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]

R1.3Reactivity 1 — What drives chemical reactions?

Energy from fuels — IB Chemistry SL Revision Notes & Topical Questions

20 min read 33 parts Lesson 3 of 8 1 quiz

📚 IB Chemistry SL 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

R1.3 Energy from fuels — IB CHEMISTRY SL notes: Write equations for complete and incomplete combustion, evaluate fossil fuels and biofuels, link CO2 to climate, calculate specific energy and describe fuel cells.Combustion — a rapid, exothermic oxidation of a fuel that needs oxyge…

Non-metals
Organic fuels
Conditions
Hydrocarbons
Carbon monoxide
Carbon content

Part 1 of 9Combustion reactions

Combustion

Combustion a rapid, exothermic oxidation of a fuel that needs oxygen and an ignition source, giving heat and light.

Metals

Metals all metals oxidise, but only reactive or finely divided ones combust, e.g. 2Mg + O₂ → 2MgO;
- copper only forms black CuO on heating.

Non-metals

Non-metals form covalent, usually acidic oxides, e.g. S + O₂ → SO₂ and P₄ + 5O₂ → P₄O₁₀.

Redox

Redox the fuel is oxidised and oxygen is reduced.

Organic fuels

Organic fuels hydrocarbons and alcohols are used because they release much energy,
- and their high activation energy makes them safe to store and transport.

Part 2 of 9Complete combustion

Conditions

Conditions a plentiful supply of oxygen, giving CO₂ and H₂O.

Hydrocarbons

C_xH_y + (x + y/4)O₂ → xCO₂ + (y/2)H₂O, e.g. CH₄ + 2O₂ → CO₂ + 2H₂O.

Alcohols

Alcohols e.g. C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O, where the oxygen in the alcohol must be counted.

Balancing

Balancing balance carbon, then hydrogen, then supply the product oxygen from O₂, using fractions and doubling if needed.

Flame

Flame a Bunsen burner gives a blue, non-luminous flame.

Part 3 of 9Incomplete combustion

Conditions

Conditions limited oxygen, common in car engines and faulty boilers, giving a yellow, luminous flame.

Products

Products water plus carbon monoxide and/or carbon (soot), and it releases less energy per mole than complete combustion.

Carbon monoxide

Carbon monoxide colourless, odourless and toxic because it binds irreversibly to haemoglobin and prevents oxygen transport, e.g. C₃H₈ + 3.5O₂ → 3CO + 4H₂O.

Soot

C₃H₈ + 2O₂ → 3C + 4H₂O, causing respiratory problems and global dimming.

Hydrogen

Hydrogen is never a product because it is preferentially oxidised to water.

Carbon content

Carbon content compounds with a higher percentage of carbon, such as benzene derivatives, burn with sootier flames.

Alcohols

Alcohols burn more cleanly than hydrocarbons because they already contain oxygen.

Part 4 of 9Fossil fuels

Origin
- coal, oil and natural gas formed over millions of years from dead organisms, so they are non-renewable and finite.

Fuel	Advantages	Disadvantages
Coal	Cheap, widely available, long-lasting, safe to store	Most CO₂ per unit energy, SO₂ and particulates, mining damage
Oil	High energy density, easily transported and refined by fractional distillation and cracking	CO₂, SO₂, CO and smog, oil spills, uneven distribution
Natural gas	Cheapest, high specific energy, burns cleanly with least CO₂	Hard and costly to store under pressure, leaks of methane

Chain length
- as hydrocarbon chains lengthen, more CO₂ and less energy per gram are released,
  - and incomplete combustion is more likely; methane releases the most energy per gram.

Part 5 of 9Specific energy and enthalpy of combustion

Heat energy

Heat energy q = mcΔT, with c = 4.18 J g⁻¹ K⁻¹ for water.

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)

Enthalpy of combustion

ΔH_c = −q/n(fuel), in kJ mol⁻¹.

Specific energy

Specific energy the energy released per unit mass of fuel, equal to |ΔH_c| divided by molar mass in kg mol⁻¹ (kJ kg⁻¹).

Energy density

Energy density the energy released per unit volume of fuel (kJ dm⁻³), equal to specific energy × density.

Experimental method

Experimental method burn a fuel in a spirit burner under a copper calorimeter of water and find the fuel mass lost.

Low experimental value

Low experimental value the result is less exothermic than the data-booklet value because of heat loss, incomplete combustion, fuel evaporation and heat absorbed by the calorimeter.

Alcohols

Alcohols release less energy per gram than hydrocarbons because they are partly oxidised, so ethanol has lower specific energy than octane.

Part 6 of 9Carbon dioxide and the greenhouse effect

Carbon dioxide and the greenhouse effect

Sources

Sources burning fossil fuels, deforestation, livestock (methane) and decomposition of carbonates.
- (especially for electricity)

Greenhouse gases

CO₂, CH₄, N₂O/NO_x and water vapour.

Mechanism

Mechanism short-wavelength radiation from the Sun reaches the Earth,
which re-emits longer-wavelength infrared, and greenhouse gases absorb it because their bond vibrations change the dipole moment.

Re-radiation

Re-radiation the gases re-emit the energy in all directions, trapping heat.

Enhanced greenhouse effect

Enhanced greenhouse effect extra warming from human-made increases in greenhouse gas concentrations, causing global warming and climate change.

Oceans

Oceans dissolved CO₂ forms carbonic acid, causing ocean acidification.

Monitoring

Mauna Loa records CO₂ since 1958, with seasonal fluctuations caused by photosynthesis.

Part 7 of 9Biofuels

Renewable resources

Renewable resources replaced as fast as or faster than they are used; non-renewable ones are finite.

Bioethanol

Bioethanol glucose from photosynthesis is fermented anaerobically by yeast, C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.

Biodiesel

Biodiesel triglycerides react with methanol by transesterification, usually with an alkali catalyst, to give methyl esters;
- excess methanol shifts the equilibrium.

Biogas

Biogas anaerobic breakdown of organic waste gives mostly CH₄ and CO₂.

Carbon neutrality

CO₂ absorbed in growth equals that released on burning, but energy used in farming, transport and processing adds CO₂.

Advantages	Disadvantages
Renewable and close to carbon neutral	Land used instead of food crops and deforestation
Can be made from organic waste, reducing landfill	Fertiliser and pesticide use and soil depletion
Create jobs and income	Lower specific energy than fossil fuels and costly engine conversion

Part 8 of 9Fuel cells

Definition

Definition an electrochemical cell that converts the chemical energy of a continuously supplied fuel and oxidant directly into electrical energy.

Operation

Operation the cell runs while fuel and oxygen are supplied, with no recharging, and energy is not stored in the cell.

Efficiency

Efficiency higher than combustion engines because chemical energy becomes electrical energy without passing through heat.

Cell	Anode	Cathode
Hydrogen, acidic (PEM)	H₂ → 2H⁺ + 2e⁻	O₂ + 4H⁺ + 4e⁻ → 2H₂O
Hydrogen, alkaline	H₂ + 2OH⁻ → 2H₂O + 2e⁻	O₂ + 2H₂O + 4e⁻ → 4OH⁻
Methanol (PEM)	CH₃OH + H₂O → CO₂ + 6H⁺ + 6e⁻	O₂ + 4H⁺ + 4e⁻ → 2H₂O

1Overall

Overall hydrogen cell 2H₂ + O₂ → 2H₂O (E = +1.23 V) and methanol cell 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O.

2Hydrogen cell advantages

Hydrogen cell advantages water is the only product, no NO_x, quiet with no moving parts.

3Hydrogen cell disadvantages

Hydrogen cell disadvantages expensive platinum catalyst, flammable and hard-to-store hydrogen with low energy density by volume,
- and mostly made from fossil fuels.

4Methanol cell versus hydrogen

Methanol cell versus hydrogen easier to store, higher energy density by volume and can come from biomass,
- but toxic, flammable, lower power and it produces CO₂.

Part 9 of 9More from the specification

Carbon dioxide

A gas that turns limewater cloudy.

Chemical formula

Symbols and numbers showing the relative number of atoms of each element.

Key terms

Term	Meaning
Complete combustion	Combustion in excess oxygen giving CO₂ and H₂O.
Incomplete combustion	Combustion in limited oxygen giving CO and/or C as well as water.
Specific energy	The energy released per unit mass of fuel.
Energy density	The energy released per unit volume of fuel.
Biofuel	A fuel made from recently living biomass.
Greenhouse gas	A gas that absorbs infrared radiation emitted by the Earth.
Fuel cell	A cell converting the chemical energy of a continuously supplied fuel directly into electrical energy.

Avoid these mistakes

Forgetting the oxygen in an alcohol when balancing — count it on the reactant side.

Stating hydrogen forms in incomplete combustion — hydrogen is oxidised to water first.

Saying biofuels are always carbon neutral — farming and processing add CO₂.

Saying greenhouse gases trap the Sun's rays — they absorb infrared emitted by the Earth.

Last updated: 14 October 2026

Part 1 of 9Combustion reactions

Part 2 of 9Complete combustion

Part 3 of 9Incomplete combustion

Part 4 of 9Fossil fuels

Part 5 of 9Specific energy and enthalpy of combustion

Part 6 of 9Carbon dioxide and the greenhouse effect

Part 7 of 9Biofuels

Part 8 of 9Fuel cells

Part 9 of 9More from the specification

Key terms

Avoid these mistakes

Marked practiceQuizzes for this lesson