D32.3 Factors Affecting Reaction Rates

Chemical Nature of the Reacting Substances

Some substances react faster than others. For example, potassium and calcium, which are next to each other in the fourth row of the periodic table, both react with water to form H2 gas and a basic solution:

2K(s) + 2H2O(ℓ) ⟶ 2KOH(aq) + H2(g)
Ca(s) + 2H2O(ℓ) ⟶ Ca(OH)2(aq) + H2(g)

As the video below shows, calcium reacts at a moderate rate, whereas potassium reacts so rapidly that the reaction is almost explosive. One factor affecting these different rates is that the reactions involve loss of electrons from potassium or calcium atoms, and potassium has a smaller first ionization energy, making loss of an electron easier. In other words, the smaller IE1 of potassium makes the activation energy of the potassium reaction lower than that of the calcium reaction.

Video: Different substances react at different rates. Left: reaction of potassium with water. Right: reaction of calcium with water. The reaction of potassium with water is faster. At the end of the video the calcium reaction is enlarged so that bubbles of H2(g) can be seen more easily.

Temperature

Chemical reactions typically occur faster at higher temperatures. At higher temperatures, the rate constant is larger, as shown by the Arrhenius equation. Therefore, assuming the concentrations of reactants are the same, a larger rate constant means a faster reaction. For example, methane (CH4) does not react rapidly with air at room temperature, but strike a match and POP!

Video: Temperature affects the rate of a chemical reaction. Natural gas coming out of a burner does not combust rapidly until its temperature is raised by a burning match.

Concentrations

Reaction 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 (SO2). Specifically, sulfur dioxide reacts with water vapor to produce sulfurous acid:

SO2(g) + H2O(g) ⟶ H2SO3(aq)

Sulfurous acid then reacts with calcium carbonate:

CaCO3(s) + H2SO3(aq) ⟶ CaSO3(aq) + CO2(g) + H2O(ℓ)

The rate of the overall reaction depends on the concentration of sulfur dioxide in the air. In a more polluted atmosphere where the concentration of sulfur dioxide is higher, calcium carbonate deteriorates more rapidly.

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: acid rain. Statues made from carbonate compounds such as limestone and marble typically weather slowly over time due to the actions of water as well as thermal expansion and contraction. However, pollutants like sulfur dioxide can accelerate weathering. (credit: James P Fisher III)

In another example, a cigarette burns slowly in air, which contains about 21% oxygen by volume. Put it in pure oxygen and the rate of the reaction accelerates, as shown in the video below.

Video: Higher concentration increases reaction rate. The flask contains pure oxygen, which reacts with paper and tobacco in the cigarette. The cigarette burns much faster in pure oxygen than in air, which contains about one-fifth the oxygen concentration .

Presence and Concentration of a Catalyst

A catalyst is a substance that increases the rate of a chemical reaction by providing an alternative reaction pathway but is not consumed by the reaction. Oftentimes, the greater the concentration of a catalyst the more the catalyst can speed up a reaction. How catalysts work will be discussed in detail later on. Watch the video below to see how a catalyst can speed up the decomposition of hydrogen peroxide to form oxygen and water.

Video: Effect of a catalyst. Manganese dioxide catalyzes decomposition of aqueous hydrogen peroxide, forming water and oxygen. Until the MnO2 is added there is no perceptible reaction. A rapid reaction occurs as soon as the MnO2 is added to the aqueous solution of H2O2. The reaction is exothermic so the temperature goes up. The white cloud is water droplets that condense when heated water vapor cools after escaping from the soda bottle.

Surface Area

If a reaction occurs at a surface, an increase in the surface area of the intersection of two phases (such as the surface of a solid in contact with a gas) can increase the rate. A finely divided solid (like a powder) has more surface area available for reaction than one large solid piece of the same substance. For example, large pieces of wood smolder, smaller pieces burn rapidly, and sawdust burns explosively. The video below shows how large pieces of iron can be held in a burner flame for a long time and hardly react, whereas iron powder blown into the flame sparkles as the tiny particles burn.

Video: Surface area affects the rate of a heterogeneous reaction. A bar of iron held in a flame does not oxidize perceptibly, but when the iron is powdered and blown into the flame from a plastic bottle, a rapid reaction with oxygen occurs.
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Chemistry 109 Fall 2021 Copyright © by John Moore; Jia Zhou; and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.