Dihydrogen, H2 Molecule

Q: What is Lane's process?

A two-stage commercial method. Steam over iron at 1025 to 1075 K oxidises the metal to Fe₃O₄ and releases dihydrogen. Water gas is then passed over the oxide to bring the iron back. The stages alternate, so the iron is never consumed.

Q: Why is dihydrogen unreactive at room temperature?

Because the H-H bond needs 435.9 kJmol⁻¹ to break, the highest value for a single bond between two identical atoms. Until something supplies that energy, no reaction can start, which is why heat, sunlight, pressure or a catalyst appears in almost every equation.

ChemistryHydrogenFor JEE aspirants

Dihydrogen, $H_{2}$ , is the ordinary form of hydrogen: two atoms held together by the strongest single bond between two like atoms. The preparation of dihydrogen always works the same way, by pushing hydrogen out of water, an acid or an alkali with a suitable metal, or by electrolysing water. This page covers the laboratory and commercial preparation of dihydrogen, its properties and uses, and the hydrogen economy. The topic is in the JEE Advanced syllabus; JEE Main and NEET have dropped it.

On this page1Occurrence2Lab methods3Lane's and Bosch4Properties5Uses6Hydrogen economy7Flowchart8Examples

Key Formulas - Quick Reference

Dihydrogen is $H_{2}$ ; the all-deuterium molecule $D_{2}$ is dideuterium and $HD$ is the mixed molecule.
With cold water: $2Na + 2 H_{2} O$ gives $2NaOH + H_{2}$ . Active metals only.
With steam: $3Fe + 4 H_{2} O$ gives $F e_{3} O_{4} + 4 H_{2}$ at about $1073 K$ .
★ Must learn Laboratory method: $Zn + H_{2} S O_{4}$ (dilute) gives $ZnS O_{4} + H_{2}$ , collected over water.
★ Must learn With alkali (amphoteric metals only): $Zn + 2NaOH$ gives $N a_{2} Zn O_{2} + H_{2}$ .
Electrolysis of acidified water: $2 H_{2} O$ gives $2 H_{2} + O_{2}$ , so the gases come off in the volume ratio $2 : 1$ .
★ Must learn Water gas: $C + H_{2} O$ gives $CO + H_{2}$ at $1270 K$ .
★ Must learn Water gas shift: $CO + H_{2} O$ gives $C O_{2} + H_{2}$ over $F e_{2} O_{3}$ and $C r_{2} O_{3}$ at $770 K$ .
★ Must learn Bond dissociation enthalpy of $H - H$ is $435.9 kJ mol^{- 1}$ , so $H_{2}$ needs heat, light, pressure or a catalyst before it reacts.
Burning one mole of $H_{2}$ releases about $286 kJ$ , more energy per gram than any other fuel.

1. Dihydrogen and Where It Occurs

The word dihydrogen is used for the molecule that contains two hydrogen atoms, whatever isotopes they are. Because hydrogen has three isotopes, three different diatomic molecules and three mixed ones are possible.

Dihydrogen, $H_{2}$ : the general term, and also the name for the ordinary mixture of $H_{2}$ , $D_{2}$ and HD found in nature.
Diprotium: the molecule made of two protium atoms only.
Dideuterium, $D_{2}$ : the molecule made of two deuterium atoms only.

1.1 Occurrence

Hydrogen is the most abundant element in the universe. About half the mass of the sun and the stars is hydrogen, and Jupiter and Saturn are mostly hydrogen. On the earth the free element is rare, found only in small traces in volcanic and natural gases, because the light molecule escapes the atmosphere. In the combined state it is everywhere, above all as water, and also in acids, alkalis, hydrocarbons and every living thing.

2. Preparation of Dihydrogen

Every laboratory method does the same job: a metal gives up electrons, and hydrogen that was combined in water, in an acid or in an alkali is set free as $H_{2}$ .

Figure 1: Four ways to make dihydrogen. Only the metal and the reagent change; the gas released is always the same.

2.1 Action of metals on water

How hot the water has to be depends on how reactive the metal is.

(a) Very active metals in cold water. Sodium, potassium and calcium react at room temperature, giving the hydroxide and dihydrogen.

2Na + 2 H_{2} O 2NaOH + H_{2}

2K + 2 H_{2} O 2KOH + H_{2}

Ca + 2 H_{2} O Ca (OH)_{2} + H_{2}

These reactions are violent and the dihydrogen released often catches fire. In the laboratory the metal is used as an amalgam with mercury, which slows the reaction to a safe rate.

(b) Moderately active metals with boiling water or steam. Magnesium, zinc and aluminium need heat, and they give the oxide rather than the hydroxide.

Mg + H_{2} O heat MgO + H_{2}

Zn + H_{2} O heat ZnO + H_{2}

2Al + 3 H_{2} O heat A l_{2} O_{3} + 3 H_{2}

(c) Less active metals with steam at high temperature. Iron and tin react only with steam passed over the red hot metal. This reaction is the basis of Lane's process in section 3.

3Fe + 4 H_{2} O 1073 K F e_{3} O_{4} + 4 H_{2}

2.2 Action of dilute acids on metals

Any metal placed above hydrogen in the activity series displaces hydrogen from dilute hydrochloric acid or dilute sulphuric acid.

Fe + 2HCl FeC l_{2} + H_{2}

Mg + 2HCl MgC l_{2} + H_{2}

The standard laboratory preparation uses granulated zinc and dilute sulphuric acid in a Woulfe bottle.

Zn + H_{2} S O_{4} ZnS O_{4} + H_{2}

Figure 2: Granulated zinc and dilute sulphuric acid give dihydrogen, which is collected over water because it is almost insoluble.

Exam Trick

Two details are asked again and again. The acid must be dilute, because concentrated $H_{2} S O_{4}$ is an oxidising agent and gives $S O_{2}$ instead of $H_{2}$ . The zinc must be impure (commercial granulated zinc), because the impurities set up tiny electrochemical couples that speed the reaction up.

Where a metal stands in the activity series decides which of these routes it can take:

Figure 3: The position of a metal in the activity series decides which route gives

H_{2}

. Everything left of H reacts with dilute acid; only the amphoteric Al, Zn and Sn also react with alkali.

2.3 Action of strong alkalis on metals

Only amphoteric metals such as zinc, tin and aluminium dissolve in hot sodium or potassium hydroxide, giving a salt of the metal and dihydrogen.

2Al + 2NaOH + 2 H_{2} O 2NaAl O_{2} + 3 H_{2}

Sn + 2NaOH + H_{2} O N a_{2} Sn O_{3} + 2 H_{2}

Zn + 2NaOH Δ N a_{2} Zn O_{2} + H_{2}

The products are sodium meta-aluminate, sodium stannate and sodium zincate. This is the one preparation that needs an alkali rather than an acid, so zinc, tin and aluminium give dihydrogen with either reagent.

2.4 Electrolysis of water

Pure water conducts very poorly, so a little acid or alkali is added. Electrolysis then gives dihydrogen of very high purity.

2 H_{2} O electrolysis 2 H_{2} + O_{2}

The gases appear in the volume ratio of two parts dihydrogen to one part dioxygen, which matches the formula of water.

Figure 4: Acidified water splits into

H_{2}

at the cathode and

O_{2}

at the anode in a

2 : 1

volume ratio, exactly the

H : O

ratio in

H_{2} O

Key idea

Every laboratory route takes hydrogen that is already combined, in water, an acid or an alkali, and sets it free with a metal, or splits water with electricity.

Quick Recall: tap to check

Why is commercial granulated zinc preferred to pure zinc?

Its impurities set up tiny electrochemical couples that speed the reaction up; pure zinc reacts very slowly.

Which metals give

H_{2}

with hot NaOH?

Only amphoteric metals: zinc, aluminium and tin (sodium zincate, meta-aluminate, stannate).

In what volume ratio do

H_{2}

and

O_{2}

form when water is electrolysed?

2 : 1

, the same ratio as in

H_{2} O

3. Commercial Preparation

3.1 Lane's process

Superheated steam is passed over iron filings heated to about $1025$ to $1075 K$ . The iron is oxidised to the magnetic oxide and the water is reduced to dihydrogen. This is the oxidation stage.

3Fe + 4 H_{2} O 1025 to 1075 K F e_{3} O_{4} + 4 H_{2}

When all the iron has been oxidised, the steam is cut off and water gas ( $CO$ and $H_{2}$ ) is passed instead. It strips the oxygen out again and returns the iron to its metallic state. This is the reduction stage.

F e_{3} O_{4} + 2CO + 2 H_{2} 3Fe + 2C O_{2} + 2 H_{2} O

Figure 5: In Lane's process the iron is never used up. Steam takes its oxygen from water, and water gas takes that oxygen away again.

Two or more furnaces are run side by side and the stages are alternated, so that one furnace is always making dihydrogen while another is being reduced back to iron. The iron itself is never used up: it only carries oxygen from the steam to the water gas.

3.2 Bosch process

Most commercial dihydrogen is made this way. It runs in three steps.

(a) Making water gas. Steam is passed over red hot coke at about $1270 K$ . The mixture of carbon monoxide and dihydrogen produced is called water gas.

C + H_{2} O 1270 K CO + H_{2}

Water gas can also be made from hydrocarbons instead of coke, a route called steam reforming, which is how most industrial dihydrogen is made today.

C_{n} H_{2n + 2} + n H_{2} O 1270 K, Ni nCO + (2n + 1) H_{2}

C_{3} H_{8} + 3 H_{2} O 1270 K Ni 3CO + 7 H_{2}

(b) The shift reaction. The water gas is mixed with more steam and passed over $F e_{2} O_{3}$ with $C r_{2} O_{3}$ as promoter at $770 K$ . The carbon monoxide is oxidised to carbon dioxide, and more dihydrogen is produced at the same time. This step is called the water gas shift reaction.

CO + H_{2} O F e_{2} O_{3} / C r_{2} O_{3} 770 K C O_{2} + H_{2}

(c) Separating the dihydrogen. The mixture is bubbled through cold water under pressure. Carbon dioxide dissolves, and dihydrogen, which is almost insoluble, escapes and is collected.

Figure 6: The Bosch process makes water gas first, then turns its carbon monoxide into

C O_{2}

, which is easy to wash out.

Exam Trick

The shift reaction is not there to make more gas, it is there to make the impurity removable. Carbon monoxide is hard to wash out; carbon dioxide dissolves in cold water under pressure. Converting $CO$ to $C O_{2}$ gives extra $H_{2}$ as a bonus.

Lane's processsteam over red hot iron,

1025

1075 K

iron oxidised to

F e_{3} O_{4}

water gas reduces it back
two stages, alternated

Bosch processsteam over red hot coke,

1270 K

gives water gas,

CO + H_{2}

shift:

CO

C O_{2}

770 K

C O_{2}

washed out in water

3.3 Electrolysis of water

Where electricity is cheap, water containing $15$ to $20%$ sodium hydroxide is electrolysed between a nickel-plated iron cathode and a nickel anode. The reactions at the electrodes are:

2 H_{2} O + 2 e^{-} H_{2} + 2O H^{-}

4O H^{-} 2 H_{2} O + O_{2} + 4 e^{-}

The dihydrogen collected at the cathode is more than $99.9%$ pure, which is why this method is used when purity matters more than cost.

Key idea

Industry makes

H_{2}

from steam, over red hot iron (Lane) or coke and hydrocarbons (Bosch), and cleans it by turning

CO

into

C O_{2}

, which dissolves in water.

4. Properties of Dihydrogen

4.1 Physical properties

Colourless, tasteless and odourless gas.
Almost insoluble in water, because the molecule is non-polar. This is why it can be collected over water.
The lightest substance known: its density is about one fourteenth that of air.
Liquefied only at high pressure and very low temperature.
Highly combustible, so it must be handled away from flames.

JEE Advanced

Ortho and para hydrogen. Each nucleus in $H_{2}$ is a proton with a spin. In ortho-hydrogen the two nuclear spins are parallel; in para-hydrogen they are opposed. Only para-hydrogen can occupy the lowest rotational level, so cooling shifts the equilibrium towards it: about $25%$ para at room temperature, $52%$ at $77 K$ and almost $100%$ at $20 K$ . The two forms have the same chemistry but different heat capacity and thermal conductivity. Liquid hydrogen made without a catalyst slowly turns into para-hydrogen, and the heat released boils part of it away, so liquefiers convert it over a catalyst such as iron oxide first.

Figure 7: Equilibrium para-hydrogen, computed from the rotational levels of

H_{2}

99.8%

at its boiling point,

52.1%

77 K

and

25.1%

298 K

4.2 Chemical properties

The $H - H$ bond needs $435.9 kJ mol^{- 1}$ to break, the largest value for any single bond between two identical atoms. Dihydrogen is therefore unreactive at room temperature, and every reaction below needs heat, light, pressure or a catalyst.

Figure 8: Dihydrogen needs heat, light, pressure or a catalyst in every case, because its bond is the strongest single bond between two like atoms.

Neutral character. Dihydrogen has no effect on litmus.
Combustion. It burns in air with a pale blue flame to form water.
$2 H_{2} + O_{2} 2 H_{2} O$
With active metals. With sodium, calcium and lithium it takes an electron and forms ionic hydrides, so here dihydrogen is the oxidising agent and hydrogen takes the $- 1$ state.
$2Na + H_{2} 673 K 2NaH$
$Ca + H_{2} 673 K Ca H_{2}$

With platinum, palladium and nickel the hydrogen atoms simply sit in the gaps of the metal lattice, a process called occlusion. Strong heating drives the gas out again.

With non-metals. The conditions differ sharply from one non-metal to another.

Non-metal	Reaction	Conditions
Carbon	$C + 2 H_{2}$ gives $C H_{4}$	$1273 K$ , methane
Carbon	$2C + H_{2}$ gives $C_{2} H_{2}$	electric arc, $3300 K$ , ethyne
Fluorine	$H_{2} + F_{2}$ gives $2HF$	even in the dark, explosive
Chlorine	$H_{2} + C l_{2}$ gives $2HCl$	sunlight
Oxygen	$2 H_{2} + O_{2}$ gives $2 H_{2} O$	burning or a spark
Nitrogen	$N_{2} + 3 H_{2}$ gives $2N H_{3}$	Fe/Mo, $673 K$ , $200 atm$

With metal oxides. Dihydrogen reduces the oxides of metals that are less active than itself, and is oxidised to water in the process.

CuO + H_{2} Δ Cu + H_{2} O

F e_{3} O_{4} + 4 H_{2} Δ 3Fe + 4 H_{2} O

With carbon monoxide. At about

700 K

and high pressure over a zinc and chromium oxide catalyst, the product is methanol.

CO + 2 H_{2} Zn/ C r_{2} O_{3} 700 K C H_{3} OH

With unsaturated hydrocarbons. Alkenes and alkynes add dihydrogen over finely divided nickel or platinum and become saturated.

C H_{2} = C H_{2} + H_{2} Ni or Pt 473 K C H_{3} C H_{3}

CH \equiv CH + 2 H_{2} Ni 473 K C H_{3} C H_{3}

Hydrogenation of oils. When dihydrogen under pressure is passed through groundnut or cottonseed oil at about

473 K

with finely divided nickel, the liquid oil hardens into the edible solid fat sold as vanaspati ghee. The process is called hardening of oils.

H_{2}

as an oxidising agentwith very active metals: Na, Li, Ca

2Na + H_{2} \to 2NaH

hydrogen goes from

0

- 1

H_{2}

as a reducing agentwith oxides of less active metals

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

hydrogen goes from

0

+ 1

Key idea

The

H - H

bond (

435.9 kJ mol^{- 1}

) is why dihydrogen needs heat, light, pressure or a catalyst; once started, it reduces oxides, makes hydrides and adds to multiple bonds.

Quick Recall: tap to check

What is the water gas shift reaction?

CO + H_{2} O \to C O_{2} + H_{2}

over

F e_{2} O_{3}

with

C r_{2} O_{3}

770 K

Which catalyst hardens vegetable oil into vanaspati?

Finely divided nickel, at about

473 K

2Na + H_{2} \to 2NaH

, is

H_{2}

oxidised or reduced?

Reduced: hydrogen goes from

0

- 1

, so

H_{2}

is the oxidising agent here.

5. Uses of Dihydrogen

Ammonia and fertilisers. The largest use by far: ammonia made from dihydrogen becomes urea, ammonium sulphate and calcium ammonium nitrate.
Hydrogenation of vegetable oils into vanaspati ghee.
Synthetic petrol and methanol manufacture.
Oxy-hydrogen torch for welding, where about $2500^{\circ} C$ is needed.
Atomic hydrogen torch for welding, where about $4000 K$ is needed. Dihydrogen is first split into atoms by an electric arc, and the atoms release that energy when they recombine on the metal surface.
Rocket fuel. Liquid hydrogen with liquid oxygen powers space programmes.

6. The Hydrogen Economy

A hydrogen economy means running transport and industry on dihydrogen instead of petrol, diesel and coal. Hydrogen is not a source of energy: it has to be made first. It is an energy carrier, and its attraction is that the loop closes with water.

Figure 9: Hydrogen is not a source of energy but a carrier. The only product of burning it is water, which goes straight back into the same loop.

6.1 Why hydrogen is attractive as a fuel

It is available in the combined state as water, which covers most of the planet.
Burning it gives only water, so there is no carbon dioxide, no soot and no sulphur dioxide.
The loop from water to fuel and back to water takes days to weeks, while fossil fuels take millions of years to form.
An engine burning hydrogen is about $25$ to $50%$ more efficient than one burning petrol.
Its heat of combustion per gram, $142 kJ g^{- 1}$ , is about three times that of petrol or jet fuel (see the chart below).
Hydrogen-oxygen fuel cells can drive motor vehicles directly, without burning anything.
It is an excellent reducing agent and can replace coal in industrial processes, with far less pollution.

Figure 10: Per gram,

H_{2}

releases

142 kJ

, about

2.6

times methane and three times petrol, because each molecule is so light (

285.8 kJ mo l^{- 1}

from only

2.016 g

6.2 The obstacles

Availability. Free hydrogen does not occur in nature, so cheap production is the first requirement. Water is the likely source, ideally split using solar energy at the site where the fuel is needed.
Storage and transport. Hydrogen is explosively flammable. It is kept in vacuum insulated cryogenic tanks, moved by road and rail tankers or pipelines, and, more safely, absorbed in alloys such as Fe-Ti, $NaN i_{5}$ , Mg- $Mg H_{2}$ and Ti- $Ti H_{2}$ that soak it up like a sponge and release it on demand.
Platinum scarcity. Hydrogen-oxygen fuel cells need platinum as the catalyst, and demand already outruns supply.

6.3 Liquid hydrogen as rocket fuel

Liquid hydrogen is already in routine use. Burning one mole of dihydrogen releases about $286 kJ$ :

2 H_{2} + O_{2} 2 H_{2} O

In a space shuttle the fuel tank holds about $1.5 \times 1 0^{6} L$ of liquid hydrogen and the oxygen tank about $5.4 \times 1 0^{5} L$ of liquid oxygen. Together they run the main engine for roughly $8.5$ minutes during lift-off, consuming liquid hydrogen at nearly $3000 L$ per second.

7. Choosing a Route: Flowchart and Mind Map

Questions on the preparation of dihydrogen usually ask which reagent a given metal needs. Three questions settle it, in the order shown below; the mind map after it puts the whole page on one screen.

Figure 11: Flowchart: three questions pick the reagent. Concentrated oxidising acids never belong on this chart, because they give

S O_{2}

NO

instead of

H_{2}

Figure 12: Mind map: the whole page on one screen. Making

H_{2}

always means taking hydrogen out of water, an acid or an alkali; using it always needs activation first.

8. Solved Examples

Solved Example 1

Why are concentrated

H_{2} S O_{4}

and pure zinc not used in the preparation of dihydrogen?

Solution:

The acid. Concentrated sulphuric acid is not only an acid, it is also an oxidising agent. It is itself reduced during the reaction, so the gas that comes off is $S O_{2}$ and not $H_{2}$ .

Zn + 2 H_{2} S O_{4} ZnS O_{4} + 2 H_{2} O + S O_{2}

The zinc. Pure zinc is non-porous, so the acid attacks only the outer surface and the reaction is very slow. The impurities in commercial zinc set up tiny electrochemical couples on the surface, which makes the metal porous and speeds the reaction up.

Answer: dilute acid keeps the product $H_{2}$ , and impure zinc keeps the rate usable.

Solved Example 2

The metal which gives hydrogen on treatment with an acid as well as with sodium hydroxide is
(A) Fe
(B) Zn
(C) Cu
(D) none of these

Solution:

Answer: (B). Zinc is amphoteric, so it reacts both ways.

Zn + H_{2} S O_{4} ZnS O_{4} + H_{2}

Zn + 2NaOH Δ N a_{2} Zn O_{2} + H_{2}

Iron reacts with acids but not with alkali, and copper lies below hydrogen in the activity series, so it reacts with neither.

Solved Example 3

Activated hydrogen is obtained by
(A) electrolysis of heavy water
(B) reaction of water with heavy metals
(C) thermal decomposition of water
(D) passing a silent electric discharge through hydrogen at low pressure

Solution:

Answer: (D). A silent electric discharge at low pressure splits the molecule into free atoms, called atomic or activated hydrogen.

H_{2} electric discharge low pressure 2H

These atoms recombine within a fraction of a second, releasing the bond energy as heat. That is exactly how the atomic hydrogen torch reaches about $4000 K$ .

Solved Example 4

6.5 g

of zinc is treated with excess dilute sulphuric acid. What volume of dihydrogen is obtained at STP? (Zn

= 65

)

Solution:

One mole of zinc gives one mole of dihydrogen:

Zn + H_{2} S O_{4} ZnS O_{4} + H_{2}

n_{Zn} = \frac{6.5}{65} = 0.1 mol \Rightarrow n_{H_{2}} = 0.1 mol

V = 0.1 \times 22.4 = 2.24 L at STP

Answer: $2.24 L$ . The acid is in excess, so zinc is the limiting reagent.

Solved Example 5

Dihydrogen burns readily in air, yet it is described as unreactive at room temperature. Explain.

Solution:

Reactivity and the energy released are two different things. Burning dihydrogen gives out a lot of energy, but before any of it is released the $H - H$ bond must be broken, and that costs $435.9 kJ mol^{- 1}$ , the highest value for any single bond between two like atoms.

Answer: the reaction is strongly favourable but slow to start. A flame, a spark, sunlight or a catalyst supplies the energy for the first bond to break, after which the heat released keeps the reaction going.

Solved Example 6

Write the two stages of Lane's process and explain why the iron is not used up.

Solution:

Oxidation stage: superheated steam over iron at $1025$ to $1075 K$ .

3Fe + 4 H_{2} O F e_{3} O_{4} + 4 H_{2}

Reduction stage: the steam is stopped and water gas is passed over the oxide.

F e_{3} O_{4} + 2CO + 2 H_{2} 3Fe + 2C O_{2} + 2 H_{2} O

Answer: the iron ends the second stage in exactly the state it started the first. It only carries oxygen from the steam across to the water gas, so the same charge of iron is used over and over and the two stages are simply alternated.

Solved Example 7

In the Bosch process, why is the water gas treated with steam over

F e_{2} O_{3}

before the dihydrogen is separated?

Solution:

Water gas is a mixture of $CO$ and $H_{2}$ , and carbon monoxide is very hard to remove from dihydrogen: it is almost as insoluble in water as $H_{2}$ itself.

CO + H_{2} O F e_{2} O_{3} / C r_{2} O_{3} 770 K C O_{2} + H_{2}

Answer: the shift reaction turns the awkward impurity into $C O_{2}$ , which dissolves in cold water under pressure and washes straight out. As a bonus, every molecule of $CO$ removed produces one more molecule of $H_{2}$ .

Solved Example 8

Calculate the heat released when

1.0 kg

of dihydrogen burns completely, and compare it with

1.0 kg

of methane. (

Δ_{c} H

H_{2}

- 285.8

C H_{4}

- 890.3 kJ mol^{- 1}

)

Solution:

Moles in one kilogram: $1000/2.016 = 496.0$ for $H_{2}$ and $1000/16.04 = 62.34$ for $C H_{4}$ .

q_{H_{2}} = 496.0 \times 285.8 = 1.418 \times 1 0^{5} kJ

q_{C H_{4}} = 62.34 \times 890.3 = 5.550 \times 1 0^{4} kJ

Answer: about $1.42 \times 1 0^{5} kJ$ against $5.55 \times 1 0^{4} kJ$ , so dihydrogen gives about $2.6$ times as much heat per kilogram.

Solved Example 9

Which pair does not give dihydrogen?
(A) Zn and dilute

H_{2} S O_{4}

(B) Cu and dilute HCl
(C) Al and aqueous NaOH
(D) Na and water

Solution:

Answer: (B). Copper lies below hydrogen in the activity series, so it cannot displace hydrogen from an acid. Zinc displaces it from dilute acid, aluminium is amphoteric and dissolves in alkali, and sodium reacts with cold water.

Practice Questions

Hydrogen has three isotopes. The number of possible diatomic molecules is
(A) 3 (B) 6 (C) 9 (D) 12Answer: (B) 6. Three identical pairs ( $H_{2}$ , $D_{2}$ , $T_{2}$ ) and three mixed pairs (HD, HT, DT).
During the electrolysis of acidified water, which gas is collected at the cathode and in what volume ratio to the gas at the anode?Answer: Dihydrogen at the cathode, dioxygen at the anode, in the ratio $2 : 1$ by volume.
Name the process in which steam and water gas are passed alternately over red hot iron.Answer: Lane's process.
Why is sodium amalgam, rather than sodium metal, used when dihydrogen is to be made from water in the laboratory?Answer: Sodium reacts so violently that the dihydrogen catches fire. The amalgam releases the sodium slowly, so the reaction stays safe.
$0.1 mol$ of aluminium dissolves completely in sodium hydroxide solution. How many moles of dihydrogen are formed?Answer: $0.15 mol$ . From $2Al + 2NaOH + 2 H_{2} O$ giving $2NaAl O_{2} + 3 H_{2}$ , two moles of Al give three moles of $H_{2}$ .
Write the balanced equation for steam reforming of methane.Answer: $C H_{4} + H_{2} O \to CO + 3 H_{2}$ , at about $1270 K$ over nickel.
Why is hydrogen called an energy carrier rather than a source of energy?Answer: Free hydrogen does not occur on earth. It must first be made, for example by electrolysing water, which uses energy; burning it only gives that energy back.

Common Mistakes to Avoid

Watch out

Using concentrated $H_{2} S O_{4}$ in the laboratory preparation. It oxidises the zinc and gives $S O_{2}$ ; only the dilute acid gives $H_{2}$ .
Assuming every metal liberates hydrogen from a dilute acid. Only metals above hydrogen in the activity series do; copper, silver and gold do not.
Confusing water gas ( $CO$ + $H_{2}$ ) with producer gas ( $CO$ + $N_{2}$ ). Water gas comes from steam over coke, producer gas from air over coke.
Writing that hydrogen is reduced in every reaction. With active metals it is the oxidising agent and becomes $H^{-}$ ; with metal oxides it is the reducing agent.
Forgetting the conditions. Hydrogenation needs nickel at about $473 K$ , ammonia needs Fe/Mo at $673 K$ and $200 atm$ , and methanol needs a zinc-chromium oxide catalyst.
Writing the reduction of copper oxide as $CuO + H_{2}$ giving CuS. The products are copper and water.
Calling hydrogen a source of energy. It has to be manufactured first, so it is an energy carrier, like electricity.
Saying dihydrogen is reactive because it burns. The $H - H$ bond enthalpy is $435.9 kJ mol^{- 1}$ , so it is unreactive until it is given heat, light or a catalyst.

Frequently Asked Questions

How is dihydrogen prepared in the laboratory?

Granulated zinc is treated with dilute sulphuric acid in a Woulfe bottle fitted with a thistle funnel. The gas is led through a delivery tube to a beehive shelf and collected over water in an inverted jar, because $H_{2}$ is almost insoluble in water.

Why is dilute and not concentrated sulphuric acid used with zinc?

Concentrated sulphuric acid acts as an oxidising agent as well as an acid. It is reduced during the reaction, so sulphur dioxide is released instead of dihydrogen. Only the dilute acid simply exchanges its hydrogen for the metal and gives $H_{2}$ .

What is water gas and how is it made?

Water gas is an equal mixture of carbon monoxide and dihydrogen, made by passing steam over red hot coke at about $1270 K$ . It is the starting point of the Bosch process and is also used as a reducing agent in the second stage of Lane's process.

What is Lane's process?

A two-stage commercial method. Steam over iron at $1025$ to $1075 K$ oxidises the metal to $F e_{3} O_{4}$ and releases dihydrogen. Water gas is then passed over the oxide to bring the iron back. The stages alternate, so the iron is never consumed.

Why is dihydrogen unreactive at room temperature?

Because the $H - H$ bond needs $435.9 kJ mol^{- 1}$ to break, the highest value for a single bond between two identical atoms. Until something supplies that energy, no reaction can start, which is why heat, sunlight, pressure or a catalyst appears in almost every equation.

What is the hydrogen economy?

It is the idea of using dihydrogen as the main fuel for transport and industry. Water is split by electrolysis, the gas is stored and transported, and burning it or using it in a fuel cell gives back only water. Hydrogen carries energy rather than supplying it.

Is the preparation of dihydrogen asked in NEET?

Not as a topic. NMC removed the Hydrogen chapter from the NEET syllabus, so Lane's and the Bosch process are not set. The reactions still help: zinc with dilute acid, metals with water and hydrogenation over nickel come up again in redox reactions and hydrocarbons, which NEET does test.

Is dihydrogen in the JEE syllabus?

Only in JEE Advanced. NTA dropped the Hydrogen chapter from JEE Main, but the JEE Advanced syllabus lists the preparation, properties and uses of hydrogen and hydrogen as a fuel, so the methods, conditions and equations on this page can all be asked there.

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