the Earth’s atmosphere, for example, is a slow reaction that can take years, whereas cellulose combustion in a fire happens in fractions of a second. Most reactions have a decreasing rate as the reaction progresses. The rate of a reaction can be calculated by measuring the changes in concentration over time.

A reaction’s rate will always be positive. The presence of a negative sign indicates that the reactant concentration is decreasing. The International Union of Pure and Applied Physics (IUPAC) recommends that the unit of time always be the second. Generally, the rate of reaction varies from the rate of concentration increase of a product P by a constant factor and by minus the reciprocal of the stoichiometric number for a reactant A. The stoichiometric numbers are included so that the defined rate is independent of the reactant or product species measured.

The nature of the reaction, concentration, pressure, reaction order, temperature, solvent, electromagnetic radiation, catalyst, isotopes, surface area, stirring, and diffusion limit are all factors that influence the reaction rate. Some people’s reactions are inherently faster than others. The number of reacting species, their physical state (solid particles move much more slowly than gas or solution particles), the complexity of the reaction, and other factors can all have a significant impact on the rate of a reaction.

The rate of reaction will increase as concentration increases, as described by the rate law and explained by collision theory. The frequency of collision increases as reactant concentration increases. The rate of gaseous reactions increases with pressure, which corresponds to an increase in gas concentration. The reaction rate increases where there are fewer moles of gas and decreases where there are more moles of gas. The pressure dependence of condensed-phase reactions is weak.

The order of the reactions determines how the reactant concentration (or pressure) affects the rate of the reaction.

As explained by collision theory, conducting a reaction at a higher temperature delivers more energy into the system and increases the reaction rate by causing more collisions between particles. The main reason that temperature increases the rate of reaction is that more colliding particles have the required activation energy, resulting in more successful collisions (when bonds are formed between reactants). The temperature effect is described by the Arrhenius equation. Coal, for example, burns in a fireplace when there is oxygen present, but it does not burn when stored at room temperature. The reaction occurs spontaneously at both low and high temperatures, but its rate at room temperature is so slow that it is negligible. The rise in temperature caused by a match allows the reaction to begin, and because it is exothermic, it then heats itself. This is true for a variety of other fuels, including methane, butane, and hydrogen.

Reaction rates can indeed be temperature independent (non-Arrhenius) or temperature dependent (Arrhenius) (anti-Arrhenius). Reactions that do not have an activation barrier (for example, some radical reactions) have anti-Arrhenius temperature dependence: the rate constant decreases as the temperature rises.

Numerous reactions occur in solution, and the properties of the solvent influence the rate of the reactions. The rate of the reaction is also affected by ionic strength.

Energy is emitted by electromagnetic radiation. As a result, it may accelerate or even cause a spontaneous reaction by providing more energy to the reactant particles. This energy is stored in the reacting particles in some way (it may break bonds, promote molecules to electronically or vibrationally excited states, etc.), resulting in intermediate species that react easily. As the intensity of the light increases, the particles absorb more energy, increasing the rate of reaction. When methane reacts with chlorine in the dark, for example, the reaction rate is slow. It can be accelerated by exposing the mixture to diffused light. The reaction will be explosive in direct sunlight.

In fact, the existence of a catalyst increases the reaction rate (in both forward and reverse reactions) by providing a lower activation energy alternative pathway. Platinum, for example, catalyses the combustion of hydrogen with oxygen at room temperature.

Because of the relative mass difference between hydrogen and deuterium, the kinetic isotope effect causes a different reaction rate for the same molecule if it contains different isotopes, usually hydrogen isotopes. The rate of reaction increases as the surface area increases in reactions on surfaces, such as those that occur during heterogeneous catalysis. This is because more solid particles are exposed and can be hit by reactant molecules.

Stirring could have a significant impact on the rate of heterogeneous reactions.

Diffusion is a constraint on some reactions. Except for concentration and reaction order, the reaction rate coefficient takes into account all of the factors that influence a reaction rate (the coefficient in the rate equation of the reaction).

First-order reaction

The first reaction is a chemical reaction in which the rate of a reaction is linearly related to the concentration of only one reactant. In other words, a first-order reaction is a chemical reaction in which the rate varies as the concentration of only one of the reactants changes. As a result, the order of these reactions equals 1.

The kinetic parameters of a chemical equation can be studied using rate law, which describes the relationship between reaction rate and reactant concentration.

The derivative of the reactant’s concentration with time is given by the differential rate law. It is written as follows for a first-order reaction:

R = – d[A]/dt = k [A]

Here,

R is said to be the reaction rate

[A] is said to be the concentration of the reactant A

k is considered as the rate constant and d[A]/dt is the derivative of [A] with time.

The integrated rate law could be used to predict how much of a reactant will remain at a given time or how long it will take for the concentration to fall to a given level (e.g., one-half). The differential form of the first-order reaction must be integrated over concentration and time in order to derive the integrated form.

The integral form is represented by,

[A] = [A]_o exp (-kt)

Here,

[A]_o is said to be the concentration at time t = 0

[A] is said to be the concentration at time t.

The concentrations can be expressed in terms of their natural logarithm using the above expression.

ln [A] = ln [A]_o – kt

This equation is a straight line equation that can be used to calculate the rate constant.

Plot the natural logarithm of the concentration versus time to see if the graph is linear to determine if a reaction is first-order. The reaction must be first-order if the graph is linear and has a negative slope.

Second-order reaction

A second-order reaction is a kind of chemical reaction that is influenced by the concentrations of one second-order reactant or two first-order reactants. This reaction proceeds at a rate proportional to the square of one reactant’s concentration or the product of two reactant concentrations. The rate at which the reactants are consumed is referred to as the reaction rate.

The reaction rate for a general chemical reaction aA + bB → cC + dD can be expressed in terms of the reactant concentrations using the equation:

rate=k[A]x[B]y

Here, k is said to be a constant

[A] and [B] are considered as the reactant concentrations;

x and y are the experimentally determined reaction orders, not to be confused with the stoichiometric coefficients a and b.

The sum of the values x and y determines the order of a chemical reaction. A reaction of second order is one in which x + y = 2. This can occur if one reactant is consumed at a rate proportional to the square of its concentration (rate = k[A]²) or if both reactants are consumed linearly over time (rate = k[A][B]).

The general form is,

2 A → products
or
A + B → products.

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FAQs

What influences the rate of reaction?

Reactant concentration, the physical state of the reactants, surface area, temperature, and the presence of a catalyst are the four main factors that influence reaction rate.

How is the rate of reaction used in industry?

The rate of reaction is the amount of time it takes for a reaction to complete. Since the ultimate goal of any industry is to make as much money as possible, industries strive to have the fastest rate possible. Increase the concentration/pressure of the reaction mixture to achieve high rates of reaction.

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