Study of the First Element - Hydrogen

The One Thing

Hydrogen is the lightest element and a highly combustible, reactive gas whose preparation, identification and uses depend on its physical properties and chemical behaviour. It is commonly prepared by metal–acid reactions or electrolysis, and it can act as either an oxidising or reducing agent in different reactions.

Reactions, Processes and Experiments

Key Terms

• Hydrogen: The lightest element, represented by H, with atomic number 1 and relative atomic mass approximately 1.
• Diatomic molecule: A molecule made of two atoms; hydrogen exists naturally as H₂.
• Laboratory preparation: Preparation of hydrogen by reacting granulated zinc with dilute sulphuric acid or dilute hydrochloric acid.
• Displacement reaction: A reaction in which a more reactive metal displaces hydrogen from a dilute acid.
• Reactivity series: An arrangement of metals according to their reactivity; metals above hydrogen can generally displace it from dilute acids.
• Electrolysis: The decomposition of a compound using electricity; water produces hydrogen at the cathode and oxygen at the anode.
• Reduction: Removal of oxygen or addition of hydrogen to a substance; hydrogen can act as a reducing agent.
• Combustion: Burning in oxygen; hydrogen burns with a pale blue flame and forms water.
• Pop test: A test for hydrogen in which the gas burns with a characteristic ‘pop’ sound when a lighted splint is brought near it.
• Diffusion: The spreading of gas particles from a region of higher concentration to lower concentration; hydrogen diffuses very rapidly because it is extremely light.
• Nascent hydrogen: Freshly produced hydrogen considered highly reactive in certain chemical reactions; it is represented as [H].
• Hydrogenation: The addition of hydrogen to unsaturated compounds, often in the presence of a catalyst such as nickel.
• Atomic number: The number of protons in an atom; hydrogen has atomic number 1.
• Electronic configuration: The arrangement of electrons in an atom; hydrogen has the electronic configuration 1.
• Valency: The combining capacity of an element; hydrogen has valency 1.
• Relative atomic mass: The average mass of an atom relative to the defined atomic-mass standard; hydrogen has a relative atomic mass of approximately 1.
• Oxidising agent: A substance that causes oxidation in another substance; hydrogen can act as an oxidising agent in some reactions.
• Reducing agent: A substance that causes reduction in another substance; hydrogen reduces certain metal oxides.
• Nascent hydrogen notation: Freshly produced hydrogen is represented as [H].
• Henry Cavendish: The scientist commonly associated with identifying hydrogen as a distinct gas in 1766.
• Antoine Lavoisier: The scientist who later named hydrogen, meaning “water former”.

Easily Confused

• Hydrogen collected over water vs hydrogen diffusion: Collection over water depends on hydrogen’s near insolubility and lack of reaction with water; diffusion refers to the rapid spreading of its very light particles.
• Hydrogen–air mixture vs hydrogen–oxygen mixture: Both may explode on ignition, but the stated hydrogen–oxygen volume ratio is 2:1.
• Combustion vs reduction: Combustion is hydrogen burning in oxygen to form water; reduction is the removal of oxygen from substances such as copper(II) oxide.
• Laboratory preparation vs electrolysis: Laboratory preparation uses a metal and dilute acid, whereas electrolysis decomposes water using electricity.
• Cathode vs anode in electrolysis: Hydrogen is collected at the cathode and oxygen at the anode.
• Hydrogen as a fuel vs hydrogen as a reducing agent: As a fuel, hydrogen burns to form water; as a reducing agent, it removes oxygen from certain metal oxides.
• Hydrogen’s point-of-use cleanliness vs overall environmental benefit: Hydrogen combustion produces water, but its overall environmental benefit depends on how the hydrogen is produced.
• Hydrogen’s use with vegetable oils vs rocket fuel: Hydrogenation adds hydrogen to unsaturated vegetable oils, whereas liquid hydrogen is used as rocket fuel, generally with liquid oxygen.

What Gets Asked

• Writing laboratory preparation equations: Questions may require either Zn + H₂SO₄(dilute) → ZnSO₄ + H₂↑ or Zn + 2HCl(dilute) → ZnCl₂ + H₂↑. A common error is omitting the dilute acid condition or failing to balance the equation.
• Identifying hydrogen from an observation: The correct test is the characteristic ‘pop’ when a lighted splint is brought near the gas. Ignoring the need to expel air can lead to an unsafe hydrogen–air explosion.
• Explaining collection over water: The explanation must refer to hydrogen being nearly insoluble in water and not reacting with it under ordinary conditions.
• Interpreting electrolysis of water: Students may be asked for the equation, electrode products and volume ratio: hydrogen at the cathode, oxygen at the anode, in a 2:1 ratio. Reversing the electrodes or the ratio loses marks.
• Describing hydrogen’s reactions: Questions may require the equations for combustion, reduction of copper(II) oxide, reaction with chlorine, or ammonia formation. The specific equations must be distinguished rather than treated as one general reaction.
• Explaining uses and properties: Answers may connect hydrogen’s low density, rapid diffusion, combustibility, reducing properties and hydrogenation reactions with uses including fuel cells, ammonia manufacture, vegetable-oil hydrogenation, hydrochloric-acid production, metal-oxide reduction, industrial cooling and rocket fuel.