Chemical Reactions

Chemical Reactions

Observations are an important factor in the determination of whether or not a chemical reaction has occurred. Some types of evidence that may indicate a chemical reaction include the following observations:

Formation of a gas: This may be indicated by fizzing or bubbling.

The reaction of carbonates or bicarbonates with acids to produce carbon dioxide gas is an example of this type of evidence.

Formation of a solid:

The formation of a solid (precipitate) is evidenced by the reaction of silver ions with chloride ions to form the insoluble salt, silver chloride.

Disappearance of a solid:

When "acid rain" destroys marble statues or limestone structures a chemical reaction is occurring. The carbonates that form marble or limestone are dissolved away by the acids resulting from environmental pollutants.

Change in color:

When copper carbonate (a bluish-green compound) is heated, carbon dioxide is given off and copper oxide (a black compound) is formed.

Heat is given off or absorbed:

When heat is given off by a chemical reaction, it is called an exothermic reaction. When heat is absorbed during the reaction, the reaction is called an endothermic reaction. Instant ice packs or instant hot packs are everyday example of endothermic and exothermic reactions.

Would you classify the following as exothermic or endothermic?

Since they give off heat, the correct classification is exothermic.

All chemical reactions can be classified as to type. There are many classification schemes possible and a chemical reaction may be placed in several categories. One classification scheme includes the following reaction types:

Combination reactions: In this reaction type two or more reactants combine to form a single product.

Decomposition reactions: In a decomposition reaction a single reactant forms two or more products.

Single replacement reactions: In a single replacement reaction, one element displaces another element from a compound and takes its place. Single replacement reactions are also known as substitution reactions.

Double replacement reactions: Double replacement reactions occur when the reacting substances "swap partners". They are also called metathesis reactions.

Combustion reactions: Combustion occurs when a substance combines with oxygen.

	Combination reactions: In this reaction type two or more reactants combine to form a single product.
	Decomposition reactions: In a decomposition reaction a single reactant forms two or more products.
	Single replacement reactions: In a single replacement reaction, one element displaces another element from a compound and takes its place. Single replacement reactions are also known as substitution reactions.
	Double replacement reactions: Double replacement reactions occur when the reacting substances "swap partners". They are also called metathesis reactions.
	Combustion reactions: Combustion occurs when a substance combines with oxygen.