Chem 1212 - 17.2 Factors Affecting Reaction Rates

A chemical reaction between two or more substances require

intimate contact between the reactants

For many chemical processes,

reaction rates are approximately doubled when the temperature is raised by 10 degrees Celsius

Compared with the reaction rate for large solid particles,

the rate for smaller particles will be greater b/c the surface area in contact with the other reactant phase is greater

Example of Physical States of Reactant

large pieces of iron react more slowly with acids than they do with finely divided iron powder (Figure 17.6). Large pieces of wood smolder, smaller pieces burn rapidly, and saw dust burns explosively.

5 factors typically affect the rates of chemical reactions:

-chemical nature of the reacting substances -state of subdivision (one large pulp vs many small particles) of the reactant -temperature of the reactants -concentration of the reactants -presence of a catalyst

Reactions that appear similar may have different rates under the same conditions,

depending on the identity of the reactants

Example of Concentrations of Reactants

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 17.7). An acidic oxide, sulfur dioxide combines with water vapor in the air to produce sulfurous acid in the following reaction: SO2(g)+H2O(g)⟶H2SO3(aq)SO2(g)+H2O(g)⟶H2SO3(aq) Calcium carbonate reacts with sulfurous acid as follows: CaCO3(s)+H2SO3(aq)⟶CaSO3(aq)+CO2(g)+H2O(l)CaCO3(s)+H2SO3(aq)⟶CaSO3(aq)+CO2(g)+H2O(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.

Chemical reactions typically occur at

higher temperature

The rate of a reaction depends

on the nature of the participating substances

As lower temperatures would

slow down chemical reactions

The presence of a catalyst

speeds up a reaction by increasing the number of effective collisions

catalyst

substances that function to increase the rate of reaction

Rate usually increases when

the concentration of one or more reactants increases

The rates of many reactions depend on

the concentrations of the reactants

When reactants are in different physical states or phases,

the reaction takes place only @ the interface between the phases

Gas burners, hot plates, and ovens are often used in the laboratory

to increase the speed of reactions that proceed slowly at ordinary temperatures.

Example of the chemical nature of the reacting substances

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.