Chemistry: Particles, Ions, Reactions and Equations
Chemistry questions on the Tottori paper ask you to explain a change at the particle level and to express it as a balanced equation. The strands are 物質の成り立ち, 化学変化, 原子・分子, イオン, 酸・アルカリ and 化学反応式.
Atoms, molecules and ions
An atom has protons and neutrons in the nucleus and electrons outside. Atomic number equals protons; mass number equals protons plus neutrons. Molecules such as H₂, O₂ and CO₂ are covalently bonded. Ionic compounds contain cations and anions: metals form positive ions (Na⁺, Mg²⁺), nonmetals form negative ions (Cl⁻, O²⁻). Solid ionic compounds do not conduct; melted or dissolved they do because ions become mobile.
Equations and reaction types
Balance equations by adjusting coefficients only; subscripts are fixed. Diatomic elements are written H₂, O₂, N₂, Cl₂. Recognize combustion, thermal decomposition, displacement, precipitation and neutralization. In neutralization H⁺ + OH⁻ → H₂O; the salt is named from the acid and base.
Acids and bases
Acids produce H⁺ in water, bases produce OH⁻. pH below 7 is acidic, 7 neutral, above 7 basic. Neutralization is complete when moles of H⁺ equal moles of OH⁻. A precipitate forms when ions combine into an insoluble compound, for example Ag⁺ + Cl⁻ → AgCl.
Worked example
2.4 g of magnesium burns completely. What mass of magnesium oxide forms? (Mg = 24, O = 16) Equation: 2Mg + O₂ → 2MgO. n(Mg) = 2.4/24 = 0.10 mol. The ratio Mg : MgO is 2 : 2, so 0.10 mol MgO forms. M(MgO) = 24 + 16 = 40, so mass = 0.10 × 40 = 4.0 g.
Traps
- Changing a subscript to balance an equation.
- Forgetting that oxygen is O₂, not O.
- Assuming every reaction goes to completion with no limiting reactant.
- Mixing up cations and anions in ionic formulas.
- Forgetting that dissolved ions, not solid ions, carry current.