Chemistry (Scientific Branch) - الكيمياء
Chemistry - الكيمياء - examines atomic structure and periodic trends, chemical bonding, thermochemistry, chemical equilibrium, acids and bases, electrochemistry, and organic functional groups. The paper rewards exact mole arithmetic and correct equation balancing more than description.
Core rules
- n = m/M; for solutions, n = cV with V in litres (dm³).
- Ionic equations must be balanced for atoms and charge; mole ratios come only from a balanced equation.
- pH = −log[H⁺]; at 25 °C, [H⁺][OH⁻] = 10⁻¹⁴, so pH + pOH = 14.
- A strong acid dissociates completely, so [H⁺] follows directly from concentration and basicity; a weak acid needs Ka, and [H⁺] ≠ c.
- Le Chatelier: changes in concentration, pressure, or temperature shift the position of equilibrium; a catalyst changes only the rate.
- Oxidation is loss of electrons and an increase in oxidation number; reduction is the reverse.
Worked example
Find the pH of 0.005 M H₂SO₄, taken as fully dissociated in both stages. Each formula unit releases two H⁺, so [H⁺] = 2 × 0.005 = 0.010 mol dm⁻³. pH = −log(0.010) = 2.00. Note the difference a diprotic acid makes: treating it as monoprotic would give pH 2.30 and lose the mark.
Traps
- Assuming [H⁺] equals the acid concentration for weak acids such as ethanoic acid.
- Ignoring the second proton of H₂SO₄ in stoichiometry questions.
- Using unbalanced equations to derive mole ratios.
- Forgetting that heating an exothermic equilibrium shifts it backwards, not forwards.
- Confusing the anode and cathode in electrolysis versus a galvanic cell.