Library Guides: Chemistry Textbook: Collision Theory and Factors Affecting Reaction Rates

Collision Theory

By the end of this section, you will be able to:

Use the postulates of collision theory to explain the effects of physical state, temperature, and concentration on reaction rates

We should not be surprised that reactant molecules (or atoms/ions) must collide with each other to form new chemical bonds leading to the formation of products. This simple premise that reactant particles should collide is the basis for Collision theory which powerfully explains many observations regarding speeds of reactions and the factors affecting them.

Collision theory is based on the following postulates:

Greater the number of collisions between reactants in a given amount of time, faster will be the reaction. In order words, the rate (or speed) of a reaction is proportional to the rate of reactant collisions:

$r e a c t i o n r a t e \propto \frac{# c o l l i s i o n s}{t i m e}$
The collision must occur with adequate energy (this minimum energy needed is called activation energy). The adequate energy permits mutual penetration of the reacting species’ valence shells so that the electrons can rearrange and form new bonds leading to new chemical species.
The reacting species must also collide in proper orientation. Reacting species must be oriented such that the atoms that will become bonded together in the product must come in contact.

Factors Affecting Reaction Rates

Five factors typically determine the speed of chemical reactions: the chemical nature of the reacting substances, the state of subdivision (one large lump versus many small particles) of the reactants, the temperature of the reactants, the concentration of the reactants, and the presence of a catalyst.

The Chemical Nature of the Reacting Substances

The rate of a reaction depends on the nature of the participating substances. Reactions that appear similar may have different rates under the same conditions, depending on the identity of the reactants. For example, when small pieces of the metals iron and sodium are exposed to air, the sodium reacts completely with air overnight, whereas the iron is barely affected. The active metals calcium and sodium both react with water to form hydrogen gas and a base. Yet calcium reacts at a moderate rate, whereas sodium reacts so rapidly that the reaction is almost explosive.

The Physical States of the Reactants

A chemical reaction between two or more substances requires intimate contact between the reactants. When reactants are in different physical states, or phases (solid, liquid, gaseous, dissolved), the reaction takes place only at the interface between the phases. Consider the heterogeneous reaction between a solid phase and either a liquid or gaseous phase. Compared with the reaction rate for large solid particles, the rate for smaller particles will be greater because the surface area in contact with the other reactant phase is greater. For example, large pieces of iron react more slowly with acids than they do with finely divided iron powder (Figure 10.2). Large pieces of wood smolder, smaller pieces burn rapidly, and saw dust burns explosively.

This figure shows two photos labeled (a) and (b). Photo (a) shows the bottom of a test tube. The test tube is filled with a dark gas, and there is a dark substance and bubbles in the bottom. Photo (b) shows a rod and bubbles in a test tube similar to photo (a), but the gas in the test tube is not as dark. — Figure 10.2 (a) Iron powder reacts rapidly with dilute hydrochloric acid and produces bubbles of hydrogen gas: 2Fe(s) + 6HCl(aq) $⟶$ 2FeCl₃(aq) + 3H₂(g). (b) An iron nail reacts more slowly because the surface area exposed to the acid is much less.

LINK TO LEARNING

Watch this video to see the reaction of cesium with water in slow motion and a discussion of how the state of reactants and particle size affect reaction rates.

Temperature of the Reactants

Chemical reactions typically occur faster at higher temperatures. Food can spoil quickly when left on the kitchen counter. However, the lower temperature inside of a refrigerator slows that process so that the same food remains fresh for days. Gas burners, hot plates, and ovens are often used in the laboratory to increase the speed of reactions that proceed slowly at ordinary temperatures. For many chemical processes, reaction rates are approximately doubled when the temperature is raised by 10 °C.

Concentrations of the Reactants

The rates of many reactions depend on the concentrations of the reactants. Rates usually increase when the concentration of one or more of the reactants increases. For example, calcium carbonate (CaCO₃) deteriorates as a result of its reaction with the pollutant sulfur dioxide. The rate of this reaction depends on the amount of sulfur dioxide in the air (Figure 10.3). An acidic oxide, sulfur dioxide combines with water vapor in the air to produce sulfurous acid in the following reaction:

{SO}_{2} (g) + H_{2} O(g) ⟶ H_{2} {SO}_{3} (a q)

Calcium carbonate reacts with sulfurous acid as follows:

{CaCO}_{3} (s) + H_{2} {SO}_{3} (a q) ⟶ {CaSO}_{3} (a q) + {CO}_{2} (g) + H_{2} O(l)

In a polluted atmosphere where the concentration of sulfur dioxide is high, calcium carbonate deteriorates more rapidly than in less polluted air. Similarly, phosphorus burns much more rapidly in an atmosphere of pure oxygen than in air, which is only about 20% oxygen.

A photograph is shown of an angel statue. While some details of the statue, including facial features, are present, effects of weathering appear to be diminishing these features. — Figure 10.3 Statues made from carbonate compounds such as limestone and marble typically weather slowly over time due to the actions of water, and thermal expansion and contraction. However, pollutants like sulfur dioxide can accelerate weathering. As the concentration of air pollutants increases, deterioration of limestone occurs more rapidly. (credit: James P Fisher III)

LINK TO LEARNING

Phosphorous burns rapidly in air, but it will burn even more rapidly if the concentration of oxygen is higher. Watch the video below to see an example.

The Presence of a Catalyst

Relatively dilute aqueous solutions of hydrogen peroxide, H₂O₂, are commonly used as topical antiseptics. Hydrogen peroxide decomposes to yield water and oxygen gas according to the equation:

2 H_{2} O_{2} (l) ⟶ 2 H_{2} O (l) + O_{2} (g)

Under typical conditions, this decomposition occurs very slowly. When dilute H₂O₂(aq) is poured onto an open wound, however, the reaction occurs rapidly and the solution foams because of the vigorous production of oxygen gas. This dramatic difference is caused by the presence of substances within the wound’s exposed tissues that accelerate the decomposition process. Substances that function to increase the rate of a reaction are called catalysts.

LINK TO LEARNING

Chemical reactions occur when molecules collide with each other and undergo a chemical transformation. Before physically performing a reaction in a laboratory, scientists can use molecular modeling simulations to predict how the parameters discussed earlier will influence the rate of a reaction. Use the PhET Reactions & Rates interactive to explore how temperature, concentration, and the nature of the reactants affect reaction rates.

Key Concepts and Summary

The rate of a chemical reaction is affected by several parameters. Reactions involving two phases proceed more rapidly when there is greater surface area contact. If temperature or reactant concentration is increased, the rate of a given reaction generally increases as well. A catalyst can increase the rate of a reaction by providing an alternative pathway with a lower activation energy.