Why does reaction rates matter in life?

General Chemistry Online! What are everyday examples of temperature effects on reaction rate?

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Collision Theory
Factors That Affect Rate
Media Attributions

Copyright © 1997-2010 by Fred Senese
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Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/kinetics/faq/everyday-kinetics.shtml

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The assertion that "a 10° temperature rise doubles reaction rates" is just a rule of thumb, not a law of nature. Like most rules of thumb, "thumbtimes it works, and thumbtimes it doesn't." In fact, relying this rule to predict the effect of temperature on the rate of a chain reaction can be a catastrophic mistake.

What is the source of the rule? The rule probably began with a back-of-the-envelope calculation based on the Arrhenius equation, first published in 1889. Arrhenius explained the variation of rate constants with temperature for several elementary reactions using the relationship

k = A exp(-Ea/RT)

where the rate constant k is the total frequency of collisions between reaction molecules A times the fraction of collisions exp(-Ea/RT) that have an energy that exceeds a threshold "activation energy" Ea at a temperature of T (in kelvins). R is the universal gas constant.

To see what temperature rise is required to change the rate constant from k1 (at T1) to k2 (at T2), take the ratio of the Arrhenius equations for each of the two temperatures:

k2k1

A exp(-Ea/RT2)A exp(-Ea/RT1)

exp

(

EaR

[

1T2

1T1

])

The calculator below solves this equation for the activation energy, given a rate factor increase (k2/k1) and an initial and final temperature. Use it to evaluate the rule of thumb. Try answering the following questions:

What activation energy must the reaction have to apply to make the rate double with a 10°C rise?
Is the same activation required to make the rate quadruple for a 20°C temperature rise?
Try a different initial temperature. Will the activation energy required be the same for any 10°C rise, no matter what the initial temperature is?

If the activation energy has a particular value, and if the temperature change occurs in the right range, and if the reaction is an elementary one that obeys the Arrhenius equation, then a 10°C rise might double the reaction rate. It's a very iffy generalization.

Can we at least say that reaction rate increases with temperature? For reactions that occur in a single step involving a molecular collision of some kind, the answer is yes. Only molecules with sufficient energy are able to react. Heat increases the average energy of the molecules and so it would be expected that reaction rate would always increase with increasing temperature.

But many reactions aren't that simple, and their rates may actually decrease with increasing temperature.

What are some examples of reactions that slow down when temperature rises? Nearly all biochemical reactions, for example, are catalyzed by rather delicate protein molecules called enzymes. Warming a biochemical reaction increases its rate as expected - up to a certain temperature. Heating beyond that point actually decreases the reaction rate and further heating can stop the reaction completely. Heat causes the enzyme to unravel or unfold, and the enzyme's shape is critical to its ability to accelerate the reaction.

Even some inorganic reactions can slow down when things heat up. Consider this reaction with intermediate B:

A B	(fast, exothermic)
B products	(slow)

Heating the reaction should drive the fast equilibrium step towards the left, depleting the amount of B. That will decrease the rate of the second reaction - so the rate of the overall reaction A

products will decrease as temperature rises.

When can relying on the rule be dangerous? Consider the formation reaction for gaseous hydrogen chloride,

H2(g) + Cl2(g)

2 HCl The reaction requires heat (or light) for initiation: heat + Cl2(g)

2 Cl(g) The extremely reactive chlorine atoms then trigger a chain reaction: Cl(g) + H2(g)

HCl(g) + H(g)
H(g) + Cl2(g)

HCl(g) + Cl(g)
Notice that chlorine atoms are consumed in one step and produced in another. The reaction keeps going without further need for the initiation step.

Heating the H2/Cl2 mixture will at first elevate the reaction rate only slightly. Once the propagation steps begin to occur at significant rates, further heating causes a sudden, tremendous acceleration in reaction rate. Instead of a doubling of rate that might otherwise have been expected, the rate may suddenly increase by orders of magnitude- resulting in a violent explosion!

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Author: Fred Senese

Jessie A. Key

To gain an understanding of collision theory.
To gain an understanding of the four main factors that affect reaction rate.

Reaction kinetics is the study of the rate of chemical reactions, and reaction rates can vary greatly over a large range of time scales. Some reactions can proceed at explosively fast rates like the detonation of fireworks (Figure 17.1 “Fireworks at Night Over River”), while others can occur at a sluggish rate over many years like the rusting of barbed wire exposed to the elements (Figure 17.2 “Rusted Barbed Wire”).

Figure 17.1 “Fireworks at Night Over River.” The chemical reaction in fireworks happens at an explosive rate.

Figure 17.2 “Rusted Barbed Wire.” The rusting of barbed wire occurs over many years.

Collision Theory

To understand the kinetics of chemical reactions, and the factors that affect kinetics, we should first examine what happens during a reaction on the molecular level. According to the collision theory of reactivity, reactions occur when reactant molecules “effectively collide.” For an “effective collision” to occur, the reactant molecules must be oriented in space correctly to facilitate the breaking and forming of bonds and the rearrangement of atoms that result in the formation of product molecules (see Figure 17.3 “Collision Visualizations”).

Figure 17.3 “Collision Visualizations.” This visualization shows an ineffective and effective collision based on molecular orientation.

During a molecular collision, molecules must also possess a minimum amount of kinetic energy for an effective collision to occur. This energy varies for each reaction, and is known as the activation energy (Ea) (Figure 17.4 “Potential Energy and Activation Energy”). The rate of reaction therefore depends on the activation energy; a higher activation energy means that fewer molecules will have sufficient energy to undergo an effective collision.

Figure 17.4 “Potential Energy and Activation Energy.” This potential energy diagram shows the activation energy of a hypothetical reaction.

Factors That Affect Rate

There are four main factors that can affect the reaction rate of a chemical reaction:

Reactant concentration. Increasing the concentration of one or more reactants will often increase the rate of reaction. This occurs because a higher concentration of a reactant will lead to more collisions of that reactant in a specific time period.
Physical state of the reactants and surface area. If reactant molecules exist in different phases, as in a heterogeneous mixture, the rate of reaction will be limited by the surface area of the phases that are in contact. For example, if a solid metal reactant and gas reactant are mixed, only the molecules present on the surface of the metal are able to collide with the gas molecules. Therefore, increasing the surface area of the metal by pounding it flat or cutting it into many pieces will increase its reaction rate.
Temperature. An increase in temperature typically increases the rate of reaction. An increase in temperature will raise the average kinetic energy of the reactant molecules. Therefore, a greater proportion of molecules will have the minimum energy necessary for an effective collision (Figure. 17.5 “Temperature and Reaction Rate”).

Figure 17.5 “Temperature and Reaction Rate.” Effect of temperature on the kinetic energy distribution of molecules in a sample
Presence of a catalyst. A catalyst is a substance that accelerates a reaction by participating in it without being consumed. Catalysts provide an alternate reaction pathway to obtain products. They are critical to many biochemical reactions. They will be examined further in the section “Catalysis.”

Reactions occur when two reactant molecules effectively collide, each having minimum energy and correct orientation.
Reactant concentration, the physical state of the reactants, and surface area, temperature, and the presence of a catalyst are the four main factors that affect reaction rate.