Factors Affecting Rates of Reaction

Whenever a reaction occurs, molecules collide with enough energy to break down or change reactants into a new species known as a product (often there is more than one product). As a result, the rate of reaction is both the rate at which the product is formed and the rate at which the reactant is depleted. Because reactions necessitate the molecules to overcome a specific energy barrier in order to collide successfully, the rate of reaction frequently indicates whether the conditions are favourable for this to occur. A slow rate of reaction, for example, may indicate that not enough collisions occur with sufficient force to break the chemical bonds of the reactants, causing the product to be produced more slowly. Whether this is known, manufacturers can investigate the best way to increase the number of successful molecule collisions in order to increase yield. It really would be difficult to monitor a specific chemical being produced or used because reactions are frequently a confusing mixture, but we can often observe obvious side effects that are easy to measure.

Overview

Chemical reactions take place at different rates depending on the nature of the reacting substances, the type of chemical transformation, the temperature, and other factors. Reactions involving atoms or ions (electrically charged particles) happen very quickly, whereas reactions involving covalent bonds (bonds in which atoms share electrons) happen much slower. The rate of a given reaction varies with temperature, pressure, and the number of reactants present. Reactions typically slow down over time due to reactant depletion. In some cases, the addition of a non-reactant substance, known as a catalyst, speeds up a reaction.

A rate is said to be a measure of how a property changes over time. The well-known rate of speed expresses how far an object travels in a given amount of time. A wage is a rate that represents the amount of money earned by a person who works for a set period of time. Similarly, the rate of a chemical reaction is a measure of how much reactant or product is consumed or produced by the reaction in a given amount of time.

Rate of a reaction

The quantity of a reactant or product that changes per unit time is referred to as the rate of reaction. Reaction rates are thus calculated by determining the time dependence of some property that is related to a reactant or product amounts. Rates of reactions that consume or produce gaseous substances, for example, can be easily calculated by measuring changes in volume or pressure. Rates of reactions involving one or more coloured substances can be monitored using light absorption measurements. Rates of aqueous electrolyte reactions can be measured using changes in solution conductivity.

The rate of a reaction is always positive and the presence of a negative sign indicates that the concentration of the reactant is decreasing. The International Union of Pure and Applied Physics (IUPAC) recommends that time be measured in seconds. In general, the rate of reaction differs from the rate of concentration increase of a product P by a constant factor (the reciprocal of its stoichiometric number) and by a factor less than the reciprocal of the stoichiometric number of a reactant A. The stoichiometric numbers are included so that the defined rate is not affected by the reactant or product species being measured.

Rate of reaction formula

Consider a standard chemical reaction:

a A + b B → p P + q Q

Small letters (a,b,p,q) denote Stoichiometric coefficients, while capital letters (A&B) denote reactants and capital letters (P&Q) denote products. The rate of reaction r occurring in a closed system without the formation of reaction intermediates under isochoric conditions is defined in the IUPAC Gold Book as:

In this case, the negative sign denotes a decreasing concentration of the reactant.

Factors affecting rates of 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 and the reaction is explosive in direct sunlight.

In reality, of course, the presence of a catalyst speeds up the reaction (both forward and reverse) by providing a lower activation energy alternative pathway. Platinum, for example, catalyses the combustion of hydrogen with oxygen at room temperature.

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).

The average rate of reaction

The average rate of reaction is described as the ratio of the change in concentration of a chemical reaction’s reactants or products to the time interval. The average rate is represented mathematically as ΔX/ Δt.

The average reaction rate can indeed be positive or negative.

When the rate of product concentration increases, the average rate of reaction is said to be positive.

When the rate of concentration of the reactant decreases, the average rate of reaction becomes negative.

First-order reaction

The first reaction is a chemical reaction in which the rate of 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.

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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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