12.2 Factors Affecting Reaction Rates

12.2 Factors Affecting Reaction Rates

The rates at which reactants are consumed vary greatly.

The chemical nature of the reactants, the temperature of the reactants, the concentration of the reactants, and the presence of a catalyst are some of the factors that affect the rates of chemical reactions.

The nature of the substances affects the rate of reaction.

Depending on the identity of the reactants, reactions that appear similar may have different rates.
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 metals react with water to form hydrogen gas and a base.
The calcium reaction is moderate while the sodium reaction is very fast.

Reactions occur at the boundary, or interface, between two phases, except for substances in the gaseous state or solution.

The rate of a reaction between two phases depends on the surface contact between them.
A finely divided solid has more surface area available for reaction than a large piece of the same substance.
A finely divided solid will react more quickly than a large piece of the same solid.

Large pieces of iron react slowly with acids, while finely divided iron reacts much more quickly.

Large pieces of wood smolder, smaller pieces burn rapidly, and saw dust burns.

The video shows the reaction of cesium with water in slow motion and a discussion of how the state of reactants and particle size affect reaction rates.

Chemical reactions happen at higher temperatures.

Food can be left on the counter.
The lower the temperature in the refrigerator, the more time it takes for the same food to remain fresh.
We use a burner or a hot plate in the laboratory to increase the speed of reactions.
An increase in temperature of 10 degC will double the rate of a reaction in a heterogeneous system.

Concentrations of the reactants affect the rates of many reactions.

When the concentration of the reactants increases, rates increase.
As a result of its reaction with sulfur dioxide, calcium carbonate degrades.

In a polluted atmosphere where the concentration of sulfur dioxide is high, calcium carbonate degrades more quickly than in less polluted air.

In an atmosphere of pure oxygen, phosphorus burns more quickly than in air, which is only 20% oxygen.

Statues made from carbonate compounds such as limestone and marble tend to weather slowly due to the actions of water and thermal expansion.

Sulfur dioxide can accelerate weathering.
The limestone degrades more quickly as the concentration of air pollutants increases.

If the concentration of oxygen in the air is higher, phosphorous will burn more quickly.

To see an example, watch this.

When hydrogen peroxide solutions are poured onto an open wound, substances in the exposed tissues act as catalysts, increasing the rate of hydrogen peroxide's decomposition.

In the case of the bottle in the medicine cabinet, complete decomposition can take months.
The minimum amount of energy required for a chemical reaction is called activation energy.
A catalyst increases the reaction rate by providing an alternative pathway.
The mechanisms of reactions will be discussed in greater detail later in the chapter.