Lesson 23 Notes

Chemical Reactions Involving Metals

Definition and examples:

A chemical reaction is a process in which the physical and chemical properties of the original substances change as new substances with different physical and chemical properties are formed. An example: The burning of gasoline, the rusting of iron, and the baking of bread are all chemical reactions.

Characteristics of Chemical Reactions:

A chemical reaction always results in the formation of a new substance. An example: The dark material on a burned match is a new substance. It is not the same substance that was originally on the match.

The substances present before the change and the substance formed by the change are the two kinds of substances involved in a chemical reaction. A substance that enters into a chemical reaction is called a reactant. A substance that is produced by a chemical reaction is called a product. So a general description of a chemical reaction can be stated as reactants changing into products.

A couple of things to remember: 1.) Changes in chemical and physical properties occur with a chemical reaction. 2.) Chemical reactions always involve a change in energy. Energy is either absorbed or released during a chemical reaction. An example: Heat energy is absorbed when sugar changes into caramel or when gasoline burns, heat energy is released.

Capacity to React:

In order for a chemical reaction to occur, the reactants must have the ability to combine with other substances to form products.

Atoms contain electrons, or negatively charged particles. Electrons are located in energy levels surrounding the nucleus. The electrons in the outermost energy level are called valence electrons. It is the valence electrons that are involved in chemical bonding. An atom forms chemical bonds with other atoms in order to complete its outermost energy level. This is the most stable condition for an atom. An atom will try to fill its outermost energy level by gaining or losing electrons, or by sharing electrons. A chemical bond formed by the gain or loss of electrons is an ionic bond. A chemical bond formed by the sharing of electrons is a covalent bond.

During a chemical reaction, atoms can form molecules, molecules can break apart to form atoms, or molecules can react with other molecules. New substances are produced as existing bonds are broken, atoms are rearranged, and new bonds are formed.

Energy of Chemical Reactions definition:

Based on the type of energy change involved, chemical reactions are classified as either exothermic or endothermic reactions.

Exothermic Reactions:

A chemical reaction in which energy is released is an exothermic reaction. A reaction that involves burning, or a combustion reaction, is an example of an exothermic reaction. The combustion of methane gas, which occurs in a gas stove, releases a large amount of heat energy. In an exothermic reaction, the reactants are higher in energy than the products are.

Endothermic Reactions:

A chemical reaction in which energy is absorbed is an endothermic reaction. The energy that is absorbed in an endothermic reaction is now stored in the molecules of the products. The decomposition of sodium chloride, or table salt, is an endothermic reaction. It requires the absorption of electric energy.

Rates of Chemical Reactions:

The burning of a log takes hours. If the log is ground into fine sawdust, the burning can take place at a high speed. If the dust is spread through the air, the burning can produce an explosion. In both of the processes, the same reaction is take place. The various substances in the wood are combining with oxygen. One reaction proceeds at a faster speed than the other one does. The differences in reaction time is a study of kinetics. Kinetics is the study of reaction rates. The rate of a reaction is a measure of how quickly reactants turn into products.

A. Collision theory:

This relates particle collisions to reaction rates. According to the collision theory, the rate of a reaction is affected by four factors: concentration, surface area, temperature, and catalysts.

B. Concentration:

The concentration of a substance is a measure of the amount of that substance in a given unit of volume. A high concentration of reactants means there are a great many particles per unit volume.

C. Surface Area:

When one of the reactants in a chemical reaction is a solid, the rate of reaction can be increased by breaking the solid into smaller pieces. This increases the surface area of the reactant. Surface area refers to how much of a material is exposed.

D. Temperature:

An increase in temperature generally increases the rate of a reaction.

E. Catalysts:

Some chemical reactions take place very slowly. The reactions involved with digesting a cookie are examples. IF these reactions preceeded at their normal rate, it could take weeks to digest one cookie. Certain substances speed up the rate of a chemical reaction. These substances are catalysts. A catalyst is a substance that increases the rate of a reaction but is not itself changed by the reaction.