Factors affecting rate of chemical reaction

Factors that can affect rate of chemical reactions

Chemical reactions are fundamental processes that underpin the intricacies of chemistry, from the simplest interactions to complex biochemical pathways. Understanding the factors affecting the rate of chemical reactions is essential for academicians who seek to manipulate reaction conditions for desired outcomes in fields ranging from industrial chemistry to pharmacology.

Meanwhile, recall that the rate of a chemical reaction refers the number of moles of reactants converted or moles of products formed over a period of time (in a specific reaction).

Read detailed explanation on the introduction to rate of chemical reaction here.

This article explores the essential factors that determine reaction rates. They include the following:

1. Concentration of reactants

2. Temperature

3. Pressure

4. Surface area of reactants

5. Catalysts

6. Nature of reactants

7. Light

8. Physical state

• Concentration of Reactants

The concentration of reactants is a primary factor influencing reaction rates. According to the law of mass action, an increase in the concentration of reactants generally leads to an increase in the rate of reaction. A higher concentration signifies that more reactant molecules are present in a given volume, thus enhancing the probability of effective collisions between them, resulting in reactions.

Please read more on collision theory and activation energy here.

As an instance, if you dissolve more sugar in water, the solution will become sweet quicker if stirred, illustrating a faster reaction rate.

An increase or decrease in the concentration of the reactants will result into a corresponding increase or decrease in effective collisions of the reactants and hence, the reaction rate.

In reactions involving gases, doubling the concentration of a gaseous reactant can substantially increase the reaction rate, provided that other conditions remain constant.

You can read on Graham's law of diffusion here.

• Temperature

Temperature plays a crucial role in determining the kinetics of chemical reactions. As temperature increases, the average kinetic energy of the molecules also increases, making them move more rapidly, thus leading to more frequent and energetic collisions. This heightened activity often results in a higher reaction rate.

In diagram A, an increase in temperature (∆H) lowered the energy barrier (activation energy or E_a) of the reaction. This consequently leads to a faster reaction rate. However, this is in contrast with diagram B (with a decreased ∆H), giving rise to an increased activation energy.

Increasing the temperature of a system can lead to an increase in the kinetic energy of the reactant particles. As a result, the number of effective collisions increases and the reaction proceeds at a faster rate. For instance, when cooking, recipes often suggest higher temperatures to ensure ingredients react quickly, yielding a faster rate of the cooked product.

On the contrary, a decrease in temperature leads to a decreased reaction rate

Please read on the concept of ions and electrolytes here.

The Arrhenius equation quantitatively illustrates this relationship, showing that the rate constant increases exponentially with temperature. This principle is notably significant in biological systems, where enzymes (the catalysts of life) often have an optimal temperature range beyond which their activity can decrease drastically.

• Pressure

For reactions involving gases, pressure can significantly influence the reaction rate.

According to Le Chatelier's principle, increasing the pressure effectively increases the concentration of gaseous reactants. This rise in partial pressure leads to an increased frequency of collisions, thereby accelerating the reaction.

Please read more on Le Chatelier's principle on pressure here.

Reaction rate influenced by pressure is particularly important in industrial settings, such as in the Haber process for ammonia synthesis, where optimal pressures are instrumental to achieving substantial yields.

• Surface Area of Reactants

The surface area of solid reactants also impacts the rate of reaction. In heterogeneous reactions, where reactants exist in different phases (solid, liquid and gas), the available surface area for collisions can be a limiting factor.

Finely divided solids such as powders expose more surface area than larger clumps, facilitating faster reaction rates. A practical application of this principle is seen in the use of powdered catalysts or reactants in industrial processes, which enhance reaction efficiency and yield.

Rate of reaction is faster with finely divided solids (powdered solids) and slower with larger clumps of solid.

Consider the diagram below:

From the above diagram, the second curve (comprising smaller pieces of marble) will have a faster reaction rate since its particles have a larger surface area. Simply put, powdered particles have the largest surface area, and consequently, the fastest rate of reaction. On the contrary, larger particles have got smaller surface area with a slower rate of reaction; (assuming all other factors are kept constant).

You can read on characteristics of solids here.

• Catalysts

Catalysts are substances that alters the rate of a reaction without undergoing permanent changes themselves. In this regard, catalysts that increases the rate of chemical reactions are termed positive catalyst, while those that decreases reaction rate, negative catalysts.

Positive catalyst work by providing an alternative reaction pathway with a lower activation energy, thus allowing more reactant molecules to collide with sufficient energy (effective collision) to surpass this barrier. On the contrary, negative catalyst increases the activation energy of a reaction, thus slowing the reaction rate. This is shown in the diagram below:

In the above diagram, the activation energy (energy barrier) was lowered in the catalysed reaction.

The use of catalysts is prevalent across various chemical industries; from petroleum refining to pharmaceuticals, where even minute amounts can have significant effects on reaction rates.

You can read on fractional distillation of petroleum and other separation techniques here.

Another common example of catalysts is the use of enzymes in biological processes, where they significantly increase the rate of reactions necessary for life without being altered themselves.

• Nature of Reactants

Using dilute Hydrochloric Acid (HCl) as a point of reference, its rate of reaction will vary when reacted with zinc (Zn), iron (Fe), and gold (Au) respectively. During its reaction with each of these elements (with all other factors kept constant), hydrogen gas is evolved rapidly with zinc, and slowly with iron. It is not evolved with gold. 

The difference in the rate of reaction is due to the chemical nature of the elements, as they naturally possess different amounts of energy. This reactivity of elements can be seen in the activity series.

Please read on the periodic table of elements here.

• Light

Certain reactions are influenced by light; for instance photosynthesis. Higher intensity of light increases the reaction rate, while lower light intensity decreases reaction rate. In fact, some reaction do not proceed in the absence of light.

Reactions that proceed in the presence of light are termed photochemical reactions. They include the following:

1. Photosynthesis.
   Please read more on autotrophic nutrition and photosynthesis here.

2. Decomposition of hydrogen peroxide (H₂O₂).

3. Reaction between hydrogen and chlorine.

4. Conversion of silver halides to grey metallic silver.

5. Reaction between methane and chlorine.

• Physical State

The physical state of the reactants also affects the reaction rate. Gases generally react faster than solids or liquids.

For example, in combustion, the gaseous reactants in the air mix and react more quickly than powdered fuels, which must dissolve before they can burn effectively.

Please read on gas laws in chemistry here.

In conclusion, by effectively manipulating these factors that alter the rate of chemical reactions, chemists can enhance reaction rates, optimize processes, and innovate new methods for synthesizing substances that benefit various industries and scientific endeavors. Understanding these principles not only enriches academic research, but also aids in practical applications that shape the world we live in.