To dissociate a molecule of hydrogen into two constituent atoms, one needs to apply energy that is greater or equal to the dissociation energy. It is a type of energy that holds the bond together, and similar or greater energy needs to be supplied to separate the atoms. For hydrogen, the dissociation energy is 104 kcal/mole.
Reactivity towards hydrogen in its atomic form is highly reactive. It reacts with several elements to form its hybrids. For example, potassium hydride, sodium hydride, etc. It also reacts to reduce metal oxides, which produces metal in this element stage. Metallic surfaces that do not have real behaviour with hydrogen to produce stable hydrides catalyze the recombination reaction of the hydrogen atoms to form molecules of hydrogen. They are thereby heated to illumination by the energy that is released during this reaction.
The reactivity of hydrogen changes with temperature. Hydrogen molecules react with numerous compounds and elements. At room temperature, the rate of reactivity is slow and negligible, but at high temperatures, the rate of reaction is extremely elevated. This is due to the heightened dissociation energy present between the atoms.
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Hydrogen Reaction With Metals
Most of the metals don't react with molecular hydrogen as both metals and hydrogen form compounds by giving up electrons. Therefore, atomic hydrogen rejects electrons offered by metallic atoms to form a compound. However, a few active metals like calcium, sodium, potassium, and magnesium react with hydrogen atoms by forcing them to accept the electrons and form ionic salt-like compounds known as metal hydrides. An example of reaction of metal with hydrogen is given below.
Gaseous hydrogen is passed over heated sodium to form sodium hydride.
2Na (s) + H2(g) → 2NaH (s).
The Reaction of Potassium Metal With Water
Potassium metal reacts with water rapidly, very rapidly to give a basic colourless solution of potassium hydroxide or KOH along with hydrogen in a gaseous state. The reaction goes on even after the solution is turned basic. The end product is a basic solution of the dissolved hydroxide in the solution. The reaction of potassium metal with water is exothermic, which means it produces heat during or after the reaction. As potassium metal reacts with water, it gets so heated that it catches fire and burns in a characteristic shade of pale lilac colour. Hydrogen gas is evolved as one of the products during the reaction.
2K (s) + 2H2O → 2KOH (aq) + H2(g)
This reaction is slower than the reaction of rubidium with water. However, it is faster than the reaction of sodium with water.
Reaction of Sodium Metal With Water
Sodium metal reacts with water instantly to form a basic colourless solution of sodium hydroxide or NaOH and hydrogen gas or H2(g). The reaction doesn't stop even after the solution turns basic. The dissolved hydroxide is responsible for turning the resulting solution basic. The reaction of potassium metal with water is exothermic, which means it produces heat during or after the reaction. As sodium metal reacts with water, it gets so heated that it catches fire and burns in a characteristic shade of orange colour.
2Na (s) + 2H2O → 2NaOH (aw) + H2 (g)
Reactivity of Hydrogen Halides
Hydrogen halides undergo an electrophilic reaction with alkenes to produce alkyl halides. This reaction is called electrophilic because the reaction is initiated due to the accumulation of an electrophilic proton. Furthermore, the addition of hydrogen halides is very regiospecific. This means the hydrogen halide only particularly reacts with the pi bond, and it can only give one of the two specific products. The result of this phenomenon is summarized by Markovinkov's rule. All the hydrogen halides, including HBr, HCl, HI, HF, can take part in the reaction. All the halides have different rates of reactions because the H-X bond becomes weak as the size of X increases. The rate of reactivity of the halides in increasing order is: HI > HBr > HCl > HF
Reactivity of Hydrogen Atoms Attached to Carbon
The reactivity of hydrogen atoms attached to atoms of carbon present in alkanes differs according to the number of carbon atoms attached to one carbon atom.
The rate of reactivity of hydrogen atoms attached to carbon in the increasing order is tertiary carbon > secondary carbon > primary carbon. The differences in the reactivity degree are due to the differences in the C-H bond dissociation. Naturally, the weaker bonds can be more easily broken than the stronger bonds.
Reaction of Alkali Metals With Hydrogen
Alkali metals are the name given to the elements present in the Group 1 of the modern-day periodic table. Following metals are included under the alkali metal: Sodium, Francium, Lithium, Potassium, Cesium, Rubidium. Hydrogen is often associated with Group 1 metals, but it rarely exhibits any similar behaviour as the rest of the metals. Alkali metals are highly reactive, and they react with dry hydrogen at extremely high temperatures to form their corresponding hydrides. The hydrogen is reduced to give the hydride H- ion.