Ch 9Hydrogen — Class 11 Chemistry, concept-first.

Hydrogen sits at the very start of the periodic table, but where it truly belongs has long been debated.

The elemental form of hydrogen, dihydrogen (), is the starting point for almost all hydrogen chemistry.

Hydrogen is the most abundant element in the universe, making up roughly 70% of its total mass, and it dominates the composition of the Sun and giant planets such as Jupiter and Saturn.

Hydrogen has three naturally occurring isotopes, differing only in the number of neutrons in the nucleus:

A number of methods exist for generating dihydrogen gas, ranging from simple laboratory reactions of metals with acids or alkalis to large-scale industrial routes built on electrolysis and hydrocarbon…

Dihydrogen is conveniently generated in the lab in two simple ways:

Several large-scale processes supply dihydrogen commercially.

Dihydrogen's behaviour as an element follows both from its physical character as a gas and from the chemistry that stems from its very strong H–H bond.

Dihydrogen is a colourless, odourless and tasteless gas that is combustible. It is lighter than air and only sparingly soluble in water.

The chemical behaviour of dihydrogen is dominated by the strength of its H–H bond, which has the highest bond dissociation enthalpy of any single bond between two atoms of any element.

- Ammonia synthesis is the single largest use of dihydrogen, feeding into the manufacture of nitric acid and nitrogenous fertilisers.

Under suitable conditions, dihydrogen combines with almost every element except the noble gases to form binary compounds called hydrides. For an element E, these are written (e.g. ) or (e.g. ).

Ionic hydrides form with most of the highly electropositive s-block elements and are stoichiometric compounds.

Dihydrogen forms molecular compounds with most p-block elements — familiar examples include , , and (hydrogen compounds of non-metals are conventionally grouped here as "hydrides" too).

Many d-block and f-block elements form this third class of hydride, though notably Groups 7, 8 and 9 do not, and from Group 6 only chromium forms a hydride ().

Water makes up a major share of all living matter — roughly 65% of the human body, and as much as 95% in some plants — making it essential to life. It is also an outstanding solvent.

Pure water is a colourless and tasteless liquid. Its physical constants, alongside those of heavy water (), are compared below.

In the gas phase, a single water molecule is bent, not linear, with a bond angle of 104.5° and an O–H bond length of 95.7 pm (Fig 9.1a).

X-ray studies of ice crystals reveal a highly ordered, three-dimensional, hydrogen-bonded lattice (Fig 9.2).

Water participates in a wide range of chemical reactions. The main categories are outlined below.

Rainwater is close to pure, carrying only traces of dissolved atmospheric gases. But as it flows over rock and soil, water's strong solvent power lets it dissolve a variety of salts.

Temporary hardness is caused specifically by dissolved magnesium and calcium hydrogencarbonates. It can be removed in two ways.

Permanent hardness comes from dissolved calcium and magnesium chlorides and sulphates — and unlike temporary hardness, boiling has no effect on it.

Hydrogen peroxide, , is an important industrial oxidant, valued in particular for treating domestic and industrial effluents as part of pollution control.

1. From barium peroxide. Acidifying barium peroxide octahydrate with sulphuric acid, then removing excess water by evaporation under reduced pressure, gives hydrogen peroxide:

In its pure state, is an almost colourless liquid with a very faint pale-blue tint. Its key physical constants are given below.

Hydrogen peroxide is a non-planar molecule. It is often pictured as an open book: two H–O–O planes hinged along the central O–O bond, at some dihedral (book-opening) angle to each other.

Hydrogen peroxide is unusual in that it can act as either an oxidising agent or a reducing agent, in both acidic and basic media, because the oxygen in sits at an intermediate oxidation state (−1) bet…

decomposes slowly on exposure to light, disproportionating into water and oxygen:

- Everyday use — as a hair bleach and a mild disinfectant; sold as an antiseptic under the trade name perhydrol.

Heavy water, , has two principal uses: as a moderator in nuclear reactors (slowing fast neutrons without absorbing them the way ordinary water would) and in exchange reactions used to study reaction m…

Burning dihydrogen releases a large quantity of heat, and comparing fuels on a mole, mass and volume basis (Table 9.5) shows just how energy-dense it is.