Introductory Concepts, Page 3

Chemical Change:

 A chemical change is a chemical process that produces one or more substances that are different from those that were present before the process occurred. The new substance(s) will be identifiable because of properties that are different from the initial substance(s).

An element or compound has its own unique set of characteristics, such as boiling point, melting point and density. If an element or compound undergoes a chemical change, then one or more new substances are created with their own unique sets of characteristics. For instance, if water is heated it will decompose into hydrogen gas and oxygen gas. The original qualities of the water are gone. Instead, two new substances now exist. Chemical changes are generally more interesting than physical changes and are more widely studied by chemists.

 Chemical Change:

  

Compound:

 A substance that is composed of two or more elements which are chemically bonded. When a compound is formed, the individual characteristics of the elements used to create the compound are destroyed and a new set of characteristics is created. The new set of characteristics will be unique to the new substance.

Compounds are represented by chemical formulas such as H2O, C6H12O6, etc. A compound formula will consist of the symbols of two or more elements with appropriate subscipts. If no subscript appears after an element symbol, then the subscript is assumed to be 1. The subscripts indicate the number of atoms of the different elements that appear in one molecule of the compound. For instance, one molecule of C6H12O6 contains 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen.

The component elements in a compound are attached to each other through electrostatic forces known as chemical bonds.

  • Bonds can either be covalent bonds or ionic bonds.
  • Each type of bond will introduce a specific set of qualities, or characteristics, into the compound.
  • A compound can not easily be separated into the individual elements.
  • The bonds that hold the structure together will resist being broken.
  • Chemical processes are required to break the bonds in a compound. In other words, chemical reactions are needed in order to split a compound into its component parts.

Coulomb's Law:

 This is a mathematical statement that will determine the force of attraction or repulsion between two charged objects. The law indicates that this attractive or repulsive force is a function of three variables:

 
  •  q1, which is the magnitude of charge on object number 1
  • q2, which is the magnitude of charge on object number 2
  • r, which is the distance between the centers of the two charged objects

 The attractive or repulsive force between the objects is referred to as a Coulombic Force or electrostatic force. The equation that is used to calculate the electrostatic force is

 The k is a constant that is needed to complete the expression.

Using the equation of Coulomb's Law, it is possible to calculate the strength of chemical bonds or the attractive forces between any two oppositely charged systems. It can also be used to calculate the attraction between the electrons around the nucleus and the nucleus, or the repulsion between the electrons on an atom or between the protons in the nucleus. Understanding the equation and its many applications is critical to assist in determining the factors that hold chemical systems together.

Sample Coulomb's Law Problems

Element:

 A substance that is composed of a single type of atoms.

  • The definitive list of the elements is the Periodic Chart.
  • The formula of an element is the symbol of the element.
  • Some elements can appear in a variety of solid states. These different solid states are referred to as allotropes.

Allotropes

  •  Allotropes are different crystal structures for a specific type of element.
  • For instance, diamonds and graphite are different crystal structures of carbon, or allotropes of carbon.

The elements can not be subdivided to any substance smaller than themselves using normal chemical procedures. A sample of an element can be reduced down to smaller components, such as protons, electrons, and neutrons, by using the extraordinary processes of nuclear chemistry. This will destroy the element, however.

Most elements are not written with chemical formulas. They are represented with the symbol of the element, alone. For instance, gold is Au, lead is Pb, neon is Ne. A limited number of elements are written with formulas that involve subscripts. Common examples would include H2, P4, and S8. Such formulas are the exception, more than the rule for elements.


Energy

 That quality which, when present in matter, can be used to perform work. There are two type of energy.

  • Kinetic Energy--Is the energy in an object that arises from the object's motion. The quantity of energy is a function of the mass and velocity of the moving object. It can be calculated by using the equation

  •  Potential Energy--Is the energy of an object that arises from its relationship to other objects in its environment. Potential Energy in an object is dependent upon the masses of neighboring objects and the distance by which the object is separated from the other objects.

 Potential Energy is dependent on environmental factors, while Kinetic Energy is independent of its environment. Potential Energy depends upon where an object is. Kinetic Energy dependents upon what an object is doing.

 Potential Energy is often referred to as the "energy of rest". Kinetic energy is often referred to as the "energy of motion". These ideas may be useful to help remember the two terms. But, they can be misleading.

  • Potential Energy is not limited to objects that are standing still.
  • Moving objects will have Potential Energy that is determined by their relationships to other objects.
  • Kinetic Energy does exist for an object at rest, but its value is zero.

 Energy is fundamentally associated with one of the driving forces that impacts on all processes. Chemical systems will always strive to release energy, no matter what form the energy is in. This basic desire is often instrumental in determining if a chemical process will, or will not, take place.

 Questions and comments should be sent to :
   kdrews@bcpl.net  

Updated September 1, 2000