Chemical Reactions

A chemical reaction is a change in matter that forms new substances. Evidences of chemical reactions include changes in color (e.g., cobalt II nitrate with hydrochloric acid), formation of precipitates (e.g., sodium dichromate with lead II nitrate), evolution of gas (effervescence, e.g., calcium with water), and evolution of heat/light (e.g., methane reacting with oxygen in a Bunsen burner).

Chemical reactions are expressed as chemical equations: reactants yield products. Reactants are substances before the reaction, and products are those formed after. Phases (solid, liquid, gas, aqueous) are indicated by symbols. Chemical equations must obey the law of conservation of mass, meaning they must be balanced by using coefficients. Examples of monoatomic and diatomic elements in equations are provided, along with a detailed example of balancing the combustion of magnesium ribbon.

There are four main types of chemical reactions: combination (synthesis), decomposition, substitution (single replacement/displacement), and double decomposition (double displacement/metathesis). Each type is explained with its general form and an example reaction, such as hydrogen and chlorine forming hydrogen chloride (combination), calcium carbonate decomposing into calcium oxide and carbon dioxide (decomposition), iron reacting with copper II sulfate (substitution), and silver nitrate reacting with potassium chloride (double decomposition).

Igniting a magnesium ribbon causes it to react with oxygen gas, producing a bright white light and forming a white powdery ash, magnesium oxide. This is a combination reaction, and the balanced chemical equation is discussed based on mass conservation.

Heating copper wire in a Bunsen burner causes it to react with oxygen, forming black cupric oxide. The initial reddish-brown copper turns black, demonstrating a chemical change.

Mixing iron filings and sulfur powder initially forms a heterogeneous mixture where iron can be separated by a magnet. Heating this mixture results in a chemical reaction, forming ferrous sulfide, which is a new compound and not attracted to a magnet. This demonstrates the difference between a mixture and a compound formed by a chemical reaction.

Heating orange mercuric oxide causes it to decompose into grayish-black metallic mercury and oxygen gas, which is confirmed by a glowing splinter brightening. Upon cooling, the mercury reacts with oxygen from the air to reform mercuric oxide, indicating a reversible decomposition.

An iron nail placed in a blue copper (II) sulfate solution results in a substitution reaction. Iron, being more active than copper, displaces copper, forming a reddish-brown copper coating on the nail and turning the solution light green due to the formation of iron (II) sulfate.

Mixing mossy zinc with concentrated hydrochloric acid produces vigorous bubbling (hydrogen gas, confirmed by a 'pop' sound with a glowing splinter) and an increase in temperature (exothermic reaction). Zinc replaces hydrogen in hydrochloric acid, forming zinc chloride. This is another example of a substitution reaction.

No visible reaction occurs when copper wire is added to concentrated hydrochloric acid because copper is less active than hydrogen, and therefore cannot displace it from the acid.

Adding sodium bicarbonate to concentrated hydrochloric acid produces fizzing (carbon dioxide gas, which extinguishes a glowing splinter) and a decrease in temperature (endothermic reaction). This double decomposition reaction forms sodium chloride and carbonic acid, which then breaks down into carbon dioxide and water.

Combining clear solutions of calcium chloride and sodium carbonate results in the formation of a white insoluble solid (precipitate), calcium carbonate. This is a double decomposition reaction where ions exchange partners to form new products, one of which is insoluble.