Measuring Rates of Reactions
Chemical reactions depend on collisions between the reacting particles, atoms, molecules, or ions. it is these collisions that will produce an activated complex that enables the reaction to take place. The rate of reaction (i.e. how fast the reaction is occurring) is affected by the number of collisions occurring and the fraction of these collisions that are effective. The following video will illustrate the method that can be used to measure the rate of a chemical reaction.
Factors Affecting Rates of Reaction
The rate of a chemical reaction depends on several factors, which are listed below.
NATURE OF THE REACTANTS
Since bonds may be broken or formed in a reaction, the nature of the bond is an important fact for affecting reaction rates.
Reactions that involve very little bond rearrangements are usually rapid at room temperature, such as the reactions of ionic substances in aqueous solutions. Reactants that involve the breaking of many strong bonds tend to be slow at room temperature, such as the reaction between hydrogen and oxygen.
CONCENTRATION OF REACTANTS:
-an increase in the concentration of one or more reactants generally increases the rate of reaction.
-and increase in the concentration of a reactant increases the frequency of collisions by increasing the number of collisions per unit time.
-in a gaseous system, an increase in pressure will result in an increase in concentration and thus an increase in the rate of reaction.
TEMPERATURE
-an increase in temperature increases the rate of all chemical reactions.
-an increase in temp. increases the speed (and thus the kinetic energy) of the particles and increases not only the number of collisions per unit time, but also of greater importance, the effectiveness of the collisions.
SURFACE AREAI
Increasing the surface area of the reactants by crushing, pulverizing or using smaller pieces increases the rate of reaction.
In heterogeneous reactions, surface area plays an important part in how fast a reaction occurs. For example, a given amount of zinc will react more readily with dilute hydrochloric acid if the surface area of zinc is increased by using smaller pieces.
CATALYSTS
Catalysts are substances that change the activation energy required for a chemical reaction, and thus change the rate of reaction. Also, a catalyst changes the mechanism of a reaction to one involving less activation energy, but does not change the overall process, and the catalyst itself is not changed. A CATALYST DOES NOT INITIATE A CHEMICAL REACTION.
Equilibrium Defined
Many reactions in closed systems are reversible. A state of Equilibrium is reached that is dynamic and only describes the overall appearance of the system. It does not describe the activity of individual particles. the word dynamic implies motion, and dynamic equilibrium is that condition where the the interaction of the particles of the reactants in one direction, is balanced by the interaction of the particles of the products in the opposite direction.
Equilibrium can thus be defined as the state in which the rate of a forward reaction is equal to the rate of the corresponding reverse reaction. Although the reaction rates for the opposing reactions are equal, a state of equilibrium may exists where the quantities of reactions are not equal. Thus equilibrium may be reached when only a small quantity of either the reactants or products are present.
Potential Energy Diagrams
All chemical reactions involve energy exchanges. Energy is either released (exothermic) or absorbed (endothermic). Reactions that are at equilibrium are no exception. Each of the reactions, either forward or reverse, is losing or gaining energy. Since the reaction is at equilibrium, however, there is no noticeable change in temperature of the reaction.
Nevertheless, it is possible to draw a graphical representation of the potential energy changes as the chemicals react in each direction.
Watch the following video to see how potential energy changes in any reaction can be illustrated.
