Changes in Matter Summary Part II

Part II Chemical Changes Summary

Greetings,

This post continues a summary of chemical changes lessons presented in this blog.  The following general topics will be reviewed; collision theory, reaction rates, potential energy changes, and equilibrium theory.

Collision Theory

Chemical species, such as molecules, need additional energy before undergoing a reaction.  This energy is thought to come from a collision between reactant molecules.The force of collision between reacting molecules is increased with increasing heat energy.  Other modes of reactant activation exist, such as microwave energy induction, ultrasound cavitation, and ultraviolet light absorption, but these methods are far less common and will not be discussed here.

Molecular Collision Factors

There is a short list of incidents which must occur in order for a chemical reaction to be initiated.  The reaction collision factors are Frequency, Force, and Orientation.  The combined effect of the factors is a mathematical product of the three and is closely related to the resulting rate of reaction.  Frequency is the total number of collisions occurring per second, which depends on reactant molecule concentration and the average speed of motion.  Speed of molecular motion is in turn effected by the average kinetic energy of the reaction 'system'.  'Force' refers to the number of collisions occurring with sufficient force to help initiate a reaction.  'Force' is also dependent on speed of molecular motion.  The orientation of colliding molecules also needs to be "just right" for a reaction to occur.   The following diagram provides images of these principles.

Reaction Rates

The rate of a chemical reaction is a measure of the speed at which a reactant is consumed.  Measuring the speed of product formation gives the same result.  Most chemical reactions are carried out in a liquid solution and so molarity is often a convenient measure for tracking the rate of a reaction.  The reaction mixture can be sampled at regular time intervals and those samples can be analyzed for reactant or product molar concentration.  The change in sample molarities over a designated time interval provides the necessary data to calculate reaction rate.

Reaction Rate vs. Heat of Reaction

It is important to understand that a reaction with a high Enthalpy (Heat of Reaction) does not automatically equate to a fast rate of reaction.  For example, the oxidation of iron to form rust (Iron Oxide) is thermodynamically-favored but is known to occur slowly.  Heat of reaction can be estimated by taking the difference in total bond energy of products and reactants.  There is no such simple formula for estimating the rate of a chemical reaction.

Reaction Rate Order

The order of a reaction is a measure of the effect of reactant concentration on the rate of a reaction.  The reaction order , which must be determined experimentally, increases with an increasing effect of reactant concentration on rate.  If the rate of a reaction doubles with a doubling of reactant concentration, then we say that the reaction is first-order in that reactant.  If we find that doubling a reactant concentration has increased reaction rate by a factor of four, than we say that the reaction is second-order in that reactant.

Potential Energy Changes

Potential energy is stored within compounds in the form of the chemical bonds holding the atoms together.  The total amount of energy (potential + kinetic) involved in a chemical change will be the same before and after the reaction.  Only the amounts of potential and kinetic energy relative to each other actually changes.  Here is a link to a diagram in a previous post showing the conservation of total energy in a chemical change.  The same diagram also shows the difference between an endothermic and an exothermic reaction.

Equilibrium Theory

We say a reaction is at equilibrium when two conditions are met: 1) The reaction is reversible and 2) The rate of the forward reaction equals the rate of the reverse reaction.  A reaction at equilibrium appears as though nothing is happening, but in reality there is a lot going on so we say "dynamic equilibrium".  A reaction system at equilibrium appears to be in a static state because the reactant and product concentrations are constant.  Changing reactant or product concentrations and changing temperature will result in a shifting of the equilibrium position according to Le Chatelier's Principle.   

That's all for now.  Have a good one!

A Publication of http://www.excellenceinlearning.biz