كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

3aPhysical Chemistry

Energetics — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

15 min read 17 parts Lesson 1 of 9 1 quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision

Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution

pH = -log[H⁺]  |  pOH = -log[OH⁻]  |  pH + pOH = 14  |  [H⁺][OH⁻] = 10⁻¹⁴

Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14

Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.

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Lesson summary

Energetics Edexcel O Level Linear Chemistry notes: know that chemical reactions in which heat energy is given out are described as exothermic, and those in which heat energy is taken in are described as endothermicExperimental values are less exothermic due to heat loss and incom…

Energetics Edexcel O Level Linear Chemistry: what to do next
Enthalpy change (ΔH)
Standard conditions
Hess’s law
Bond energy
Activation energy

Part 1 of 6Key ideas

Key ideas

Experimental values are less exothermic due to heat loss and incomplete combustion.

Mean bond energies give approximate values.

Energy profile diagrams show ΔH and Ea.

Part 2 of 6Syllabus points

know that chemical reactions in which heat energy is given out are described as exothermic, and those in which heat energy is taken in are described as endothermic

describe simple calorimetry experiments for reactions such as combustion, displacement, dissolving and neutralisation

calculate the heat energy change from a measured temperature change using the expression Q = mcT

calculate the molar enthalpy change (H) from the heat energy change, Q 3.5C draw and explain energy level diagrams to represent exothermic and endothermic reactions 3.6C know that bond-breaking is an endothermic process and that bond-making is an exothermic process 3.7C use bond energies to calculate the enthalpy change during a chemical reaction

practical: investigate temperature changes accompanying some of the following types of change: - salts dissolving in water - neutralisation reactions - displacement reactions - combustion reactions.

Part 3 of 6Process & sequence

Measure mass of water and temperature rise.

2What the symbols mean:

q = m c ΔT (c = 4.18 J g⁻¹ K⁻¹).

What the symbols mean: E, Q or q = energy transferred (J) · m = mass (kg; g in chemistry calorimetry) · c = specific heat capacity (J kg⁻¹ °C⁻¹; water 4.18 J g⁻¹ °C⁻¹) · ΔT or Δθ = temperature change (°C or K)

Find moles of limiting reactant.

4What the symbols mean:

ΔH = −q ÷ n (kJ mol⁻¹).

What the symbols mean: ΔH = enthalpy change (kJ mol⁻¹) · q = heat energy measured (J, divide by 1000 for kJ) · n = amount of the limiting reactant (mol); negative sign = exothermic

Reactants
Break bonds (+)
Atoms
Make bonds (−)
Products

Hess cycles

ΔHr = ΣΔHf(products) − ΣΔHf(reactants)

ΔHr = ΣΔHc(reactants) − ΣΔHc(products)

ΔHr = Σ bonds broken − Σ bonds made

Part 4 of 6Summary

Summary

Exothermic reactions have negative ΔH.

Hess’s law: ΔH is independent of route.

Part 5 of 6Keep going

Energetics Edexcel O Level Linear Chemistry: what to do next

These Energetics Edexcel O Level Linear Chemistry notes covered the key ideas, worked examples and exam technique for this topic. Check your Energetics Edexcel O Level Linear Chemistry understanding with the practice questions below, then reread any Energetics Edexcel O Level Linear Chemistry point you were unsure about before moving on.

Practice: Energetics Edexcel O Level Linear Chemistry practice questions

Previous lesson: 2h Chemical tests — Notes

Next lesson: 3b Rates of reaction — Notes

Course: Edexcel Linear – OL (chemistry) — all units

Official syllabus: Pearson Edexcel International GCSEs

Part 6 of 6Compare: Compare

Standard enthalpy changes
Enthalpy change	Symbol	Definition	Sign	Example	Note
Reaction	ΔH⦵r	Molar quantities in equation react	±	—	Depends on equation
Formation	ΔH⦵f	1 mol compound from elements in standard states	Usually −	C + O₂ → CO₂	Elements = 0
Combustion	ΔH⦵c	1 mol burns completely in O₂	Always −	CH₄ + 2O₂ → CO₂ + 2H₂O	—
Neutralisation	ΔH⦵neut	1 mol water formed from H⁺ + OH⁻	−	≈ −57 kJ mol⁻¹	Strong acid/alkali
Atomisation	ΔH⦵at	1 mol gaseous atoms from element	+	½Cl₂ → Cl	—
Hydration / solution	ΔH⦵hyd / sol	Ions into water / solid dissolves	− / ±	—	A2 extension

Key terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Enthalpy change (ΔH) · Exothermic · Endothermic · Standard conditions · Hess’s law · Bond energy · q = mcΔT · Activation energy

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).

Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

Avoid vague words

“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.

Do not extrapolate a graph line beyond the data or to zero unless asked.

Six-mark extended responses are marked on content AND a logical, linked line of reasoning.

Last updated: 18 October 2026