Chapter 7 · Section 4Textbook pages 135–137Fig 7.2

Hydrogen as a Fuel and Calorific Value ഹൈഡ്രജൻ ഒരു ഇന്ധനം

Which gives more heat: a gram of LPG, a gram of petrol or a gram of hydrogen? Chemists compare fuels with one number, the calorific value. On this page you read the textbook's fuel graph (Fig 7.2) bar by bar, discover why hydrogen is called the fuel of the future, and also why your kitchen still does not run on it.

📖 What you need (page 3 of this guide)

Hydrogen burns in oxygen to give water and heat: 2H₂ + O₂ → 2H₂O + heat; it burns with a "pop" (page 3).
Hydrogen is less dense than air.
Heat energy is measured in joules (J); 1 kilojoule (kJ) = 1000 J. A fuel ഇന്ധനം is a substance burnt to get heat.

1. Calorific value കലോറിക മൂല്യം

⭐ Calorific value (page 135)

ഒരു ഇന്ധനത്തിന്റെ യൂണിറ്റ് മാസ് പൂർണമായി ജ്വലിക്കുമ്പോൾ സ്വതന്ത്രമാകുന്ന താപോർജമാണ് ആ ഇന്ധനത്തിന്റെ കലോറിക മൂല്യം (Calorific value).

The heat energy released when unit mass of a fuel burns completely is the calorific value കലോറിക മൂല്യം of that fuel. It is usually given in kilojoules per gram (kJ/g).

🔑 Reading the unit

"Hydrogen: 143 kJ/g" means 1 g of hydrogen, burning completely, gives out 143 kJ of heat. 2 g give 2 × 143 = 286 kJ, and so on: heat = mass × calorific value. The word "completely" matters – a fuel that burns in too little air gives less heat (and makes smoke or carbon monoxide).

🔥 Heat from burning – try a fuel and a mass

Fuel Mass burnt

2. The fuel graph (Fig 7.2)

ചില ഇന്ധനങ്ങളും അവയുടെ കലോറിക മൂല്യവും ഉപയോഗിച്ചുള്ള ഗ്രാഫ് നൽകിയിരിക്കുന്നു. The graph shows some fuels and their calorific values. Tap a bar (or a button) to read it.

📊 Fig 7.2 – calorific values of some fuels (kJ/g)

Order Show

Values exactly as in the textbook. Petrol is mostly octane, and coal is mostly carbon, so the book gives their values in brackets.

Questions on Fig 7.2 (page 136).

1. കലോറിക മൂല്യം ഏറ്റവും കുറഞ്ഞ ഇന്ധനം ഏത്? Which fuel has the lowest calorific value?

2. കലോറിക മൂല്യം ഏറ്റവും കൂടിയ ഇന്ധനം ഏത്? Which fuel has the highest calorific value?

3. ഹൈഡ്രജൻ വായുവിൽ ജ്വലിക്കുമ്പോൾ ഉണ്ടാകുന്ന ഉൽപന്നം ഏത്? What is the product formed when hydrogen burns in air?

1. Carbon monoxide (10 kJ/g). 2. Hydrogen (143 kJ/g). 3. Water (H₂O): 2H₂ + O₂ → 2H₂O + heat.

ഗ്രാഫ് വിശകലനത്തിൽ നിന്ന് മറ്റ് ഇന്ധനങ്ങളെ അപേക്ഷിച്ച് ഹൈഡ്രജന് കലോറിക മൂല്യം കൂടുതലാണെന്ന് ബോധ്യപ്പെട്ടുവല്ലോ. – From the graph we are convinced that hydrogen has a higher calorific value than the other fuels.

The same heat from different fuels

Turn the question round: how many grams of each fuel must burn to give the same amount of heat? The lower the calorific value, the more fuel you need.

⚖️ How much fuel for the same heat?

Heat needed

mass needed = heat needed ÷ calorific value. 1430 kJ is roughly the heat needed to bring about 4½ litres of water from room temperature to the boil (if no heat were wasted).

3. Hydrogen as a fuel: merits and limitations

ഹൈഡ്രജൻ ഒരു ഇന്ധനമായി ഉപയോഗിക്കുന്നതുകൊണ്ടുള്ള മേന്മകൾ എന്തെല്ലാം? What are the merits of using hydrogen as a fuel?

✅ Merits മേന്മകൾ

ഹൈഡ്രജൻ വായുവിൽ ജ്വലിക്കുന്നതിന്റെ ഫലമായി ജലം മാത്രം ഉണ്ടാകുന്നതിനാൽ മലിനീകരണ സാധ്യത വളരെ കുറവാണ്.When hydrogen burns in air only water is formed, so the chance of pollution is very low – no smoke, no soot, no carbon dioxide.
ഉയർന്ന കലോറിക മൂല്യംHigh calorific value – 143 kJ/g, the highest in Fig 7.2.

⚠️ Limitations പരിമിതികൾ

ഹൈഡ്രജൻ സ്ഫോടനത്തോടെ കത്തുന്ന വാതകമാണ്.Hydrogen is a gas that burns explosively.
സംഭരിച്ചു വയ്ക്കുന്നതിനും വിതരണം ചെയ്യുന്നതിനും പ്രയാസമാണ്.It is difficult to store and to transport (distribute).

ഈ മേന്മകൾ ഉണ്ടായിട്ടും ഹൈഡ്രജൻ ഗാർഹിക ഇന്ധനമായി ഉപയോഗിക്കുന്നില്ല. ചില പരിമിതികളാണ് ഇതിന് കാരണം. പരിമിതികൾ പട്ടികപ്പെടുത്തുക. – In spite of these merits, hydrogen is not used as a household fuel. Some limitations are the reason. List the limitations – then sort all the statements below.

⚠️ A mixture of hydrogen and air explodes when lit. Never fill balloons or bottles with hydrogen at home, and keep flames well away from any hydrogen source. This is exactly why it is not a kitchen fuel.

🗂️ Merit or limitation? Sort the statements

The first four are from the textbook; the last two are extra facts that fit the same columns.

ഈ പരിമിതികൾ മറികടന്നാൽ ഹൈഡ്രജൻ സാർവത്രിക ഇന്ധനമായി മാറും. ഫോസിൽ ഇന്ധനങ്ങളുടെ ലഭ്യതക്കുറവ്, പരിസര മലിനീകരണം തുടങ്ങിയ പ്രശ്നങ്ങൾ പരിഹരിക്കാൻ ഇതിലൂടെ കഴിയും. – If these limitations are overcome, hydrogen will become a universal fuel. Problems such as the shortage of fossil fuels and environmental pollution can then be solved.

🔭 Go further – hydrogen at work today

Rockets: the cryogenic engines of ISRO's rockets burn liquid hydrogen with liquid oxygen. These engines are developed at the Liquid Propulsion Systems Centre, Valiamala, Thiruvananthapuram.
Fuel cells: a fuel cell combines hydrogen with oxygen from the air to make electricity directly, and its only waste is water. Hydrogen fuel-cell buses and cars are already being tried in several countries, including India.
Storing it safely is the big challenge: hydrogen is kept as a gas squeezed to very high pressure in strong tanks, or as a liquid at about −253 °C.

4. In-text questions (page 137)

? 1. നിങ്ങൾക്കറിയാവുന്ന ഹൈഡ്രജൻ സംയുക്തങ്ങൾ ഏതെല്ലാം? Which hydrogen compounds do you know? Tap every compound that contains hydrogen.

Hydrogen compounds: water (H₂O), ammonia (NH₃), methane (CH₄), hydrogen chloride / hydrochloric acid (HCl), sulphuric acid (H₂SO₄), sodium hydroxide (NaOH), methanol (CH₃OH), glucose (C₆H₁₂O₆), LPG (butane, C₄H₁₀), ethanol (C₂H₅OH) … Any formula containing H is a hydrogen compound. CO₂, NaCl, CaCO₃ and MgO contain no hydrogen.

? 2. താഴെ പറയുന്ന സന്ദർഭങ്ങളിൽ ഹൈഡ്രജന്റെ ഏത് സവിശേഷതയാണ് പ്രയോജനപ്പെടുത്തിയിരിക്കുന്നത്. Which property of hydrogen is made use of in each situation?

a. ഹൈഡ്രജൻ നിറച്ച ബലൂണുകൾ വായുവിൽ ഉയർന്നു പൊങ്ങുന്നു. Balloons filled with hydrogen rise up in air.

b. ഇന്ധനമായി ഉപയോഗിക്കുന്നു. It is used as a fuel.

a. Hydrogen is less dense than air (the lightest gas). b. Hydrogen burns and has a high calorific value, giving a lot of heat per gram, and only water is formed.

? 3. ഇന്ധനമായി ഹൈഡ്രജൻ ഉപയോഗിക്കുമ്പോഴുള്ള സാധ്യതയെക്കുറിച്ച് കുറിപ്പ് തയ്യാറാക്കുക. Prepare a note on the possibilities of using hydrogen as a fuel.

Hydrogen – the fuel of the future. Hydrogen has the highest calorific value of all common fuels – 143 kJ/g, about two and a half times that of methane and three times that of petrol. When it burns, the only product is water (2H₂ + O₂ → 2H₂O + heat), so it causes very little pollution and gives out no carbon dioxide. It can be made from water, which is plentiful. Because of these merits, hydrogen could replace fossil fuels, which are running short and pollute the air. But today hydrogen is not used as a household fuel, because it burns explosively and is difficult to store and transport. It is already used in rockets and in fuel cells for buses and cars. If safe and cheap ways to make, store and distribute it are found, hydrogen will become a universal fuel and help solve the problems of fuel shortage and pollution.

⚠️ Common mistakes

Defining calorific value as "the heat given by a fuel" ✗ – it is the heat from unit mass (1 g) burning completely.
Saying hydrogen gives carbon dioxide when it burns ✗ – hydrogen contains no carbon; it gives only water.
Giving "high calorific value" as a limitation ✗ – it is a merit. The limitations are: burns explosively, hard to store and transport.
Mixing up the property used in balloons (less dense than air) with the property used in fuels (burns with high calorific value).

🌐 Explore online needs internet · opens in a new tab

📄 Hydrogen Wikipedia
The most abundant element in the universe – properties and uses.
📄 Henry Cavendish Wikipedia
Discovered hydrogen in 1766 and called it "inflammable air".
📄 Hydrogen economy Wikipedia
Hydrogen as a clean fuel – its advantages and problems.
📄 Heat of combustion Wikipedia
Calorific values of fuels in kJ/g.

5. Check yourself

🧭 Summary

Calorific value കലോറിക മൂല്യം = heat released when unit mass of a fuel burns completely (kJ/g). Heat = mass × calorific value.
Fig 7.2: hydrogen 143, methane 56, petrol (octane) 48, coal (carbon) 33, ethanol 30, methanol 23, carbohydrates 16, carbon monoxide 10 kJ/g. Highest – hydrogen; lowest – carbon monoxide.
Hydrogen burns to give only water.
Merits: only water forms (little pollution); high calorific value. Limitations: burns explosively; difficult to store and transport. So it is not a household fuel yet.
Balloons rise – less dense than air. Fuel – burns with a high calorific value.

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