Reaction Rate

The reaction rate, also known as the rate of reaction, is the rate at which a chemical reaction occurs, and is proportional to the increase in product concentration per unit time and the decrease in reactant concentration per unit time. The speed at which the reaction proceeds varies a lot. 

Rate of reaction examples: The oxidative rusting of iron under Earth's atmosphere is a gradual process that can take several years, while the combustion of cellulose in a fire occurs in fractions of a second. The rate of most reactions decreases as the reaction progresses. The rate of a reaction can be calculated by tracking changes in concentration over time.

The rate of a chemical reaction is often expressed in terms of the concentration (amount per unit volume) of a substance produced in a unit of time or the concentration (amount per unit volume) of a reactant consumed in a unit of time. It can also be expressed in terms of the number of reactants consumed or products produced in a given amount of time.

This article will study reaction rate, rate of chemical reaction, to define rate of reaction, and rate of reaction formula.

Factors Affecting Rate of Reaction

The nature of the reaction, concentration, strain, reaction order, temperature, solvent, electromagnetic radiation, catalyst, isotopes, surface area, stirring, and the diffusion limit are all factors that affect the reaction rate. Some reactions occur more quickly than others. The rate of a reaction is greatly affected by the number of reacting species, their physical state (solid particles move much more slowly than gases or those in solution), the complexity of the reaction, and other factors.

1. Concentration of Reactant

As defined by the rate law and explained by collision theory, the reaction rate increases with concentration. The number of collisions increases as the concentration of reactants rises. The rate of gaseous reactions increases as pressure rises, which is similar to a rise in gas concentration. While there are fewer moles of gas present, the reaction rate increases, and when there are more moles of gas present, it decreases. The pressure dependency is poor for condensed-phase reactions.

2. Electromagnetic Radiation

As a consequence, electromagnetic radiation can speed up or even make a reaction spontaneous by adding more energy to the reactant particles. This energy is stored in the reacting particles in one way or another, resulting in intermediate species that are easy to react. The particles gain more energy as the strength of light increases, and thus the rate of reaction increases.

3. Catalysts

By offering an alternative pathway with lower activation energy, the presence of a catalyst increases the reaction rate (in both forward and reverse reactions). At room temperature, platinum, for example, catalyzes the combustion of hydrogen with oxygen.

4. Isotope

Because of the relative mass difference between hydrogen and deuterium, the kinetic isotope effect induces a different reaction rate for the same molecule if it has different isotopes, typically hydrogen isotopes. The rate of reaction increases as the surface area increases in reactions on surfaces, such as during heterogeneous catalysis. This is due to the fact that more stable particles are exposed and can be struck by reactant molecules.

5. Stirring

For heterogeneous reactions, stirring can have a significant impact on the rate of reaction.

6. Diffusion

Diffusion is a limiting factor in certain reactions. The reaction rate coefficient takes into account all variables that influence a reaction rate, excluding concentration and reaction order (the coefficient in the rate equation of the reaction).

7. Temperature

The average kinetic energy of the reactants is measured by temperature. The kinetic energy of the reactants increases as the temperature rises. In other words, the particles are moving faster. Since the reactants are moving faster, further collisions will occur at a faster pace, raising the chances of reactants forming into products and therefore increasing the rate of reaction. A ten-degree increase in temperature causes the reaction rate to double. The temperature dependence of each reaction rate coefficient k is typically given by the Arrhenius equation:

k=Ae\[^{-Ea/RT}\]

8. Pressure

The concentration of gases increases as pressure rises, resulting in a faster rate of reaction. The reaction rate increases as the number of gaseous molecules decreases and decreases as the number of gaseous molecules increases.

As a result, it's easy to see how pressure and concentration are related, and how they both influence reaction rates.

Average Rate of Reaction

Now consider the following reaction to achieve a better understanding.

For the given reaction below:

A → B

A reactant A undergoes a chemical reaction to produce a product B in this reaction. The concentration of any reactant or product is usually defined as [reactant] or [product]. As a result, A's concentration can be expressed as [A] and B's concentration as [B]. The start time, t=0 should be the time when the reaction starts.

Let's take a look at the following situation:

At t = t₁,

The concentration of A = [A]₁

The Concentration of B = [B]₁

At t = t₂,

The concentration of A = [A]₂

The concentration of B = [B]₂

In the time interval between t₁ and t₂, we want to know the rate at which A (reactant) disappears and the rate at which the product B appears. As a result

Rate of Disappearance of [A] = \[\frac{[A]_{2} - [A]_{1}}{t_{2} - t_{1}}\] = \[\frac{-\Delta A}{\Delta t}\]

The negative sign indicates that the concentration of A is decreasing.

Rate of Appearance of [B] = \[\frac{[B]_{2} - [B]_{1}}{t_{2} - t_{1}}\] = \[\frac{\Delta B}{\Delta t}\]

Since A is the only reactant in the reaction and B is the only product produced, and since mass is conserved, the amount of A that has disappeared in the time interval t will be the same as the amount of B that has formed in the same time interval. So we can conclude that

The reaction rate = – A's rate of disappearance equals B's rate of appearance.

Rate of Reaction = \[\frac{-\Delta A}{\Delta t}\] = \[\frac{\Delta B}{\Delta t}\]

The above reaction shows that the disappearance of A is equal to the appearance of B.

Instantaneous Rate of Reaction

The rate of reaction at any given time is known as the instantaneous rate of reaction.

Assume that the time t has a very small value and is approaching zero. Now we have an infinitesimally small t, which is a very short time period and can be thought of as a single point in time. The instantaneous rate of reaction would be the average reaction rate.

Instantaneous Rate of Reaction = \[\frac{-dA}{dt}\] =  \[\frac{dB}{dt}\]

Did You Know?

The power dependence of rate on all reactant concentrations can be described as the order of the reaction. The rate of a first-order reaction, for example, is solely determined by the concentration of one species in the reaction. The following are some features of a chemical reaction's reaction order.

1. The number of species whose abundance directly affects the rate of reaction is defined by reaction order.

2. All the exponents of the concentration terms in the rate expression can be added to get it.

3. The stoichiometric coefficients corresponding to each species in the balanced reaction have no effect on the reaction order.

4. A chemical reaction's reaction order is often determined by reactant concentrations rather than product concentrations.

5. The order of reaction can be expressed as an integer or as a fraction. It is also possible for it to have a value of zero.

The power-law form of the rate equation is commonly used to calculate the reaction order r = k[A]^x[B]^y is the expression for this form of the rate law.