Romania Clasa a IX-a Chemistry (natural sciences)
Chapters: 9
1. Classifying reactions in inorganic chemistry
Types of chemical reactions
- Oxidation Number: Rules, Calculation and Types of Redox – Oxidation number is a pretend charge. We imagine that shared electrons go fully to the atom that pulls harder. A few simple rules give the number for any atom, and all numbers in a species add up to its charge. If an atom’s oxidation number goes up, it is oxidised; if it goes down, it is reduced. With this we can sort redox reactions into combination, decomposition, displacement and disproportionation.
2. Reactions with change of oxidation number
Redox reactions · Electrochemical applications
- Redox Reactions: From Oxygen to Electron Transfer – Oxidation first meant adding oxygen or removing hydrogen. Reduction meant the opposite. Today we use a bigger idea: oxidation is losing electrons and reduction is gaining electrons. Both always happen together, so we call them redox reactions. A more active metal gives electrons to the ion of a less active metal.
- Redox Reactions and Electrode Processes – If we keep the oxidation half and the reduction half in separate beakers and join them with a wire and a salt bridge, the electrons travel through the wire. This makes a cell. Each metal in its ion solution is an electrode with its own potential. The hydrogen electrode is set at 0.00 V. Listing electrodes by standard potential gives the electrochemical series, and E°cell = E°(cathode) − E°(anode).
3. The electron shell of the atom
Electron configuration
- Electronic Configuration and the s, p, d, f Blocks – The periodic table is really a map of electron filling. The period number is the highest shell (n) being used. The block is the subshell that receives the last electron: s-block (groups 1–2, ns¹⁻²), p-block (groups 13–18, ns² np¹⁻⁶), d-block (groups 3–12, (n−1)d¹⁻¹⁰ ns⁰⁻²) and f-block (lanthanoids and actinoids, (n−2)f¹⁻¹⁴ (n−1)d⁰⁻¹ ns²). Group: s-block = number of ns electrons; p-block = 10 + ns + np electrons; d-block = ns + (n−1)d electrons.
4. Metallic and non-metallic character
Periodic trends
- Periodic Trends in Properties – Two forces decide almost every trend: the pull of the nucleus (effective nuclear charge) and the distance of the outer shell. Across a period the nuclear pull grows while the shell stays the same, so atoms shrink, ionisation enthalpy rises, electron gain enthalpy becomes more negative and electronegativity rises. Down a group a new shell is added, so atoms grow and these values fall. Cations are smaller and anions bigger than their atoms. Valence follows the outer electrons; metallic reactivity is highest at the bottom left and non-metallic reactivity at the top right.
5. Chemical bonds
Ionic bond · Covalent bond · Complex compounds and metallic bond
- Kossel-Lewis Approach and the Ionic Bond – Atoms join so that each gets a stable outer shell of 8 electrons (an octet), like a noble gas. Kossel said atoms can give or take electrons to make ions (ionic bond). Lewis said atoms can also share pairs of electrons (covalent bond). Lewis structures show these electrons as dots and lines. Formal charge (V − L − B/2) helps pick the best Lewis structure. Ions pack into a crystal, and the energy released is linked to the lattice enthalpy.
- Bond Parameters, Resonance and Polarity – A covalent bond is described by four numbers: bond length (distance between nuclei), bond angle (angle between bonds at an atom), bond enthalpy (energy to break 1 mol of bonds) and bond order (number of shared pairs). Higher bond order means a shorter, stronger bond. When one Lewis structure cannot describe a molecule, the real molecule is a resonance hybrid of several structures. Unequal sharing makes a bond polar; the dipole moment μ = q × d measures it, and the shape decides whether bond dipoles cancel.
- Chemical Bonding: Ionic, Covalent and Metallic Bonds – Atoms join together (bond) to become more stable. Only their outer electrons take part. Most atoms are most stable with 8 outer electrons: the octet rule. There are three main ways to reach it. In an ionic bond, a metal gives electrons to a non-metal, making oppositely charged ions that attract. In a covalent bond, two non-metals share pairs of electrons to make molecules. In a metallic bond, metal atoms release outer electrons into a shared 'sea' that holds positive ions together. The difference in electronegativity (how strongly an atom pulls shared electrons) tells us which kind of bond forms. The type of bond explains melting points, whether a substance conducts electricity, and whether it dissolves in water.
6. Intermolecular interactions
Intermolecular forces
- Intermolecular Forces – Intermolecular forces are weak attractions BETWEEN molecules. From weakest to strongest (for similar-size molecules): London dispersion < dipole–dipole < hydrogen bond. Stronger forces mean higher melting and boiling points, higher viscosity and surface tension, and they decide what dissolves in what.
7. Solutions
Aqueous solutions · Strong acids and bases; pH
- Solutions: How Things Dissolve and How Much Can Dissolve – A solution is a uniform mixture of a solute dissolved in a solvent. In water (an aqueous solution) the solute breaks into particles too small to see, so it never settles and passes through filter paper. Concentration tells how much solute is present (mass % = solute ÷ solution × 100). Solubility is the most that can dissolve in 100 g of solvent at a given temperature; beyond it the solution is saturated. Evaporation, crystallisation and distillation separate solutions.
- The pH Scale: How Strong Is an Acid or a Base? – pH is a number from 0 to 14 that tells how acidic or basic a solution is. 7 is neutral, below 7 is acidic and above 7 is basic. Each step down means ten times more H⁺ ions.
8. The gaseous state
Gases
- Perfect Gas Equation PV = nRT and Work in Compressing a Gas – A gas is made of countless tiny molecules flying about. Their hits on the walls make pressure. For a low-density gas, three simple laws hold: at fixed temperature, P × V stays constant (Boyle); at fixed pressure, V grows in step with kelvin temperature (Charles); at the same P and T, equal volumes hold equal numbers of molecules (Avogadro). Put together they give the perfect gas equation PV = nRT = N k T. One mole holds Avogadro's number, 6.022 × 10²³, of particles. Pushing a piston in does work on the gas; the work equals the area under the P–V graph, and at fixed temperature W = nRT ln(V₁/V₂).
9. Stoichiometric calculations
Stoichiometry
- Stoichiometry, Limiting Reagent and Concentration Terms – Stoichiometry means measuring the amounts of substances in a reaction. A balanced equation acts like a recipe: its numbers give the mole ratio of reactants and products. With it you can change any mass into moles, use the ratio, and change back to the mass of any other substance. The reactant that runs out first is the limiting reagent; it decides how much product forms. For solutions, concentration is given as mass percent, mole fraction, molarity or molality.