Understanding Chemical Change
A chemical reaction involves a chemical change in which substances (reactants) transform into new substances (products) with entirely different properties. Unlike physical changes (like ice melting), chemical changes are generally irreversible. Common observations that indicate a chemical reaction include a change in state, change in color, evolution of a gas, or a change in temperature.
Balancing Chemical Equations
According to the Law of Conservation of Mass, matter can neither be created nor destroyed in a chemical reaction. Therefore, the number of atoms of each element must remain the same before and after the reaction. Balancing equations involves adjusting the stoichiometric coefficients (the numbers in front of the molecules) so that the reactants and products have identical numbers of each type of atom. For example, the unbalanced equation for the rusting of iron is Fe + H₂O → Fe₃O₄ + H₂, which balances to 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
Types of Chemical Reactions
- Combination Reactions: Two or more substances combine to form a single product (e.g., C + O₂ → CO₂).
- Decomposition Reactions: A single compound breaks down into two or more simpler substances, often requiring heat, light, or electricity (e.g., 2H₂O → 2H₂ + O₂).
- Displacement Reactions: A more reactive element displaces a less reactive element from its compound (e.g., Zn + CuSO₄ → ZnSO₄ + Cu).
- Double Displacement Reactions: Exchange of ions between two compounds forms new compounds, often resulting in a precipitate (e.g., AgNO₃ + NaCl → AgCl + NaNO₃).
Oxidation and Reduction
Oxidation is the gain of oxygen or the loss of hydrogen (or loss of electrons). Reduction is the loss of oxygen or the gain of hydrogen (or gain of electrons). These reactions always occur together and are called redox reactions. The substance that gets oxidized acts as a reducing agent, and the substance that gets reduced acts as an oxidizing agent. Real-world examples of redox reactions include corrosion of metals and the rancidity of fats in food.