Let us organize the knowledge of chemical changes learned in the eighth grade and deepen our understanding by connecting oxidation, reduction, exothermic reactions, and endothermic reactions with familiar technologies and phenomena.
Basic Knowledge
Let us organize the classifications and relationships of chemical changes.
From the viewpoint of energy
・Exothermic reactions: combustion, oxidation of iron (hand warmers), neutralization, reactions between metals and acids
・Endothermic reactions: photosynthesis, sodium hydrogen carbonate + citric acid, barium hydroxide + ammonium chloride
From the viewpoint of oxidation and reduction
・Oxidation: a substance gains oxygen → combustion, rusting, combination
・Reduction: a substance loses oxygen → smelting, reduction of copper oxide
・Oxidation and reduction always occur simultaneously (an oxidation-reduction reaction)
From the viewpoint of changes in substances
・Combination: two or more substances → one substance (example: Fe + S → FeS)
・Decomposition: one substance → two or more substances (example: 2H2O → 2H2 + O2)
・All of these are chemical changes (the types of substances change)
Chemical change (a change in which the types of substances change. Includes decomposition, combination, oxidation, and reduction)
Law of conservation of mass (the total mass does not change before and after a chemical change)
Law of definite proportions (in the same compound, the mass ratio of the elements is always constant)
Oxidation-reduction reaction (oxidation and reduction occur simultaneously. A cell also involves an oxidation-reduction reaction)
Exothermic and endothermic reactions (a classification of chemical changes based on the direction of heat transfer)
Chemical cell (a device that extracts electrical energy from an oxidation-reduction reaction)
Further Study
Knowledge of chemical changes is directly connected to modern technology.
① Ironmaking: Iron is extracted by reducing iron ore (Fe2O3) with coke. In a blast furnace, CO acts as a reducing agent.
② Renewable energy: Fuel cells (hydrogen is oxidized → electricity), solar cells (light energy → electricity)
③ Food and medicine: Antioxidant foods (prevent oxidation), hydrogen peroxide solution (a disinfectant. O2 is produced by decomposition)
④ Environmental problems: An increase in CO2 (combustion of fossil fuels) → global warming. Chemical changes involving carbon have become a global-scale problem.
When these ideas are integrated, chemical changes can be understood through three main principles: “Atoms are only rearranged and the total amount does not change (conservation of mass); energy always enters or leaves during the change (exothermic or endothermic); and when oxygen is exchanged, oxidation and reduction occur simultaneously.”
- Three main principles of chemical changes: ① conservation of mass ② exothermic/endothermic ③ oxidation-reduction
- Combination (2 substances → 1 substance) and decomposition (1 substance → 2 or more substances) are opposite changes
- Oxidation and reduction always occur simultaneously
- Cell = extracts electricity through an oxidation-reduction reaction
- Ironmaking = reduction of iron ore (reducing agent = CO derived from coke)
- The only emission from a fuel cell is water (low environmental impact)
- Combustion of fossil fuels → increase in CO2 → global warming (chemical changes cause environmental problems)
Points to Note
① Even when energy enters or leaves during a chemical change, the law of conservation of mass always holds. ② Distinguishing physical changes from chemical changes: determine whether the type of substance changes. ③ Oxidation-reduction and exothermic-endothermic are independent concepts, although many oxidation reactions are exothermic (combustion is a clear example). ④ Ironmaking and smelting are carried out through reduction. “Extracting iron by smelting iron oxide” means “reducing iron oxide.”
Practice
- Choose all applicable descriptions from “oxidation,” “reduction,” “exothermic,” and “endothermic” for the following changes: ① combustion of iron ② heating copper oxide with carbon ③ photosynthesis
- What is used as the reducing agent when iron is extracted from iron ore in ironmaking?
- Explain how to distinguish chemical changes from physical changes, and give one example of each.