Summary of "التفاعلات والمعادلات كيمياء صف عاشر متقدم الفصل الثاني 2026"

Overview of the unit

The unit contains three lessons:
1. What a chemical reaction (chemical change) is; how to write equations (word and symbolic); how to balance them.
2. Classification / types of chemical reactions.
3. Reactions in aqueous (water) solutions: precipitation, reactions of solutions, and activation in water.
Emphasis: mastery of Chapter 1 material (element symbols, valencies, common polyatomic ions, formula writing) is essential to succeed in this unit.

What is a chemical reaction (chemical change)

A chemical reaction is a process in which atoms in one or more substances are rearranged to form new substances. Bonds in the reactants are broken and new bonds are formed in the products. The bonds involved may be ionic, covalent, or metallic depending on the elements.

Example:

Metallic sodium + chlorine gas → ionic sodium chloride Na (metallic bonding) and Cl2 (covalent bond) are broken; a new ionic bond forms in NaCl.
Reactions can convert toxic elements into harmless compounds (e.g., chlorine gas → NaCl).

Observable evidence that a chemical reaction has occurred

Common signs that a chemical reaction has taken place:

Change in color (e.g., Mg ribbon: silver → MgO white powder when burned)
Change in temperature (exothermic or endothermic)
Smell or gas evolution (new odor or gas produced)
Other signs: precipitate formation, bubbles, light emission, etc.

Representation of reactions and common symbols

Reactants are written on the left of the arrow; products on the right. The arrow means “react to produce.”
Use plus signs (+) to separate multiple reactants or products.
State symbols:
- (s) solid
- (l) liquid
- (g) gas
- (aq) aqueous (solution)
Diatomic elements (when elemental, gaseous): H2, O2, N2, F2, Cl2, Br2, I2.
Reversible reactions: ⇌ indicates the reaction can proceed both directions. A single arrow (→) indicates it proceeds effectively one way under the given conditions.
Reaction conditions (heat, catalyst, etc.) are often written above or below the arrow (e.g., Δ for heat).

Types of equation notation

Word (verbal) equations: substances named in words (useful for initial description).
Symbolic (formula) equations: chemical formulas plus state symbols.
Balanced chemical equations: symbolic equations with coefficients that satisfy the law of conservation of mass (equal numbers of each type of atom on both sides).

Law of conservation of mass and balancing equations

Law: mass (and the number of atoms of each element) is conserved in a chemical reaction — atoms are neither created nor destroyed.
A balanced chemical equation shows the smallest whole-number ratio of amounts of reactants and products that satisfies conservation.
Coefficients (integers placed before formulas) are used to balance an equation. Do NOT change subscripts (changing subscripts changes the identity of the substance).

Note: Always use coefficients to balance; never change subscripts.

Step-by-step methodology for writing and balancing chemical equations

Read the description and write the reaction in words first (if given verbally).
Convert the word equation into a symbolic equation:
- Use correct element symbols.
- Write elemental diatomic gases as X2 where appropriate (H2, O2, Cl2, etc.).
- Use correct formulas for polyatomic ions (e.g., OH-, SO4^2-, NO3-, PO4^3-) and combine using valencies to make neutral compounds.
- Add state symbols (s, l, g, aq) as specified.
Count atoms of every element on both sides of the unbalanced equation.
Use coefficients to balance each element:
- Start by balancing elements that appear in only one reactant and one product.
- If a polyatomic ion remains unchanged on both sides, treat it as a single unit.
- Use the smallest whole-number coefficients; if necessary, use common multiples to reconcile differing counts.
Recount all atoms to verify both sides match for every element.
Reduce coefficients to the simplest whole-number ratio (divide by a common factor if possible).
Do NOT change subscripts to balance.
Include reaction conditions above/below the arrow if required (e.g., Δ for heating).

Worked examples (symbolic and balanced forms)

Sodium + chlorine:
- Unbalanced: Na (s) + Cl2 (g) → NaCl (s)
- Balanced: 2 Na + Cl2 → 2 NaCl
Magnesium combustion:
- Unbalanced: Mg (s) + O2 (g) → MgO (s)
- Balanced: 2 Mg + O2 → 2 MgO
Hydrogen gas + bromine gas → hydrogen bromide:
- Unbalanced: H2 (g) + Br2 (g) → HBr (g)
- Balanced: H2 + Br2 → 2 HBr
Carbon monoxide + oxygen → carbon dioxide:
- Unbalanced: CO + O2 → CO2
- Balanced: 2 CO + O2 → 2 CO2
Potassium chlorate decomposition (on heating):
- Unbalanced: KClO3 (s) → KCl (s) + O2 (g)
- Balanced: 2 KClO3 → 2 KCl + 3 O2 (Δ above arrow)
Aluminum + bromine → aluminum bromide:
- Unbalanced: Al (s) + Br2 (l) → AlBr3 (s)
- Balanced: 2 Al + 3 Br2 → 2 AlBr3
Sodium hydroxide (aq) + calcium bromide (aq) → calcium hydroxide (s) + sodium bromide (aq):
- Symbolic: NaOH (aq) + CaBr2 (aq) → Ca(OH)2 (s) + NaBr (aq)
- Balanced: 2 NaOH + CaBr2 → Ca(OH)2 + 2 NaBr
Combustion/oxidation of carbon disulfide:
- Balanced: CS2 + 3 O2 → CO2 + 2 SO2
Ferric chloride (aq) + sodium hydroxide (aq) → ferric hydroxide (s) + sodium chloride (aq):
- Balanced: FeCl3 + 3 NaOH → Fe(OH)3 + 3 NaCl
Zinc metal + sulfuric acid (aq) → zinc sulfate (aq) + hydrogen gas (g):
- Balanced: Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)
Phosphoric acid + calcium hydroxide → calcium phosphate + water:
- Balanced: 2 H3PO4 + 3 Ca(OH)2 → Ca3(PO4)2 + 6 H2O

Practical notes and exam guidance

Always start by writing the word equation, then convert to symbolic form, then balance.
Memorize common element symbols, valencies, diatomic gases, and common polyatomic ions from Chapter 1.
Include state symbols when asked and place reaction conditions (e.g., Δ) above the arrow when specified.
Use coefficients only when balancing; do not alter subscripts.
For exam problems: carefully count atoms, adjust coefficients using least-common-multiple strategies when needed, and verify final counts.
Repetition and steady practice are emphasized to internalize formula-writing and balancing skills.

Speakers / sources

Primary speaker: the chemistry lecturer delivering the lesson.
Secondary references mentioned:
- The course textbook (“the book” / “the textbook”).
- Chapter 1 material (element symbols, valencies, polyatomic ions) as prerequisite material.
- Contextual references to the Egyptian curriculum and local naming conventions.