Rate of a Chemical Reaction: Factors Affecting Reaction Rate

The study of reaction rate and its determinants is called chemical kinetics. Kinetics helps in understanding the chemical reaction mechanisms, which deal with the speed and movement of electrons and atoms. The knowledge of reaction mechanisms is useful to find the effective ways by which a reaction can be caused to occur. Besides explaining reaction mechanisms, kinetics also finds its applications in psychology, geology, engineering, biology, and cosmology. As a study of reaction rates, it lays down the various factors that affect the rate of a chemical reaction. But before understanding the determinants, know the underlying process of a chemical reaction. 

Key Terms: Rate of Reaction, Chemical Reactions, Temperature, Reactant, Atoms, Chemical kinetics, Energy, Integrated Rate Equations

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What happens in a Chemical Reaction?

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A chemical reaction is a process of converting substances, or reactants, into other different substances, or products. This is achieved by rearranging the atoms present in the reactants so that they can form new substances. The reactants can be both compounds or elements. And while both chemical and physical reactions involve changes, there is an essential difference between the two.

Physical changes such as water evaporating into vapour do not change the chemical property of water. But if the water in any of its forms combines with another substance, a new substance will be formed. It can then be said that a chemical reaction has occurred. The rate of reaction gives useful insights into the formation of various products.

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What is a Reaction Rate?

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In the simplest of terms, the speed at which reactants are converted into products is known as reaction rate. It refers to the change in the concentration of reactants per unit time. The rate of occurrence of chemical reactions varies at different ranges; some being instantaneous while others take a certain amount of time to occur.

The rate or speed of a chemical reaction is estimated by dividing the change in a substance's concentration by the interval of time in which the change occurs. Thus, the reaction rate can be expressed as:

rate= Δ concentration/Δ time

The Collision Theory of reactivity states that reactions result from the effective collision of the reactant molecules. And for the reactant molecules to break and form bonds, they must have a proper space. After forming bonds, the atoms rearrange themselves to form the product molecules. This process occurs in different time frames.

Time scales can cause the reaction rate to vary greatly. Some reaction rates are high and fast, such as the fireworks. But other reaction rates, like the rusting of iron, may take a much longer time. Apart from time, other factors determine reaction rates

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Factors Determining the Rate of a Reaction

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There are certain factors that determine the rate of a reaction:

1. Temperature
2. Catalyst
3. Reactant Concentration
4. Chemical nature of Reactant
5. Reactant Subdivision rate

Temperature

Reaction rate increases by two times with each 10° rise in temperature. And this is because a rise in temperature increases the average kinetic energy that the reactant molecules have. In other words, the heating of a substance causes the molecules to move faster. As a result, the activation energy, which is the minimum required energy for a reaction to start, of the molecules decreases. This, in turn, causes the molecules to collide with high energy. Note that there should be enough energy for the particles to collide and start the reaction.

Catalyst

Catalysts are substances that remain non-participatory in a reaction and yet accelerate their rate. Catalysts, therefore, provide an alternate pathway through which a chemical reaction can occur with less energy and create new products. Some catalysts help increase different types of reaction rates. Other catalysts work with only a specific reactant molecule. Such catalysts include the enzymes present in the bodies of human beings.

Reactant Concentration

The Rate of Reaction is related to the concentration of reactants. As the number of molecules in a reactant increase, its concentration also increases. And a higher concentration means the reactants will collide more in a given period. The result of the collision is eventually a higher reaction rate.

Reactant chemical nature

The nature of reactants causes reaction rates to vary under the same conditions. For example, both sodium and calcium combine with water to form a base and hydrogen gas. But the reaction of calcium is moderate, whereas the reaction of sodium is rapid enough to cause explosions.

Reactant subdivision state

The surface contact of a heterogeneous mixture of reactant molecules, other than those in solution or the gaseous state, restricts the rate of reaction. If, for instance, a gas reactant mixes with a solid reactant such as a metal, only the molecules that are closer to the surface of the metal will collide with the gas molecules. Similarly, a solid that is evenly divided has enough surface area for a reaction than an uncut piece of that solid.

Reactant Subdivision State

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Things to Remember

• Rate of a chemical reaction is the change in concentration of a reactant or product per unit time. 
• Factors that affect rate of reaction are:
  • Surface area of a solid reactant.
  • Concentration or pressure of a reactant.
  • Temperature.
  • Nature of the reactants.
  • Presence/absence of a catalyst.
• Reaction rates can vary dramatically. For most reactions, the rate decreases as the reaction proceeds.
• A catalyst speeds up a chemical reaction, without being consumed by the reaction. 
• Catalysts increase the reaction rate by lowering the activation energy for a reaction.
• Optimal pH increases enzyme rate of reaction while less than optimal pH decreases it.

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