Chemical Change

A chemical change is a change which produces a new substance. The new substance will have properties that are different from the original substances. Burning is an example of a chemical change. The gases and ashes have different properties from the original substance before burning. The original substance cannot be restored from the gases and ashes.

Chemical reactions are classified by the energy changes which accompany them:

A. exothermic reaction--any chemical change which releases energy (heat), or gives off more energy than it absorbs.

B. endothermic reaction--any chemical change which absorbs energy


The unit for measuring heat is called the joule. One thousand joules is a kilojoule (kj).

Chemical reactions involve the forming or breaking of chemical bonds. The two primary types of chemical bonds are:

A. ionic bonds--the attraction of two oppositely charged ions.

B. covalent bonds--bonds which form when two or more atoms share electrons.

Bond dissociation energy is the measure of how strong chemical bonds are. It is the amount of energy required to break a chemical bond. Bonds with low bond dissociation energy are easier to break than bonds with high dissociation energy.

The original substances in a chemical reaction are called reactants. The new substance is called the product. If in a chemical reaction one or more of the reactants are used up, the reaction is called a complete reaction. When some amount of each of the reactants remains, the reaction is described as incomplete.

When a solid is formed in a chemical reaction between two liquids, the solid is called a precipitate.

Another way to classify chemical reactions is as: synthesis, decomposition, single replacement, or double replacement.

A. synthesis reaction--any chemical reaction in which a single compound is formed from a combination of two or more elements or compounds.

B. decomposition reaction--any reaction in which a compound is broken down into two or more elements or compounds. Decomposition reactions usually require the input of energy. Therefore, they are usually endothermic.

C. single replacement reaction--a reaction in which an element in a compound is replaced by another element.

D. double replacement reaction--a reaction is which an element in one compound is exchanged with an element in another compound.

Chemical equations summarize chemical reactions. The total mass of the reactants consumed in a chemical reaction will equal the total mass of the products generated. This is the law of conservation of mass. It states that mass is never lost in a chemical reaction. The mass of the product is always equal to the mass of the reactants. This law causes chemical equations to be balanced.

To measure the amount of reactants and product, a unit called the mole is used. The mole is the number of atoms in exactly 12 grams of the carbon-12 isotope. It works out that a mole of any element is equal to the number of atoms needed to make a mass in grams equal to the atomic mass of that element. For example; one mole of oxygen is the number of oxygen atoms it would take to equal 15.999 grams of oxygen. One mole of gold is equal to the number of gold atoms it would take to produce 196.966 grams of gold.

To calculate the number of atoms in a mole, Avogadro's number is used. Avogadro's number is 6.02 X 1023.

Chemical reactions occur at different rates or different speeds. The reaction speed, or reaction rate, is the ratio of the measure of how far the reaction goes over the time required for the reaction. A match burning is a very fast reaction. Iron rusting is a very slow chemical reaction.

Factors which influence the rate of chemical reactions:

A. the chemical properties of the reactants

B. the amount of exposed surface area of solids

C. temperature--increasing the temperature increases the rate of a chemical reaction. Decreasing the temperature decreases the rate of a chemical reaction. Higher temperature causes the electrons of atoms to move faster. This creates more energy. Kinesiology is the study of movement. Movement energy is called kinetic energy. Fast moving electrons collide with each other, increasing the chance of a chemical reaction. Generally, an increase of 10 degrees Celsius produces a doubling of speed of reaction.

D. concentration of reactants--the speed of a chemical reaction can be controlled by changing the concentration of the reactants. This is the equal of adding fuel to a fire. The more fuel added, the greater the speed of the reaction. Increasing the concentration of reactants increases the rate of reaction. Decreasing the concentration of reactants decreased the rate of reaction. Concentration is defined as the ratio of the amount of a solute to the amount of solvent it is in.

The amount of energy needed to start a chemical reaction is known as activation energy.

Catalyst--a catalyst speeds up a chemical reaction by lowering activation energy. However, the catalyst is itself is not changed by the reaction. A catalytic converter in an automobile uses the catalyst--platinum to cause unburned gases to react with oxygen to make combustion more complete. This results in less gas being released as exhaust pollution.

Most chemical reactions which take place in living things require catalysts called enzymes. Many vitamins are enzymes or co-


Inhibitors--block or prevent the chemical reaction. Many drugs work by blocking a chemical reaction in the body. For example, antihistamines block the reaction of chemicals called histamines.


1. ore--a natural source of metal

2. oxide--compounds composed of metal and oxygen

3. refining--the process of removing a metal from its ore

Chemical Change Study Sheet