Hydrogen ICSE Class 8: Preparation, Bosch Process, Properties, Uses, Oxidation and Reduction

What Hydrogen is about

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Chemistry: your school sets and marks the exam. Hydrogen sits in the Theme: Hydrogen theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

Hydrogen from water: electrolysis

Water is a compound of hydrogen and oxygen, so hydrogen can be obtained from it by passing electricity through it.

Three terms to learn:

• Electrolysis: the chemical decomposition of a compound (in the molten state or in solution) by passing an electric current through it.
• Electrolyte: a compound that conducts electricity in the molten state or in solution and is decomposed by the current. Acidified water acts as the electrolyte here.
• Electrodes: the two plates or rods through which current enters and leaves the electrolyte. The one joined to the positive terminal of the battery is the anode; the one joined to the negative terminal is the cathode.

Pure water conducts electricity very poorly, so a few drops of dilute sulphuric acid are added (acidified water). When current passes:

• hydrogen collects at the cathode (negative electrode);
• oxygen collects at the anode (positive electrode);
• the volume of hydrogen is twice the volume of oxygen.

$2H_{2}O\stackrel{\text{electricity}}{\to }2H_2+O_2$

This is a decomposition reaction caused by electricity.

Worked example

In the electrolysis of acidified water, 30 cm³ of gas collects at the cathode. Name the gas and find the volume of gas at the anode.

Try it first, then show the solutionSolution

1. Hydrogen collects at the cathode (negative electrode).
2. Hydrogen and oxygen form in the volume ratio 2 : 1, so oxygen = 30 ÷ 2 = 15 cm³ at the anode.

Answer: Hydrogen; 15 cm³ of oxygen

Laboratory preparation from zinc

Reactants: granulated zinc and dilute hydrochloric acid or dilute sulphuric acid.

$Zn+2HCl\to ZnC{l}_2+H_2$

$Zn+H_{2}S{O}_4\to ZnS{O}_4+H_2$

Apparatus: granulated zinc is placed in a flat-bottomed flask (or Woulfe's bottle) fitted with a thistle funnel and a delivery tube. Dilute acid is poured in through the thistle funnel.

Precautions and reasons:

• The lower end of the thistle funnel must dip below the acid, or the gas escapes through the funnel.
• The apparatus must be airtight, because a mixture of hydrogen and air explodes when lit.
• Hydrogen is collected by downward displacement of water, because it is only sparingly soluble in water. (It could also be collected by downward displacement of air, as it is lighter than air, but over water it is purer and you can see when the jar is full.)
• Before collecting or lighting the gas, let the first bubbles escape: they are a mixture of hydrogen and the air that was in the flask.
• Test for purity: collect the gas in a test tube and bring it to a flame. Pure hydrogen burns quietly with a soft 'pop'; a mixture with air gives a loud pop or squeaky explosion.

Worked example

Why is hydrogen collected over water, and why must the thistle funnel dip into the acid?

Try it first, then show the solutionSolution

1. Hydrogen is only sparingly soluble in water, so very little dissolves and it can be collected by downward displacement of water.
2. If the funnel end is above the acid, hydrogen escapes through the funnel instead of passing through the delivery tube.

Answer: Hydrogen is almost insoluble in water; the dipped funnel stops the gas escaping.

Why zinc, and why not dilute nitric acid

Why granulated zinc is preferred:

• Sodium and potassium react violently (even with water), so the reaction cannot be controlled and is dangerous.
• Calcium reacts too vigorously, and it is costly.
• Magnesium reacts quickly and is costly.
• Iron reacts slowly, and the hydrogen is impure because iron usually contains sulphur and other impurities.
• Lead forms an insoluble coating of lead chloride or lead sulphate on its surface, which stops the reaction.
• Copper is below hydrogen in the reactivity series, so it does not give hydrogen with dilute acids at all.
• Zinc reacts at a steady, moderate rate, gives fairly pure hydrogen and is cheap. Granulated zinc (with a small amount of impurity such as copper) reacts faster than very pure zinc and has a larger surface area.

Choice of acid:

• Dilute hydrochloric acid or dilute sulphuric acid is used.
• Dilute nitric acid is not used: it is an oxidising agent. It oxidises the hydrogen formed to water, and itself is reduced to oxides of nitrogen such as nitric oxide, so hardly any hydrogen is obtained.
• Concentrated sulphuric acid is also avoided: it gives sulphur dioxide instead of hydrogen.

Worked example

Give two reasons why copper and sodium are both unsuitable for preparing hydrogen in the laboratory.

Try it first, then show the solutionSolution

1. Copper lies below hydrogen in the reactivity series, so it does not displace hydrogen from dilute acids.
2. Sodium is so reactive that it reacts violently with dilute acids, causing an explosion risk.

Answer: Copper does not react; sodium reacts violently.

Bosch's process (large-scale hydrogen)

Industry needs hydrogen in huge amounts (for example to make ammonia). Bosch's process makes it cheaply from steam and coke.

Step 1: Making water gas. Steam is passed over red-hot coke at about 1000 °C. The mixture of carbon monoxide and hydrogen formed is called water gas.

$C+H_{2}O\stackrel{1000^{\circ }C}{\to }CO+H_2$

This step is endothermic, so the coke cools and must be reheated from time to time.

Step 2: Converting CO to CO₂. Water gas is mixed with more steam and passed over a catalyst (iron(III) oxide, with chromium(III) oxide as a promoter) at about 450 °C. Carbon monoxide is changed to carbon dioxide, and more hydrogen forms.

$CO+H_2+H_{2}O\to C{O}_2+2H_2$

Step 3: Removing CO₂. The gases are passed through water under pressure (about 30 atm). Carbon dioxide is fairly soluble under pressure and dissolves; hydrogen, which is sparingly soluble, does not.

Step 4: Removing remaining CO. The gas is passed through ammoniacal copper(I) chloride solution, which absorbs carbon monoxide. Hydrogen is left behind.

Worked example

In Bosch's process, how are carbon dioxide and carbon monoxide removed from the hydrogen?

Try it first, then show the solutionSolution

1. Carbon dioxide: the mixture is passed through water under pressure, in which CO₂ dissolves.
2. Carbon monoxide: the gas is passed through ammoniacal copper(I) chloride solution, which absorbs CO.

Answer: CO₂ by water under pressure; CO by ammoniacal copper(I) chloride.

Properties of hydrogen

Physical properties:

• Colourless, odourless and tasteless gas.
• The lightest substance known: about 14 times lighter than air.
• Sparingly soluble in water.
• Neutral to litmus (no effect on moist red or blue litmus).
• Can be liquefied only at very low temperature.

Chemical properties:

• Combustible, not a supporter of combustion: hydrogen burns in air with a pale blue flame, forming water, but a burning splinter pushed into a jar of hydrogen goes out.

$2H_2+O_2\to 2H_{2}O$

• A mixture of hydrogen and air (or oxygen) explodes when lit.
• Reducing agent: hot hydrogen removes oxygen from the oxides of less reactive metals.

$CuO+H_2\to Cu+H_{2}O$ (black copper oxide turns reddish-brown; water droplets form)
$PbO+H_2\to Pb+H_{2}O$
It does not reduce the oxides of very reactive metals such as sodium, calcium, magnesium, aluminium and zinc.

• Combines with non-metals: with chlorine to give hydrogen chloride, $H_2+C{l}_2\to 2HCl$; with nitrogen (iron catalyst) to give ammonia, $N_2+3H_2\rightleftharpoons 2N{H}_3$; with sulphur to give hydrogen sulphide, $H_2+S\to H_{2}S$.
• Combines with very reactive metals: $2Na+H_2\to 2NaH$ (sodium hydride).

Worked example

Dry hydrogen is passed over heated black copper(II) oxide. State two observations and write the equation.

Try it first, then show the solutionSolution

1. The black solid turns reddish-brown (copper forms).
2. Droplets of a colourless liquid (water) collect on the cooler part of the tube.
3. $CuO+H_2\to Cu+H_{2}O$

Answer: Black to reddish-brown; water droplets; $CuO+H_2\to Cu+H_{2}O$

Uses of hydrogen

Each use links to a property:

• Hydrogenation of oils: hydrogen is passed through hot vegetable oil with nickel as a catalyst, turning the liquid oil into a semi-solid fat (vanaspati ghee).
• Making ammonia (Haber's process) for fertilisers: $N_2+3H_2\rightleftharpoons 2N{H}_3$.
• Making hydrochloric acid from hydrogen chloride, and other chemicals such as methanol.
• Fuel: liquid hydrogen is a rocket fuel; hydrogen burns to give only water, so it is a clean fuel.
• Oxy-hydrogen flame (about 2800 °C): for welding and cutting metals.
• Extracting metals: as a reducing agent for some metal oxides.
• Balloons: being the lightest gas, it was once used to fill balloons and airships; today helium is preferred because hydrogen is highly flammable.

Worked example

Name the property of hydrogen used in (i) filling weather balloons (ii) the oxy-hydrogen torch.

Try it first, then show the solutionSolution

1. (i) Hydrogen is the lightest gas, much lighter than air.
2. (ii) Hydrogen burns in oxygen, giving out a large amount of heat.

Answer: (i) Lightness (ii) It is combustible and burns with great heat.

Oxidation and reduction

At Class VIII, oxidation and reduction are defined using oxygen and hydrogen.

	Oxidation	Reduction
Oxygen	addition of oxygen	removal of oxygen
Hydrogen	removal of hydrogen	addition of hydrogen

• An oxidising agent gives oxygen or removes hydrogen. It is itself reduced.
• A reducing agent removes oxygen or gives hydrogen. It is itself oxidised.
• Oxidation and reduction happen together in the same reaction, so such a reaction is called a redox reaction.

Example 1: $CuO+H_2\to Cu+H_{2}O$

• CuO loses oxygen: reduced. H₂ gains oxygen: oxidised.
• Oxidising agent: CuO. Reducing agent: H₂.

Example 2: $H_{2}S+C{l}_2\to 2HCl+S$

• H₂S loses hydrogen: oxidised (to sulphur). Cl₂ gains hydrogen: reduced (to HCl).
• Oxidising agent: Cl₂. Reducing agent: H₂S.

Worked example

In $PbO+H_2\to Pb+H_{2}O$, name the substance oxidised, the substance reduced, the oxidising agent and the reducing agent.

Try it first, then show the solutionSolution

1. PbO loses oxygen, so PbO is reduced; H₂ gains oxygen, so H₂ is oxidised.
2. The oxidising agent is the one that is reduced (PbO); the reducing agent is the one that is oxidised (H₂).

Answer: Oxidised: H₂; reduced: PbO; oxidising agent: PbO; reducing agent: H₂.

Chemistry in real life: Hydrogen

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Preparation of Hydrogen: Electrolysis, Laboratory Method and Bosch's Process
• Properties and Uses of Hydrogen; Oxidation and Reduction

Hydrogen formula sheet

Electrolysis of acidified water

$2H_{2}O\to 2H_2+O_2$

H₂ at the cathode, O₂ at the anode; volume ratio 2 : 1.

Laboratory preparation

$Zn+2HCl\to ZnC{l}_2+H_2$

Or Zn + dilute H₂SO₄; never dilute HNO₃.

Bosch: water gas

$C+H_{2}O\to CO+H_2$

Steam over red-hot coke at about 1000 °C.

Bosch: shift step

$CO+H_2+H_{2}O\to C{O}_2+2H_2$

Iron(III) oxide catalyst, chromium(III) oxide promoter, about 450 °C.

Burning

$2H_2+O_2\to 2H_{2}O$

Pale blue flame; hydrogen and air explode when lit.

Reduction of copper oxide

$CuO+H_2\to Cu+H_{2}O$

CuO reduced, H₂ oxidised.

Hydrogen: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Hydrogen

The question patterns to expect, and the method that works for each.

Tricks to remember Hydrogen

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Cathode catches hydrogen

  Both start with a hard sound: 'Cathode Catches H'. Twice as much, because H₂O has two H for one O.

  Use it when: Which gas forms at which electrode, and the 2 : 1 ratio.

• Oxidation: Oxygen in, hydrogen out

  Oxidation: oxygen in, hydrogen out. Reduction is the opposite: oxygen out, hydrogen in.

  Use it when: Deciding oxidation or reduction.

• Goldilocks zinc

  Sodium too fast, copper too slow (none), lead gets a coat, iron gives dirty gas; zinc is just right.

  Use it when: Reasons for choosing zinc.

• Bosch = Coke + Steam, then more Steam

  Water gas first (1000 °C), then shift with steam over iron oxide (450 °C), then wash out CO₂ with water and CO with copper(I) chloride.

  Use it when: The four steps of Bosch's process in order.

• Agent does the opposite

  The oxidising agent is reduced; the reducing agent is oxidised.

  Use it when: Naming agents correctly.

Hydrogen: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 7-mark question split into 2 + 2 + 3 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Hydrogen

The slips examiners see most often, and what to write instead.

• ✗Mistake: Putting hydrogen at the anode

  Hydrogen collects at the cathode (negative electrode); oxygen at the anode.

  ✓Do this: Cathode Catches H.

  Usually costs: 1 mark, and the diagram labels.

• ✗Mistake: Writing the hydrogen-oxygen volume ratio as 1 : 2

  Water is H₂O: two hydrogen for one oxygen, so hydrogen is double.

  Wrong working

  30 cm³ of oxygen with 15 cm³ of hydrogen

  Right working

  30 cm³ of hydrogen with 15 cm³ of oxygen

  ✓Do this: Hydrogen is always the bigger volume.

  Usually costs: 1-2 marks.

• ✗Mistake: Saying hydrogen supports combustion

  Hydrogen burns itself but puts out a burning splinter pushed into it.

  ✓Do this: Combustible, but not a supporter of combustion.

  Usually costs: 1 mark.

• ✗Mistake: Calling hydrogen the oxidising agent in $CuO+H_2\to Cu+H_{2}O$

  Hydrogen takes oxygen, so it is oxidised and is the reducing agent.

  ✓Do this: The agent is the opposite of what happens to it.

  Usually costs: 1 mark.

• ✗Mistake: Forgetting the second step of Bosch's process

  Water gas is only half hydrogen; the shift with steam over a catalyst gives more hydrogen and turns CO into CO₂.

  Wrong working

  Stopping at water gas, $C+H_{2}O\to CO+H_2$, and calling it pure hydrogen

  Right working

  $CO+H_2+H_{2}O\to C{O}_2+2H_2$

  ✓Do this: Learn both equations with their temperatures.

  Usually costs: 2 marks.

• ✗Mistake: Drawing the thistle funnel above the acid

  The gas would escape through the funnel.

  ✓Do this: Always show the funnel end dipping into the acid.

  Usually costs: 1 mark on the diagram.

Practice questions on Hydrogen

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Which acid is NOT used in the laboratory preparation of hydrogen?
   (a) dilute hydrochloric acid
   (b) dilute sulphuric acid
   (c) dilute nitric acid
   (d) none of these

   Hint

   Which acid is an oxidising agent?

   Answer

   (c)

2. Q2 · 1 mark · Very short answerExam level

   Name the catalyst used in the hydrogenation of vegetable oils.

   Hint

   A transition metal.

   Answer

   Nickel.

3. Q3 · 2 marks · Short answerExam level

   State two physical properties of hydrogen.

   Hint

   Colour, density, solubility.

   Answer

   Any two: colourless and odourless; the lightest gas (about 14 times lighter than air); sparingly soluble in water; neutral to litmus.

4. Q4 · 2 marks · Short answerExam level

   What is water gas? How is it made?

   Hint

   Bosch step 1.

   Answer

   A mixture of carbon monoxide and hydrogen, made by passing steam over red-hot coke at about 1000 °C: $C+H_{2}O\to CO+H_2$.

5. Q5 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): Hydrogen is collected over water.
   Reason (R): Hydrogen is lighter than air.
   (a) (A) is true and (R) is false.
   (b) (A) is false and (R) is true.
   (c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

   Hint

   Collection over water depends on solubility.

   Answer

   (d)

6. Q6 · 2 marks · Short answerHarder exam level

   In an electrolysis of acidified water, 24 cm³ of oxygen is collected. What volume of hydrogen is collected, and at which electrode?

   Hint

   Ratio 2 : 1.

   Answer

   48 cm³ of hydrogen, at the cathode.

7. Q7 · 2 marks · Short answerHarder exam level

   Give two reasons why iron is not used for preparing hydrogen in the laboratory.

   Hint

   Rate and purity.

   Answer

   Iron reacts slowly with dilute acids, and the hydrogen is impure because iron usually contains sulphur and other impurities.

8. Q8 · 3 marks · Short answerHarder exam level

   In $CuO+C\to Cu+CO$, name the substance oxidised, the substance reduced and the reducing agent.

   Hint

   Who gains oxygen?

   Answer

   Carbon is oxidised (gains oxygen); CuO is reduced (loses oxygen); carbon is the reducing agent.

9. Q9 · 3 marks · Short answerHarder exam level

   Write balanced equations for hydrogen reacting with (i) chlorine (ii) nitrogen (iii) sulphur. Name the products.

   Hint

   Non-metal + hydrogen.

   Answer

   (i) $H_2+C{l}_2\to 2HCl$, hydrogen chloride (ii) $N_2+3H_2\rightleftharpoons 2N{H}_3$, ammonia (iii) $H_2+S\to H_{2}S$, hydrogen sulphide.

10. Q10 · 2 marks · Short answerHarder exam level

    9 g of water is completely electrolysed and gives 1 g of hydrogen. (i) Find the mass of oxygen formed. (ii) If 50 cm³ of hydrogen collects in the cathode tube, what volume of oxygen collects in the anode tube at the same time?

    Hint

    Conservation of mass; then the 2 : 1 volume ratio.

    Answer

    (i) 9 − 1 = 8 g of oxygen. (ii) H₂ : O₂ = 2 : 1, so oxygen = 50 ÷ 2 = 25 cm³.

11. Q11 · 2 marks · Short answerHarder exam level

    How much hydrogen can 26 g of zinc give with excess dilute acid, if 65 g of zinc gives 2 g of hydrogen?

    Hint

    26 is two-fifths of 65.

    Answer

    26 × 2 ÷ 65 = 0.8 g.

12. Q12 · 3 marks · Short answerHOTS / competency

    A student uses concentrated nitric acid with zinc and collects a brown gas instead of hydrogen. Explain what went wrong, and suggest a correct pair of reactants.

    Hint

    Nitric acid oxidises.

    Answer

    Nitric acid is an oxidising agent: any hydrogen is oxidised to water and the acid is reduced to oxides of nitrogen (the brown gas is nitrogen dioxide). Use granulated zinc with dilute hydrochloric or dilute sulphuric acid.

13. Q13 · 3 marks · Short answerHOTS / competency

    A jar of gas makes a burning splinter go out, but the gas itself catches fire at the mouth of the jar with a pale blue flame. Identify the gas and explain both observations. What does this tell you about using it in a fire extinguisher?

    Hint

    Combustible but not a supporter.

    Answer

    Hydrogen. It does not support combustion, so the splinter goes out inside; it is combustible, so it burns where it meets air at the mouth. Since it burns, it can never be used to put out fires.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Which electrode collects hydrogen in electrolysis of water?Show answer

   The cathode (negative electrode).

2. 2.Volume ratio of hydrogen to oxygen from water?Show answer

   2 : 1.

3. 3.Which metal is used in the laboratory preparation of hydrogen?Show answer

   Granulated zinc.

4. 4.Why is dilute nitric acid not used?Show answer

   It is an oxidising agent and oxidises the hydrogen to water.

5. 5.What is water gas?Show answer

   A mixture of carbon monoxide and hydrogen.

6. 6.Catalyst in the second step of Bosch's process?Show answer

   Iron(III) oxide (with chromium(III) oxide as promoter).

7. 7.Colour of the hydrogen flame?Show answer

   Pale blue.

8. 8.Catalyst for hydrogenation of oils?Show answer

   Nickel.

9. 9.Oxidation in terms of hydrogen?Show answer

   Removal of hydrogen.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you define electrolyte, electrode and electrolysis?
• ☐Can you draw and label the laboratory set-up for hydrogen?
• ☐Can you give a reason against each of Na, Cu, Pb and Fe?
• ☐Can you write both Bosch equations with temperatures and catalyst?
• ☐Can you describe what happens when hydrogen is passed over hot CuO?
• ☐Can you name the oxidising and reducing agents in any given equation?

Hydrogen: frequently asked questions